# Correlative infrared optical coherence tomography and hyperspectral chemical
imaging

Ivan Zorin  Research Center for Non-Destructive Testing GmbH, Science Park 2,
Altenberger Str.69, 4040 Linz, Austria  Corresponding author:
<EMAIL_ADDRESS>Rong Su  Manufacturing Metrology Team, Faculty of
Engineering, University of Nottingham, Nottingham NG8 1BB, UK  Bettina Heise
Research Center for Non-Destructive Testing GmbH, Science Park 2, Altenberger
Str.69, 4040 Linz, Austria  Bernhard Lendl  Institute for Chemical
Technologies and Analytics, TU Wien, Getreidemarkt 9, 1060 Vienna, Austria
Markus Brandstetter  Research Center for Non-Destructive Testing GmbH, Science
Park 2, Altenberger Str.69, 4040 Linz, Austria

###### Abstract

Optical coherence tomography (OCT) is a high-resolution three-dimensional
imaging technique that enables non-destructive measurements of surface and
subsurface microstructures. Recent developments of OCT operating in the mid-
infrared (MIR) range (around 4 µm) lifted fundamental scattering limitations
and initiated applied material research in formerly inaccessible fields. The
MIR spectral region, however, is also of great interest for spectroscopy and
hyperspectral imaging, which allow highly selective and sensitive chemical
studies of materials. In this contribution, we introduce an OCT system (dual-
band, central wavelengths of 2 µm and 4 µm) combined with MIR spectroscopy
that is implemented as a raster scanning chemical imaging modality. The fully-
integrated and cost-effective optical instrument is based on a single
supercontinuum laser source (emission spectrum spanning from 1.1 µm to 4.4
µm). Capabilities of the in-situ correlative measurements are experimentally
demonstrated by obtaining complex multidimensional material data, comprising
morphological and chemical information, from a multi-layered composite
ceramic-polymer specimen.

_K_ eywords mid-infrared • optical coherence tomography • spectroscopy •
supercontinuum source • hyperspectral imaging • multimodal system •
correlative imaging • non-destructive testing

## 1\. Introduction

Optical coherence tomography (OCT) is a well-established non-invasive
measurement technique that allows to obtain and visualize three-dimensional
structural information of morphologically complex samples. Operating in the
visible and near-infrared (NIR) ranges, OCT-based diagnostic methods and
systems have demonstrated outstanding efficiency and capabilities in various
biomedical scenarios and ophthalmology [1]. Nevertheless, the focus of
interest within the OCT community is consequentially extending, covering non-
destructive testing (NDT) applications in industrial metrology and material
sciences [2, 3]. Significant advances in OCT technology have been enabled by
the rapid evolution of bright and spatially coherent mid-infrared (MIR)
supercontinuum light sources [4] that allowed to extend the operating
wavelength range and reduce scattering effects [5]. Thus, recently reported
results on the expansion of OCT into the MIR [6, 7] spectral region verified a
hypothesis of an enhanced penetration depth into turbid materials. These
solutions opened up opportunities for sub-surface dimensional measurements in
the fields of, for instance, additive manufacturing [8], pharmaceutical
industry [9, 10], cultural heritage diagnosis and art preservation [11, 12,
13]. In general, a further increase in the central wavelength causes reduction
of the axial resolution that is correlated with the coherence length and,
thus, proportional to the ratio of squared wavelength and spectral bandwidth.
However, a rational extension of the bandwidth, which shortens the coherence
length, is also limited in practice since this may cause a degradation of the
point-spread-function due to dispersion [14]. Therefore, in contrast to the
terahertz range [15], the 3 µm - 7 µm part of the MIR spectral region
represents a reasonable compromise, in particular, for industrial applications
where high spatial resolution is a necessity.

Since their recent commercialization, MIR supercontinuum sources have been
significantly optimized in terms of their noise performance, thereby
attracting significant attention within the infrared (IR) spectroscopy
community. Due to the broadband emission and peculiar properties of the
radiation, these sources have already launched intensive applied research in
the field. Through the particular advantages over conventional thermal
emitters, their efficiency was demonstrated in attenuated total reflection
spectroscopy [16], Fourier-transform infrared (FTIR) spectroscopy [17], MIR
microspectroscopy[18, 19, 20, 21, 22], gas [23], environmental [24], and
stand-off spectroscopic measurements [25, 26]. Operating in the so-called
functional group region, where most of the molecules display distinct
absorption lines, MIR supercontinuum sources allow to considerably extend the
light-matter interaction length, thereby lowering detection limits compared to
state-of-the-art FTIR instruments [27, 17]. Due to the prevailing brightness
[28] and high beam quality, supercontinuum radiation enables diffraction-
limited and sensitive chemical mapping realized in reflection geometry [20,
18], thus, representing a novel and attractive tool for chemical analysis.
Moreover, studies of noise properties, emission parameters and extension of
the broadband spectral coverage, which is far superior to quantum cascade
lasers, are the subjects of ongoing research [29, 30, 31, 32, 33, 34, 35, 36].
Therefore, stable sources covering the entire MIR spectral range are expected
to become commercially available soon.

The possible combination of these two NDT techniques – OCT and MIR
spectroscopy – in one system is challenging and of particular interest.
Spectroscopic-OCT, for instance, is a well-known and advantageous solution,
which allows getting depth-resolved spectral information [37, 38]. The method
employs a single detector array and it is based on numerical splitting of the
bandwidth in a spectral-domain OCT (SD-OCT) configuration [14] into several
sub-bands before performing the Fourier transform of the individual
interferograms. This approach is feasible, however, impractical for a system
that operates in the MIR range. One of the reasons is the absence of sensitive
high-resolution detector arrays (focal plane arrays) – the crucial component
of spectroscopic- and SD-OCT – operating in this spectral range; this
technical gap is considered in detail in [6]. However, the fundamental problem
is an insufficient spectral and spatial resolution due to the contradictory
requirements in the spectroscopic-OCT system: a high spectral resolution
requires a narrow spectral bandwidth but confines the axial resolution of SD-
OCT. For example, in order to resolve CH (carbon hydrogen functional chemical
group) stretching vibrations at around 3320 nm (center wavelength,
corresponding to 3010 cm-1 center wavenumber), a spectral resolution of 50 nm
of the spectroscopic mode (i.e. OCT sub-band after numerical splitting) would
result in a spatial resolution of around 100 µm in the axial direction for the
regular OCT imaging mode. For state-of-the-art OCT systems with low-NA
(numerical aperture) focusing optics, such a resolution is inexpedient, since
a maximum imaging depth, which is determined by the confocal gate, is on
millimiter scale [14, 39].

In this paper, we propose a reasonable and new solution for the combination of
these techniques that allows to achieve both high morphological and chemical
resolution. Since the raster scanning approach is identical for OCT and
spectroscopic imaging, we implement a multimodal system integrated in a single
measurement head. In contrast to Spectroscopic-OCT, the modalities use
separate detection arms, therefore, the resolution matching condition
described above is eliminated. The SD-OCT system uses a single supercontinuum
laser source and operates in dual-band mode (reported in [8]), i.e. within the
sub-bands of 1.85 µm - 2.1 µm (NIROCT) and 3.7 µm - 4.2 µm (MIROCT). It
enables high-resolution imaging in the NIR regime and penetration-enhanced
measurements in the MIR band. The MIR spectroscopy modality supplements the
morphological information obtained by OCT with correlative spatially-resolved
spectra that are integrated over depth. The operational window of the
spectroscopic system is 3.1 µm - 4.4 µm (corresponds to 2275 cm-1 \- 3225
cm-1). Being presented as a hyperspectral cube, the spectroscopic measurements
reveal specimen features hidden for OCT; they enable chemical differentiation
of constituent materials and analysis of their distribution. The detection
systems are based on low-cost pyroelectric detectors [6].

## 2\. Multimodal infrared OCT and spectroscopy system

### A. Infrared supercontinuum source

The multimodal OCT and spectroscopy system reported in this study is based on
a commercial pulsed supercontinuum light source from NKT Photonics (SuperK
Compact Mid-IR). The supercontinua are deterministic and generated in a
fluoride ZrF4-BaF2-LaF3-AlF3-NaF fiber (ZBLAN, step-index profile) pumped in
the spectral band around 2050 nm (first zero dispersion wavelength) by a high-
power erbium-ytterbium- and thulium-doped fiber amplifier. The broadband
emission covers the range from 1.1 µm to 4.4 µm. Figure 1 shows the spectrum
of the source measured using an FTIR spectrometer (Vertex 70, Bruker Optics);
the bands employed for OCT and spectroscopic imaging are indicated. Edge-pass
(1.65 µm cut-on wavelength) and neutral density filters were used to prevent
oversaturation of the Mercury Cadmium Telluride (MCT) detector. A specific
spectral sensitivity of the detector was applied to reconstruct the displayed
spectrum.

Figure 1: Spectrum of the supercontinuum source measured using an FTIR
spectrometer; the sensitivity curve of the MCT detector is used to correct
distortions of the spectral shape; the ranges exploited for OCT and
spectroscopic imaging are indicated.

The beam of the laser is specified by the quality factor M2 $\leq$ 1.1; the
transverse profile is the fundamental (TEM00) Gaussian mode. A parabolic
mirror is used as an output collimator. The outgoing beam diameter is in the
range of 3 mm - 6 mm, measured using band-pass filters at 2 µm and 4 µm
wavelengths correspondingly.

The repetition rate of the source is 2.5 MHz. The corresponding supercontinua
pulse duration is below one nanosecond. The total average power is about 490
mW, which, in conjunction with the high quality of the beam, provides superior
brightness - an essential feature for active imaging systems [40]. Relative
pulse-to-pulse energy fluctuations were characterized using a monochromator
(reported in [17]); the measurements yield the mean value of around 5.8 % over
the exploited spectral range. The default operating frequency of the
pyroelectric detectors depends on the sample reflectivity and magnitudes of
the surface and bulk scattering and is in the range of 40 Hz - 100 Hz.
Therefore, due to the prevailing averaging, the pulse-to-pulse intensity
fluctuations [41, 42] are negligible.

### B. Experimental system

The principal optical arrangement of the multimodal IR OCT and spectroscopy
system is shown in Figure 2.

IR supercontinuumsourceChopperFPTFLinearArrayGratingMotorizedmirrorXY-
StageSampleSphericalMirrorLensOpticsPellicleShutterMirrorFilterSMOCT Figure 2:
Scheme of the OCT and spectroscopic imaging system (SM), detection systems of
the modalities are indicated.

The experimental setup consists of a free-space Michelson interferometer, a
measurement head (sample scanning system, focusing optics, and chopper), OCT
and spectroscopy detection systems (indicated in Figure 2). The arms of the
interferometer are formed by an IR pellicle beam splitter. In the reference
arm, an unprotected gold mirror is used. Due to optimal dispersion and
transmission profiles, a BaF2 lens (50 mm focal length) is employed in the
sample arm to focus the probing beam onto the specimen. The measurement head
also includes a mechanical chopper to perform light intensity modulation as
required for pyroelectric detectors to operate [43, 44]. The modulation system
is internally synchronized with both modalities.

The detection systems of the two modalities are optically separated,
therefore, the beam exiting the interferometer is reflected by a motorized
mirror (synchronized with a shutter) either to a wide-band Fabry-Pérot tunable
filter spectrometer (FPTF, LFP-3144C, InfraTec GmbH) or to a grating-based OCT
spectrometer. The mirror is replaceable by e.g. an IR beamsplitter, thus,
allowing to perform independent measurements, but in trade-off with a
magnitude of the signal. Besides, the flexibility of such a solution is
limited due to the specification lag of optics and filters in this spectral
range, a standard splitting ratio for the MIR beam splitters is 50:50 and
invariant due to the current technology state.

The spatial resolution of the imaging system is identical for both modalities
and determined by the quality of the beam and focusing optics, and, in
general, is wavelength dependent. To specify the maximum and minimum
resolution, a 1951 USAF resolution test target was measured at spectral
extremes (around 2 µm and 4 µm) by means of the OCT detection modality (see
details in section D); the results yield the spatial resolution (Rayleigh
criterion) between 39.37 µm (3rd group, width of 5th element) and 11.05 µm
(5th group, width of 4th element).

### C. Spectroscopy modality

The compact and low-cost FPTF spectrometer used in the spectroscopy modality
(SM) is based on a fully-integrated MEMS-based Fabry-Pérot etalon with an
electrically controllable cavity length. Acting as a filter, it allows to
precisely tune the transmitted spectral band that satisfies the resonance
condition within the wavelength range of 3.1 µm to 4.4 µm (corresponds to 2275
cm-1 \- 3225 cm-1 in wavenumbers). Figure 3 shows the transmission behavior
with respect to the applied voltage, recorded using an FTIR spectrometer
(Vertex 70, Bruker Optics). The characterized spectral resolution is defined
by the full-width at half maximum (FWHM) of the transmitted band yielding 50
nm - 75 nm (corresponds to $\approx$ 40 cm-1).

Figure 3: Characterized spectral resolution (50 nm - 75 nm at FWHM) of the
Fabry-Pérot tunable filter; transmission at different control voltages,
measured using an FTIR spectrometer equipped with a thermal emitter;
absorption of CO2 around 4235 nm is observed.

The mechanical time constant of the filter is 2 ms - 20 ms. A pyroelectric
LiTaO3 point detector (2$\times$2 mm2 area) that is located behind the tunable
filter is used to record spatially resolved reflectance spectra. Thus, the
chopper used in the OCT detection system switches optimally between the modes
without causing delays, asynchronization or the need to deactivate the
modulator when changing to another measurement cycle. The detector detectivity
of 3.6$\times$106 $\mathrm{cm\sqrt{Hz}/W}$ is specified for 10 Hz modulation
frequency at a temperature of 25 °C.

To perform the correlative spectroscopic measurements, the reference arm of
the interferometer is blocked by a shutter. Therefore, a strong reference
field is excluded to protect the system from possible optical damage. The
mirror directs the collected light into the FPTF-based spectrometer, where it
is focused onto the point detector using suitable optics. The spectrum of the
probing beam, which is reflected by the sample, is measured by spectral tuning
of the filter transmission.

### D. Optical coherence tomography modality

The OCT measurement cycle is triggered, when the shutter opens and the SD-OCT
modality is activated by the motorized mirror. Therefore, in-situ structural
and spectral information is obtained. A Czerny-Turner type spectrometer is
used to record spectral interferograms for the OCT modality. It is based on a
blazed diffraction grating (300 lines/mm) and a pyroelectric linear array
(PYROSENS, DIAS Infrared GmbH); a spherical mirror (100 mm focal length) is
used to focus the light onto the detector. The array has 510 rectangular
shaped pixels with a dimension of 20$\times$500 µm2, optimally suited for
mirror-based design with non-toroidal surfaces [6].

The dual-band operation mode of SD-OCT is implemented using the grating
spectrometer based on a single array. Thereby, the detection of the overlapped
diffraction orders of the sub-bands with doubled center wavelengths is
exploited. Ghost spectra in the NIR region (1850 nm - 2150 nm, second
diffraction order) overlap with the MIR part (3750 nm -4250 nm, first
diffraction order), thus, the required band could be accessed using
corresponding band-pass filters (indicated in Fig. 2), while the second
spectral region is suppressed. The axial resolution of the dual-band OCT
system was characterized in [8] and is around 17 µm and 37 µm within the NIR
and MIR ranges correspondingly; the characterization of the lateral resolution
(around 11 µm and 39 µm respectively) is demonstrated in Figure 4.

Figure 4: Spatial resolution of the imaging system characterized in the
extreme spectral sub-bands of the multimodal system (center wavelengths of (a)
2 µm and (b) 4 µm) using a 1951 USAF resolution test target; performed using
OCT modality (en-face images), a neutral density filter (optical density of
0.3) was used for evaluation.

(b)(a)

The OCT A-scan rate (depth scan at a fixed lateral position) is theoretically
limited by spectra acquisition speed, i.e. defined by the detection and
modulation system. In practice, however, a reasonable signal-to-noise ratio is
observed at chopping frequencies of 40 Hz - 100 Hz depending on the sample
reflectivity and scattering [8]. Due to the unique features of the
pyroelectric detectors, namely their sensitivity only to alternating radiation
[45, 43], the chopper is set in the sample arm of the interferometer. Thereby,
a DC component or spectral offset usually subtracted in OCT imaging is
naturally eliminated.

The post-processing algorithm employed in SD-OCT imaging is based on the
method reported in [6] and was upgraded since the reference spectrum
(represents a constant signal component) is undetectable for the pyroelectric
detector, thus, fewer steps are implemented. They include interpolation of the
spectral interferogram to be equidistant in wavenumbers, filtering of a signal
with further multiplication by a Gaussian-window. The last step involves a
discrete Fourier transform of the processed interferograms for generating
images in the spatial domain. A more detailed description of the principles of
OCT techniques can be found in [14].

## 3\. Sample preparation

In order to demonstrate the capabilities of multimodal morphological and
chemical imaging, a sample based on various practically relevant industrial
materials that exhibit distinct structural and spectral features was prepared.
Due to the enhanced penetration of MIROCT into scattering media, a ceramic
stack was selected as a basic assembly. The specimen was formed by two single-
crystal Al2O3 plates (0.4 mm thick), one was positioned on top of another. The
top surfaces of the plates have a mean roughness 0.5 µm < $R_{a}$ < 1.2 µm,
thus, causing strong surface scattering in the VIS and NIR ranges, where
standard OCT systems are operating. Since these interfaces were positioned
first to the incident light, strong specular reflection at these boundaries is
reduced. The rear surfaces of the plates are epi-polished with $R_{a}$ < 0.2
nm.

The spectral and spatial diversification of the stack was implemented
employing different visible-transparent, non-scattering polymer films that
were inserted in the gap between the plates. Polyethylene terephthalate (PET),
polystyrene (PS), and polypropylene (PP) were selected to be accessed and
measured through the turbid ceramic media. The films have specific spectral
signatures in the exploited range: PET exhibits absorption lines induced by
Phenyl-H and CH2 bond stretching [46]; for PP a slightly wider band is
observed due to symmetric and asymmetric stretching vibration of CH3 and CH2
functional groups, and a CH shoulder-type band [47]. PS has distinctive and
relatively strong absorption bands at higher frequencies (3082 cm-1 \- 3026
cm-1) due to the aromatic CH stretching. Besides, asymmetric and symmetric
stretching vibrations of methylene groups CH2 are also observed in the
absorption spectrum of PS [48]. The thicknesses of the different polymer films
were the same (around 50 µm). Figure 5 illustrates the schematic three-
dimensional model of the resulting composite sample; the constituent layers
and details are indicated.

Al2O3Al2O30.4 mm0.4 mmPSPETPPRough top surfacePolished bottom surfaceRough top
surfacePolished bottom surface Figure 5: Schematic model of the multi-layered
composite test sample; 50 µm thick polymer films (shown in blue color) –
polyethylene terephthalate (PET), polystyrene (PS), and polypropylene (PP) –
are embedded in between the alumina ceramics; the top interfaces of the
ceramic plates are rough, the bottom surfaces are epi-polished.

## 4\. Results

(a) B-scan of the top section; 2 µm OCT system

(b) C-scan; 4 µm OCT system

(c) En-face image of the polymer films inserted between the plates (denoted);
4 µm OCT

Figure 6: Tomographic imaging of the multi-layered ceramics-polymer stack by
means of the dual-band IR OCT modality: (a) B-scan (high resolution 2 µm OCT)
of the top section of the sample, the ceramic plate and polymer films are
revealed, the air gap between the ceramics and films is partially detectable
(visualized and indicated in the inset); (b) C-scan of the sample obtained by
means of 4 µm OCT, the complete structure is accessed; (c) en-face image of
the polymer films embedded in between the ceramic stack (4 µm OCT) revealing
morphological information but no chemical information of the polymers (types
of the polymers are denoted).

PSPETPP0.4 mm0.4 mmAl2O3B-scan positionRough top surfaceAir gapPolymer films

In order to demonstrate the performance of the system, correlative NIROCT,
MIROCT and MIR spectroscopic measurements (area scan, 8$\times$6 mm2) of the
ceramic-polymer stack were carried out. The experimental results allow to
visualize the structure and enable in-situ identification of the embedded
polymers.

The dual-band OCT measurements - depicted in Figure 6 \- reveal the entire
three-dimensional volume of the multi-layered sample. The top surfaces of the
plates cause no image artefacts due to the high roughness and absence of
specular reflection. The polymer films that are positioned at a depth of
around 400 µm are effectively detected and can be displayed using an en-face
MIROCT image as shown in Figure 6(c). The thicknesses of the plates and films
can be calculated for defined refractive indexes using high-resolution NIROCT
cross-sectional images (B-scans); a typical B-scan is presented in Figure
6(a).

The air gap between the rear polished surface of the ceramic plate and the
polymer films is on the same scale as the axial resolution and can be detected
depending on the actual topology (i.e. at certain positions where it can be
resolved). The rough top interface of the bottom plate is not detectable for
NIROCT due to strong surface scattering.

Nevertheless, the complete sample morphology, which was obtained using OCT
modality, does not allow differentiation of polymers due to the equal
thickness and the absence of any characteristic scattering, the magnitudes of
back-reflections from the different polymer interfaces are equal.

Using the MIR spectroscopy modality, spatially resolved and OCT-co-registered
reflection spectra were collected, thereby a hyperspectral cube has been
recorded; the acquisition time per single spectrum was around 3 sec. In order
to calculate absorbance using the Beer-Lambert law [49], a single reflection
spectrum from the blank plates stack (no polymers inserted) was recorded as a
background. In practice, the same hyperspectral data set can be used to
retrieve background spectra selecting particular spatial coordinates, where
e.g. no polymers are presented. As post-processing steps, a baseline
correction, denoising based on principal component analysis and spectral
smoothing (Savitzky-Golay filter) were applied using an open-source python-
based software (Orange [50]). The correlative MIR spectroscopic measurements
are visualized as false-color absorbance images integrated within different
spectral regions that are characteristic for chemical differentiation (Figure
7). A small and varying air gap between the polymer films and rear polished
surface of the plate ($R_{a}$ < 0.2 nm) causes interference due to multiple
reflections. These effects introduce fringes in the spectroscopic images and
can be also observed as an equidistant modulation in each reflection spectrum,
frequencies of the modulation depend on the thickness of the air gap.

Figure 7: Correlative MIR hyperspectral images of the multi-layered ceramic-
polymer stack, visualized as false-color absorbance images integrated within
different spectral regions (resulting in a single absorbance value for each
position); the polymer films can be differentiated and identified; the
clearly-visible interference patterns (fringes) are introduced by the multiple
reflections within the thin air gap between the polymer films and the rear
polished surface of the ceramic plate.

3125 cm-1-2976 cm-12940 cm-1-2858 cm-12858 cm-1-2702 cm-1PSPETPPPSPETPPPSPETPP

As demonstrated in Figure 7, due to the material-dependent spectral response
the evaluation of the absorbance spectra enables unambiguous differentiation
of the polymers embedded in the turbid environment. Besides, each individual
pixel of the recorded hyperspectral cube contains the full absorbance spectrum
at the pixel’s position. Spatially averaged spectra for each polymer are shown
in Figure 8. Effects due to thin-film interference average out in this
representation. Therefore, the measurements enable identification of the
polymers that display different signatures in the exploited spectral
bandwidth.

(a) Absorbance spectra of the polymer films embedded in the ceramic stack,
averaged over the scanned areas; experimental system, MIR spectroscopy
modality (resolution 40 cm-1)

(b) Reference measurements; absorbance spectra of the blank polymer films, a
commercial FTIR spectrometer equipped with a thermal emitter (resolution 4
cm-1)

Figure 8: Spectra evaluation of the embedded polymers: (a) spatially averaged
absorbance of the films and (b) reference FTIR measurements of the polymers
confirming the accuracy of the spectroscopic investigation.

In order to verify the acquired spectra, reference spectra of the polymer
films were measured using a high-resolution (4 cm-1) FTIR instrument (Lumos,
Bruker Optics) that was operated in reflection mode to provide accurate data
on the spectral positions of the observed absorption bands. Thus, reflectance
spectra were recorded for the blank films outside the ceramic stack; back-
reflected light was collected from an area of 100$\times$100 µm2 and
absorbance was calculated using a measured background spectrum (mirror). The
normalized spectra – depicted in Figure 8(b) – are in a good agreement with
the results obtained with the developed multimodal system. Due to the lower
resolution, the FPTF-based spectrometer is not able to resolve individual
vibrational modes, however, the position of the bands, the envelope and trends
are observed: PET and PS demonstrate absorption in the range beyond 3000 cm-1;
the width of absorption bands for PP and PET are very well consistent with
FTIR measurements, especially in the range around 2800 cm-1.

Taking into account the spectral features and selecting a suitable spectral
range to integrate absorbance (according to specific chemical signatures, as
done for images in Figure 7), the chemical analysis of the components and
their spatial distribution can be visualized for complex and spectrally
diverse samples. The obtained data effectively supplement the structural
information obtained by the OCT modality and reveal details that are commonly
inaccessible. We would like to emphasize that the spectra were collected in
reflection mode, through the 400 µm thick ceramic plate with a rough surface
(corresponding to 800 µm, double propagation, thus, the contrast of the images
can be affected. The spectral shape of the bands can also be influenced by
artefacts due to interference and varying magnitude of scattering.

Additionally, the observed interference effects (known as an autocorrelation
term within OCT community [14]) that, at first glance, introduce unwanted
distortions in the spectra and hyperspectral images can be elegantly exploited
to reconstruct the thicknesses of underlying layers using methods well-known
in IR spectroscopy. Due to the significantly wider spectral bandwidth of the
MIR spectroscopy modality (Fig. 1), the reconstruction enables sub-OCT spatial
resolution (using the standard definition for OCT [14], of approximately 5
µm). Acquired reflection spectra carry distinct and singular frequencies
(usually referred to by the term “fringes”) originating from constructive and
destructive interference due to multiple reflections between the highly
reflective polished ceramic interface (bottom surface of the top plate) and
top interface of the polymers. Other possible reflections were not detected as
they are significantly weaker, suppressed by e.g. strong surface scattering;
the sensitivity of the SM-based reconstruction approach is weaker than that of
OCT due to the absence of the strong reference field, i.e. the coherent
amplification. Besides, despite the higher resolution, the maximum thickness
(imaging depth) that can be accessed using this indirectly formed system of a
common-path interferometer and the FPTF spectrometer (as a detector) is
limited. The limitations are imposed by the lower spectral resolution of the
latter, i.e. they are governed by the Nyquist-Shannon-Kotelnikov theorem; so
this extension is well suited and feasible especially for measurements of
relatively thin layers.

Thus, the thickness $d$ of the air gap between the top ceramic plate and the
polymer films at the particular spatial coordinates can be obtained as follows
[51]:

$d=\frac{N}{2n(\nu_{1}-\nu_{2})},$ (1)

where N is a number of fringes observed in the spectral bandwidth between
$\nu_{1}$ and $\nu_{2}$ (in wavenumbers), $n$ is the refractive index ($n=1$
for air) of the media between the interfaces causing interference.

PSPETPP Figure 9: Thickness map of the air gap between the top ceramic plate
and the polymer films; the profile is retrieved using the hyperspectral data,
i.e. spectral fringes originated from constructive and destructive
interference due to the multiple reflections within the air gap.

Figure 9 depicts the spatially resolved thickness map of the air gap between
the top ceramic plate and the polymer films. The profile was reconstructed
using (1). In prior to fringes counting according to [51], an interference-
free background spectrum was subtracted from the raw spectra (for each pixel)
to increase contrast and reveal pure interference patterns. Then the number of
peaks was determined using a robust peak finder algorithm [52]; a calculated
period of the frequencies was used to access the exact fractional number of
fringes.

Thereby, the developed orthogonal modalities integrated into the single
optical instrument do not only complement the data while operating with
chemical and structural analysis of materials, but can successfully enhance
and correct each other. The present demonstration reveals the potentials of
the multimodal systems to obtain more detailed information about the samples
under study – which is of great interest for various scenarios.

## 5\. Conclusion and Outlook

In this contribution, we have introduced in-situ correlative near- and mid-
infrared (NIR, MIR) optical coherence tomography (OCT) and MIR hyperspectral
chemical imaging; relevant key-aspects of this unique combination were
discussed in details. The developed multimodal optical system has been
evaluated and experimentally demonstrated, to the best of our knowledge, for
the first time. The metrogological capabilities of the presented solution were
verified by investigating a complex specimen with distinct morphology as well
as chemical signatures.

The two modalities (OCT and MIR hyperspectral imaging) were united in a single
measurement head and complement each other, revealing sample details that
would usually be hidden for a stand-alone technique. A single supercontinuum
laser has been employed as a light source covering parts of the NIR and MIR
region (1.1 µm - 4.4 µm emission range).

The OCT modality operated in spectral-domain configuration (SD-OCT) exploiting
a dual-band detection scheme (2 µm and 4 µm center wavelengths), reported in
[8], which enabled high-resolution and penetration-enhanced imaging for
inspection of highly scattering samples. Using a cost-effective pyroelectric
array in the MIR band, the system able to resolve sub-interfaces and detect
structures embedded in a benchmark ceramic-based composite sample.

The MIR spectroscopy modality has been implemented in the attractive spectral
region (3.1 µm - 4.4 µm), where functional organic groups display valuable and
strong absorption bands. The detection system is based on a low-cost
electrically tunable Fabry-Pérot filter equipped with a pyroelectric point
detector. The spectral resolution of around 50 nm - 75 nm ($\approx$ 40 cm-1)
allowed to uniquely distinguish and chemically identify the 50 µm thin polymer
films (polyethylene terephthalate, polystyrene, and polypropylene) embedded in
a highly scattering ceramic stack. Therefore, the spectroscopic imaging
modality is particularly advantageous and feasible, as it is independent on
the OCT modality; thus, no matching condition between spectral and axial
resolution has to be considered.

The correlative morphological and hyperspectral imaging data set delivered by
the developed system provides multidimensional information of the sample under
investigation. Thereby, the emission spectrum of the single supercontinuum
source is effectively and expediently exploited. Moreover, this kind of source
is, in fact, an enabling technology for the presented dual-modality
correlative imaging setup. Due to the current developments in the field of MIR
supercontinua generation, further expansion of spectral coverage into the MIR
fingerprint region, enhanced brightness, as well as decreasing prices of this
novel type of laser-based light sources, are expected. Therefore, we
anticipate a substantial application potential of supercontinuum laser sources
for both OCT and spectroscopy in the fields of non-destructive testing,
biomedical optics and beyond.

## Funding

Horizon 2020 Framework Programme (722380); Province of Upper Austria
(Innovative Upper Austria 2020); EFRE Urban Innovative Actions (IWB2020);
Österreichische Forschungsförderungsgesellschaft (FFG) (871974).

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11institutetext: S. Ambily 22institutetext: Indian Institute of Astrophysics,
Koramangala 2nd block, Bangalore, 560034, India
Laboratory for Atmospheric and Space Physics, University of Colorado, UCB 600,
Boulder, CO, 80309, USA
22email<EMAIL_ADDRESS>33institutetext: Mayuresh Sarpotdar
44institutetext: Satellite Research Centre, Nanyang Technological University,
Singapore 55institutetext: Joice Mathew 66institutetext: Advanced
Instrumentation and Technology Centre, RSAA, Mount Stromlo Observatory,
Australian National University, Canberra, Australia 77institutetext: Binukumar
Gopalakrishnan Nair 88institutetext: Jayant Murthy 99institutetext: Margarita
Safonova 1010institutetext: Rekhesh Mohan 1111institutetext: Indian Institute
of Astrophysics, Koramangala 2nd block, Bangalore, 560034, India
1212institutetext: A. G. Sreejith 1313institutetext: Space Research Institute,
Austrian Academy of Sciences, Schmiedlstrasse 6, Graz, Austria
Laboratory for Atmospheric and Space Physics, University of Colorado, UCB 600,
Boulder, CO, 80309, USA 1414institutetext: K. Nirmal 1515institutetext:
Deutsches Elektronen-Synchrotron (DESY), Zeuthen, Germany 1616institutetext:
Vinod Kumar Aggarval 1717institutetext: S. Nagabhushanam 1818institutetext:
Sachin Jeeragal 1919institutetext: Crucible of Research and Innovation, PES
Institute of Technology, Bangalore, India

# The Near Ultraviolet Transient Surveyor (NUTS): An ultraviolet telescope to
observe variable sources

S. Ambily  Mayuresh Sarpotdar  Joice Mathew  Binukumar G. Nair  A. G. Sreejith
Nirmal K  Jayant Murthy  Margarita Safonova  Rekhesh Mohan  Vinod Kumar
Aggarval  S. Nagabhushanam  Sachin Jeeragal

(Received: date / Accepted: date)

###### Abstract

Observing the ultraviolet (UV) sky for time-varyiable phenomena is one of the
many exciting science goals that can be achieved by a relatively small
aperture telescope in space. The Near Ultraviolet Transient Surveyor (NUTS) is
a wide-field ($3^{\circ}$) imager with a photon-counting detector in the near-
UV (NUV, 200 – 300 nm), to be flown on an upcoming small satellite mission. It
has a Ritchey–Chrétien (RC) telescope design with correction optics to enable
wide-field observations while minimizing optical aberrations. We have used an
intensified CMOS detector with a solar blind photocathode, to be operated in
photon-counting mode. The main science goal of the instrument is the
observation of transient sources in the UV, including flare stars, supernovae,
and active galactic nuclei. NUTS’s aperture size and effective area enable
observation of relatively unexplored, brighter parts of the UV sky which are
usually not accessible to larger missions. We have designed, fabricated, and
assembled the instrument, and the final calibrations and environmental tests
are being carried out. In this paper, we provide the scientific motivation and
technical overview of the instrument and describe the assembly and calibration
steps.

###### Keywords:

Variable sources Ultraviolet astronomy Small satellites Wide field imager UV
sky surveys

##  1 Introduction

CubeSats and small satellites present a unique opportunity for space-based
astronomy by providing a platform for technology development while exploring
specific science goals that are difficult to achieve with large missions. This
is especially true for the ultraviolet (UV) wavelength range where there are
few missions that are currently active: the UV spectrographs on the Hubble
Space Telescope (Woodgate et al., 1998; Green et al., 2012); the Ultraviolet
Imaging Telescope (UVIT)111Currently (2021) only the FUV channel is
operational. on AstroSat (Tandon et al., 2017); and the supplementary UV
telescopes on the Swift (Roming et al., 2005); and XMM-Newton (Mason et al.,
2001) missions. The last few years have seen an increased interest in the UV
region222The UV spectrum is usually subdivided into a number of ranges (by the
ISO standard ISO-21348): Vacuum UV: $10-200$ nm; Extreme UV: $10-121$ nm; Far
UV: $121-200$; and Near UV: $200-400$ nm. with a number of CubeSat missions
under development (Fleming et al., 2018; Shkolnik et al., 2018; Fleming et
al., 2019; Cenko, 2019), each of which seeks to address a set of distinct
science goals. CubeSats also provide a platform for technological advancements
for the future that were traditionally carried out using relatively low-cost
sounding rockets and high-altitude balloon experiments (France et al., 2016;
Sreejith et al., 2016). There are several technological challenges to be
addressed for future UV missions, such as improving the performance of the
detectors in the Far-UV (FUV), developing high-reflectivity mirror coatings
for the vacuum UV, and fabricating low-noise large-area microchannel plates
(The LUVOIR Team, 2019; Gaudi et al., 2020). CubeSats offer an excellent
opportunity to improve the technology readiness levels of these activities by
providing actual spaceflight opportunities at faster time scales and a
fraction of the cost of full-fledged satellite missions.

Space missions such as GALEX (Martin et al., 2005), FUSE (Moos et al., 2000),
and the suite of UV instruments on the HST have demonstrated the fascinating
science that can be done with UV observations. Although these missions have
studied the sky in both FUV and NUV through both photometric and spectroscopic
observations, many areas in the UV sky are not yet well-studied. For example,
the GALEX mission has imaged around 80% of the sky, with a limiting magnitude
of 28 AB (Bianchi et al., 2017). However, due to detector limitations
(Morrissey et al., 2007), it has not been able to observe the brightest and
most interesting regions of the sky such as the Milky Way plane and the
Magellanic Clouds. A small dedicated telescope can survey those areas that are
typically left out by large missions with broader science motivations (Brosch,
Balabanov & Behar, 2014).

We, the small-payloads group at the Indian Institute of Astrophysics (IIA),
have been working on building and testing payloads targeted at astronomical
and atmospheric observations in the UV (Mathew et al., 2017; Chandra et al.,
2020). We have used high-altitude balloons to test the sub-system prototypes
and later develop them into full-fledged space payloads (Sreejith et al.,
2016; Mathew et al., 2018a). The Near Ultraviolet Transient Surveyor (NUTS) is
one such payload and is an NUV imaging telescope that we plan to fly on a
small satellite. The instrument is a 150-mm Ritchey–Chrétien (RC) telescope
covering the wavelength range of 200 – 300 nm using a solar-blind
photocathode. The instrument has a peak effective area of 12 cm2 and can
detect objects as faint as 21 AB (Oke & Gunn, 1983). With a wide field of view
(FOV: $3^{\circ}$), we will search for variable UV sources such as flare
stars, supernovae, and other transient events with a spatial resolution of
$13^{\prime\prime}$. These observations will be used to generate calibrated
light curves of interesting transient phenomena with a cadence of up to 2 ms.
Because the aperture and the effective area of the telescope are small, our
primary targets are bright regions of the UV sky where larger missions are not
able to observe due to detector limitations.

Mission parameter | GALEX | UVIT NUV | ULTRASAT | NUTS
---|---|---|---|---
| Morrissey et al. (2007) | Tandon et al. (2017) | weizmann.ac.il/ultrasat/ | This work
Aperture (mm) | 500 | 376 | 330 | 146
Collecting area (cm2) | 1950 | 852 | 856 | 141
Detector size | 4K$\times$4K | 512$\times$512 | 4 $\times$ 4K$\times$4K | 1K$\times 1$K
Pixel scale (′′/pix) | 1.5 | 3 | 5.4 | 4.3
Spatial resolution (′′) | 5.3 | $1.2-1.6$ | 13 | 13
FOV (deg2) | 1.21 | 0.18 | 200 | 7.07
Wavelength range (nm) | $177-283$ | $200-300$ | $220-280$ | $200-300$
Brightness limit (AB) | 9 | 9 | 16 | 7
Weight (kg) | 280 | 230 | 400 |  $\sim$4.5
Approximate cost ($$)$ | 150M | 6M | 100M | 0.2M
Current status | Decommissioned | Non-functional | In prep. | In prep.
Table 1: Comparison of missions parameters in NUV channel. The cost figures
are taken from official releases, and may not include the launch and operation
costs in some cases.

##  2 Scientific Motivation

Despite its scientific potential, time domain astronomy in the UV has not been
studied extensively (Sagiv et al., 2014; Mathew et al., 2017). Traditionally,
transient observations from space are carried out in the high energy domains
of X-Rays and gamma rays. In addition, ground-based surveys such as the
Palomar (Law et al., 2009) and Zwicky (Bellm et al., 2019) Transient
Facilities, and the Vera C. Rubin Observatory Legacy Survey of Space and Time
(LSST, Ivezić et al. (2019)) are also capable of covering large swaths of the
sky to look for variable sources. It is important to complement these with
space-based UV observations, as in many cases the UV transient can be observed
ahead of time and can act as an alert for observations from the ground (Sagiv
et al., 2014). The UV sky background is faint, making a majority of variable
sources in the night sky detectable with a small space-based observatory
(Safonova et al., 2014). The potential of a wide-field UV survey for
transients detection and observations has been demonstrated by the GALEX time
domain survey that discovered and classified over a thousand variable UV
sources (Gezari et al., 2013). However, although UV space missions of the past
and present, such as GALEX or SWIFT, have resulted in the discovery of many
interesting UV transients, a dedicated program is not feasible with such
shared resources but is ideal for a small satellite mission.

A comprehensive overview of the transient astronomical sources that can be
observed in the UV range can be found in Sagiv et al. (2014) and Wang et al.
(2019). We have also outlined a number of scientific objectives of our balloon
program in Mathew et al. (2018a). A dedicated small imaging payload can survey
the night sky for bright transient sources such as supernovae (Welsh et al.,
2011; Wils et al., 2010; Ganot et al., 2016), Active Galactic Nuclei (Wang et
al., 2019), flare stars (Welsh et al., 2007), and gravitational wave
counterparts (Ridden-Harper et al., 2017). Surveying the UV sky for such
transients, along with the follow-up observations in ground-based optical and
radio wavelengths, are critical steps that can help answer key astrophysical
questions.

Some of the most exciting transients are supernova explosions which arise from
the death of massive stars and are associated with a prominent high-energy
emission. Observations of supernovae play a vital role in understanding the
star formation rates and chemical evolution of the universe (Rabinak & Waxman,
2011). Determining the physical properties of massive stars prior to the
explosion is a critical step towards understanding and constraining the
current models of stellar evolution. The radiation from early stages of
exploding supernovae called shock-breakout flares, as well as the following
cooling phase, are observable in the UV wavelengths (Ganot et al., 2016; Nakar
& Sari, 2010). A modest NUV camera can contribute greatly to the early
detection and observation of supernovae explosions (Sagiv et al., 2014).
Observing these UV flare parameters such as duration and amplitude helps in
estimating the pre-explosion conditions of their progenitor stars such as the
radius and chemical composition (Rabinak & Waxman, 2011).

Stellar flares are another poorly understood astronomical transients that can
greatly benefit from UV observations. Late-type stars such as K and M dwarfs,
having a high amount of activity in their corona and strong magnetic fields
associated with them, flare quite frequently in the UV with a $10^{2}$ to
$10^{3}$ times increase in intensity (Welsh et al., 2007; Brasseur et al.,
2019). Such stars are interesting candidates for exoplanet science, as many of
these stars host close-in exoplanets making them viable for transit
observations (Howard et al., 2012; Dressing & Charbonneau, 2015). Detection
and characterization of terrestrial planets are limited to cool stars for the
next few years due to observational constraints. But to characterize planet
properties through transit observations, it is critical to understand the
stellar activity variations and separate those from planetary signals.

M dwarf flares, especially in the far and extreme UV, are also important from
a habitability perspective, as the high energy associated with these flares
can impact the planetary atmospheric composition and thereby the habitability
(Rugheimer et al., 2015; Johnstone et al., 2015). It is also proposed that the
flares are associated with an intense NUV radiation, which can trigger the
formation of important biomolecules on the planet (Ranjan et al., 2017; Rimmer
et al., 2018). With $3^{\circ}$ FOV and a low-noise photon-counting detector,
we will be able to detect such flares with a variability of a few milliseconds
up to a few minutes (Brasseur et al., 2019). Our instrument is sensitive down
to 21 AB mag, enabling us to detect even some of the faint M dwarfs. As a
dedicated mission, we can also do multiple observations of the same target to
obtain its light curves with high photometric accuracy.

##  3 Instrument Overview

The instrument design for NUTS was carried out keeping in mind the scientific
requirements of a wide FOV with good spatial resolution while meeting the
mechanical constraints of small satellites. We are nominally proposed to make
use of the microsat platforms of the Indian Space Research Organization
(ISRO), which gives us a constraint of fitting the science instrument in a
roughly 12U volume ($300\times 200\times 200$ mm) with no moving parts. The
tight mechanical constraints and the low UV flux levels of our observation
targets limit the number of reflections in the optical path for the desired
performance. A summary of the instrument parameters is given in Table. 2.

Instrument | Wide-field UV imager
---|---
Telescope type | Ritchey-Chrétien
Primary aperture | 146 mm
Field of view |  $3^{\circ}$ (circular)
Operating bandwidth | 200 – 300 nm
Detector | MCP with solar-blind coating
Pore size |  $10\,\mu$m
Instrument Power |  $5.5$ W
Volume (L $\times$ W $\times$ H) |  $300\times 160\times 160$ mm
Weight |  $4.5$ kg
Spatial resolution | $13^{\prime\prime}$
Time resolution | 2 ms full frame, 10 $\mu$s windowed
Peak effective area | 12 cm2
Mean effective area | 9 cm2
Sensitivity | 21 AB (with SNR of 5 in 1200 sec exposure)
Bright Limit | 7 AB
Table 2: Summary of instrument parameters

###  3.1 Optomechanical design

The optical design consists of a 150-mm primary mirror and a 64-mm secondary
mirror with two 30 mm corrector lenses. Both of the mirrors are hyperbolic to
eliminate the off-axis aberrations such as coma and spherical aberrations
across the wide FOV, resulting in an RC telescope design. However, the RC
design and relatively large FOV introduce additional optical aberrations such
as field curvature and astigmatism, which are then corrected by a set of two
Fused Silica lenses. The optical layout and performance (spot diagram) are
shown in Figs. 1 and 2, respectively.

The primary and secondary mirrors were manufactured and delivered by
Wavelength Opto-electronics, Singapore. They also fabricated and assembled the
corrector lens assembly, with its optomechanical components. The mirrors are
made of Zerodur substrate with a reflective Aluminum coating and a protective
MgF2 layer, giving a broad bandpass of 120 – 900 nm. The set of two lenses,
used to correct the aberrations, are made from fused silica with a 90%
transmission efficiency down to 195 nm. In addition, both sides of the lens
are coated with an anti-reflection MgF2 coating to suppress the reflection and
thus maximize the transmission (up to 95%) in the NUV band. We have also
minimized the optical aberrations such that 80% of the encircled energy falls
within four pores of the MCP ($\sim$20 $\mu$m).

Figure 1: Optical layout of the instrument.

Figure 2: Optical performance of the system for various field positions with
the field angle (in degrees) and image position (in mm) indicated above and
below the spot, respectively. The three marker colors blue, green, and red
denote the instrument wavelengths of 200 nm, 250 nm, and 300 nm, respectively.

The mechanical design was carried out keeping in view the spacecraft
requirements such as light weight, small volume, and the structural stability
to withstand launch-load vibrations. The overall design consists of the
optical system, the detector unit, and the electronics box. The optical system
comprises primary and secondary mirrors cells, holders for the correcting
lenses, and the telescope tube that houses all these elements. Aluminum
6061-T6 is used for the tubular structure of the telescope, due to its high
strength-to-density ratio, low outgassing properties, easy machinability, and
low cost. The mounts for mirrors and lenses are made of Invar36 as its very
low thermal expansion properties match with the Zerodur material. The optics
are glued to their mounts using space grade adhesives, 3M® 2216B. The MCP
enclosure, which is attached to the lens cover, houses the MCP and the fiber
optics taper. One side of the taper is glued to the phosphor side of the MCP
and the other to the CMOS. The electronic box consists of the a series of
three PCBs and the high voltage supply box. An exploded CAD drawing of the
instrument is shown in Fig. 3.

The instrument is attached to the spacecraft with an interface flange at the
center of gravity (CG) for better mechanical stability. From our vibration
analyses and launch provider specificatios (Mathew et al., 2017; Kumar et al.,
2012), we have determined that the natural frequency must be above 100 Hz, and
the instrument should be able to withstand shock loads of up to 5 g. We have
also taken into account the manufacturing and alignment tolerances and
temperature fluctuations that affect the final Point Spread Function (PSF) to
ensure the performance for even the widest FOV. A baseline performance
estimate ensuring that 80% encircled energy falls within a $20\,\mu$m radius
was adopted, and the tolerance analysis was done using Zemax Optics Studio.
The tolerance limits were derived (Table 3) using the telescope focus as the
compensator, which can be adjusted with the help of shims during the
calibration. From here, we also concluded that an operating temperature of
$15^{\circ}$ – $25^{\circ}$C is optimal for the optomechanical unit to give
the desired performance, which can be achieved by both passive (multi-layer
insulation) and active (heaters) methods.

Tolerance type | Tolerance term | Value
---|---|---
Manufacture | Radius of Curvature (Mirror) | 1 %
| Radius of Curvature (Lens) | 0.1 %
| Thickness |  50$~{}\mu$m
| X-Y decenter |  50$~{}\mu$m
| X-Y tilt | $60^{\prime\prime}$
| Surface roughness |  $\lambda$/6
Alignment | X-Y decenter |  50$~{}\mu$m
| X-Y tilt | $60^{\prime\prime}$
Table 3: Manufacture and alignment tolerances for the instrument. Figure 3:
CAD model of the mechanical structure Figure 4: Diffraction-encircled energy
for on-axis and off-axis fields. The colours indicate different fields, with
the percentage of energy distribution (in $y$ axis) as a function of the
distance from the spot center (in $x$ axis).

###  3.2 Detector and electronics

Photon-counting detectors using MCPs have been the standard for UV payloads
because of their low readout noise, large detector area, radiation tolerance,
and long-wavelength rejection (Kimble et al., 2003; Joseph, 1995). We have
achieved significant savings in cost by using a phosphor-screen anode as the
readout, and by focusing the image of the anode onto a CMOS sensor as shown in
Fig. 5. We have developed and tested a laboratory model of the detector
(Ambily et al., 2017) for some of our balloon payloads (Sreejith et al., 2015;
Mathew et al., 2018a). The detector for NUTS is a modified version of this
with a solar blind photocathode made of Cesium Telluride to address the red
leak and dark current issues of the S20 photocathode. As most of the UV
targets are relatively faint, the detector system works in the photon-counting
mode, where it records the position and arrival time of each photon with an
accuracy of $13^{\prime\prime}$ and 2 ms, respectively. The key part of the
detector is the MCP340 assembly from Photek333http://www.photek.com/. It is a
40-mm Z-stack MCP with a solarblind photocathode deposited on a fused Silica
input window and a P46 phosphor deposited on the anode. The gain of the MCP is
varied by the voltage at the MCP output, which automatically adjusts the
screen voltage, maintaining a constant voltage difference between the Anode
and MCP_Out terminals. The resulting electron cloud from the MCP output
electrode is accelerated towards the phosphor screen anode (with a peak
response at 530 nm), and the resulting photons are focused on the CMOS surface
by a custom-made fiber taper, also manufactured by Photek.

Figure 5: Block diagram of the detector system. The electrons from the MCP are
converted to green photons by the phosphor screen at its output. This light is
coupled to a CMOS chip using a fiber taper.

Since we are interested in time-domain astronomy, it is important to have a
CMOS image sensor and readout electronics that can support high frame rates.
The LUPA1300-2 (NOIL2SM1300A) is a 1.6 MP sensor from ON Semiconductor that
meets our requirements of faster readout and a large sensor size. It has 14
$\mu$m pixels with $18\times 14$ mm size, which matches the output diameter of
the taper, and gives rise to fewer aberrations when interfaced with the MCP.
It also supports high-speed clocks of up to 315 MHz rendering a maximum frame
transfer rate of up to 500 fps. The sensor further supports windowed and sub-
sampled readouts, which means we can achieve a temporal resolution of 2 ms for
the full resolution of 1.3 MP and 10 $\mu$s for the smallest window. We have
designed a custom readout board to give the necessary bias and clock lines for
the sensor. It also interfaces the synchronization clocks and pixel values to
the FPGA board. The data acquisition and processing unit is a field-
programmable gate array (FPGA) board, which is a generic board we have
developed to be used with multiple CMOS and CCD sensors (Sarpotdar et al.,
2016). It is based on a MIL-grade Spartan-6Q FPGA from Xilinx444Xilinx Inc.:
http://www.xilinx.com/ with a wide operating temperature range. This digital
readout board also performs the necessary image processing operations for the
photon-counting mode, including computing the centroids of photon events to
sub pixel accuracy (Ambily et al., 2017). The board contains the necessary
peripherals for storage of the intermediate data (SDRAM, Flash, and SD Card)
and interfacing with other spacecraft electronics (UART, I2C, and JTAG).

####  3.2.1 Photon event centroiding

The electron cloud from the MCP is roughly Gaussian in shape (Joseph, 1995;
Hutchings et al., 2007). To accurately determine the $X-Y$ position of the
photon on the photocathode, we need to determine the centroid of the resulting
photon cloud on the CMOS to sub-pixel accuracy. The centroiding algorithm was
implemented using a $5\times 5$-pixel sliding window, to first detect a
possible photon even which would appear as a local maximum in the window. We
estimate the background level in each window from the lowest corner pixel
value (Hutchings et al., 2007; Ambily et al., 2017), which is then used to
flag hot pixels and multiple events. Once the actual photon event locations
are identified, the subpixel or fractional position values are computed by a
weighted mean algorithm. In the centroiding mode, the output data for
telemetry is in the form of packets. Each packet contains a frame ID, event
ID, integer and fractional values of centroid coordinates, and the intensity
value of the central pixel.

Figure 6: Fine alignment setup using Zygo interferometer. The mechanical axis
of the system was first established with a theodolite, which was then used by
the interferometric setup to align the primary and secondary mirrors with the
optical axis of the telescope.

##  4 Assembly and Calibration

The assembly, alignment and initial calibration of the instrument were
performed at the M. G. K. Menon Laboratory (MGKML) for Space Sciences at IIA.
We have employed the procedures and techniques described in Mathew et al.
(2019) for the assembly and calibration of the Lunar Ultraviolet Cosmic Imager
(LUCI, Mathew et al. (2017)). This facility was also used by the UVIT payload
on AstroSat (Kumar et al., 2012) and includes the necessary equipment for
calibrating NUV instruments while ensuring cleanliness. We have access to
class 10000 cleanrooms with laminar flow tables providing a class 1000 or
better local environment.

###  4.1 Optical alignment and calibration

We have performed the coarse alignment of the telescope using a theodolite and
a pair of cross-hair targets on a vibration controlled optical table. The
theodolite has a precision of up to 0.01′′, which was used to establish the
mechanical axis of the system. This was used to align the optical axis of the
system with the help of a Zygo interferometer and a reference flat mirror. As
shown in Fig. 6, this setup was used to align the primary and secondary optics
of the system to the opto-mechanical axis.

The calibration steps include the measurement of system throughput, and the
optical performance estimates, using the same equipment and methods described
in Mathew et al. (2019). UV light from a deuterium lamp is coupled to a
collimator setup through an Acton monochromator, which will be used along with
NIST-calibrated photodiodes for the effective area measurements. The optical
performance will be estimated by imaging a set of pinhole masks of varying
sizes from different points in the field.

Once the full ground calibration and qualification of the payload are
completed, we plan to commence the environmental tests at the MGKML. The
instrument will undergo testing for vibration performance, thermal vacuum
tests including thermal balance and thermal cycling, and EMI/EMC tests. Being
a UV telescope, NUTS is extremely sensitive to contamination, and all the
components are cleaned using bake-out, and ultrasonic and solvent cleansing.
The instrument and individual flight components are also kept in a nitrogen
purge during and after the tests.

##  5 Performance estimates

We have estimated the system throughput and instrument effective area from the
components level tests, by using the mirror reflectivity values, and the
measured values for lens transmission and detector quantum efficiency. The
total system response in terms of effective area in cm2 can be expressed as

$A_{\rm eff}=A_{\rm coll}\times R_{\rm PM}(\lambda)\times R_{\rm
SM}(\lambda)\times T_{\rm l}^{2}(\lambda)\times{\rm QE}(\lambda)\,,$ (1)

where $A_{\rm coll}$ is the effective geometrical collecting area, $R_{\rm
PM}$ and $R_{\rm SM}$ are the reflectivity of the primary and the secondary
mirrors respectively, $T_{\rm l}$ is the lens transmission, and QE$(\lambda)$
is the quantum efficiency of the detector. The effective area plot for the
instrument is shown in Fig. 7, which shows that the instrument has a peak
effective area of 12 cm2.

Figure 7: Estimated effective area of the instrument (black solid line) on the
left axis. On the right axis, the mirror reflectivity values are measured and
provided by the manufacturer (blue dashes), lens transmission was measured in
the lab (purple dots), and the detector quantum efficiency (red dot-dashes)
was estimated from the manufacturer’s numbers for the solar blind photocathode
and from our lab measurements.

We have also used the UV sky simulator (Safonova et al., 2013) to model the UV
sky as could be seen by the instrument. The tool takes into account diffuse
and point sources as well as the instrumental background estimates. The point
sources accounted for here are the bright stars from the Hipparcos catalog,
whose flux in $200~{}\textendash~{}300$ nm range and the count rates are
estimated from their model spectra. The diffuse components include the NUV
background, zodiacal light (for January in this case), and airglow
contribution. These diffuse observations are taken from GALEX, which means
that the galactic plane remains unobserved. The diffuse background in these
regions is estimated by applying cosecant law (Murthy et al., 2010). The
estimated flux values are converted to instrument counts using the effective
area curve from Fig. 7, assuming zero dark current. These numbers are combined
to generate an output file which gives the total count rate in a given
$3^{\circ}$ field. The all-sky map as could be seen by NUTS is given in Fig.
8.

To determine the feasibility of using NUTS for temporal variability studies,
we also looked at previous observations of transient events in archival data,
particularly from the GALEX mission. We made use of the observations from the
variability catalogs (Welsh et al., 2005; Wheatley et al., 2008) and time
domain survey (Gezari et al., 2013), as well as additional catalogs of novae
outbursts (Wils et al., 2010) and M dwarf flares (Welsh et al., 2007). In
addition, there have been new archival studies of short timescale UV
variability (de la Vega & Bianchi, 2018; Brasseur et al., 2019) using the
gPhoton package (Million et al., 2016). These catalogs were used to determine
the median NUV flux levels of these events, which were then converted to their
equivalent counts as would be seen by NUTS. The image in Fig. 9 shows one
subset of such observations, where the estimated counts for NUTS are shown
across the sky and shows that we would have the possibility of detecting UV
transients across various pointing directions and magnitude ranges.

Figure 8: Simulated UV Sky as seen by NUTS. The color bar represents the
estimated counts, which include the diffuse background, zodiacal light, and
stellar contributions taken from archival observations.  Figure 9: Variable
sources in GALEX catalogs as seen by NUTS, scattered along their RA and DEC.
The colormap represents the estimated counts. The NUV magnitudes from these
variability catalogs were converted to flux using GALEX conversion factors.
These flux values, along with the estimated throughput of NUTS, were used to
determine the photon counts as seen by the instrument in 1200 sec.

##  6 Launch and Operations plan

We are proposing to fly NUTS on the small satellite platform (PS4-OP) of the
Indian Space Research Organization (ISRO) to a polar low-Earth orbit (PS4-OP,
2019). The platform provides the electrical and data interfaces via an IEEE
1553 interface, in addition to a 28 V power supply for the electronics box.
The Attitude Determination and Control System (ADCS) system includes a star
tracker capable of providing a pointing accuracy of $30^{\prime\prime}$ and
stability of about $1^{\prime\prime}$ per second. The mechanical interface is
provided as a flange at the center of gravity of the instrument. The
spacecraft is also estimated to actively maintain the instrument temperature
within $\pm 10^{\circ}$C, with the help of temperature sensors and heaters
attached to the instrument body.

Our default observation mode as a transient survey telescope, is to stare at
the UV sky in an anti-solar direction. The science observations happen during
the orbital night while maintaining the telescope’s Moon and Earth avoidance
angles at $40^{\circ}$ and $25^{\circ}$ respectively. Each pointing will cover
a 7 square degrees field and last 1200 seconds, which gives us a limiting AB
magnitude of 21. Further calculations are done to ensure that we avoid
brighter targets ($<7$th AB) to preserve detector life. The expected data rate
for the instrument is 100 kbps, with data consisting of the photon event lists
logged during the observations. The events are registered as individual
photons on the detector which will be assembled into a data stream containing
the $X-Y$ position and pulse height and transmitted to the ground, along with
time tags. As we are going to be in LEO and observations occur only in
eclipse, we have about 25 minutes of observing time per orbit. This amounts to
a maximum science data volume of $\sim$32 MB per day, with a 5-byte data
packet per photon event. We plan to implement further data compression
algorithms to reduce the data volume to around 5 to 10 MB/day. NUTS is also
capable of doing longer observations of any potentially interesting
transients, by pointing the telescope for multiple orbits.

One of the crucial tasks for the mission now is the development of the data
pipeline and in-flight calibration techniques. After receiving the photon
lists at the ground station, an automated pipeline will convert it into a 2D
image. The pipeline software will draw on the previous software developed at
IIA for missions such as TAUVEX (Mohan, 2008) and UVIT (Murthy et al., 2017).
The photometric calibration of the instrument will also follow the procedures
established by previous UV space missions such as HST (Bohlin, 1986), GALEX
(Morrissey et al., 2007), and UVIT (Rahna et al., 2017). We will use a set of
well studied standard stars, such as the UV-bright white dwarfs LB227 and HZ4,
for the in-flight calibration of the instrument (Bohlin et al., 2014). The
ground and on-orbit calibrations will help us identify the photometric errors
as a function of NUV magnitude, which we would then use to set the criteria
for a transient event (say, $>5\sigma$). These open source software would be
made available to the community, to generate calibrated light curves from the
photon event lists.

##  7 Summary and Conclusions

We have described the science motivation, instrument design, and calibration
procedures for the wide-field NUV imager, NUTS. The mission attempts to fill
the gap in available UV resources and would demonstrate the capabilities of a
wide field camera for UV transient surveys. Currently, we have designed,
fabricated, and assembled the components, and the final calibrations and tests
are being carried out currently. Once in orbit, the instrument will stare at
large areas of the sky and the resulting data will be monitored in real-time
to initiate follow-up observations for interesting transients. This will also
be followed up by ground-based optical telescopes, such as e.g. the Himalayan
Chandra Telescope (HCT) of the Indian Astronomical Observatory (IAO), Leh,
Ladakh.

##  8 Acknowledgments

Part of this research has been supported by the Department of Science and
Technology (Government of India) under Grants IR/S2/PU-006/2012 and
EMR/2016/001450/PHY. This project was funded in part by the Austrian Science
Fund (FWF) [J 4596-N]. MS acknowledges the financial support by the Department
of Science and Technology (DST), Government of India, under the Women
Scientist Scheme (PH), project reference number SR/WOS-A/PM-17/2019. We thank
Mr. S. Sriram and all the staff at the M. G. K. Menon laboratory (CREST) for
helping us with optics assembly and storage of payload components in the
cleanroom environment.

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