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Preliminary design plan

Near Infra-Red Hyper Spectral Imager version 7

Fred Sigernes1,2, Marie Bøe Henriksen 2, Sivert Bakken 2, Joseph Garrett 2, Eirik Selnæs Sivertsen 2, Roger Birkeland 2, Torbjørn Skauli 3 and Tor Arne Johansen 2

1 The University Centre in Svalbard (UNIS), Norway
2 Norwegian University of Science and Technology (NTNU), Trondheim, Norway
3 University of Oslo, Oslo, Norway

Abstract
This page is constructed to design a Near Infra-Red Hyper Spectral Imager version 7 (NIR HSI v7). The aim is to find a suitable instrumental design as payload for the next generation HYPSO-3 satellite.

Created: 23 October 2023 - Last update: 21 February 2024

Source illumination
Fig. 1. Solar illumination adopted from [5].

First, the source illumination in the NIR region can be identified in Figure 1. The wavelength region 700 - 1100 nm is less intens compared to the visible part of the spectrum and dominated by two atmospheric absorption bands (O2 and H2O).

Optical design
The design is inspired [1] by the pushbroom HSI v6 onboard the HYPSO-1 satellite [2-4]. The equation for a head on illumination of a transmitting grating is essential for the construction

n λ = a sin β,

where n is the spectral order, λ is the wavelength, a the groove spacing, and β the diffracted angle.

Fig. 2. Optical diagram of Hyper Spectral Imager version 7 (NIR HSI v7) using a NIR blazed grating from Thorlabs

The optical diagram is shown in Figure 2. A 300 lines/mm blazed grating is the key element. Blaze angle is 31.7 degrees. The efficiency is above 55% for the wavelength region 700 - 1100 nm. The effective aperture may be set to D0 = D1 = 18 mm which corresponds to ~ F/2.8 The input slit width is fixed with no magnification or demagnification of the height (h = 10 mm). Field Of View (FOV) along slit equals 11.4o. A slit width of w = 50 µm will result in a first order (n = 1) Full Width Half Maximum (FWHM) spectral bandpass of 3.33 nm.

All components are off-the-shelf. Three 50 mm focal length NIR objectives from the company Edmund Optics (EO) is used in combination with a standard blazed transmitting grating from Thorlabs. The detector is a Black Silicon CMOS sensor from the company SiOnyx, LLC.

Front lens - slit - collimator assembly
The front lens focuses light from infinity onto the entrance slit plane. Light through the slit is collimated by the collimator lens to produce a parallel light beam that will illuminate the grating. These two lens objectives are chosen to be identical to preserve image quality through the system. The same mechanical solution as presented in [3] is used. See Figure 3.

Fig. 3. Mechanical solution for front lens, slit and collimator lens assembly. Note that brass should not be used in space due to outgassing.

Instrumental enclosure
The above parts are sufficient to create a prototype hyper spectral imager. All parts may now be embedded into a 3D printed design or a metal housing according to the angles and positions defined in the optical diagram. Note that a right-angle prism can be used between the collimator and the grating to make the design more compact in size. See figure below.

Fig. 4. Instrumental sketch: Compact version of HSI v7 using a right-angle prism.

Fig. 5. Left: 3D printed press fit base. (1) is font lens, (2) slit housing (tube), (3) collimator lens, (4) right-angle prism, (5) transmitting grating, (6) detector lens, and (7) Sionyx development board. Right: Assembled instrument. (1) is camera head, (2) front lens, and (3) press fitted 3D housing by six 60 mm long M4 bolts and nuts. Size and dimensions are according to Fig. 4.

Test of the SiOnyx sensor
The camera sensor evaluation kit (XRB-1350-PDK) was tested with a Schneider Kreuznach F/1.4 CCTV 17 mm focal length lens. A front cut-off filter HOYA R72 was used to cut light below 740 nm. The camera was mounted under a dome at the observatory to a motorized pan-tilt unit from the company Directed Perception (PTU-D47-70). This setup enables us to quickly point the camera to any sky target of interest using the SvalPoint tracking system.

Snapshot
A snapshot image of a low intensity (1-2 kR) red colored post noon auroral arc was identified as soon as we turned the system on. See image below. Default automatic mode of camera was used. The gain was 3200 with an exposure time of just 11 msec running at 90 FPS.

Fig. 6. SiOnyx camera snapshot of post noon auroral arc at 12:55 UT, 9th of January 2024. Exposure time is 11 msec with maximum gain at 3200. Framerate is 90 FPS. The image is resized by a factor of 50%.

Movie
A 60 second video timelaps is shown here.

Note that the video is scaled down 4 times to reduce storage and the real duration is only 16.7s. Nevertheless, the above auroral experiment indicates that the sensor is low light sensitive and a promising candidate for our hyperspectral imager.

Wavelength Calibration
Fig. 7. HSI v7 Wavelength Calibration. The measured Hydrogen gas discharge tube spectrum (blue curve) is the average response across the entire slit of the recorded spectrogram. The red curve shows the synthetic Hydrogen spectrum with a triangular instrumental line function that corresponds to a Full Width Half Maximum (FWHM) equal to 3.3 nm.

The assembled instrument was wavelength calibrated using a hydrogen gas discharge spectral tube and an UV/NIR cut-off filter EO #89795. The first order Hα line and the second order Hβ line was identified, and the spectral range is calculated to be from the visible red into NIR (618 - 1243 nm). This procure was applied since the sensor is highly sensitive in the visible part of the spectrum [6].

A red colored filter is needed to cut the second order visible part of the spectrum. A mounted filter (EO #46-545) should be installed on the front lens. This must be done prior to sensitivity calibration.

Prototype parts list

Item Part / links Description Qty Cost $
1 EO VIS-NIR 50 mm 50mm C VIS-NIR Series Fixed Focal Length Objective * 3 1785
2 EO 2nd order filter M30.5 x 0.5 mounted Red filter 1 54
3 Thorlabs SM1A10 Adapter ring SM1 - C- mount internal 2 45
4 Thorlabs SM1M10 SM1 lens tube 1 inch long with internal threads 1 17
5 Thorlabs S50LK Fixed high precision mounted slit 1 122
6 Thorlabs Spacer Rings Thorlabs C-mount 0.25-2mm space ring kit 1 121
7 Thorlabs GTI25-03A-NIR (25 x 25) mm 2 Blazed Trans. grating (300 grooves/mm) 1 118
8 Thorlabs right-angle prism N-BK7 Right-Angle Prism, Uncoated, L = 25 mm 1 65
9 Sionyx RD board Black Silicon sensor (12.3 x 9.9) mm2 1 790
10 3D printer material PRUSA Jet Black PETG filament 1 27
Total 13 3144

Table 1. Detailed part list NIR HSI v7. * Possible lens candidate: KOWA 50 mm.

Summary
A pushbroom Near Infra-Red Hyper Spectral Imager design (NIR HSI v7) is described using off-the-shelf components. The spectral range is approximately 600 - 1200 nm with a spectral resolution less than 4 nm. Spatial resolution and senstivity is expected to be comparable to the HSI v6 on board the HYPSOP-1 satellite. Total part cost of prototype is estimated to be less than 4k USD.

References
  1. Fred Sigernes, Mikko Syrjäsuo, Rune Storvold, João Fortuna, Mariusz Eivind Grøtte, and Tor Arne Johansen, Do it yourself hyperspectral imager for handheld to airborne operations, Opt. Express 26, 6021-6035 (2018), https://doi.org/10.1364/OE.26.006021
  2. M. E. Grøtte, R. Birkeland, E. Honore-Livermore, S. Bakken, J. L. Garrett, E. F. Prentice, F. Sigernes, M. Orlandic, J. T. Gravdahl, T. A. Johansen, Ocean Color Hyperspectral Remote Sensing with High Resolution and Low Latency - the HYPSO-1 CubeSat Mission, IEEE Trans. Geoscience and Remote Sensing, Vol. 60, pp. 1-19 (2022), https://doi.org/10.1109/TGRS.2021.3080175
  3. M. Henriksen, E. Prentice, C. van Hazendonk, F. Sigernes, and T. Johansen, Do-it-yourself VIS/NIR pushbroom hyperspectral imager with C-mount optics, Opt. Continuum 1, 427-441 (2022), https://doi.org/10.1364/OPTCON.450693
  4. S. Bakken, M. B. Henriksen, R. Birkeland, D. D. Langer, A. E. Oudijk, S. Berg, Y. Pursley, J. L Garrett, F. Gran-Jansen, E. Honore- Livermore, M. E. Grøtte, B. A. Kristiansen, M. Orlandic, P. Gader, A. J. Sørensen, F. Sigernes, G. Johnsen and T. A. Johansen, HYPSO-1 CubeSat: First Images and In-Orbit Characterization, Remote sensing, 15(3), 755 (2023), https://www.mdpi.com/2072-4292/15/3/755
  5. Post by Harron, All you need to know about Solar Radiation, http://synergyfiles.com/2016/05/solar-radiation/
  6. Mark Crawford, Black silicon is ready to revolutionize photoelectronics, SPIE, The international society for optics and photonics, 08 December 2008.