Short Wave Infra-Red Hyper Spectral Imager version 7 (SWIR HSI v7)

Fred Sigernes^1,2, Marie Bøe Henriksen ², Sivert Bakken ², Joseph Garrett 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

First, the source illumination in the SWIR region can be identified in Figure 1. The wavelength region 1000 - 1600 nm is roughly less than 50% in intensity compared to the visible part of the spectrum and dominated by two atmospheric water absorption bands.

The design is inspired [1] by the pushbroom HSI v6 onboard the HYPSO-1 satellite [2-4]. All components are off-the-shelf. Three 50 mm focal length SWIR objectives from the company Kowa is used in combination with a Volume Phase Holographic (VPH) grating from Wasatch Photonics.

The detector is an industrial camera using an InGaAs (iridium gallium arsenide) image sensors from the company Pembroke Instruments.

The optical diagram is shown in Figure 2. A 250 lines/mm Volume Phase Holographic (VPH) grating is the key element. The efficiency is above 50% for the wavelength region 1000 - 1600 nm. The effective aperture may be set to D₀ = D₁ = 24 mm which corresponds to ~ F/2. The input slit width is fixed with no magnification or demagnification of the height (h = 10 mm). A slit width of w = 50 µm will result in a first order spectral bandpass (FWHM) of approximately 4 nm.

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.

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 90-degree mirror can be used between the collimator and the grating to make the design more compact in size.

Item	Part / links	Description	Qty	Cost $
1	Kowa LM50HC-SW	50 mm SWIR C-mount Objective	3	4347
2	Thorlabs SM1A10	Adapter ring SM1 - C- mount internal	2	44
3	Thorlabs SM1M10	SM1 lens tube 1 inch long with internal threads	1	17
4	Thorlabs S50LK	Fixed high precision mounted slit	1	120
5	Thorlabs SM1RC	Slip Ring - SM1 tubes	1	27
6	Thorlabs Spacer Rings	Thorlabs C-mount 0.25-2mm space ring kit	1	119
7	WP-250/1250-xx	Volume Phase Holographic (VPH) grating	1	625
8	Pembroke Instruments	SWIR NIT InGaAs sensor (12.8 x 10.24) mm2	1	29 065
9	3D printer material	PRUSA Jet Black PETG filament	1	27
		Total	11	34 391

Table 1. Detailed part list HSI v7 SWIR.

A pushbroom Short Wave Inra-Red Hyper Spectral Imager design (SWIR HSI v7) is described using off-the-shelf components. The spectral range is 1000 - 1600 nm with a spectral resolution less than 5 nm. Spatial resolution will be comparable to the HSI v6 on board the HYPSOP-1 satellite. Total part cost of prototype is estimated to be close to 35k USD.

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/