Since we are building a hyperspectral imager, it is indispensable to have a spectrometer against which we can check the spectral accuracy of our imager. While we can’t budget for a commercial SWIR spectrometers, we actually have most of the components (worth ~90% of the total cost) that it takes to build it. In doing so, we can build our optical and opto-mechanical knowledge that would directly transfer to our hyperspectral imaging payload design, while also giving Science a new instrument with which they can validate their measurements. The Data Processing team will write a GUI to display the spectrum from the camera image, and test their keystone & smile correction algorithm approach on real hardware.
The spectrometer design is actually quite similar to what we’ll be building for our hyperspectral imaging payload. While the optical design of a spectrometer is not difficult at all, it is the construction of the optomechanical housing which poses a more involved engineering challenge. While the optical engineering team simply specifies the size and shape, as well as positioning of optical elements (lenses, mirrors, gratings, filters, etc.) relative to each other, it is the optomechanical engineering team’s task to ensure these components are held in place notwithstanding temperature changes and vibration. In addition, the housing should be designed with consideration to fabrication and assembly, and also subsequent adjustment / alignment of optical components within.
Take for example, a camera lens. In order to provide a sharp focus and good imaging quality, the spherical lens surfaces are precision ground and polished to often lambda / 4 accuracy, which for optical wavelengths (~0.4-0.7µm) works out to be <150nm. As a consequence, the tolerance on positioning components is fairly tight - an axial displacement of 50µm (0.05mm) would reduce the optical system performance by 50% ! Since it is impractical and uneconomical to specify such tight tolerances on the mechanical housing, the optical system is typically designed to have a focus adjusting cam barrel which moves a group of lenses relative to the sensor so the operator can manually restore focus if it drifts. The field of optomechanics is quite interesting as it calls engineers to develop a robust and high-precision (and mostly-static) mechanism that enables the optical performance of the system!
https://exclusivearchitecture.com/03-technical-articles-CLT-03-lens-barrel.html
What is a spectrometer - explain the optics and the optical components inside it to an optomechanical engineer
Special considerations for our spectrometer
A high-level overview of the spectrometer prototype project was discussed today. This prototype will help UTAT become more familiar with the challenges in designing a folded optical system and managing the transfer from optics to optomechanics. We seek to have the system designed by end of June and assembled in August 2026.
The spectrometer will be composed of a custom-machined mechanical housing, which holds two spherical mirrors and a planar ruled grating in a Czerny-Turner configuration. Input light will be fed via linear fiber bundle patch cable through a slit, and collected by , with a field stop controlling the Numerical Aperture (NA) of light collected by the spectrometer. Coming from the mirror-grating-mirror system, light will be imaged onto a camera sensor that will be attached externally to the spectrometer housing.
Considering our interest in both the visible (VIS) and short-wave infrared (SWIR) range, we will provision for the diffraction grating to be exchangeable - one grating for the VIS range, and one for the SWIR range. Mirrors will not need to be readjusted because they are achromatic. Differences in the wavelength-to-angle dispersion can be addressed by selecting appropriate gratings, e.g., 300 l/mm for a 400nm blaze would produce equivalent fields as a 100l/mm for a 1200nm blaze (with a minor reduction in diffraction efficiency when the incident angle is kept the same). The lab already has camera sensors - which typically make up ~50% of the cost of a commercial spectrometer - for both ranges. Since these cameras are needed for other purposes, they will have to be detachable and readily interchangeable using appropriate mechanical adapter plates.
The project will have a design and simulation component, as well as a manufacture and assembly component. For the simulations, we will: