Prism
This page summarizes the optical design and laboratory-measured spectral performance of the Prism slitless spectrometer for the Wide Field Instrument (WFI), as reported in Bray et al. (2024)
Prism Specifications (On-axis)
Parameter | Value |
|---|---|
Bandpass | 760–1800 nm |
Resolving Power (R) | ~100–180 (2-pixel resolution element) |
Throughput (peak) | ~91% |
PSF Quality | Not diffraction-limited |
Beam Deviation | Zero at 1550 nm |
Optical Design
Two-element transmissive design:
P1: S-TIH1 (dispersing element)
P2: CaF₂ (correcting optic)
No anti-reflection coatings or gratings.
Optical path is aligned to produce zero beam deviation at 1550 nm.
Figure 1. Zemax diagram of the Prism assembly. The highly wedged P1 element provides dispersion, while P2 cancels angular deviation.
Dispersion Scale
Measured using a comb filter tailored to the Prism dispersion profile.
Smooth variation across field and band; model match is good with ~0.7% shorter spectral trace due to slight glass index difference.
Three camera configurations used to cover full bandpass
Figure 2. Prism dispersion scale vs. wavelength across field positions. Differences from model are small and consistent.
Ghost Image Analysis
Two ghost images detected, caused by internal reflections (no grating involved).
Largest ghost is located ~70–76 mm from parent trace.
Integrated brightness is only 0.018–0.028% of parent flux.
Ghost irradiance is ~10⁻⁷ relative to main spectral trace—below background requirements.
Figure 3. Comparison of Prism science trace (left) and ghost image (right) for a blackbody source. Ghost is faint and spatially well-separated.
PSF and Encircled Energy
PSF is not diffraction-limited, especially off-axis.
Measurements performed at circle of least confusion using LLTF laser input.
Results consistent with optical model within ~20% variation.
Best-focus determined manually by rotational symmetry of the PSF.
Dispersion Clocking
On-axis dispersion aligned with –Y direction.
Measured clocking angle: –3.6 ± 3.9 mrad, consistent with model (0 mrad).
Prism dispersion direction remains consistent across field.
Bandpass Edge Calibration
Measured using spline fits due to ripple in transmission near edges.
Edge positions shift with field angle and polarization (e.g., up to several nm off-axis).
Blue edge has a steeper slope, by design, for improved wavelength localization.
Throughput
Measured at 29 wavelengths using photodiodes and integrating spheres.
Excellent agreement between model and measured values.
Peak throughput exceeds 0.91 across most of the 800–1600 nm range.
Test Setup
Calibration performed using the Ellipse Test Bed:
Large off-axis elliptical mirror (F/7.1)
Tunable supercontinuum lasers and comb filters
Motorized hexapods for tip/tilt and focus control
Data acquisition via custom LabVIEW scripts
References
- Bray et al. 2024, JATIS, 10(1), 014003