1. Grisms Michael Sholl Space Sciences Laboratory 29 March 2003 Practical implementation for SNAP

2. Dispersive Elements

3. NICMOS Grisms Resolving power: 200 Central wavelengths: 0.967, 1.414, 2.067μm Bandpass: 0.8-1.2, 1.1-1.9, 1.4-2.5μm Interference filters made by OCA Spacing: 44.82, 30.66, 20.97 lines/mm Wedge angles: 5.8, 5.8, 5.9 Image taken through wide-band filter for calibration (slitless) No collimating optics (f/45 beam) Materials: fused silica and CaF (high-λ throughput) 1cm size

4. GRISM Design Parameters Interference filters are used to prevent order overlap Resolving power: 100-200 (λ/Δλ)  easy to achieve --Increases with wedge angle --Increased by more rulings (larger grism, or smaller spacing) Overall bandpass: 0.35-1.7μm --fused silica acceptable for throughput Beyond 2.5μm, would use CaF, ZnSe or sapphire --Individual grism bandpass limited by order overlap Use interference filter on grism front surface Must use several grisms to span bandpass Practical constraints --Desire one interference filter design for all grisms (performance degraded off-axis  constrains wedge angle) --Replica gratings produced in discrete line spacings  vary wedge angle slightly to achieve desired R ~f/22 beam: collimating optics required for spatial resolution? 5cm x 5cm field of view?

5. Potential GRISM Designs Goal: ~200 resolving power (easy to go down to 100) Span 0.35-1.7 μm bandpass Grating lengths: 1cm (size of collimated beam) (tertiary-focal plane: ~ f/11) Materal: fused silica Pixel Size: 10.5um Prism/focal plane angle: 3deg λ (μm)Wedge (°)d (μm)/ (lines/mm) RGFD1 (cm) GFD1. 5 (cm) GFD2 (cm) 0.44 (0.35-0.55)2.422.8/43.82011116.522 0.64 (0.5-0.8)3.522.8/43.82017.611.315.1 0.88 (0.7-1.1)4.822.8/43.82015.58.211.0 1.36 (1.0-1.7)7.422.8/43.82033.65.37.1

6. GRISM Configuration 02 In front of focal plane Lens axes normal to slant ray Out of plane dispersion

7. GRISM Configuration 03 In front of focal plane No collimating optics Out of plane dispersion

8. GRISM Configuration 11 Behind Focal Plane Good stray light Images focal plane Out of plane dispersion

9. Large field of view: 5cm x 5cm Back side of focal plane: requires large lenses to collimate and refocus Front side of focal plane --stray light worse (need to quantify) --can we do without collimating optics? --quantify spot size and shift with grism Grating lengths: size of f/11 beam at grism Prism/focal plane angle: 3deg

10. Grism in front of focal plane 5cm by 5cm grism Distance from focal plane: 11cm 2.4° wedge angle Line spacing: 22.8µm Design central wavelength: 0.44 µm Inplane dispersion

11. Image degradation/shift Configuration1-σ X (μm)1-σ Y (μm)Shift X(μm)Shift Y(μm) Standard1.312.3000 Prism only12.528.121480 Grism1.312.3000

12. Conclusions: Grisms may be added to SNAP focal plane Decision on R TBD Decision on # of pixels/Δλ TBD Stray light analysis TBD