CoDR Optics Supplement: Overview

1. CoDR Optics Supplement:Overview • Extended Performance, 0.31m to 1.1m • Throughput Estimates (best data) • Ghosting (simple discussion) • New issues: -- New “full-aperture” ADC design (2.5º prisms) -- Differential distortion -- Effects of displaced focal surface

2. ExtendedPerformance (0.31--1.1m) • Modeled both full- and sub-aperture designs -- there’s virtually no difference between the two for dispersion correction. • Full correction at z=60 requires increasing prism separation by 20% over current designs (planned). • Fully-corrected residuals are 0.07" rms and 0.22" pk-to-pk. • Correcting to 0.31m (vs 0.32) is easy; 1.1m most difficult. • UV PSFs are sim- ilar to 0.4m PSFs.

3. Corning provided numbers for UV-grade Fused Silica: result is about 1% loss for 70mm of glass at nearly all wavelengths. Sol-gel/MgF2 coatings appear fine (≤1% loss per surface), although performance below 0.4m is still calculated, not measured -- but this should simply require reducing thickness of the coatings by 20% over the 0.4--1.4m case. (0.4--1.1m GMOS Sol-gel measured by J. Stilburn.) LLNL has facilities capable of coating the prisms. Throughput Estimates: ≥95% (“est.” assumes 1% loss per surface)

4. Throughput, cont’d • Tuning of coatings will trade off performance in the UV vs IR. Do we want to push coatings to 0.31m (extinction)? What throughput is needed at 0.31m?

5. Ghost Images (Simple Discussion) • Six surface pairs potentially produce ghosts. • Ghosts involving non-parallel surfaces (4 pairs) will miss the mirror/grating (14.5°/7.25° > 5.6°). • Parallel surfaces (2 pairs) will produce ghosts at a level 10-4, but these rapidly become out-of-focus and much lower contrast. • Only the inner prism surfaces are likely to be a problem, and even there when the ghost level is above 10-6, the ghost image will appear within the PSF wings.

6. Spot locations measured at 9 locations and three orientations, with prism separation 850mm (full correction at Z=60), as well as null case. For each case, telescope was re-pointed to position Spot B at the field center; then a least-squares fit to translation and rotation performed. Differential Distortion Spot locations: E G I A B C D F H

7. Differential Distortion At non-zero angles, rotation is present (up to 0.001°)

8. (end presentation)

9. Effects of Displaced Focal Surface • Telescope must be re-pointed (by 50", handled by guider) • Displacement of curved focal surface introduces focus tilt. (OK for LRIS if tel. focus is adjusted) • It produces dominant differ-ential distortion. (tolerable, if pointing adjusted) • Moves edge of vignetted region around edge of LRIS field (potential flat-fielding problem) • Can we correct with M2??

10. Spot sizes for null-ADC and no ADC are virtually identical Spot sizes measured at 9 locations and three orientations, with prism separation 850mm (full correction at Z=60) Spot Sizes Spot locations: E G I A B C D F H

11. ADC Spot rms-Diameters (") (calculated at 5000A)

12. ADC Spot 80%-Diameters (") (calculated at 5000A)