ICON FUV Optical Design Requirements

1. ICON FUV Optical Design Requirements Tim Miller, Stephen Mende, Harald Frey, Tom Immel Sep 25 2013 DRAFT Collection of requirements on the optical design captured from various emails, conversations

2. ICON FUV Overview Slide from Sholl, clean up • 135.6nm channel • Block 130.4nm <1% • 155.0nm channel • 152.5 to 162.5 (31 July 2013) • Wavelength blocking • Czerny-Turner configuration • Block 130.4nm • Pass 135.6nm & 155.0nm • “50 or 80% EE within a pixel would be acceptable” • ±20º scan, dispersion direction (±10º scan mirror rotation along slit) • Dispersion used for wavelength discrimination • Grating ruling density: 4000lpmm • 6x32mm entrance slit • Detector • Sealed tube microchannel plate detectors • 256x256 100 pixels across sensor • 24º circular FOV • 3mm MgF2 at detector • 24mm diameter circular detector • 23mm usable • Liens • Design requires external review • Tolerance analysis • Stray light analysis • MgF2 dispersion model needs to be verified at ICON FUV wavelengths • Design file: • ICON_c7 ZGM_48 MgF2 aped.zmx

3. Fill in the rest…

4. Bandpass • SW channel: • center at 135.6 (AO) • suppress 130.4 (O) < 1% • LW channel: • suppress 164.1 (O) to >10% • avoid 149.3 (NI) • collect as much of N2 LBH light as possible • ~153-162 available, design capability leads to center at 157 nm • ~138-147 too close to SW channel, not feasible

5. Bandpass 164.1 (O)

6. Bandpass 149.3 (N) 164.1 (O)

7. Spatial Resolution required • SW channel: • Used in day, night • req vertical resolution = 4km (night most stringent) • req horizontal resolution = 20km (night most stringent) • Channel affected by steering • Effectively rotates scene, so FUV horizontal imaging must be good enough to meet vertical imaging req * cos(steering angle) • LW channel: • Used in day only • req vertical resolution = 10km, better (4km) desired • req horizontal resolution = 500km • Not affecting by steering • exception: desired occasional 20° offset from zero (see steering range chart)

8. Steering Range • To meet L2 requirement for coverage efficiency >90%: • By Tom’s 9/17/13 charts: • Steering ±20° achieves 78% coverage, violates L2 req • Steering ±30° gets to 97% coverage, incl. occasional coverage gap over Atlantic • Steering ±40° gets to 100% coverage • To meet desire to look where MIGHTI looks: • Diagnostic function only, not a requirement • Steer to ±40° • To meet desire to keep sun out of daytime FOV: • this is occasional situation: ±3 weeks out of year around summer solstice, for ~1/8 of orbit • corresponds to only a few percent of coverage efficiency hit • steer 20° off-nominal in daytime, for these occasions only • may be asymmetric in + or - , but not too different Takeaway: reqt is ±30°, goal is ±40°

9. Glint FOV • Driven by goal to avoid direct LOS from glint source to physical steering mirror; • same concept as solar avoidance, which drove baffle length • daytime concern only • no steering in daytime, except desired occasional 20° offset from zero (see steering range chart) • Instantaneous FOV of scene: 24° • Glint FOV: beyond scene FOV, depends on baffle geometry • currently = ? glint FOV scene FOV

10. Scene FOV • Requires 24deg diameter • Possible that horizontal FOV could be less • up to 25% less, estimated • steering range may have to increase to cover the change

11. Spot Size in FOV • in FOV, small spot size is more important at the center of the horizontal FOV, than at the horizontal edges • unclear why? FOV

12. BACKUP

13. Mike Sholl, Sept 5, v.51 v.51 design does not yet have clear apertures trimmed (trivial to model)

14. Photon Engineering, Aug 27

15. Requirements Discussion • LW channel is only used during the day • Can afford enough light loss to add a mirror to LW path, i.e. a fold • LW channel can allow worse horizontal imaging than vertical • Can afford worse horizontal spot size, i.e. simpler optics in LW path