Optical Design Studies to Date

Keck ADC Studies culminate in Nelson & Mast (2000/2002) Report

• Adopted Linear ADC design as most feasible.
• Coarsely explored optimal parameters using ZEMAX.
• Found acceptable image quality with a design using 5-deg fused silica prisms with separation up to 680 mm, parallel prisms on the telescope optical axis, with inner faces perpendicular to axis.
• Ghosting/reflections not a problem
• Mast (2002) report shows tolerances of optical elements which are "easy" to meet.

Major differences between current design and Nelson & Mast

• They place ADC 6.5-m in front of focal plane (ghosting?)
• They were designing for the full 20-armin daimeter Keck field (possible LRIS improvement by varying parameters?)
• They were designing for 0.4--1.3 micron wavelength range (going down to 0.32 microns more difficult)

Throughput and Coatings

• Want the highest through-put we can achieve. Two components:
• There is some uncertainty of the transmission of fused silica; it appears to be around 0.99 per 10mm. However, the thickness of our prisms (70mm total on any optical path) makes the exact value important:

         Trans/10 mm        Trans/70 mm
            0.985             0.90
            0.99              0.93	**
            0.995             0.965
  

• Best coatings (4 surfaces) are Sol-Gel over MgF2. There are measurements of this at near 0.995 over almost the whole range we want (0.32--1.1 microns). Even at ends of range, transmission is near 0.99. This gives:

         Trans/surf        Total (4 surfaces)
            0.99              0.96
            0.995             0.98
  For comparision, MgF2 is:
            0.98              0.92
  

• Disadvantages of SolGel: "soft" (but can be "hardened"); developing technology; lifetime?; possible high cost.
• Total expected throughput: around 0.90.