The Cassegrain ADC is designed to operate over the wavelength
range 3200 Angstroms to 10,000 Angstroms, completely covering the
useful operating range of LRIS.
The first figure below shows the different refraction for LRIS in
without any dispersion compensator. At zenith (zenith
distance=0°, elevation=90°, airmass=1.0) the light is not
refracted and thus the dispersion is zero. At an zenith distance
of 60° (elevation of 30° , airmass=2.0) the differential
refraction between light at the UV and IR limits is over 3 arcsec,
and at zenith distance of 72° the differential refraction is
nearly 6 arcsec.
The following figure shows the effect of enabling dispersion
compensation with Cass ADC. The differential refraction is
negligible from zerith to zenith distance of 55° and is still
well under 1.0 arcsec at 60° over the full wavelength range
from 3200 to 10,000 Angstroms.
The designed operating range for the Cass ADC is from zenith
(elevation 90°) to airmass=2.0 (elevation 30°). As shown
in the preceding figure, the dispersion over the entire operating
range of LRIS is under 1.0 arcsec over this entire range. Cass
ADC can be used at elevations below 30° but will be unable to
correct fully for the effects of differential refraction.
However, the image quality produced with Cass ADC should still be
far superior to that without.
The Cass ADC was designed to deliver throughput of 95% or greater
over the full operating wavelength range of LRIS, with higher
throughput in the red. Complete information is contained in a technical
note by Drew Phillips of UCO/Lick.
Due to its excellent SolGel anti-reflection coatings, the Cass ADC
shows very little ghosting. This technical
note from Drew Phillips describes the expected ghosting
The Cass ADC module modifies the telescope focal plane slightly.
The Keck I telescope's control system (DCS) accounts automatically
for these effects:
Both corrections have been tested and appear to work as expected.
- Pointing. The presence of the Cass ADC optics
changes the pointing of the telescope slightly. The image shift
varies as a function of the prism separation and must be
continuously calculated by the DCS.
- Focus. To keep the telescope focal plane
coincident with the LRIS slitmask plane, the secondary mirror
position must be changed. Again, this correction is a
function of the prism separation and thus DCS must regularly
compute and apply the needed correction.
Plate Scale Change
The Cass ADC changes the image scale at the LRIS focal plane by a
small but measurable amount (less than 1 arcsec across the 8
arcmin field of view). If not accounted for, this effect would
case slit losses due to target/slit misalignment for slitmask
observers. Hence, two implications for those using slitmasks with
- Observers should not use LRIS masks designed for use
without the Cass ADC module when observing with LRIS-ADC.
- Observers using slitmaks must download a revised version of
the slitmask design software for LRIS. Judy Cohen has
released version 3.09 of her AUTOSLIT package, and Drew
Phillips has created a similarly corrected version of his
FABMASK software for LRIS mask design.
Tests with the LRIS polarimeter module and the Cass ADC
shows that the impact of the Cass ADC on polarimeter observations
is very small. Full details are provided in