Comparison to measured IRCAL results

The background and limiting magnitudes for the existing camera and AO system were measured and reported in [12] and are reproduced in Table 2. The model had to be adjusted to include off-axis emission because the IRCAL cold stop is 18% oversized. Strehl ratio for the $ K$ band was derived from [13] and scaled for other bands using the relation:

$\displaystyle S = \tr {exp}\left[-\left(\frac{\lambda_0}{\lambda}\right)^2 \log\left(\frac{1}{S_0}\right)\right]$ (5)

where $ S_0$ is the reference Strehl (0.42 in LGS mode and 0.65 in NGS mode in the $ K$-band) and $ \lambda_0$ the central wavelength of the reference filter. Filter curves were derived from those displayed in the IRCAL manual on the Lick Observatory website.3

Table 2: Mt. Hamilton measured sky background by the Gemini instrument and IRCAL. IRCAL J-band sky background was not reported in [12] but estimated later. The values above are from the IRCAL manual on the Lick Observatory website.
Filter Gemini sky b/g IRCAL sky b/g Total
  mag arcsec$ ^{-2}$ mag arcsec$ ^{-2}$ Throughput
J 16.1 16.0 0.08
H 14.3 14.4 0.16
K 13.0 9.3 0.13
Ks   10.3 0.14

Gemini in Table 2 is an older infrared camera and the measurements for it are taken from [14]. The sky backgrounds predicted by the ShaneAO model with no dust coating the optics and a 3% dust coating for the existing system (IRCAL) are in Table 3. The predicted values are in good agreement with the measured values.

Table 3: Mt. Hamilton predicted sky background and throughput numbers by the model of the current Shane AO system with IRCAL.
Filter dust=0.0 dust=0.03 Avg trans
  mag arcsecs$ ^{-2}$ mag arcsecs$ ^{-2}$ dust=0.03
J 15.80 15.80 0.112
H 13.80 13.77 0.131
K 9.75 9.31 0.136
Ks 10.53 10.10 0.139

Compared to Keck, the dust fraction had to be increased to 3% to better match the IRCAL system's magnitudes and throughput. Since Mauna Kea (the site for the Keck telescopes) is at $ \sim 3.5$ times the altitude of Mt. Hamilton, increasing the dust fraction for IRCAL by a similar factor seems reasonable. Coating degradation (2.5% for Aluminum and 1% for other surfaces) is the same for both comparisons.

Given the variation in NIR sky emission, the numbers in Tables 2 and 3 are in very good agreement. Further validation is provided by comparing limiting magnitudes for point sources using a natural guide star (NGS) as measured and reported in [12] and available online4 versus those predicted by our model in Table 4. The unusually low measured throughput in the $ J$ band is not reproduced by the model but the decrease in throughput between $ J$ and $ H$ bands is.

Table 4: Point source limiting magnitudes (Vega system): IRCAL measured and predicted by the model (300s Fowler sample - 16 reads, SNR=5)
Filter IRCAL IRCAL model
  measured predicted
J 21.8 21.46
H 20.5 20.45
K 17.8 18.15
Ks 18.3 18.34

Srikar Srinath 2013-10-09