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Backgrounds (Sky and Thermal)

Sky frames were taken from the 13th-17th of November, 10th December and were reduced using the following primitives:

Subtract Dark Background

Load Wavelength Calibration

Assemble Spectral Datacube

Interpolate Wavelength Axis

The sky flux was then calculated as the median value within each spectral channel, divided by the integration time per coadd (ITIME keyword).


Y_direct mode skies were obtained on the 13th and 17th. On the 13th the flux was ~0.25 ADU/s, and on the 17th the flux was at a significantly higher level of ~1.75 ADU/s. The only apparent difference between the skies were the exposures times, (~30s on the 13th and 9s on the 17th). Proximity to the Moon can be discounted to explain the high counts on the 17th, as it was more than 20 degrees away at the time. Based on the IR sky background, the sky flux at Y should be similar to or slightly higher than J, suggesting that the measurement on the 13th is incorrect. No Y_coron skies were obtained.


J and H bands

J and H counts in the H-direct mode were consistent over the the Nov run. Adding the lyot mask and apodizer but not occulter didn’t decrease the sky level in either filter. This is surprising and not yet understood given the throughput measurements below. No measurements in the H-coron mode were taken on the 13th, although measurements taken on the 14th in the H-coron mode show a significant drop in flux. 

K1 and K2 bands

For the sky frames obtained in K1_coron and K2_coron obsmodes, the flux levels remained roughly the same at <1 ADU/s for K1 and <2.5 ADU/s for K2 on the three different dates. The flux level in the direct obsmode varied significantly between nights.

Average and standard deviation of the median sky count across the band for various obsmode configurations

Filter Mode Date Background Levels
Y Y_direct 17th Nov 2013 1.80±0.01
J J_direct 17th Nov 2013 1.62±0.18
H H_coron 13th Nov 2013 0.62 ± 0.06
H H_direct 13th, 15th, 17th Nov 2013 0.62 ± 0.14
K1 K1_coron 16th, 17th Nov, 10th Dec 2013 0.42 ± 0.11
K1 K1_direct 13th Nov 2013 0.84 ± 0.01
K1 K1_direct 15th Nov 2013 7.08 ± 0.39
K1 K1_direct 17th Nov 2013 3.02 ± 0.01
K2 K2_coron 16th, 17th Nov, 10th Dec 2013 1.20 ± 0.13
K2 K2_direct 13th Nov 2013 2.23 ± 0.05
K2 K2_direct 15th Nov 2013 8.41 ± 0.51
K2 K2_direct 17th Nov 2013 4.12 ± 0.17

ADU/s for sky frames taken in coron (left), unblocked (middle), and direct (right) obsmodes.



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