Sky Frames Continued

I have also examined files that were taken at the same setting for different objects throughout night one of commissioning.  The setting used was wave number 1548.46 cm^(-1).  Below is the list of files I examined along with information about the observing.

acma.flat.30028.fits  
   altitude=38kft
   issues with boresite
   TELEL=40.6

*changed modes*

aboo.flat.30055.fits
   assuming altitude=41kft (last noted altitude in notes)
   TELEL=54.4

bgem.flat.30060.fits
   altitude=43kft 
   TELEL=29.1

alyr.flat.30068.fits
   altitude=43kft
   TELEL=38.8

*changed modes*

mucep.flat.30085.fits
   altitude=43kft
   TELEL=34.6

I created MCDS 7 frames for each objects observation.  By examining the MCDS frames, there is an obvious shift in the spectral direction for acma.flat.30028.fits compared to the other frames (see image below).  I do not know why this is.  It was taken earlier in the night at a lower altitude compared to the other frames.

Figure 1:  The zoomed-in frames are from left-to-right, top-to-bottom:  alyr.flat.30068.fits,   aboo.flat.30055.fits,  acma.flat.30028.fits,   bgem.flat.30060.fits,  mucep.flat.30085.fits.  The green arrow is placed in the same position on each file.  The green arrow is offset from the line for file acma.flat.30028.fits.

Next I made difference images from all of the objects.  There will obviously be a big offset for any image subtracted with acma.flat.30028.fits due to the offset.  Below is the figure that shows a zoomed in view of the difference images.

Figure 2: The frames are from left-to-right, top-to-bottom: (28-85), (28-55), (28-60), (28-68), (55-60), (55-68), (55-85), (60-68), (60-85), (68-85)

The residuals that are seen could be due to several different things, e.g., air mass difference, seeing variability.  I am not sure the best way (or if it is possible) to isolate the cause.  It does not appear to be due plane exhaust, though, since the telescope elevation does not seem to correlate with the residuals.

Examination of Sky Frames

Looking for sky variations to try to quantify how much things change as a function of time and as a function of elevation.  Using the sky frames taken near the end of commissioning night 1.

1376cm-1; high-med; staremode; N6 S0 N5 D0
sky.sci.30076.fits elevation=40 deg, 16 repeats
sky.sci.30077.fits elevation=20 deg, 10 repeats
sky.sci.30078.fits elevation=25 deg, 8 repeats
sky.sci.30079.fits elevation=25 deg, 4 repeats

To complete the analysis, I first made each frame into an MCDS 7, which meant that sky.sci.30076.fits had 16 frames to examine, sky.sci.30077.fits had 10 frames to examine, etc...  Below is a picture of an MCDS 7 frame for each image.

Figure 1 ( Top left: sky.sci.30076.fits; Top right: sky.sci.30077.fits; Bottom left: sky.sci.30078.fits; Bottom right: sky.sci.30079.fits)

To examine any variations with respect to time, I subtract each subsequent MCDS frame from the previous one for each image.  This means that for sky.sci.30076.fits there are 15 data points, for sky.sci.30077.fits there are 9 data points, etc...  The frames visually subtract very well (see Figure 2 below).  When examining the median value across a 200x200 pixel section in the middle of each difference image, the values appear consistent with zero (see Figure 3 below).  One interesting feature is how the frames have larger offsets at the beginning of the exposure.  This may point to some sort of reset issue that takes time to become stable, but the difference in counts is still very small.

 

Figure 2 ( Top left: sky.sci.30076.fits; Top right: sky.sci.30077.fits; Bottom left: sky.sci.30078.fits; Bottom right: sky.sci.30079.fits)

Figure 3 (Comparison of difference in counts between subsequent frames taken at the same elevation.)

The next comparison is with respect to changing elevation.  For this analysis I compared:

sky.sci.30076.fits - sky.sci.30077.fits (elevation 40 - elevation 20)

sky.sci.30076.fits - sky.sci.30078.fits (elevation 40 - elevation 25)

sky.sci.30078.fits - sky.sci.30077.fits (elevation 25 - elevation 20)

sky.sci.30078.fits - sky.sci.30079.fits (elevation 25 - elevation 25)

I only examined the first 4 MCDS images from each frame since sky.sci.30079.fits has only 4 repeats.  Figure 4 shows the difference images.  What can be easily seen is a small offset between the different elevation.  The difference between the same elevations does not appear to have an offset.  The offset is around ~5 pixels.  I examined the median value in a 50x50 pixel area on the bright line that is shown near the top of each image in Figure 4.  Those results are displayed in Figure 5.

Figure 4 (Top left: elevation 25 - elevation 20; Top right: elevation 25 - elevation 25; Bottom left: elevation 40 - elevation 20; Bottom right: elevation 40 - elevation 25)

Figure 5 (Comparison of difference in counts between frames taken at different elevations.)