Troubleshooting guide

It computes running difference images from which it computes the FWHM. You can use this to
monitor the seeing during a sequence of observations. To use this for focus runs, e.g.,
• 4mguest@flmn-4m-1a{19} singleimage.pl
• 4mguest@flmn-4m-1a{20} relative.offset.kpno.pl 10 0
• Ask the operator to increment the focus (50 – 100 units).
Repeat these three steps as necessary, offsetting the telescope back and forth between each image.
Once you've taken 2 or more images, epar fwscan to make certain it is set up for the 4-m, then run
fwscan. Here's some sample output:
fl> fwscan p182e.j 1 9
#Main Bkgd 1st
#Frame Frame Average Median Quartile #stars
1 2 0.85" 0.83" 0.81" 51
2 1 0.89" 0.88" 0.85" 49
3 2 0.82" 0.82" 0.77" 42
4 3 0.88" 0.87" 0.81" 45
5 4 0.91" 0.92" 0.82" 51
6 5 0.79" 0.80" 0.74" 39
7 6 0.86" 0.85" 0.79" 48
8 7 0.76" 0.76" 0.70" 45
9 8 0.82" 0.81" 0.78" 43
• fwcheck differences two frames, computes the FWHM, displays the difference frame in buffer 4 of
ds9, and finishes with an imexam cursor on the image. To call it:
fwcheck <image root name> <background image number> <source image number>
• Some notes on using imexam, in the standard manner:
• After an image has displayed in ds9 type imexam with no arguments at the cl> prompt
2
. You can keep
imexam open as you continue to take focus images.
• Use the r, a, and e keys to obtain information on the image quality.
• We usually use the Moffat fit number (the second fit number from the right after hitting the a or r
keys) for characterizing the image quality.
• FWHM of 2 – 2.5 pixels over the entire chip are pretty good for the 4-m (0.63 – 0.79 arcsec).
• Optimize focus on stars out about 350-500 pixels in radius from the center of the chip.
• Stay away from the left edge of the chip, however, as it is slightly more out of focus than the rest of
the array.
7. Variation of focus with temperature. The telescope changes focus due to temperature variations; the focus
value will increase with decreasing temperature. The relation we have been using is:
∆T(+1 C) = -130 focus units
2 Note that if you type in the image name, IRAF will have to redisplay the image and possibly will not choose a useful
image scaling; it will also disable the scaling buttons in ds9. It is generally better to use the ds9 File/Open menu in order
to display an image rather than type display in IRAF.
FLAMINGOS@4-m, Ver. 2.39, 2013 April 23 Page 16 of 47