Troubleshooting guide
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 good enough (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; as the
temperature drops the focus will increase. The relation we have been using is:
∆T(+1 C) = -130 focus units
We frequently monitor the middle dome temperature, but there now is also a truss temperature monitor that
the telescope operator can read for you. It's advisable to watch the temperature every 30 – 60 minutes, and if
necessary adjust the focus on the TV camera.
NOTE: If the focus changes a large amount (2000 units) from one night to the next, verify that the ADC prisms
within the guider are out of the beam. They have been known to move into the beam without being expressly
commanded to do so.
You are now ready to start observing!
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 ds9's file open in order
to display an image rather than type display in iraf.
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