The focus_fine script runs exactly the same as focus_coarse, except it increments by 0.1 mm instead of 0.2 mm. (The focus will therefore be decreased by 0.3 mm at the beginning instead of 0.6 mm.) As of October 2, 2011 a 0.5 mm increment focus script, focus_first, is available and can be quite useful on the first night of a run. It is in /scr6/home/lfc/scripts and will need to be copied to your working directory.
If you require a different exposure time you’ll need to edit the focus scripts. Change the first line in the script (focus 5 focus_fine eg.)to the exposure value you want.
Image / Seeing Analysis
You can measure the seeing and do some simple statistics with the iraf command imexamine.
Put the cursor over a star and hit the r key. A radial plot should appear with three numbers in the lower right
corner of the window. The FWHM (middle number) is a good approximation of the profile fit in pixels.
In unbinned mode, a pixel is 0.18". In binned mode, a pixel is 0.36".
The primary mirror is astigmatic. Use this to your advantage. With the cursor over a star, hit the “e” key and look at the contour plot. When the telescope focus is a little too low, you will see that stars are elongated either left to right, or up and down. When the telescope focus is a little too high, the axis flips. When the telescope focus is just right, the star will be round. As you inspect stars in your science field later in the night, you can use this information to bump focus a little as stars start to elongate one way or the other.
If the seeing is bad, say greater than 2.0", all bets are off; everything is round.
Switching between some filters requires a focus offset. For example, the Sloan r', i', and z' filters are parfocal, but the Sloan g' and u' filters are 1.5 to 1.7 millimeters higher. Consult the chart on the observing station desk for filter focus offsets.