Focusing the telescope

 When the sky is sufficiently dark, move the telescope to a bright (magnitude 6-9) star relatively near the first object, in order to confirm the chip orientation and focus the telescope. The observing assistant has an on-line list of SAO stars which they can use to easily accomplish this task. Note that twilight flats are not really possible with the Fabry-Perot, due to the wavelength-dependent response of the etalon.

The telescope should be focused using the same filter that is to be used for the object observations. See Appendix B or the Fabry-Perot instrument web page for approximate focus settings of the Palomar telescopes with the Fabry-Perot system. Use the focus command to obtain a focus observation:

The resulting image will contain seven 10-second images of each star, separated by 20'' in the N/S direction. The final two images will be separated by twice the specified amount, e.g., 40'', to aid in interpreting the image. The focus program will pause between each image, to allow the observer to change the focus, as well as to offset the telescope.

Typically, you will want to increment the telescope focus by about 20/0.30 units per exposure at the 60-inch/200-inch telescope. The exposure time will be 1-10 seconds per exposure depending on the brightness of the selected star; be careful not to saturate the detector. The initial focus setting should be about 50/0.75 units below the expected focus. The focus should always be adjusted in the same direction in order to avoid backlash in the focus drive mechanisms. Because of the downward trend in temperature over the course of a night, the focus setting should probably be stepped in the positive direction. This includes approaching the initial focus value from about 30/0.45 units in the proper direction.

Examine the resulting image to determine an approximate value for the optimal telescope focus. If the best focus setting is not obvious from the resulting image, FIGARO plotting and fitting routines may be used to measure stellar FWHM values. The image may also be transferred to one of the Unix machines, where a fitting tool such as IRAF's IMEXAMINE may be used to determine the optimal focus.

It will probably be useful to repeat this procedure on a finer scale around the best focus position. Then set the telescope focus to the optimal position, remembering to approach the value from the proper direction.