Daytime calibrations

 The majority of Fabry-Perot calibrations do not require a dark sky, and can therefore be obtained during the daytime:

• phase calibration
  At least one monochromatic calibration cube is needed during the run, for calibrating the response of the etalon across a range of etalon spacings. If time allows, obtain one each day, to monitor the instrument response over the entire run. The easiest way to do this is with the standard fpseq command. You should obtain a sequence consisting of more frames than a single free spectral range of the etalon, e.g., ~50 frames with the Palomar ET-50 etalon. Use a deltaz value appropriate for Nyquist sampling, e.g., 16 for the ET-50 etalon. Be sure to specify the appropriate filter for the calibration lamp being used.

  The @calcubes script can be used to obtain calibration cubes using the fpseq command, with reasonable default values. If you take more than one calibration cube, it is recommended that the cubes be started at a range of initial etalon settings, e.g., 10, 230, 450, etc. Be sure that there is sufficient overlap for comparisons between the cubes, however.

• flatfield calibration
  At least one dome flatfield should be obtained during the run through each filter used; three or more per filter is preferable. If you change or move a filter (say, because of the two-filter limitation at the 200-inch telescope), you will need to obtain new flatfields.

  The @fpflat script should be used to obtain a flatfield image. This script slowly increments the etalon spacing through an entire free spectral range, integrating the detector at each spacing. A single readout is performed at the end of the etalon scan. This has the effect of averaging out the wavelength-dependent nature of the etalon response. (This is the reason that sky flats are not really possible with the Fabry-Perot system.) As with the calibration cubes, it is recommended that multiple flatfield images be obtained with a range of initial positions.

  At the 60-inch telescope, the dome flat lamp power supply should be set to the maximum safe level noted on the power supply (600). At the 200-inch telescope, the ``lowlamp'' should probably be used, as the ``highlamp'' will probably saturate the detector.

  If a different etalon or blocking filter is used, and you intend to flux-calibrate your observations, you may want to obtain an entire flatfield cube, rather than just an image. This cube is identical to the monochromatic calibration cube, except a white light source (i.e., the flat lamp) is used. During the reduction phase, a flatfield cube will allow the transmission profile of the filter to be removed from the data. The flatfield cube should be taken as any other cube, using the fpseq command.

• bias calibration
  Unless you are using the engineering CCD configuration (camera #0) to enable reading an overscan region, take a number of bias frames using the @bias or @consec scripts. These scripts average several (7 or 9) bias reads of the chip into a single bias frame. You should take at least one bias frame in the evening, and one in the morning (and maybe more), in order to monitor the CCD read-out system, which may be affected by temperature changes, for example. Note that the system almost always contains small light leaks, so that bias frames will contain a small amount of light-induced flux (i.e., a few counts), if they are obtained with dome or flat lamps turned on. It is therefore recommended that bias frames be taken in a darkened dome. This is easier at the 200-inch telescope, where the lamps may be switched on and off within a script; manual intervention is required at the 60-inch telescope. The use of an overscan region, even with the bias frames, will allow you to assess the presence and importance of any light leaks.

• dark calibration
  In order to monitor any dark current in the CCD, you may want to take dark frames with exposure times similar to the object exposure times, using the dark command. Note that this command always names its output darkn.dst, where n is the camera port in use, so you probably want a rename command following each dark command in a script:
  
  
  CCDs #15 and #16 have reasonably low dark currents (~1 ADU/minute), so dark frames are not absolutely necessary, unless you are taking very long exposures, or are concerned about the performance of the CCD.

Normally, you will want to run a series of calibrations in the afternoon and/or during dinner. In addition, you will probably want to leave calibrations running when you go to bed in the morning. If you leave calibrations running during the daytime, be sure to leave a sign on the door to the dome, and perhaps on the instrument control computer, to notify the Palomar crew of the approximate time that the calibrations should finish.

If you are observing at the 60-inch telescope, it is often useful to take calibration cubes in the afternoon, and flatfield cubes in the morning. This is due to the fact that the lamps at the 60-inch telescope must be operated manually. Calibrations can be mixed more easily at the 200-inch telescope, where all of the lamps can be controlled via software, and can therefore be switched on and off in a script. On the other hand, the times during which calibrations can be obtained are more restricted at the 200-inch telescope, since the dome lights must be on during public viewing hours, generally 9:00 am to 4:00 pm. This makes calibrations at the 200-inch telescope very demanding, particularly during the shorter winter days.