The recommended way to measure the best focus value for FAST is to use the iraf task specfoc.
First, you need to take a sequence of HeNeAr exposures with monotonically
increasing focus values in the range that includes the expected focus
(probably where you found it). The telshell alias "fastfoc" does this for you,
or if you prefer, you can do it manually. If you wish to use
"fastfoc" at the telshell type:
fastfoc fstart step reps expt bin where: fstart is the initial focus value step is the focus step reps is the number of steps expt is the exposure time bin is the binning (1, 2 or 4) example: fastfoc 750 50 10 12 2The alias runs a script that takes the exposures and at the end, it prints out the iraf specfoc command you need to run, with appropriate arguments. It also creates the file "foclis" in the current data directory that contains the list of names of the exposures it takes. You can use this alias multiple times in succession, if you need to change the range of focus.
Second, run specfoc. In the example shown below, the values range from 850 to 1050 in steps of 25 units.
To use specfoc, nmisc must be loaded in iraf. The specfoc parameter file should look like this:
images = "@foclis" List of images
(focus = "750-1200x50") Focus values
(corwidth = 20) Correlation width
(level = 0.5) Percent or fraction of peak for width measureme
(shifts = yes) Compute shifts across the dispersion?\n
(dispaxis = 2) Dispersion axis (long slit only)
(nspectra = 1) Number of spectral samples (long slit only)
(ndisp = 1) Number of dispersion samples
(slit1 = INDEF) Lower slit edge
(slit2 = INDEF) Upper slit edge\n
(logfile = "logfile") Logfile
(flpar = no) flush pfile on assign?
(mode = "ql")
foclis is a file containing the names of the HeNeAr exposures in your focus sequence, in the exact sequence of monotonically increasing focus values.
In general, you should not need to change the other parameters of specfoc. Please refer to the iraf help on specfoc for additional details.