focus_spectro

Compute the proper spectrograph focus from a series of arc exposures, using a gaussian fit of the auto-correlation peak.

Parameters:

-in|cat	focus.cat Input catalogue or single image
-dev	ASCII Output device or filename (see Graphical output)
-seq	start,step Linear focus ramp to be used if the focus keywords are missing
-range	3. Compute the focus range at the 3%-level

Returns:

Error status (in particular, status=5 if focus position is outside the input range)

The input frames should have a FLOAT `xCFOCUS' (x=B or R) keyword giving the position of the spectrograph moving element (camera on the red channel, collimator on the blue one). If this keyword is not there, you can use the option -seq start,step to describe the focus ramp actually used.

The value used for the best-focus determination is <Sig> (RMS of SigX and SigY). It appears that the focus position is not very dependant on the quantity used (e.g. intensity).

Output devices:

The default output device is `ASCII', i.e. stdout:

 #   Focus    Intens     SigX     SigY    <Sig>   d<Sig>
 1   750.0  2.39e+10    1.242    1.574    2.005    0.041
 2   800.0  2.53e+10    1.205    1.506    1.928    0.039
 3   850.0  2.66e+10    1.174    1.459    1.872    0.038
...

If the Snifs project has been compiled with DISLIN library support, you have more choices (see Graphical output)

Note:

Use with the -quiet standard option for minimal output.

Todo:

Test (native SNIFS format) multiple-extension input frames

Test different zones in the arc frame (size and position, but beware of 0th and 2nd orders)

Fit multiple zones in the arc frame and derive a CCD-tilt

Check out why the Y-error bars are so different between the red and blue channels. The formal error estimates derived with nllsqfit_bnd seem theoritically correct.

Warning:

The auto-correlation method cannot deal with chromatic focus (since everything is naturally all mixed up in the auto-correlation peak)

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