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Focusing a Spectrograph with specfocus

  Frank Valdes has provided a very useful routine to aid us in determining the best focus of a spectrograph using a series of exposures of comparison lines. The assumption is that the collimator or camera focus has been changed between each of the exposures. The routine works by cross-correlation, and provides an easy measure of the full-width-at-half-maximum (fwhm) of the images. In addition, one can specify that the dispersion axis be broken into multiple samples to determine if the best focus is wavelength dependent, and/or the spatial axis can be broken into multiple samples to determine if the best focus varies across the spatial axis. We show in Fig. 12 the simplest use of the program. The dispersion axis goes along columns ( dispaxis=2), and the slit covers columns 5 through 50. In addition, there is a very sophisticated graphical display that helps in interpreting the results in the case of multiple sampling along the dispersion/spatial axis; read the help page for specfocus. Currently specfocus may be loaded from the nlocal package.


next previous contents
Next: Determining the gain and readnoise ... Previous: Analyzing Direct Focus Frames ...
jbarnes@noao.edu
Tue Feb 14 07:48:00 MST 1995