...
Select a star with coordinates x,y on a full frame left amplifier single and take subframe images with a succession of focus settings to find the setting with the best image quality. There is no slew to locate the star, or any kind of centering up, thus the term "inplace focus". There is a focus setting value used as a starting point called focbase; the script first sets the focus to that value. The routine then "acquires" the star by defining a standard sized (400x400 on LMI) subframe around the given position and taking a single image. Analysis is performed on this image- if the star is sufficiently bright, in terms of instrumental mag. and maximum pixel value, and the fwhm is reasonable, the subframe is redrawn to place the psf in the center of the image. The If the image appears reasonable, but either too bright or faint, the exposure time will be adjusted within certain limits and the exposure retried. If this is still out of specification, the focus script ends without a result. Otherwise the routine then initiates a "focus sweep" in which the focus setting is moved in steps from a value considerably lower than the focbase to one considerably higher. At each setting a "focus image" is taken , and another analysis is performed. If the brightness and fwhm meet similar criteria as those for acquisition, the focus is moved to the next step. Otherwise the image is retried; when retries are exhausted, the script usually aborts. This would happen, for example, if clouds covered the star in midsweep. The current version of the focus script will also adjust the exposure time during the focus sweep, prior to a retry, but with narrower limits than during the acquisition.
When the focus sweep is finished, the focus step with the smallest fwhm (or other figure of merit) is selected to be the new focus setting. If the focus setting is at the top or bottom of the sweep, the focus sweep is repeated with the center of the second sweep defined to be the best focus step found in the first.
...
- minimum fwhm
- maximum ratio of peak pxl to flux ("fom")
- minimum of parabola parabolic fit to fwhm values
- maximum of parabola parabolic fit to fom values
The estimates are printed in the lois log and in the focus log; the final focus setting adopted, however, is that based on the minimum fwhm criterion. You are not obligated to accept this focus. The setfocus command and user button can be used to select another focus. Evidence is gathering that the parabolic fit to fwhm is actually the best value obtained by the script for LMI, which is not true for other instruments that use a script like this. If the focus script fails, or is aborted, or is still at the top or bottom of the sweep after a second sweep, the focus setting is restored to focbase.
...