...
There is no slew to locate the star, or telescope motion to center it up, whence comes the term "inplace focus". The focus setting value used as the center of the sweep is called focbase; the script first sets the focus to that value. The routine then "acquires" the star by defining a standard sized (200x200 at 2:2 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, but not saturated, and the fwhm is reasonable, the subframe is redrawn to place the psf in the center of the image. 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.
When the acquisition is complete, the routine 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 terminates without a result. This could happen, for example, if clouds covered the star in midsweep. The focus script can also adjust the exposure time up or down after a measure taken during the focus sweep, prior to a retry, but within narrower limits than those used at acquisition. On LMI, there is one retry available during acquisition, and also one per individual measure within the sweep.
...
- On LMI, the instrumental magnitude increases a bit at the wings of the focus sweep- almost certainly means the object apertures for photometry are too small.
LMI Definitions Applicable To The Focus Script
These are all in the LMI TCL definitions file. It is not recommended for observers to modify these.