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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 with a resultfailure status.
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 kinds of 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 with a resultfailure status. 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.
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The focbase mentioned above, along with the focstep, the size of the focus step, are arguments to inplacefocus, with reasonable defaults. The number of focus measures is usually 9, including the central value, but that can be redefined if necessary with the focstep argument as well.
The method for taking images for measurement at each focus step is also selectable. You may take a single, or n multiple singles, or on some instruments, a subframe basic occultation with n frames (ie NAXIS3=n). In either case, if multiple frames or singles are used, the fwhm or other figure of merit for that step is an average of the analysis values obtained for the group. On LMI the basic occultation option is not supported with the focus script; such exposures can be taken, but are subject to image contamination due to lack of an optically masked transfer area.
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- minimum fwhm
- maximum ratio of peak pxl to flux (figure of merit, or fom)
- minimum of parabolic fit to fwhm values, if within the focus sweep ragerange
- maximum of parabolic fit to fom values, if within the focus sweep range
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