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Overview
The focus script is a Tcl script that runs under LOIS. In the case of LMI, it is available after LOIS has been started and initialized in the usual way. There may be an inplacefocus button under the User Buttons tab on the LMI UI to run the script but the preferred way to run the focus is through the imager. It is necessary for the TCS and AOS to be running and the usual connections via JOE established to perform the focus changes that this script will make.
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Selecting "OK" saves the parameters and we will be ready to initiate the focus sweep. "Reset to Defaults" and "Clear All Values" are self explanatory but the "Redo Focus Run" should be used with care. This button is used to repeat a focus run (same star and therefore same x & y coordinates) with different parameters. The button saves the repetition of time by not repeating the initial step of taking a full frame image.
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02:52:10 Level_9:FOCUS_PARAMS=800.0,18.58|830.0,16.85|860.0,20.44|890.0,15.47|920.0,16.34|950.0,17.75|980.0,20.83|1010.0,21.41|1040.0,22.30
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Upon reception of this message, the imager will generate a modal dialog box with three tabs. The first tab is the focus plot with the parabolic fit, the second one is the residual plot, and the third tab is the tabular data:
The fitted focus position is pre-filled in the editable text box at the bottom panel. Clicking "OK" will send this value to the AOS. Of course the observer has the option to change this value and send some thing else to the AOS. Clicking the "Cancel" button will send the focus base to the AOS.
The first tab has some annotations and if p-value < 0.9, the font color becomes red. This is an indication that the fit is most likely unreliable. The third tab has a table of the data used in the plot & one can copy the data by highlighting and using either ctrl-c or command-c. This is probably unnecessary for the experts because the IDL stuff and the related files are still accessible if a user feels more comfortable with those.
Method
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 (now 300x300 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 with a failure status.
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