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The estimates are printed in the lois log and in the focus log; the final focus setting adopted, however, is always 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 slowly gathering that the parabolic fit to fwhm is actually the best value obtained by the script, 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.
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This is provisional and will be made more automatic later.
It is also necessary to move to the desired filter with the LOUI filter wheel view first, since LOIS cannot move the LMI filter wheel, at the moment.
A star is acquired and a full frame test single (binned 2:2 on LMI) taken. The star is moved to the center as needed, and an exposure time suitable to provide good input to the focus sweep is estimated. This needs to be long enough to provide adequate counts at each end of the range. The focbase is selected as a focus setting not too distant from the actual correct value. The step size is usually already known as a function of the instrument. It is usually best to use the full 10 steps in the sweep as provided by default.
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- A focbase that is some distance from the correct value.
- A focstep that is too small.
- Bad seeing.
A good choice for a star upon which to focus is one
- On or near the meridian.
- At an elevation of 60-70 degrees.
- About 11th magnitude.
Since the AOS corrects for zenith distance automatically, the focus obtained at the above position should hold good elsewhere.
ARGUMENTS LIST
If the user button is used for this command, there should be neither quotes nor curly braces embedded in the fields for these arguments. The arguments are single strings without embedded blanks, or empty strings (all blanks) in each button field. In addition, the exptime, focstep title and flags fields accept strings with multiple tokens, ie, embedded blanks.
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