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B. Focus Procedure


Observers should begin the night with the focus set to the correct starting point for the appropriate instrument set-up. The two relevant set-ups are with and without the focal reducer lens. The approximate starting foci for the two set-ups are given in Table 4.

Table 4 - Approximate Focus Points
Set Up Target Focus
Without Focal-Reducer 2030
With Focal-Reducer 2160

If you set to the correct target focus, your initial images should be close to proper focus. If the high-signal core of a stellar image is asymmetric, you can improve the focus. If the image core looks horizontally elongated, the focus is too high. If the elongation is vertical, the focus is too low. Make small changes to the focus value (no more than a few counts) until you obtain symmetric image cores.


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Updated: 2006 July 26 [pbe]