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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.
|Set Up||Target Focus|
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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