David Dunn wrote,
I have seen this just looking at the moon itself. If you are running much power, you will need to refocus
just to adjust for the curvature of the moon.
If you are using a fast f/5-6 mirror, it may just be coma. The angular diameter of the in-focus "sweet" spot in the apparent field of view depends on focal ratio. The higher the ratio, the larger the in-focus sweet spot; the lower, the smaller the sweet spot. You usually do not notice the coma in deep sky fields because the dim stars do not provide a sufficient visual cue. The coma becomes easily seen with a large extended object like the Moon. For an f/9 mirror, the sweet spot can extend out to 3/4's of the apparent field of view. For a f/5-6 mirror in a DOB light bucket, the sweet spot may be less than 1/2 the apparent field of view. The coma problem inherent in fast f/5-6 light buckets is why the Teleview Paracorr "field flattener"/"coma corrector" is popular. http://www.cloudynights.net/lab/accessories/paracorr.pdf#search='corrector%20Paracorr' Suiter did a good article on the relationship between the size of the sweet spot to focal ratio in _Sky & Telescope_ in the late 1980s. I could not easily locate my copy but the article is referenced in the bibliography of Suiter's _Star Testing_ book. - Canopus56 __________________________________ Do you Yahoo!? Yahoo! Mail - Find what you need with new enhanced search. http://info.mail.yahoo.com/mail_250