Joe, 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. Now that I think about it, I have also seen it when viewing the Sun. Dave -----Original Message----- From: utah-astronomy-bounces+david.dunn=albertsons.com@mailman.xmission.com [mailto:utah-astronomy-bounces+david.dunn=albertsons.com@mailman.xmissio n.com] On Behalf Of Joe Bauman Sent: Wednesday, February 09, 2005 9:38 AM To: Utah Astronomy Subject: [Utah-astronomy] Question on focus Hi all, When I checked out the Sky & Telescope web site below (from Patrick's News), it had a great animation of the moon occulting Jupiter, on the third page. What I can't understand is why Jupiter is in sharp focus while the moonscape is slightly fuzzy. It isn't possible that there could be a real difference in focus between the two, I believe -- granted, Jupiter is thousands of times farther out, but for all practical purposes the focus for both should be at infinity. Or is it because the telescope was tracking on Jupiter, making it relatively stable, but the moon had some lateral motion for each exposure, causing blurring? Just curious. Also, if anyone has a minute, can you tell me why it's necessary to focus a telescope at all? You would think that, like an old Instamatic camera, everything beyond a few miles out would be at the "infinity" focal point. I know that's not true, but I do not understand why. Thanks, Joe