You really need to do a star test and look at a planet like Saturn or Jupiter under good seeing conditions. The main problem is proper collimation and you need a center dot or reinforcement label perfectly centered on the primary to collimate it properly with a Cheshire eyepiece. See : http://www.amateurastronomy.com/tools.html. An f4.5 telescope is very sensitive to collimation. When my 10.1 " f4.5 is out of collimation everything looks mushy. When it is in collimation with good seeing I have observed Mars and Saturn at 500x with good detail and clarity. That said, I have owned two Coulter mirrors - one outstanding optically (my 10" f4.5) and the other awful (a 6" f/8 mirror in a kit). -----Original Message----- From: utah-astronomy-bounces@mailman.xmission.com [mailto:utah-astronomy-bounces@mailman.xmission.com] On Behalf Of Mark Shelton Sent: Monday, January 24, 2011 2:10 PM To: Utah Astronomy Subject: [Utah-astronomy] Mirror Hi, all I have a question. I just acquired a 13.1 F4.5 Coulter dob. I am thinking about just using the mirror out of it and making my own version of a Obsession looking dob. Is there a good way to tell if the mirror is worth the effort to make a dob built around it? I have it out of the telescope and I can not see any defects so far in the mirror. ( have not looked really close at it yet.) The Mirror sets in a kind of sling in the tube. Very Strange setup. I guess it was the way they made them in early 1980 or so. If some one has a mirror cell to fit this size of mirror I would love to hear from you. I really do not know what I should be looking for to check the mirror. Any Suggestions would be appreciated. Mark _______________________________________________ Utah-Astronomy mailing list Utah-Astronomy@mailman.xmission.com http://mailman.xmission.com/cgi-bin/mailman/listinfo/utah-astronomy Visit the Photo Gallery: http://www.slas.us/gallery2/main.php Visit the Wiki: http://www.utahastronomy.com