I have found the barlowed laser technique pioneered by Nils Olof Carlin (Sky and Telescope, January 2003), to be as good as the star test and better than a Cheshire. A laser collimator alone will only work if your telescope is perfectly squared off, which in practice rarely occurs. Jim Kendrick sells the barlowed laser but you can easily make one for yourself as Carlin describes in his article. I collimated several large fast Newtonians at the Bryce Canyon Astronomy Festival using my barlowed laser and the improvement was dramatic. I have seen examples of horribly collimated telescopes using the laser alone. Clear Skies Don Colton -----Original Message----- From: utah-astronomy-bounces+djcolton=piol.com@mailman.xmission.com [mailto:utah-astronomy-bounces+djcolton=piol.com@mailman.xmission.com] On Behalf Of Chuck Hards Sent: Sunday, July 23, 2006 6:48 PM To: Utah-Astro Subject: [Utah-astronomy] Design considerations After sitting on the optics for over 20 years, I'm finally about to start building my 17.5" f/4.5. I want to avoid collimation each time I set-up; due to the fast focal ratio it must be dead-on accurate. A laser collimator alone is insufficient and a star-test is mandatory for fine-tuning. The optical axis must coincide with the focuser axis to within a fraction of a milimeter and no commercial laser can accomplish this (that I'm aware of). I am a veteran of star-testing but have never done it with a Newtonian of this size- I've always been able to have my fingers on the collimation screws while looking through the focuser. Also the telescopes were driven, a big help at the high magnifications needed to see diffraction patterns big enough to be useful. I know many large scope owners only use the laser and call it "good enough". For deep-sky work at low and moderate magnifications, they are right, especially if the focal ratio is only moderately fast. But for maximum lunar & planetary detail, and splitting the closest double stars, "dead-on" collimation is mandatory on an f/4.5 scope. I've also thought about carrying an artificial star with the big scope, so a star-test could be performed before true darkness falls, and the scope need not be tracking- but this still means another chunk of time must be devoted to set-up each and every time. All this means I have all but made up my mind to make the scope with a standard tube, keeping the optics in-place at all times. It will still fit in my SUV (why I bought the thing in the first place, no kidding) and can be set-up quickly and easily by one person thanks to some design details- in less than five minutes (tops). I also like a "tubed" scope better for aesthetic reasons. I've never liked the look of trusses. I think I can get excellent contrast with a tubed design, as another bonus. If I ever build a larger scope, then certainly a truss-tube will be mandatory, but I can avoid it on this one. The scope will have a 3.5" ma secondary and a low-profile 2" r&p focuser. Active primary cooling, probably in the form of a blower across the face of the mirror. I already have all the hardware, optics, etc., and will be using composite materials for the tube and possibly the mount. A Dobsonian mount will be built initially but I'm designing a compact split-ring equatorial in Solidworks, for the future. The short f-ratio also means a tall ladder will not be needed. A step-stool is all that's needed for a person of average height. Observers over six feet may need nothing to stand on. So the reason for this post is one last chance for input from folks with different observing habits or ideas. I am pretty sure I've covered all the bases but it never hurts to step back and go over it all one more time before spending (more) money. Thanks! __________________________________________________ Do You Yahoo!? Tired of spam? Yahoo! Mail has the best spam protection around http://mail.yahoo.com _______________________________________________ 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.utahastronomy.com