Rich It would have to be machined accurately to very high tolerances; much higher than practical for any commercial or home built telescope. As the article mentions even if the laser beam coming out of the eyepiece misses the "exact center" (i.e. the precise diagonal offset center) of the diagonal by only 1mm you have introduced a 2mm error which will not allow good images in a fast scope. For your f/6 scope the tolerances are much more forgiving. As implied above if you were to precisely offset the diagonal and mark a dot on the diagonal using Dave Kriege's template to get an accurate offset center dot and then centered doted the mirror; you would probably be OK with a laser if it precisely hit that center dot on the diagonal and then hit the center dot on the primary and returned by the same path. In order to get this to happen you would probably have to adjust the focuser, truss tubes etc. in order to get everything precisely square. Its much easier and just as effective to use the sight tube, chesire, and autocollimator since as explained in the article the telescope does not have to be precisely square with these tools. To use the laser the telescope must be precisely square to a very high tolerance. Clear Skies Don Colton -----Original Message----- From: utah-astronomy-bounces@mailman.xmission.com [mailto:utah-astronomy-bounces@mailman.xmission.com]On Behalf Of Richard Tenney Sent: Tuesday, August 12, 2003 1:02 PM To: Visit http://www.utahastronomy.com for the photo gallery. Subject: Re: [Utah-astronomy] Lasers and collimation questions I read the S&T article when it came out but never fully understood it -- call me dense (I usually deserve it), if the equiptment is accurately machined and you have a good, sturdy focuser, I still can't comprehend why it wouldn't render a very accurate picture of the optical path. I also found that using a barlow renders the holographic pattern on my laser useless! I suspect that technique is intended for the simpler single beam designs. Oh well. I suppose I ought to revisit the article and give it a more careful reading... Rich --- Chuck Hards <chuckhards@yahoo.com> wrote:
Rich, it's not the Cheshire or sight tube that does the collimation, it's the person using them! Different people can end up with different results using the same equipment. Patience & experience is the key no matter what system you use.
The star-test is a reality check. Many folks don't use a high-enough power to clearly see the diffraction pattern, or try it in less than ideal conditions. The star test, using image artifacts as it does, is subject to bad seeing. Even using an artificial star doesn't eliminate bad seeing if the air is turbulent or inhomogenous along your line-of-sight. And if your optics are not top-notch, you won't get a good diffraction pattern no matter how good the seeing is.
What's good enough is largely dependant on f/ratio. The faster the scope, the more critical collimation is. Most Dobs are f/6 & under, and as you'd expect are the worst offenders. F/4 systems must be dead-on to get diffraction-limited performance at the center of the field.
For most visual use, a laser is fine. Don cited the recent S&T article by Nils Olof Carlin, and this would be a good starting point for laser users. Nils advocates using a Barlow to increase the sensitivity of the laser, and this can go a long way to eliminate the pitfalls.
Chuck
--- Richard Tenney <retenney@yahoo.com> wrote:
The (holographic) laser collimator I use (Laser Max) gets my 16-inch close enough for my taste, only takes a minute to do, and has saved me from having to center-dot my mirror. The times that I've done a star test afterwards has shown a good pattern. Is the star test the best confirmation of collimation, or does the cheshire do a better job?
It would be interesting some time to line up a few different scopes of various focal lengths, try different lasers, and compare collimation results with traditional collimation tools and star testing (along with one or more of you collimation pros!). Maybe I've been kidding myself all this time...
-Rich
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