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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