Ed Jones ........... The best data is an interferogram, the best analysis is from an honest optician.
Ed "You have to grow old, you don't have to grow up."
Quote: So, Dave got a much better result using his Baader commercial corrector..... Which again brings me to the the image of M22. The cluster is some 17 arcminutes wide, or or about 0.28 degrees. If you measure its core, it's easy to see that the whole picture subtends about 2 degrees of the sky. Two degrees at 96 inches of focal length (almost 2.5 meters) would be 85 mm across (3.3 inches). That wouldn't even fit on a 60X60 large camera format film, let alone CMOS/CCD processors which for most SLRs are barely 15 by 24 mm frames and 50x50 mm processors are prohibitively expensive. So, unless I am mistaken about the M22 size, and I have checked, it is truly amazing that this picture was taken with a 16 inch f/6 scope, even if the totla image field covers 1 degree in all (which would require a 50 mm CCD processor. That's hardly "amateur" stuff.
20" homebuilt truss dob 6" watson refractor
Quote:The secondary can really only be finished/figured in combination with the primary - at the correct separation/focal distances.
Quote: The secondary can really only be finished/figured in combination with the primary - at the correct separation/focal distances. It is normal to do it this way by any optician - which of your two opticians will take the blame if the SA is not correct!?
25" is a high-end piece of no doubt expensive optics and that figure on the primary needs to be integrated into an overall corrected system.
Quote:So - how close does the focal length need to be to prevent an impact on the SA? Plus or minus 5". I have full faith that Mr. Royce can come closer than +-5 inches.
Quote:In the interest of full disclosure, the tolerances are actually more generous than I have stated. I was just being conservative with the +-5 inches.
Quote:I guess the moral of this story is to do your primary first on a wsDK and measure its correction carefully. Then you have tons of latitude with making your secondary.
Living at a dark site, I make scope when I have the inclination. I also do woodworking, cycling and other outdoor activities
Quote:Dall Kirkham 16cm F3.8/13.5 was done from equations in James Muirden's book from the 1970's. He pointed out that a slight oblating figuring would be needed on the spherical secondary after assembly and he was spot on.
Quote:The 14.5" seems familiar, but a lot of photons have passed through the exit pupil since that one. Seems like I was helping a guy out that was considering a modified DK. I got that so-called Baader design from a post over on the Yahoo Astrograph group. Still not sure about it, but it does work beautifully on my 16" f/6 Newt. I think when you get into wide fields and lower focal ratios, though, no generalized commercial corrector is going to work well, and a custom design is required.Mike
SharpStar 65EDQ (same as AT65EDQ) 9.25" Homemade dobsonian with 1/13 wave self-made mirror. AT6RC/Celestron ASGT http://www.astrobin.com/users/tyblakesley/