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Refractor Vs Reflector Curve Accuracy

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Posted by Robert May on April 7, 2001 03:42:36 UTC

Refractors are easier in some ways because all of the curves are spherical for most designs. It also ends up that the errors tend to be a lot less effective in making errors in the wavefront and after you get the scope together, as long as you have a surface that you can modify, you can correct for the errors of the lens afterwords. The refractor is a bit more difficult in other ways such as the glasses not being homongonus enough in smoe cases but you can take care of the gentler problems with surface changes. The other problem is to make sure that the lens has no wedging in the lens which will make for a fair number of problems. The wedge measurements can be a bit of another difficulty and, while it's not difficult to measure and correct for, it's something that you don't have to do with a mirror.
The reflector, on the other hand, usually gets coated before fully testing on a bright star. This really isn't all that good of a thing to do as you can do a lot of testing without any aluminum on the main mirror. I've split the Double Double with a 6" parabolic mirror that was uncoated (secondary was coated tho) and was able to get a nice Airy Disk from Vega nearby.
Now, as to spherometers, they are really pretty easy to build with the info on the web now. I've built several and do have a 4" one for sale if you desire. bobmay@nethere.com for further info. The curves on the Mak corrector really aren't that critical and the way to test them is to place them next to the primary and go for the double of the parabola (you are doing a double pass on the corrector) and that isn't hard to do.

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