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Focal Reducer

Started by cadillac, 03/06/2005 01:02PM
Posted 03/06/2005 01:02PM Opening Post
Hello!
Question:Is the Meade focal reducer F/3,3 is compatible with celestron Nextar Gps STC???

Thanks!
Posted 03/07/2005 02:38AM | Edited 03/07/2005 02:49AM #1
Hello Martins,

the .33 Meade works on all SCTs.
However, keep in mind that it will create SEVERE vignetting, especially if you use a large CCD detector. Even if you don't have a large chip, you must take into account the fact that, at f/3.3, you will HAVE TO use flat fields for your image reduction. Ron wodaski, in his very good book, says that a "burnt" center is a problem to be expected in your pictures whenever imaging below f/5 and I found this to be true in my experience.

I have the .33 reducer for my 11" Celestron, but I used it only once because the vignetting was so severe that my selfguide CCD (SBIG) could not "see" any star and so I had to image unguided (no biggy at such a short focal ratio, but still annoying). The final image was so badly vignetted that I had to crop it since I could not get rid of the problem even with flat fields (although I must admit that it was my 1st experience with flat fields, so the problem may have been mainly mine).

My advice is to stick to the .67 reducer (the Meade and Celestron are both OK, but there also Astro-Physics) or get a variable reducer that will allow you to get to f/5 (such as the Alan Gee II by Baader or the Optec Maxfield). These latter ones are a little more expensive, but they give you a much wider flexibility and also a better image quality.

Hope this helped.

Ciao
Luca

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