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One For Roland Christen

Started by klawson, 06/14/2005 03:46PM
Posted 06/14/2005 03:46PM | Edited 06/14/2005 10:52PM Opening Post
I don't know if this has been asked of you before but, if you were an "average" amateur astronomer like the rest of us and didn't have access to an interferometer, what would you use to test compound instruments for optical quality? I know that some don't care but I like to know the optical quality of my instruments. I have read and re-read your "essays" at: http://voltaire.csun.edu/roland/index.html many times and love the detailed info on telescope performance ( your CCD/MAK photos are flat incredible!) but what can the "average" amateur telescope owner do to know his or her optical quality?

At present I use a 250 line glass grating inside focus showing 4 bands and the out of focus star test to test my instruments. I use the airy disk (when seeing allows)as my final test. It's all I got smile

Scopejunkie


40 year old restored C14 with recirculating cooling w/Skywatcher DOB GOTO Conversion (CPC14)

Synscan DOB mounted Sky-Watcher Evostar 150mm/1200mm ED Refractor

"Rescued" Craig's List Orion 120mm/1000mm Refractor


USAF Vet




 
Posted 06/14/2005 04:11PM #1
The best method to know the exact wavefront error of your scope is to use the Roddier method. It requires that you take two images of an out-of-focus star (inside and outside focus). The images must be defocused to approximately 150 pixels. The camera must record them in 16 bit Fits format. Once you have these images, you can post them in the full 16 bit Fits format (only 150x150 pixels, please), and I can run a Roddier analysis. I would need to know the focal length and aperture of the telescope, and the size of the pixels used (the Fits header will provide that).

Roland Christen
Posted 06/15/2005 05:31AM #2
If you think your scope will hold its alignment looking down you could try autocollimation against an oil flat.

Ed Jones
Posted 06/15/2005 10:37AM #3
After looking at the responses to my questions I'm beginning to get the feeling that the poor amateur astronomer is left at the mercy of manufacturers concerning the optical quality of their compound instruments. I know that gross errors are easily identified, but getting an actual wavefront analysis seems to be beyond the "average guys" ability as far as having the right equipment is concerned. Everyone can do a ronchi test as well as the star test but as Roland has stated in his articles, the star test can be easily misinterpeted on compound instruments and the ronchi seems useless as well leaving the poor telescope owner with no where else to go as far as knowing the quality of the instrument he owns.

Scopejunkie


40 year old restored C14 with recirculating cooling w/Skywatcher DOB GOTO Conversion (CPC14)

Synscan DOB mounted Sky-Watcher Evostar 150mm/1200mm ED Refractor

"Rescued" Craig's List Orion 120mm/1000mm Refractor


USAF Vet