 RonBurgundy
sage
Reged: 06/16/09
Posts: 252
Loc: Philadelphia
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Hey everyone! I would really like to test the wavefront accuracies on my telescopes. I have a refractor and an SCT. I'm not interested in the individual components of the SCT--I would like to know the wavefront accuracy at the end of the optical path... How can I go about doing this? Will it be the same method for both telescopes? Or do the different optical designs warrant different types of testing? Thanks for the help! 
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Kipp Ginsburg
8" LX200-ACF
Orion 120mm F/5.0 Piggybacked Refractor
Meade UWA Set [4.7mm-30mm]
DSI-II
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DAVIDG
Carpal Tunnel
Reged: 12/02/04
Posts: 1985
Loc: Hockessin, De
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You' ll need a quality optical flat that is the same size or larger then the aperture of the telescope you wish to test and an interferometer. The simplest one to make is a Bath type.
- Dave
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Homemade 'scopes 8"f/7,6" f/5", 6"f/4, 4.25" Schief. 60mm Coronagraph,60mm H-alpha system, 4.25" White-light Solar Newtonian,solar spectroscope, 4.5" f/16 Schupmann Medial refractor, 14 Stellafane awards 7 in optics
Engineering = Taking what you have and making what you need.
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Mark Harry
Post Laureate
Reged: 09/05/05
Posts: 3120
Loc: Northeast
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Never mind that, get a 300 lpi grating, mount it in an eyepiece barrel, and do a startest. It's fine enough to make a close call on the total system error of the scope for a heck of a lot less money than fooling around with IF. Heck of a lot easier to learn, also. IFs are a task for even experienced individuals, and I know of only 2 others that -really- know all the ins and outs of them- Unless you want to spend a lot of time learning about how to build and work with one. My 2 cents worth.
Mark
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So many projects, so little time!
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RonBurgundy
sage
Reged: 06/16/09
Posts: 252
Loc: Philadelphia
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Gosh that sounds much easier. As for the star test--would it help if I did it with the CCD? Is there a way I could better measure the results with the stock program for the DSI? Thanks!
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Kipp Ginsburg
8" LX200-ACF
Orion 120mm F/5.0 Piggybacked Refractor
Meade UWA Set [4.7mm-30mm]
DSI-II
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Mike I. Jones
Carpal Tunnel
Reged: 07/02/06
Posts: 1571
Loc: Fort Worth TX
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Dave's answer answers your question directly - if you REALLY want a true wavefront analysis on your scopes, including P-V, RMS and Zernike modal analysis coefficients, you'd need the flat and interferometer.
But Mark is also right - if you're just curious about your scope correction rather than needing the true wavefront analysis and equipment required, the Ronchi test gives you a good qualitative assessment.
For a little more precision, photograph stars with your DSI through a 100 lpi or 200 lpi Ronchi grating, inside and outside focus an equal distance (enough out of focus to put maybe 4-5 Ronchi bands in the pupil at max travel). Straight lines at several locations either side of focus means good optics, case closed. Also, rig a 10X microscope objective to magnify star images and photograph an on-axis star either side of focus to look for asymmetry and ellipticity or other non-uniformities in the ring structure.
Side note: You can get a wave rating from the Ronchi test but you need precision focal setting readout and software that can be written but is not readily available. The software does diffraction-based Ronchi calculations and iterates the wavefront profile until the Fourier optics Ronchi patterns match the measured data. Can be done, but not off-the-shelf capability for sure. The Bath setup is easier than that.
However, if the optics make really straight Ronchi lines in your photographs, and star images are symmetric and round either side of focus, you're probably in the 1/10 wave range or better. No need to quantify it further unless you just want to.
Not-straight Ronchi lines mean we'll probably be visiting here more 
Mike
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mark cowan
Vendor (Obsidian Optics)
Reged: 06/03/05
Posts: 2156
Loc: salem, OR
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The Roddier test using defocused star-test images seems to deliver a pretty good metric on wavefront quality if done with care. And it's simple to do, maybe not simple to do right, but I've not used it. Check it out at Projet Roddier.
Best,
Mark
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RonBurgundy
sage
Reged: 06/16/09
Posts: 252
Loc: Philadelphia
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Quote:
The Roddier test using defocused star-test images seems to deliver a pretty good metric on wavefront quality if done with care. And it's simple to do, maybe not simple to do right, but I've not used it. Check it out at Projet Roddier.
Best,
Mark
Looks good....except I'm not bilingual 
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Kipp Ginsburg
8" LX200-ACF
Orion 120mm F/5.0 Piggybacked Refractor
Meade UWA Set [4.7mm-30mm]
DSI-II
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mark cowan
Vendor (Obsidian Optics)
Reged: 06/03/05
Posts: 2156
Loc: salem, OR
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Try google or alta-vista, just search for "translate french english" and follow the simple steps. There have been threads here on "Roddier" as well so you might search on that. 
Best,
Mark
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MitchAlsup
super member
Reged: 08/31/09
Posts: 184
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If you are (or become) proficient at star testing, it is easy to read the optics to 1/4 wave and not all that difficult to read down to the 1/10 level. Suiter, in his book, says that certain kinds of aberations can be read down to 1/50 wave.
A ronchi rulling makes localizing where the aberation is on the optic significantly easier.
Teh SCT should be a snap to read teh spherical term because the 33% CO is just perfect for reading 4th order spherical.
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HiggsBoson
Pooh-Bah
Reged: 02/21/07
Posts: 1118
Loc: Kal-li-fornia
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Suiter’s book is an outstanding reference. I am just starting to learn to use the star test. I am impressed with the clarity of his writing and the examples provided. If I could just get steady skies without 100% humidity.
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Michael
ATM: 6" F/9 Newtonian Travel Scope
ATM: 12.5" F/4.5 Real Soon Now...
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RonBurgundy
sage
Reged: 06/16/09
Posts: 252
Loc: Philadelphia
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Quote:
If you are (or become) proficient at star testing, it is easy to read the optics to 1/4 wave and not all that difficult to read down to the 1/10 level. Suiter, in his book, says that certain kinds of aberations can be read down to 1/50 wave.
A ronchi rulling makes localizing where the aberation is on the optic significantly easier.
Teh SCT should be a snap to read teh spherical term because the 33% CO is just perfect for reading 4th order spherical.
I can only HOPE that I have to test certain abberations down to 1/50 wave.... Longshot  ...... I just got the book on Amazon. Thank you everybody! I'll let you know some results when I get to them.
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Kipp Ginsburg
8" LX200-ACF
Orion 120mm F/5.0 Piggybacked Refractor
Meade UWA Set [4.7mm-30mm]
DSI-II
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