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SCT corrector plate help

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#1 offshore79

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Posted 02 December 2005 - 03:14 PM

:confused: I just received an old 8" daynamax (I know) as a parts scope to get a corrector plate for my dynamax that I already started work on. I looked at this and it looks like a flat to me. I carefully put a starrett parallel across both sides and I dont see a difference and no air gap between the plate and the parallel on either side. Should I see a noticeable difference between the sides or an air gap? Has anyone taken measurements across an 8" corrector plate? If so, what kind of results did you get.
Also, if given a Ronchi test with the corrector in place, should the resulting image look like a parabolic?
Thank you.

#2 wh46gs

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Posted 02 December 2005 - 06:52 PM

I just received an old 8" daynamax (I know) as a parts scope to get a corrector plate for my dynamax that I already started work on. I looked at this and it looks like a flat to me. I carefully put a starrett parallel across both sides and I dont see a difference and no air gap between the plate and the parallel on either side. Should I see a noticeable difference between the sides or an air gap



If it is ~f/2 spherical primary w/.866 neutral zone at the corrector (usual choice, best photographic correction) the curved side is highest at the center, deepest at the .866 zone - around 1/27 mm - and about 1/32 mm bellow the center level at the edge (this assumes spherical secondary, putting required corrector power at ~71% of the full power needed for correcting primary's spherical aberration).

For the neutral zone at .707 radius (best visual correction), center and edge are at the same level, and the deep is at the .707 zone, about 1/60 mm.

Also, if given a Ronchi test with the corrector in place, should the resulting image look like a parabolic?



If it is at the final focus, after the secondary, yes - it should be nearly corrected for spherical aberration. For the primary/corrector alone, it will likely be roughly 30% undercorrected.

Vlad

#3 offshore79

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Posted 02 December 2005 - 08:04 PM

Vlad, thank you for your response.:bow:

Yes, it is an F/2 spherical primary. Overall FL is 2110mm.
I hate to say this but I'm terrible at my conversions but the way I interpret this is that using a parallel, I would probably not see a visual difference in depth from zone to zone and would probably need to use a very accurate (tesa digital maybe) height gage to see the shape in the values? :thinking:


I took some quick ronchi tests with and without the corrector. They were the best I could do taking the shots by hand. I used a 50 lpi screen and when viewed by eye can get the lines down to 3 or 4 and the results are much more pronounced. The difference is no doubt - sphere/paro

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#4 wh46gs

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Posted 02 December 2005 - 10:16 PM

It is probably just unfamiliar terminology. Schmidt corrector (curved side) has convex central area that gradually changes into concave toward the edges. The point between the convex and concave section is essentially flat, determining "neutral zone" height.

I doubt it would be much of a sight even if you'd find some way to make those tiny curves visible. Worry not - they are there. If not, the scope would have some 16 waves of spherical aberration.

Vlad

#5 offshore79

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Posted 03 December 2005 - 08:13 AM

Could you then say by the photo's that the corrector is a correct corrector? :lol:

#6 wh46gs

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Posted 03 December 2005 - 11:15 AM

No, it is far too insensitive. You should try using Ronchi
(preferably 100 or more lines) with a telescope as a whole, focused at a real star, or a distant artificial star. If the scope is corrected, you'll get straight lines. Approximate Ronchi sensitivity is given by w~120n/FN, with "w" being the detectable correction error in units of 550nm wavelength, "n" being the number of intercepted line pairs, F the F# and N the number of lines per inch. With 50lpi and 4 lines intercepted at f/10.5 you'd still have only little better than 1 wave sensitivity. Using good Barlow to multilpy the F# would accordingly increase test sensitivity. With 100lpi, 2.5 intercepted lines and 2x Barlow, test sensitivity would increase to ~1/7 wave.

Vlad

#7 offshore79

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Posted 04 December 2005 - 07:35 PM

It must be a corrector. I just had it out and saw a crescent shaped Venus for the first time. Wow :shocked: then :thewave:
I would have spent more time adjusting but it was about 10 degrees out side.
Any way, thanks for the help guy's.
Posted some photo's of the scope if your interested. User Gallery. (scope_wcorrector). It's probably the only burgundy
Dynamax in existence.


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