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post your obstruction

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#26 C8jim

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Posted 13 January 2011 - 11:55 AM

Just a little input/question.
When we speak of "obstruction', I think we are talking about the amount of light being blocked from the optical train, correct?
So, I printed out the AT6RC shot, and measured it.
Yes, it is 50% of the diameter or radius, but that has little to do with the performance of the "scope.
What does count is the amount of light being blocked by the obstruction.
When the calculations are done, light blockage is 25.14 %.
The areas of the 2 object need to be calculated, not the diameter.
Rebuttals expected, let the fun begin.
C8 Jim

#27 Jared

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Posted 13 January 2011 - 01:24 PM

You are right about the effect on light grasp, Jim. It's proportional to the area of central obstruction not diameter. However, the effect on MTF and contrast is almost linear with the % obstruction by diameter.

#28 C8jim

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Posted 13 January 2011 - 02:15 PM

Jared,
Information I seem to have missed.
Sorry for any feathers I may have "ruffled".
I have not had a chance to try out my 6RC yet,
Weather just will not give me a break.
Hope this does not turn out to be too
big a problem, have to wait and see.
Thanks,
C8 jim

#29 jean-jack

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Posted 07 April 2011 - 08:19 AM

obstruction of a synta mak-cass 5" f/11,8 (127mm-1500mm).
> 40%

Attached Thumbnails

  • 4502269-obst.synta127.1500.jpg


#30 rguasto

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Posted 07 April 2011 - 08:39 AM

Nice scope.


I would think the obstruction is on the credit card.:jump:

#31 Eddgie

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Posted 07 April 2011 - 11:29 AM

For visual use the obstruction is more important than when imaging and processing, where you can artificually enhance contrast.

Contrary to most peoples rants against central obstruction, very little detail is lost because of a central obstruction being present.

What happens is that for the detail that IS present, a central obstruction will reduce the contrast of that detail.

For example, a medium gray (50%) bank on a planet might only appear to be 40% gray in a refractor, and 30% gray in an obstructed scope.

Both the unobstructed and obstructed scope loose contrast from the original, but the extra contrast loss of the obstructed scope will appear to make a fairly dark cloud band on jupiter to appear as very light gray.

Both the refractor and the obstructed scope still easily and clearly show the band, but in the unobstruted scope, it will appear richer (not in detail but in color) and darker in the unobstructed scope as compared to the obstruced scope.

A very small amount of detail visible in the unobstruceted scope might not be visible in the obstructed scope, but the vast majority of detail will be visible in both.

It will just look more washed out in the obstructed scope.

I can show this easily when comparing my EdgeHD 8" to my 6" APO. Almost all of the detail visible in the 6" APO is easily visible in the 8" SCT, but in the 8" SCT, the cloud bands don't seem to stand out as easily. They appear lighter gray, and as a consequence, the view doesn't seem quite as lively.

When I put the C14 next to the 6" APO, the difference is much more pronounced, and the bigger scope (beceause it has more clear aperture) shows those same bands as in the 6" APO, but they all appear darker and more pronounced.

So, all scopes loose contrast, but obstructed scopes, or scopes with poor optics, loose much more contrast than scopes with no obstruction or with excellent optics of the same aperture. The view simply looks richer and more detailed in the higher quality or unobstructed scope, but 98% of the detail visible in these scopes is still visible in the obstructed scope or the scope with less than perfect optics. The detail is mostly all there, but it just looks pale and washed out.


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