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Old 28-05-2021, 02:46 PM
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Stonius (Markus)
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Is central obstruction by diameter or area?

Hi all,


I'm reading about planetary observing and central obstructions as it relates to contrast.


I have an RC 10" and a 400mm newt, so I was interested to see how the central obstruction affects the image.


The book I'm reading ('Planetary Astronomy' various authors) mentions various designs and their central obstructions, including Newts with 20-25% central obstructions, which seems a lot to me.



That only looks right if you do it by diameter, not area which is how I understood % obstruction to be calculated.


For example, my 400mm newt has a 104mm obstruction, which works out to 26% by diameter, but only 6.76% by area.


By contrast, my RC is 254mm with a 112mm obstruction which works out to 44% by diameter, and 19% by area.


The difference is huge. a 19% obstruction is workable for planetary according to the book. 44% is not.


So I'm guessing I had it wrong all along. Can anyone confirm?



Markus
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Old 28-05-2021, 03:46 PM
Startrek (Martin)
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Markus,
There have been any technical papers written over the years on the effects of the CO especially with planetary imaging ( most of it way over my head ). What I can extract out of it all is that when it comes to Newts and planetary imaging , factors like poor Collimation, poor quality optics ( mirrors ) , poor anti reflective coatings inside the OTA , poorly designed spiders etc....... play a bigger role in determining the quality and resolution of your planetary images than the size of the CO with respect to the focal length and aperture.
Maybe some folk on IIS that have a better technical understanding can shed more light on this seemingly complicated topic

Martin
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Old 28-05-2021, 03:49 PM
Saturnine (Jeff)
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The central obstruction of a secondary is referred to as a percentage of diameter , usually. Not sure why the area percentage is not the more often quoted as you'd expect the area to be more relevant to affecting the contrast. Probably is a physical law pertaining to light waves that I'm not qualified to offer any insights into.
For planetary viewing or imaging, less than 25% is often quoted as desirable and >20% as an even better option. Even large SCTs' with central obstructions over 35% seem to be able to capture good detail though. A lot of great images posted to the ALPO site are taken with C11 and C14s'. Comes down to seeing conditions as much as anything else but an optimised scope in great seeing is going to make the most of the optics used compared to something with a larger central obstruction.
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Old 28-05-2021, 04:52 PM
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multiweb (Marc)
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Some good reading here.
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Old 29-05-2021, 11:16 AM
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Stonius (Markus)
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Quote:
Originally Posted by multiweb View Post
Some good reading here.

Yes, that's a good site, though a little dense for me. I think the book has a lot of the same information. At least, a lot of the same graphs at any rate!


Cheers
Markus
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Old 29-05-2021, 11:28 AM
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Camelopardalis (Dunk)
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A lot of top notch planetary imagers, here and elsewhere, use bog standard SCTs, which in theory are far from ideal.

Since you have a large newt already, roll with that why wouldn’t you there are things you can do to shore up the stability of your mirrors.
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