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test of smaller aperture is better when seeing is bad

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#101 PowerM3


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Posted 28 October 2019 - 10:09 AM

This thread seems pretty polarized in opinions on the matter. I firmly believe that the people that live in areas of general good seeing have no idea of how BAD seeing can be in general and that is the reverse for people that live in areas of generally bad seeing.


So yes the people that enjoy generally good seeing will of course almost always benefit from a larger aperture. Where I live I cannot use over a 5" scope to its full potential on anything more than a handful of nights a year. I have compared all kinds of scopes on the same night side-by-side and on the average night of seeing here in the NW a good 5" APO is about as good a planetary view as you will hope to get. I recently spent a night comparing a SV130EDT to a Meade 8" SCT and a 7" Meade MAK on Jupiter and Saturn. The seeing on this night was not great, but also not bad for my area(I suspect that you fine folks in Florida and other areas that enjoy good seeing would concider it as "bad"). The APO provided hands down the more contrasty and EASY to pick out the fine details view. That's not to say that I than could not go to the larger compound scopes and after some study not see the same detail, but it sure was much harder to see them.


In the above comparison all the scopes where allowed to cool for at least an hour. Witch brings up another can of worms... I don't think that people realize that an SCT is almost impossible to get to thermal equilibrium in some areas of the country throughout the night. It just never gets to its sweet spot in performance. It certainly does not if you take if out for my average 1-2 hour long evening viewing session of the planets(yes I know about insulation, but the vast majority of people to not insulate although I do want to give this a shoot at some point). 


Anyhow, not trying to step on any toes just another data point! smile.gif

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#102 gnowellsct



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Posted 28 October 2019 - 10:52 AM

It looks like the blue meanie Meade machines have successfully executed a classic encirclement maneuver around that g11/SV.  

#103 Astrojedi



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Posted 01 November 2019 - 02:28 PM

It's not that simple. It is all but certain that that mirror (and any other) is not a perfect surface. It has surface deviations randomly distributed over it. Depending on whether the mask limits it to a better or worse area of the mirror, performance will vary. It is likely that masked aperture will have better optical quality than unmasked (for one, will be unobstructed), but it is possible it will be worse. It may also be of a similar quality, but it can't be assumed.

Very true but you have a better chance of doing an apples to apples comparison with an aperture mask vs. using a totally different scope with potentially a different design.


To completely isolate the impact of aperture in various seeing conditions you absolutely have to eliminate all other variables including optical quality, optical design, mechanicals, quality of baffling, internal tube surfaces, thermal characteristics etc. etc. In my view the most effective way of doing that is using aperture masks.


As I mentioned in my earlier post I spent quite a bit of time observing with my 140mm apo refactor with a variety of aperture masks. I never noticed more detail in the smaller aperture scenario in any seeing conditions. Now this does not mean the theory incorrect or that this is not possible... I am sure many here speak from experience. But it just does not seem to be a result which is easy to replicate as is evidenced by the comments on this thread.

Edited by Astrojedi, 01 November 2019 - 02:28 PM.

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