Quote:
Originally Posted by Stephen65
I was using my 10" and the ASV's 25" reflector. The 25" gathers about 6 times as much light, but that doesn't make targets 6 times brighter, it makes them noticeably brighter, but my scope had better sharpness and contrast. Where the 25" comes into its own is with the faint targets that are on the edge of a 10" scope's capacity.
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Hi Steven,
You are making some assumptions here which are not quite correct, based on a small amount of exposure with large aperture scopes. I have no doubt that you have correctly reported what you saw on that night at that time, however, that is not necessarily representative of the situation all the time.
Scopes in the 15" and larger aperture range suffer logarithmically increased image deterioration as a result of less than ideal conditions. This is because of the size of the "air cells" in relation to the aperture of the scope. Those deteriorating factors include "poor seeing" and the mirror not being at "thermal equilibrium". The thermal equilibrium issue is further compounded by the fact that the cooling time of a large mirror increases logarithmically in relation to its thickness/diameter/mass ratios. In simple terms, the bigger the scope the more it is affected by bad seeing, the longer it takes for the mirror to properly cool and the more noticeable the effects of a non cooled mirror. These effects are far more noticeable as magnification is increased.
I am sure you are aware that as the scopes' aperture increases its resolving power increases and the diameter of the airy disk (the size of the star you see) decreases as a result of the increased resolution.
For instance the size of the airy disk and Dawes Limit (resolving power) for some example apertures are:-
4 inch scope, 2.8", 1.1"
8 inch scope, 1.4", .57"
10 inch scope, 1.1", .46"
12.5 inch scope, .88", .36"
15 inch scope, .73", .31"
18 inch scope, .60", .25"
25 inch scope, .44", .18"
NB: The scope aperture is followed by the Airy disk diameter followed by the Dawes Limit.
What this means is that you need progressively better conditions for the scope to perform at its best and approach its resolution limit and give it's tightest star images.
As you can see from the data above a 4" scope will deliver its tightest stars with 3" seeing and best resolution in seeing that is just worse than 1". Further, being a small aperture instrument it will cool quickly. A 10" scope sits in limbo. It will deliver its tightest star images with seeing just worse than 1" and it will resolve to its theoretical limit with .5" seeing. An 18" scope needs .6" seeing to deliver its tightest stars. A 25" scope needs .44" seeing to deliver its tightest stars.
Why you perceived that the 10" scope gave sharper images with better contrast than the 25" scope was that the seeing and thermal equilbrium conditions were better than needed for the 10" to do its best, but clearly not close to the level needed for the 25" scope to do its best. I might add that this is a common occurence. Particularly early in the night while the large scope is still cooling and the seeing is still settling. Unfortunately it can sometimes take many hours, even all night for scopes in the 18" to 25" range to thermally stabilise if the temperature is dropping rapidly all night. This is specifically the reason several of the premium mirror makers worldwide are moving towards slightly thinner large mirrors.
You can take it to the bank that when the 25" scope is well collimated, properly cooled with good seeing, it will absolutely smash the 10" scope into the weeds in every single aspect, not just its ability to go deep and pop faint fuzzies. You will see lunar and planetary detail that it is physically impossible to see in a 10" scope. You will also split double stars that are beyond the physical limits of a smaller scope. This of course assumes both scopes to be of similar optical quality. I have seen this countless times. I regularly observe with my 18" Obsession beside a friends 25" Obsession and I regularly use smaller scopes from 4" to 12" aperture of high quality, including Astrophysics and Takahashi APO refractors.
Cheers,
John B
