resolution vs seeing in large aperture RCs

[Somnium]

Hi all, i have just been thinking about some of the larger RCs that are becoming popular, particularly now that GSO have entered in the game. in particular i am looking at a comparison between the planewave CDK 12.5 and the CDK 14. given that these scopes have virtually the same FL, the only difference is a small f ratio (7.2 compared with 8) and the extra 1.5 inches of aperture. when it gets to these types of aperture sizes, are you really getting any better practical resolution out of it? unless you are on top of a mountain and can remove a large portion of the atmosphere.38' of resolution is surely going to be more than the atmosphere will allow. so is resolution not really a factor in these scopes? or is the larger aperture really for allowing less time in gathering data and better ability to pick up faint nebulosity? interested to have the thoughts those with more experience than me.

[AlexN]

All else being equal there is no substitute for the light gathering power of aperture. and on that one fabled night where the seeing is Damn near perfect, he (or she) with the biggest light bucket wins.

[Atmos]

As a general rule of thumb, unless you have absolutely perfect seeing conditions, on top of a mountain at some 15,000 feet, resolution wise you never need to go past 10" aperture. I would actually be interested in whether the "Adaptive Optics" offered by SBIG (AOX) really helps much in bringing down the resolution considering that it isn't a true Adaptive Optics.

In a more practical sense, the CDK 14" is better than the CDK 12.5" because of the extra aperture. It works out to be a ~25% increase in photon capture so ~25% shorter imaging times for the same image.

[Shiraz]

For DSO imaging, the resolution provided by any reasonable quality scope above about 6 inches aperture will be limited by the atmospheric seeing in Australia - to about 1.5-2 arcsec. Bigger scopes cannot do much better unless they have true adaptive optics and even then, that only works at near IR and for quite small fields of view.

Bigger scopes gather more photons and can provide better SNR - provided the pixel size is matched to the focal length. It is pointless to have a large aperture if the angular size of the pixels is so small that sensor noise overpowers the small amount of signal that actually gets into each pixel.

[Somnium]

what i am getting at is what exactly is it about the bigger bucket. if the seeing wont allow you to get below .39" in resolution for DSO imaging what benefit does that extra aperture give? is a 20" f4 going to pick up more detail than a 10" f8 shooting for twice the time? or are there really circumstances where .39" of resolution really would be the limiting factor?

[Somnium]

thanks Colin and Ray, that is what i was thinking

[gregbradley]

My CDK17 is quite sensitive to seeing conditions.

But with the reducer its a potent photon catcher and can get deep faint material very quickly as seen in my recent Helix Nebula image.

Greg.

[Shiraz]

for systems with equal sampling etc, f8 is going to require 4x the time of an f4.

[Atmos]

In Aus there is no reason to have a pixel scale better than 0.7 arcsec/pix, the 0.39 is the resolution that the telescope is capable of achieving, not what it is actually going to get.

Case in point, a 10" has a resolution around the 0.5" where as the 20 is closer to 0.25" but if they both have the same focal length the 20" slaughters the 10" because it captures images 4x faster and is able to go deeper (capture fainter objects). Putting a KAF-11002 with its 9 micron pixels will give a resolution of ~0.91 arcsec/pixel which would be pretty much perfect for normal every day imaging.

[Somnium]

your right , sorry Ray



i have seen your work, and that image was fantastic, picked up a lot of faint detail.

[gregbradley]

KAI11002 gives .93 arc secs per pixel on a 10 inch F8 scope. Half that with the 20 inch F8 (.46).

Greg.

[Somnium]

so in my quest to resolve distant galaxies has come down to punching a 12.5" hole through the atmosphere directly in-line with my scope ...

[Shiraz]

while you are at it, find a way through clouds as well. :)

[Somnium]

ahh clouds cant form in no atmosphere ... so my solution resolves that issue too :)

[Atmos]

True, but a 10" F/8 will be the same as a 20" F/4 :P

[Slawomir]

And there are of course "illegal" tricks like drizzle... ;)