Scopes with central obstructions affected more by seeing?

[Whippy]

Up until Monday evening, I've only really imaged with refractors. But the recent arrival of a 6" Ritchey Chretein has changed all that, and Monday was clear so I thought I'd give it it's first light. The conditions weren't great, pretty murky but fairly still. I plumped for M63 as a target and despite 3 hours of 10 minute subs, I have a LOT of noise and certainly not an image I'd be happy with.

I did remember reading something quite a while back about scopes with CO's being more affected by seeing conditions and combined with the refractor's almost mystical ability to apparently 'punch' through bad seeing, this got me thinking. Has anyone else found this or is this one of those urban myths probably propergated by refractor fans?

Tony..

[NickK]

Could be the focal length - a longer focal length is likely to be hit harder by bad seeing.

[Whippy]

Indeed it does, but we're only talking about 1300mm focal length here and I've imaged with my ED120 (f/l 900mm) in worse conditions with great results. I'm sure I've read something about this but I can't think where!

Tony..

[Psychobilly]

Any of this any use....

Effects of central obstruction in telescopes

Courses in Astrophotography

Billy...

[Whippy]

I Haven't seen the Damian Peach article before, I'll have a read. Cheers .

Tony..

[michael.h.f.wilkinson]

What about the focal ratio? The ED 120 is F/7.5, the RC F/9. More noise may simply mean less exposure per sub. You need 44 % more exposure to get the same number of photons per pixel.

[Whippy]

True Michael, I had a session on M63 a couple of years ago on my old WO ZS110 (f7) with 7 mins subs (and less total exposure time) and TBH, the effort with the RC isn't nowhere close. Plus I was using a much weaker LP filter on the RC so hence my thoughts about the CO... Read through Damian Peach's article that Billy linked to and it alludes to it but there's no definitive answer, I was hoping some of the more experienced amongst us might have shared that experience.

Don't get me wrong, I'm sure the RC will do the job and next time out I'll have a reducer on it to bring it down to f7.2 but as it stands, imaging at f9 isn't fun!

Tony..

[brianb]

I have a LOT of noise and certainly not an image I'd be happy with.

I did remember reading something quite a while back about scopes with CO's being more affected by seeing conditions

Noise isn't affected by turbulence. When you've got enough focal length for the diffraction pattern to be well resolved (i.e. f/20+ depending on the pixel pitch of the sensor you're using) or are observing visually at high magnification you may find that a central obstruction causes the effect of atmospheric turbulence to be slightly increased, though it's much more likely that the trouble is tube currents - which systems with central obstructions are more prone to, because the light passes two or three times along the tube.

Seeing turbulence, or tube currents, will smear the "pinpoint" stars out into blobs - in a long exposure image it's impossible to tell the difference between an image softened by bad seeing and an image which is badly focused.