8" scope for galaxies - classical cassegrain or Ritchey-Chrétien?

[Rallemikken]

I'd like some advice regarding 8" cassegrains vs. Ritchey-Chrétien at same aperture.   Nearing seasons end for the third time since I sunk down in this hole, and I'm already planning for next winter. My rig is a HEQ5  and a SW 200PDS, 8" newt. Works fine, have an obsy. Wind no problem.  Bortle 4,  almost 3. I've decided to stick to my old trustied Canon cameras (five in total).  Have found out that these DSLR's are better suited for well defined targets than thin clouds of dust and gas. Anyway, I'd like a new and shiny scope, and I'm considering some more focal lenght for all those tiny galaxies that more or less are out of reach today.  My first thougt was the 8" Ritchey-Chrétien.  I have an 6" f/4 newt for nebulas and such, and a focal lenght of 1600 would be a natural progression in my current setup (600mm and 1000mm). My question is: Will a 8" cassegrain be useful at all for me? Not world champion in guiding, but not that bad either. I'm aiming at things smaller than Bode's, but don't want to be stuck with a scope to long. Not that interrested in planets and the moon, my new scope will be for DSO's. I've done some attempts with a 2,5X barlow on my 200PDS with mixed results. I assume a native focal lenght will be better, but it will be a trade-off regarding number of frames on any given night. I don't like to expose longer than 2-3 minutes, I've found out that the number of useable frames in the stack is vital. Do anyone have any experience on this usecase? Havn't seen many use their casses for galaxies and other faint, small targets, mostly planetary.

[vlaiv]

You already have 8" scope, and if it works and you are satisfied with that one, why change it?

Why did you use barlow at all? You can get "closer" to target - but without any additional detail. There is not much point in making image larger without detail - you can do that in software to the same effect.

With 1000mm you should be very near optimum sampling rate with range of pixel sizes (even with small pixels you can bin data).

[Rallemikken]

12 minutes ago, vlaiv said:

With 1000mm you should be very near optimum sampling rate with range of pixel sizes

Don't know the maths behind this so well, but I'm aiming at targets smaller than 15 arcminutes. Mostly below 10. When I do that with the 200PDS, it's extremely difficult to pull out much detail in post-procesing. I'd hope a longer scope could help.

[vlaiv]

11 hours ago, Rallemikken said:

Don't know the maths behind this so well, but I'm aiming at targets smaller than 15 arcminutes. Mostly below 10. When I do that with the 200PDS, it's extremely difficult to pull out much detail in post-procesing. I'd hope a longer scope could help.

Ok, here is a bit of math to help you out and explain things a bit more.

If you are aiming for targets in range of 10-15 arc minutes, which is 600 to 900 arc seconds (times 60) in realistic conditions - say you are shooting at 1.5"/px - you will get target size of 400-600px.

I'm guessing that you'll consider such target to be very small, right? And you want to make it bigger? Thing is - you can't. I mean - you can get larger target - but you won't get detail and sharpness.

Detail and sharpness is related to star FWHM you can achieve in your images, and that depends on several things:

1. seeing

2. mount performance (tracking / guiding RMS)

3. aperture size (we are considering diffraction limited performance)

Changing 8" newtonian with 8" RC or CC won't change anything in above 3 points. Only thing where it could make a difference is actual telescope performance - how sharp the image it gives - and even then it is one of the three things and very very minor contribution.

If you are unhappy with star shapes across the field or something like that and you want to change the scope because of it - then fine, but if you want to make your targets bigger - it simply won't work like that.

Target size depends on sampling rate and sampling rate should depend on how sharp image are you able to get with depending on above 3 parameters. There is actual math for number crunching - but we don't have to do that. Experience tells us following - realistic resolution that you'll be able to achieve with 8" scope on non premium mount is 1.5"/px. With excellent seeing such combination might be able to deliver 1.3-1.4"/px. With premium mount it might go down to 1.2"/px - and in theory down to 1.0"/px should be possible - but with everything perfect - perfect mount, perfect skies and so on ...

Even if you can achieve almost impossible and get sharpness for 1"/px - you'll still have galaxies that are from 600px to 900px in size on your image.

 

[Rallemikken]

41 minutes ago, vlaiv said:

Changing 8" newtonian with 8" RC or CC won't change anything in above 3 points.

Well explained. Not easy to grasp, though.... Most other things makes sense, but in this case it's harder to see.  I'll do some reading-up and considering taking better care of the photons I capture. Bigger aperture is not on the table, means new mount, not budget for that. Some sort of calculator or guide on this issue would be great. Scope here, camera here, bortle value here, general conditions; good, average or poor. Then an answer, and a few picture samples of what can be expected. Let me play around with sensor and pixel size, find the limitations of my setup. Maybe a dedicated astro camera instead of a new scope? Things to wrap my brain around during summer.

[ONIKKINEN]

@vlaiv is wise and has given good advice. I f i were you i would stick with the 8'' newtonian and maybe look into a better quality coma corrector if you happen to have one of the cheaper ones (Baader, Maxfield 0.95, Skywatcher 0.9 etc) as they reduce the maximum sharpness you will get out of the scope.

Vlaiv's advice was difficult to swallow the first half a dozen times i read it, but time and time again it has been proven to be right at least in my conditions. 1.5'' resolution is already a very demanding target and more than half of the time the skies will just not allow it no matter how much quality glass is in the scope. In my typical seeing a resolution of 2''/pixel would be ideal, with 1.5'' being a nice good seeing night target. 1'' however i am not sure if i have ever had the conditions to reach.

5 hours ago, Rallemikken said:

Maybe a dedicated astro camera instead of a new scope?

Definitely, makes all the difference. This wont give you sharpness any more than your DSLRs, but it will improve signal to noise ratio drastically allowing you to get a much better image in the same amount of time.