Focal Length Useability

[glend (Glen)]

I am pondering the wisdom of upgrading my 8" Classical Cassegrain ( which I am happy with), to the 10" version, which would bring the f12 focal length from 2400mm to 3000mm. Sure I get more light gathering aperture and resolution as well. However, I am concerned about diminishing useability when native focal length increases. I am acknowledging that this scope is primarily a planetary, and double etc star splitter, but can still poke around very bright nebula like Orion, Tarantula, etc.
Of course focal length useability is impacted by Seeing conditions, and I am concerned that I may not find the addition focal length can be used from my coastal location. Perhaps simply using higher powered quality EPs with the 8" CC could achieve near the same result ( granted that the aperture would not be gathering as much light). As an example, recent Mars observations with the CC8 get hard to push beyond 266X without losing detail, the same 9mm EP would yield 333X with the CC10, assuming Seeing permits. A 6mm EP could return a theoretical 500X on a CC10 but likely never achievable in my environment. I have been using the Scope Calculator to explore the numbers,


https://skyandtelescope.org/observin...pe-calculator/

But it assumes perfect Seeing.

I am leaving cost considerations out of the discussion.
For all of you long focal length users, do you have opinions on this topic?

[Atmos (Colin)]

I’d suggest that a combination of seeing and optical quality trump the move from 8” to 10”.
In saying that, my decision to get the Mewlon 250 over the M210 was purely a mechanical decision. Given that the M210 is HALF the weight of the M250 I do sometimes think it would be nicer but I love the non-moving primary and the addition of the built in corrector and the better secondary collimation design.

If both the 8” & 10” CC are of similar optical quality (you won’t know unless you compare them side by side) then the 10” will punch a bit more. A better optically 8” will beat an inferior 10” under the same seeing.

[Merlin66 (Ken)]

Glen,
Visual or imaging?
Visual.... limited by the seeing. I only get average/ less than average seeing down here on the Bellarine. This limits me visually to around x250 mag max.
Imaging...well "lucky imaging" can overcome seeing (momentarily) and the camera pixel (sampling) needs to be considered.


(With say 3 arcsec seeing I can still get "good" imaging with a sampling of 0.8 arcsec/pixel)

[icytailmark (Mark)]

from experience with a c14 focal length comes down to


1) How good your mount is tracking and good polar alignment.



2) size of sensor on camera


3) weather conditions


for example keeping a planet in the sensor at 3000 FL is pointless if you don't have good collimation, accurate tracking. Also make sure your finderscope is aligned to the telescope each and every time.

[glend (Glen)]

Originally Posted by Merlin66

Glen,
Visual or imaging?
Visual.... limited by the seeing. I only get average/ less than average seeing down here on the Bellarine. This limits me visually to around x250 mag max.
Imaging...well "lucky imaging" can overcome seeing (momentarily) and the camera pixel (sampling) needs to be considered.


(With say 3 arcsec seeing I can still get "good" imaging with a sampling of 0.8 arcsec/pixel)

Ok I guess I should have been more clear, this is visual. I have another scope for EAA, frame stacking, at f5.

Your 250X is pretty much in agreement with my recent experience with the CC8 ( with 266X being a practical limit on test nights).

[glend (Glen)]

Originally Posted by icytailmark

from experience with a c14 focal length comes down to


1) How good your mount is tracking and good polar alignment.

2) size of sensor on camera

3) weather conditions

for example keeping a planet in the sensor at 3000 FL is pointless if you don't have good collimation, accurate tracking. Also make sure your finderscope is aligned to the telescope each and every time.

Mark, my scope is pier mounted in my observatory, on a CGX with perfect polar alignment. CC8 collimation is perfect,
No camera involved with the CC8, or possible 10. Weather conditions equals Seeing as far as I am concerned, I am not out there on dodgy night's.

[icytailmark (Mark)]

seeing conditions and light pollution determines how far you can push your focal length in viewing anything. There is a point where the object starts becoming fuzzy because of conditions.

[Camelopardalis (Dunk)]

Glen, the human optical workflow (retina through to brain) seems to include a persistence function.

What I mean by this is that if you look at an object at high magnification, variations in the seeing will show you glimpses of details that, on average, you can’t see all the time. This is the premise for lucky imaging of course.

In my experience, once you get a glimpse of something, it becomes easier to see it again. This requires patience and careful study of an object, and it’s best to be nice and comfortable at the eyepiece.

I do - occasionally - stick an eyepiece in my Edge11 (at 2800mm FL), although I only look at planets and the Moon at high magnification. The experience is always satisfying, and it is easy to notice when the seeing isn’t up to much, or even when it is. It serves to remind me what I’m missing with the camera shoved up the tube

My strategy with seeing is this...if it looks a bit turbulent with one eyepiece, dial it down a bit...meaning, go for a longer FL eyepiece. There’s always something to see!

[astro744]

It comes down to exit pupil. A 250mm aperture will give a 1mm exit pupil at 250x whereas a 200mm aperture would give 0.8mm exit pupil at 250x magnification. The object will be brighter at the 1mm exit pupil.

Focal length has nothing to do with it since all it means is that to get 250x with 3000mm focal length you need a 12mm eyepiece whereas 9.6mm is needed to get the same at 2400mm focal length. Of course if seeing permits you can get the same 0.8mm exit pupil at 312x in the larger aperture using a 9.6mm eyepiece.

[multiweb (Marc)]

I have a C11 and the CN-212. They both have similar FL. One is F/10 the other F/12.9. I have pushed the CN-212 to 8.2m FL. It depends on the quality of the optics. Mirrors and barlow. Visually it would be hard. With a camera it's much easier. The smaller the aperture the less affected by seeing it is and quicker to reach temperature equilibrium. I think you could push your C8 to 4m or more quite comfortably. You need a quality barlow with no light scattering or chromatic aberrations. The hardest thing with increased FL is tracking so your mount smoothness, stability and how stiff it is.

[sharpiel]

Originally Posted by Atmos

I’d suggest that a combination of seeing and optical quality trump the move from 8” to 10”.
In saying that, my decision to get the Mewlon 250 over the M210 was purely a mechanical decision. Given that the M210 is HALF the weight of the M250 I do sometimes think it would be nicer but I love the non-moving primary and the addition of the built in corrector and the better secondary collimation design.

If both the 8” & 10” CC are of similar optical quality (you won’t know unless you compare them side by side) then the 10” will punch a bit more. A better optically 8” will beat an inferior 10” under the same seeing.

I bet that mewlon 250 is amazing.

[mental4astro (Alexander)]

Glen,

I am assuming your question is wholly about visual as your opening post is only referencing visual.

I have and had scopes ranging in focal length from 2000mm to 3100mm. This sort of long focal length in itself already means they are high magnification instruments and not expected to give expansive views of the whole of M42, etc. You won't be using very much under 66X to 100X with such scopes.

The only limit I find is eyepiece selection to match scope focal length with magnification range. From the eyepieces I used with my 8" SCTs to the 3100mm focal length Mak is I've added two eyepieces. I've added a 12mm that sits between the 14mm and 10mm I already had. Doesn't sound like much, but sometimes a mm or two can make the difference when seeing is just a little too touchy to go to a 10mm. The other is a 7mm as 5mm is used even less with this scope.

The 12mm I also found was the exit pupil sweet spot for this scope. During Sydney's lockdown I managed to do a planetary nebula marathon from home. You wouldn't think it, but with a 9" Mak I pinned 24 planetaries from my home in Sydney over two nights - doesn't sound like much but we are talking about inner Sydney and sketching each and every one.

Practical limits? No, I find no real difference between the them. As has been already mentioned seeing is the main limiting factor and it then does not matter the focal length. Yes you are reducing the maximum TFOV, but it is nothing that has concerned me. Really, just how many objects are larger than 1deg in size? Not very many. And if you have a goto mount, you are not really needing to pan to find an object at low magnification, are you?

When I went from the SCT's to the Maks, it was entirely about optical quality. I understood that with Maks it meant longer focal length, but it really was not anything that concerned me from the very start at all. It only meant adapting my eyepiece case a little and really, big deal.

Alex.

[glend (Glen)]

Originally Posted by mental4astro

Glen,

I am assuming your question is wholly about visual as your opening post is only referencing visual.

I have and had scopes ranging in focal length from 2000mm to 3100mm. This sort of long focal length in itself already means they are high magnification instruments and not expected to give expansive views of the whole of M42, etc. You won't be using very much under 66X to 100X with such scopes.

The only limit I find is eyepiece selection to match scope focal length with magnification range. From the eyepieces I used with my 8" SCTs to the 3100mm focal length Mak is I've added two eyepieces. I've added a 12mm that sits between the 14mm and 10mm I already had. Doesn't sound like much, but sometimes a mm or two can make the difference when seeing is just a little too touchy to go to a 10mm. The other is a 7mm as 5mm is used even less with this scope.

The 12mm I also found was the exit pupil sweet spot for this scope. During Sydney's lockdown I managed to do a planetary nebula marathon from home. You wouldn't think it, but with a 9" Mak I pinned 24 planetaries from my home in Sydney over two nights - doesn't sound like much but we are talking about inner Sydney and sketching each and every one.

Practical limits? No, I find no real difference between the them. As has been already mentioned seeing is the main limiting factor and it then does not matter the focal length. Yes you are reducing the maximum TFOV, but it is nothing that has concerned me. Really, just how many objects are larger than 1deg in size? Not very many. And if you have a goto mount, you are not really needing to pan to find an object at low magnification, are you?

When I went from the SCT's to the Maks, it was entirely about optical quality. I understood that with Maks it meant longer focal length, but it really was not anything that concerned me from the very start at all. It only meant adapting my eyepiece case a little and really, big deal.

Alex.

Thanks Alex for that info. Yes, visual only. My comment about poking around Orion was not related to expansive views, I have other scopes for that, but rather Trapezium star splitting. I agree that EP sweet spot sits around 12mm, on this scope. I agree that pushing that extra mm on EPs can degrade the view. Cheers.

[mental4astro (Alexander)]

Ah, you do have other scopes! I wasn't aware. But it is an important point you make, and one that I was thinking about. I thought about bringing it up.

If one does have other scopes, say rich field short focal length scopes, then the question about going from 2000mm or 2500mm to 3000mm shouldn't be a concern. It really is only a concern if the scope's focal length begins to reach beyond 4000mm, even 5000mm. Then such a focal length needs even more specialized consideration, which you touched upon, namely where you have the scope set up. If seeing is not the best at the best of times, then such extreme focal lengths is a concern. But not 3000mm. I've seen a big cassegrain come up for sale recently with a focal length of some 7500mm - a VERY specialised scope!

If you don't have other, shorter focal length scopes, then it could be a concern. Maybe. Such long focal length scopes are not typically sought by newcomers to astro. So most people who are contemplating such long fl scopes, these people will already have an understanding of what these scopes offer and would be happy to make it their only scope.

And for photo, there are a totally different set of considerations.

Alex.