24 replies to this topic

[Eric H]

The only choices, please do not add to the list, do not consider mount. I am interested in hearing about the telescopes only for photography purposes...

 

108mm aperture @ f4.8, 518 Focal length, approx 17.5lbs. Petzval design

 

or

 

132mm Aperture @4.9, 655 Focal length, approx 32lbs. Triplet design with reducer.

 

 

How do you weigh factors like resolution, light gathering ability? Does 137mm more focal length really matter or is it the resolution that means more? Does the 24mm of aperture count for a lot more light?

 

What factors do you consider for these two scopes?

[Sheremy01]

I would go for the second one. I say buy the biggest aperture you can the first time or you will always regret it. I speak from experience 

Edited by Sheremy01, 07 April 2025 - 03:37 PM.

[Mike W]

The only choices, please do not add to the list, do not consider mount. I am interested in hearing about the telescopes only for photography purposes...

 

108mm aperture @ f4.8, 518 Focal length, approx 17.5lbs. Petzval design

 

or

 

132mm Aperture @4.9, 655 Focal length, approx 32lbs. Triplet design with reducer.

 

 

How do you weigh factors like resolution, light gathering ability? Does 137mm more focal length really matter or is it the resolution that means more? Does the 24mm of aperture count for a lot more light?

 

What factors do you consider for these two scopes?

What type of A/P? The only advantage of larger aperture in A/P is resolution on small objects MAINLY. Smaller scopes are MUCH easier on beginners. Also matching scope to sensor is more important. Go to "Nebula Photos" website for info on this. (Nico Carver)

Edited by Mike W, 07 April 2025 - 04:05 PM.

[Eric H]

What type of A/P? The only advantage of larger aperture in A/P is resolution on small objects MAINLY. Smaller scopes are MUCH easier on beginners. Go to "Nebula Photos" website for info on this. (Nico Carver)

This would be for larger nebulae. I have other scopes for tiny stuff.

[Mike W]

This would be for larger nebulae. I have other scopes for tiny stuff.

Scope on pier?

Portable to dark site?

These are factors that will greatly affect your ability to gather more usable data to stack. Really look at Nicos site, the answers you seek you will find there!

[Eric H]

Scope on pier?

Portable to dark site?

These are factors that will greatly affect your ability to gather more usable data to stack. Really look at Nicos site, the answers you seek you will find there!

I asked not to consider these, just the two telescopes please.

[ris242]

>>Does the 24mm of aperture count for a lot more light?

 

Your f stop is similar so the only gain is resolution.

[N-1]

...at the expense of FoV

[Eric H]

>>Does the 24mm of aperture count for a lot more light?

 

Your f stop is similar so the only gain is resolution.

So the question should be...does 24mm really equate to much more resolution? In the end, are the scopes close enough to not really see a difference in the final image besides a slight FOV change?

Edited by Eric H, 07 April 2025 - 04:43 PM.

[ris242]

So the question should be...does 24mm really equate to much more resolution? In the end, are the scopes close enough to not really see a difference in the final image besides a slight FOV change?

 

 

use this link and see what fits in the FOV with your camera.  you need to decide what suits you........and your subjects.

 

 

https://astronomy.to.../field_of_view/

[72Nova]

If it’s for larger nebula, and you already have scope(s) for smaller stuff, I’d much prefer the 518mm focal length scope that weighs half as much.

[Eric H]

The difference in resolution with my camera would be 1.5"/pixel with the 108 or 1.18"/pixel with the 130. Really not much between them I guess.

[Mike W]

I asked not to consider these, just the two telescopes please.

No, you said "What factors do you consider for these two scopes" and the answer is for A/P not much difference in performance but considerable difference in mount, etc. Hey, it's your money.

[Jared]

I disagree with several of the other responders who said that the only advantage to the larger scope is increased resolution, presumably since the focal ratio is the same. This is simply not true. That's one possible advantage, but not the only one. Larger telescopes collect more photons from your target object in a given amount of time. Only the irradiance will be the same since the focal ratio is the same. If you are shooting a subject where the extra resolution matters, then the 5" scope will win on resolution for a given pixel size, with very similar SNR binned 1x1. However, if you are imaging a subject where you are less worried about resolution or where you are oversampled for the conditions, you can downsample the image from the 5" to get better SNR than the 4" can produce at the same sampling rate. With the larger telescope, you have the choice of either having the higher spatial resolution or a deeper image for a given integration time. If you were to re-sample by a factor of 0.8, the 5" scope would yield a deeper image with the same arc seconds per pixel as the 4" scope for a given pixel size.

 

Assuming similar optical quality and assuming the mount was capable of carrying the larger scope and assuming you didn't care about the extra mass and difficulties transporting/setting up the larger scope, and assuming cost wasn't a factor, I would absolutely take the 5" scope. It will yield either better resolution (even on large nebulae) or better SNR. The only down-side is you will lose some field of view. For some subjects that will be really annoying, while for others it will actually be beneficial. Depends on the subjects.

 

The biggest issue is that the assumptions I listed are rarely correct. Generally, cost matters, bulk matters, and the moment arm of inertial from the larger scope matters. Also, with the 4" scope you know the reducer/flattener was designed with the particular objective in mind, so they will be well matched and you won't need to worry about back-focus at all. With the 5" scope and the reducer, depending on the particular models the reducer/flattener may or may not be perfectly matched to the objective, and you will need to get the back-focus exactly right which can be a hassle. It's only a one-time hassle, but still a hassle.

 

If it helps at all with your decision... I have a 130mm refractor of excellent quality and a 110mm refractor of equally excellent quality. I use both scopes for astrophotography, though the 130mm tends to get pulled out more frequently for visual use. For astrophotography, I choose the 110mm about 95% of the time over the 130mm. It's MUCH smaller and lighter. I can carry it with its tripod and mount to my most frequent observing site in a single trip from the car. The 5" requires a hand truck and a significantly heavier mount. I know you didn't want to consider factors other than the optics themselves, so I provided that answer first.

 

Based on optics alone? I would take the 5" if the reducer/flattener was well matched. The only real downside is the narrower field of view.

 

If I had to account for portability, cost, mounting, etc.? I would take the 4" in a heartbeat. The ease of use advantage for the 4" can't be overstated.

[Jared]

The difference in resolution with my camera would be 1.5"/pixel with the 108 or 1.18"/pixel with the 130. Really not much between them I guess.

This is actually fairly significant, at least for objects where resolution matters. Not all objects need or will benefit from finer sampling, but you would definitely be able to see the difference between 1.5"/pixel and 1.18"/pixel under even average skies. It's not a huge difference, but you would definitely be able to tell.

[Jared]

If it’s for larger nebula, and you already have scope(s) for smaller stuff, I’d much prefer the 518mm focal length scope that weighs half as much.

He already has a Redcat 71 as well for even wider fields, so this would be for "moderate" fields of view, I think.

[Jared]

If it’s for larger nebula, and you already have scope(s) for smaller stuff, I’d much prefer the 518mm focal length scope that weighs half as much.

He already has a Redcat 71 as well for even wider fields, so this would be for "moderate" fields of view, I think.

[hfjacinto]

All else being equal each scope will frame an object differently. Basically the 132 will show a more cropped image compared to the 108.

But not else is equal, the 132 has to have a reducer to get the fast focal ratio while the 108 doesn’t need it. If this was visual the 132 would be my choice but for AP and if always shooting with the reducer, the 108 makes much more sense. The 108 should have a flatter field due to the petzal design, it’s also lighter. But some petzals can’t be used for visual. The 132 is twice the weight so you need a more robust mount. But the 132 can be used visual.

[Eric H]

Thanks all. Yes, I thought about the extra aperture collecting more photons for a given exposure. It’s about 1.49 times more if I calculated that correctly. So a bit more gain there and a bit more resolution. But, in real world, ok seeing for the most part would I really see much difference? I guess I’m trying to justify the 108 as a medium or mid range size scope. I also know a lot of folks say a 130 kinda hits a sweet spot. Maybe an AP 130 GTX will pop up for sale. That would end my debate in a heartbeat.

[Jared]

Thanks all. Yes, I thought about the extra aperture collecting more photons for a given exposure. It’s about 1.49 times more if I calculated that correctly. So a bit more gain there and a bit more resolution. But, in real world, ok seeing for the most part would I really see much difference? I guess I’m trying to justify the 108 as a medium or mid range size scope. I also know a lot of folks say a 130 kinda hits a sweet spot. Maybe an AP 130 GTX will pop up for sale. That would end my debate in a heartbeat.

Yes, you would see the improvement. Much difference? No. You don't need to justify the 108mm. My 110mm is, quite literally, my favorite portable scope. I have had top quality refractors between 60mm and 155mm. I still own top quality refractors of 60mm, 80mm, 110mm, and 130mm. The 110mm gets by far the most use of the four for imaging. 130mm comes in a distant second for me. It would probably be more of a toss up if I didn't have a 12" scope permanently housed in a remote site, of course... 

 

So, yes, you can see the difference between a 132mm scope and a 108mm scope in terms of either resolution or SNR under average seeing conditions. It's not a huge difference, but you won't struggle to tell the data sets apart. That being said, the odds of the 108mm scope being used MUCH more frequently just because of the better portability are quite high. 

 

Ignoring whether your mount can handle the larger scope, my recommendation would be to get the 132mm if you are going to be mostly imaging from your back yard, and to stick with the 108mm if you are going to be mostly driving to your observing site. That's how I would make my decision. Obviously, your list of priorities may not match mine.

[Jared]

All else being equal each scope will frame an object differently. Basically the 132 will show a more cropped image compared to the 108.

But not else is equal, the 132 has to have a reducer to get the fast focal ratio while the 108 doesn’t need it. If this was visual the 132 would be my choice but for AP and if always shooting with the reducer, the 108 makes much more sense. The 108 should have a flatter field due to the petzal design, it’s also lighter. But some petzals can’t be used for visual. The 132 is twice the weight so you need a more robust mount. But the 132 can be used visual.

Why would the 108mm have a flatter field? I'm not certain what refractor and reducer the OP is looking at, but there are plenty of examples of refractors with add-on reducers that yield exceptionally flat fields able to cover anything up to and including a 43mm imaging circle. Things like the AP130, the Tak TOA130, Skywatcher's Esprit 120mm, and the William Optics FLT-132 all come to mind as scopes with dedicated reducer/flatteners that should yield fields every bit as good as a five element Petzval of similar quality. You've got to get the back spacing correct, but that's a one time pain.

 

I definitely agree with you on the need for a robust mount, though. The 132 is likely to weight nearly twice as much as the 108mm, and will have a much larager moment arm of inertia due to the combination of weight and length. 

[ris242]

Assuming the 108 is already owned, I would think they are a little too similar.

 

 

[Eric H]

Assuming the 108 is already owned, I would think they are a little too similar.

 

 

Screenshot 2025-04-08 143228.png

It’s not. It’s on order but weighing options.

[Wildetelescope]

This would be for larger nebulae. I have other scopes for tiny stuff.

Then you have your answer.  The 108 Petzval will give you the wider FOV and flat field.   Done.

 

JMD

[akdwivedi]

It’s not. It’s on order but weighing options.

your signature already shows ultracat 108... I guess you are asking us to validate if you have made the right choice :-).  take some pics and post them here. We all will benefit from your review of Ultracat.