Same target, different scope. - Forums

I've had two clear nights in a row, which is unusual. Because they're so short and don't really get dark at the moment, I thought I'd do something I've often wanted to do, and shoot the same target with each of my imaging scopes; an 81mm doublet refractor (f/6, 495mm) and a 200mm reflector (f/5.6, 1120mm).

I've processed as similarly as I could, and I've cropped the refractor's image to match that of the Newt, and here they are:

I think it's obvious which is which, not least because of the diffraction spikes.

Is the significantly greater amount of effort needed to get a decent image out of the Newtonian worth it? It's very subjective, obviously.

I just thought it quite interesting!

The newt image is obviously a much better image even at the same image scale.

I agree with Andrea. The Newt has captured better color in the stars and the difference in SNR away from the core is evident.

The big advantage of the Newtonian is its light grasp, not necessarily its resolution. That'll become evident once you go after fainter targets. M13 is quite bright.

Ah yes, light grasp! It's meant to be clear again tonight, so I'm going to try with my Coma Corrector 3 which gives me 760mm focal length at f/3.8 (I think) and see what that looks like (I suspect I might need the more expensive Corrector PH for an APS-C sensor, but we'll see).

I agree with both of you, but I'm still surprised quite how well the little scope does.

Here's the same target, shot using my 8" Newt but this time with a cheap Coma Corrector 3. I've rotated and scaled it to match the original shot and the results are just what I expected looking at the spot diagram so it's very apparent that the much more expensive Extender PH I used initially corrects for coma MUCH better than the Coma Corrector 3. Only to be expected, but interesting to see for real.

Might I treat myself the Vixen's Corrector PH, which is meant to correct for a full-frame camera? Probably.

I agree with others in that the newt image is superior. The colors and detail are so much better.

The problem I've had with it in the past is the collimation wasn't very stable, despite the scope's reputation of not requiring frequent collimation. However, now I've put back the original fixings (a previous owner had used Bob's Knobs) it's as good as its reputation suggests.

Very happy with it now.

I know the Corrector PH would get me down to f/3.8 and double my integration times (or however it's best described) compared to my extender (f/5.6) but surely that would also make collimation even more critical? It's not like f/5.6 is "slow" anyway.

To be fair, that refractor is about $1000 cheaper than that Newt. I would love to see a shoot out with a similarly priced 130mm refractor on the same mount.

Piers Palmer:
The problem I've had with it in the past is the collimation wasn't very stable, despite the scope's reputation of not requiring frequent collimation. However, now I've put back the original fixings (a previous owner had used Bob's Knobs) it's as good as its reputation suggests.

Very happy with it now.

I know the Corrector PH would get me down to f/3.8 and double my integration times (or however it's best described) compared to my extender (f/5.6) but surely that would also make collimation even more critical? It's not like f/5.6 is "slow" anyway.

The scope would still be a f/4 one so in that regard collimation requirements are even.

Oh, ok! Now I've mastered a barlowed laser method it's not tricky anyway. Interesting!

Ashraf AbuSara:
To be fair, that refractor is about $1000 cheaper than that Newt. I would love to see a shoot out with a similarly priced 130mm refractor on the same mount.

There's about £400 difference in the price, so depending on which nation's $ we're talking about, yes.

I'd quite like a 130mm refractor to have a go with!

Ashraf AbuSara:
To be fair, that refractor is about $1000 cheaper than that Newt. I would love to see a shoot out with a similarly priced 130mm refractor on the same mount.

You would find pretty hard to get a 5" refractor of the Vixen quality at the price of a R200SS.

andrea tasselli:
Ashraf AbuSara:
To be fair, that refractor is about $1000 cheaper than that Newt. I would love to see a shoot out with a similarly priced 130mm refractor on the same mount.

You would find pretty hard to get a 5" refractor of the Vixen quality at the price of a R200SS.

In the US, the Astro-tech AT130EDT Apo triplet retails for $1799 brand new. With a flattener / reducer it will be about $2000. The Vixen R200SS retails for $2400.

Not on par with the Vixen quality I'm talking of besides being a poor-man APO-ish. At any rate Vixen products are cheaper here (Uk) and similar EDs are more expensive.

andrea tasselli:
Not on par with the Vixen quality I'm talking of besides being a poor-man APO-ish. At any rate Vixen products are cheaper here (Uk) and similar EDs are more expensive.

*Not sure what you mean. I want to see a picture shoot out between two similarly priced scopes. The quality is in the pudding (aka the results).

Would would a triplet 130mm scope weigh? I know the Askar 130 is about 10kg, which is almost twice what my R200SS weighs. I'd also be giving up a lot of aperture, although I'm not sure how significant that is.

My SXD2 mount is rated for 15kg capacity. I think with Japanese stuff, the limits are realistic, but at the moment I'm well within it, coming in at about 8kg fully kitted-out.

I'd like to see a comparison too, but to my simple mind I'd be giving up aperture, weight before I even started.

Ashraf AbuSara:
*Not sure what you mean. I want to see a picture shoot out between two similarly priced scopes. The quality is in the pudding.

I very much doubt that any is forthcaming any time soon (side by side with the same camera model at the same time). Not that I'd need one as in my mind any good 8" newt is going to thrash the 5" in all but the widest of field, if ever.

Piers Palmer:
Would would a triplet 130mm scope weigh? I know the Askar 130 is about 10kg, which is almost twice what my R200SS weighs. I'd also be giving up a lot of aperture, although I'm not sure how significant that is.

My SXD2 mount is rated for 15kg capacity. I think with Japanese stuff, the limits are realistic, but at the moment I'm well within it, coming in at about 8kg fully kitted-out.

I'd like to see a comparison too, but to my simple mind I'd be giving up aperture, weight before I even started.

*It's a good point, a triplet 130mm fully kitted out would be 12-13 kg, which should stay within you mounts limits.

andrea tasselli:
Ashraf AbuSara:
*Not sure what you mean. I want to see a picture shoot out between two similarly priced scopes. The quality is in the pudding.

I very much doubt that any is forthcaming any time soon (side by side with the same camera model at the same time). Not that I'd need one as in my mind any good 8" newt is going to thrash the 5" in all but the widest of field, if ever.

*I'd imagine it will all depend on the target and seeing conditions. But I find it hard to believe that the 8 inch Newt will do any "thrashing" of a 5inch refractor.

Again, would be nice to demonstrate. You might be surprised.

Ashraf AbuSara:
*I'd imagine it will all depend on the target and seeing conditions. But I find it hard to believe that the 8 inch Newt will do any "thrashing" of a 5inch refractor.

Again, would be nice to demonstrate. You might be surprised.

You're welcome to try and report back. Side by side, same mount, same camera, same integrations.

If I win the lottery this weekend (and let's face it, I enter every week so I'm bound to before long) then I'll gladly carry this comparison out!

andrea tasselli:
Ashraf AbuSara:
*I'd imagine it will all depend on the target and seeing conditions. But I find it hard to believe that the 8 inch Newt will do any "thrashing" of a 5inch refractor.

Again, would be nice to demonstrate. You might be surprised.

You're welcome to try and report back. Side by side, same mount, same camera, same integrations.

Somebody did this with a top end 130mm refractor against an 11inch top end SCT for smaller targets.

https://www.astrobin.com/bswgq6/0/

Newtonians are quite different than SCTs so I can't extrapolate anything from that comparison. But the point is some findings might be surprising.

Aperture and its related sibling, SNR, is king. In seeing-limited conditions how much light per square sky area you put in for unit time per pixel is the key factor (all other being equal) so there you go: an 8" newt with 35% obstruction has nearly twice the light gathering power a 5" refractor at the same image scale. That's all it matters (as long as the required field is covered).

andrea tasselli:
Aperture and its related sibling, SNR, is king. In seeing-limited conditions how much light per square sky area you put in for unit time per pixel is the key factor (all other being equal) so there you go: an 8" newt with 35% obstruction has nearly twice the light gathering power a 5" refractor at the same image scale. That's all it matters (as long as the required field is covered).

No one is debating the theoretical merits of purely aperture or SNR. But in practice that is not the only consideration. As you noted seeing limitations are also important. At the end of the day, displaying the actual difference between a similarly priced refractor and a newtonian in an apples to apples comparison could help guide decisions about OTA choice as the additional issues you may have to deal with either OTA types need to be factored in (CA, collimation, weight, etc). I don't think it is an outlandish idea. Some comments suggest otherwise:

https://www.cloudynights.com/topic/799804-refractor-vs-newtonian-vs-cat-for-imaging/?p=11524322

I have an 8 inch SCT but I don't have a Newtonian. Maybe one day.

Ashraf AbuSara:
No one is debating the theoretical merits of purely aperture or SNR. But in practice that is not the only consideration. As you noted seeing limitations are also important. At the end of the day, displaying the actual difference between a similarly priced refractor and a newtonian in an apples to apples comparison could help guide decisions about OTA choice as the additional issues you may have to deal with either OTA types need to be factored in (CA, collimation, weight, etc). I don't think it is an outlandish idea. Some comments suggest otherwise:

https://www.cloudynights.com/topic/799804-refractor-vs-newtonian-vs-cat-for-imaging/?p=11524322

I have an 8 inch SCT but I don't have a Newtonian. Maybe one day.

Thanks for the link. I see they offer little in terms of hard facts (unlike the previous of your posts) but tales of so-I-say. That is why I left CN long time ago as just a talking shop of litle substance. Personally I find refractors not much appealing for imaging purposes except in the smaller diameters but then I'd tend to gun for photo-objectives in any case as dual purpose lenses. Obviously to each their own...

andrea tasselli:
The newt image is obviously a much better image even at the same image scale.

The colours in particular are much better.