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Head to Head: 4.5" APO refractor vs 8" SCT Edge HD -- Guess which?

Astrophotography Cassegrain DSO Equipment Imaging Refractor SCT
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#1 ant-man

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Posted 21 September 2021 - 11:24 AM

Alright: here's a head to head comparison of AT115EDT (f/7) with Hotech Field Flattener vs Celestron Edge HD 800, with 0.7x focal reducer (so f/7). 

 

I’ll let you guess which is the 8” Schmitt-Cassegrain, and which one is the 4.5” APO.
- Same target
- Similar capture time (around 20x600s exposures for each filter, both at f/7)
- I’ve matched the sizes, colors, brightness on screen so they both show a very similar field of view.

- Exactly the same processing on AstroPixel Processor. Simple integrate (with dark, flat, and darkflat calibration), and then combine RGB. That's all. 

- Same camera and filters: asi2600mm-p with Antlia 3nm SHO filters, with OAG guiding on a CEM40 mount. 

- I can't guarantee that the seeing, transparency, etc are all the same, so these could be a factor

 

Which is the SCT, which is the APO? Which do you like better (this might be obvious)?

 

I think more importantly, this is a real world comparison to what two different scopes are able to perform with respect to the same field of view. I had this question before, so I had to find out for myself, since it is so difficult to actually get answers. Some prefer SCTs, some prefer APOs. It's just the way it is. But I like a head to head comparison because it gives some level of objectivity (there are other factors, of course, that are not accounted for) to our preferences. 

 

Anyway, here it is. I will post the answer some time later! (If I don't forget). 

 

Tel 1 Screen Shot 2021-09-21 at 9.48.23 AM.jpg

Tel 2 -  -Screen Shot 2021-09-21 at 9.45.28 AM copy.jpg

 

Hmm. Maybe the images as jpgs did not turn out very well. I'll put up links to dropbox.

Telescope 1 -- https://www.dropbox....cope 1.png?dl=0
Telescope 2 -- https://www.dropbox....cope 2.png?dl=0


Edited by ant-man, 21 September 2021 - 11:30 AM.

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#2 eyeoftexas

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Posted 21 September 2021 - 11:31 AM

To my (untrained) eye, the stars in Telescope #2 are more bloated than in Telescope #1.  I believe there is better contrast and definition of textures in Telescope #1.  I would thus say that the answer to question, "which do you like better" is Telescope #1.

(Note the opinions were based on the jpeg files in the original post)

 

Because I have an AT115EDT, I'm pulling for it to be #1. 


Edited by eyeoftexas, 21 September 2021 - 11:32 AM.


#3 deepwoods1

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Posted 21 September 2021 - 11:40 AM

Oh, I'll take a guess. Top is APO, bottom is SCT. With regards to testing, the other night had the pleasure of being able to compare a SCT 8", a Mak 6", and a 100mm APO just the other night. While the APO had a more contrasty view, and it could resolve close to the Mak, the SCT showed more detail. This was just a visual test. All produced very pleasing images in their own right. Clear and steady skies....



#4 David Boulanger

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Posted 21 September 2021 - 11:49 AM

#1 is the refractor.



#5 KungFood

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Posted 21 September 2021 - 11:49 AM

Also an AT115 owner, hoping #1 is the Apo. Logic tells me it is because of the higher contrast between the dark dust clouds and the BG glow, but my logic is often not logical.



#6 idclimber

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Posted 21 September 2021 - 11:55 AM

I think #1 is the refractor, because it has tighter stars. 


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#7 OldManSky

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Posted 21 September 2021 - 11:57 AM

My guess would also be AT115 #1, SCT #2.

But that's largely based on star size/shape, which could also be affected by focus, seeing, etc.

 

:)



#8 Professor2112

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Posted 21 September 2021 - 12:31 PM

Well I’m glad the consensus seems to be what I was thinking at first glance.  The most obvious is the contrast, then the tighter stars of telescope #1.  APO all the way.  If it is indeed the edge.. I need to get me an edge.  



#9 idclimber

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Posted 21 September 2021 - 12:55 PM

On a second look, there is a detail that shows the SCT is #1 and we are all wrong. The detail I am referring to is not in the actual images though. Look at the scroll bar. 


Edited by idclimber, 21 September 2021 - 12:58 PM.

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#10 Sparks-M16

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Posted 21 September 2021 - 01:40 PM

I think I'm rooting for Telescope #1 to be the SCT.  It is a slightly nicer photo.  And I think idclimber is onto something with the scrollbar size.



#11 idclimber

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Posted 21 September 2021 - 01:59 PM

Assuming the SCT is properly focused, collimated and the seeing conditions are above average, it should have a lower FWHM (in arc seconds) than a smaller refractor. 

 

For my conditions I figure my maximum focal length with my current 2600mm to be right around 1000 to perhaps 1200mm. Using an SCT at twice that focal length means I am oversampled. As such I started binning the subs when using that scope (12" LX200). As such I am contempating ordering a SVX152 soon. 

 

I did a similar comparison of a finished image of M51 taken with my SCT and 102mm refractor earlier this year. The SCT is superior but not by the amount I would have predicted before doing the image. Images still on my Astrobin. 



#12 aa6ww

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Posted 21 September 2021 - 03:03 PM

I would say #1 is the  SCT because it does have tighter stars because it has better contrast. Odd logit to me since I've never done Astrophotography or care to. Odd because I'm seeing #2 has washed out for colors which tells me that scope is trying harder to pull in light, which is why the stars are more bloated, not because of the optical qualities of either telescope. 

 

..Ralph



#13 ant-man

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Posted 21 September 2021 - 03:04 PM

Here’s the answer:

 

Telescope #1 is the Celestron Edge HD 8” with 0.7x Reducer!!!

Telescope #2 is the AT115EDT with Hotech SC Field Flattener. 
 

When I saw the results I was surprised myself, expecting (or maybe WANTING) the APO to be the winner. But the image from the SCT is much nicer, more resolution given a particular field of view, and actually for the same field of view have a greater contrast. 
 

I’m a bit surprised pretty much everyone thought #1 is the APO. Even some of my Astro friends thought the same. But it’s really hard to expect a refractor with 4.5” aperture to beat a scope with almost twice the aperture diameter (granted there’s a central obstruction, but still over twice the area of light coming through. 
 

But it is also interesting that even though the 4.5” apo lags a bit behind for a given FOV, it has much, much more image FOV! At the end of the day, you have more than 4x the area that you are imaging at the same time. So the image quality is behind the SCT, but you are imaging so much more and making the pixels count. I think this is a good lesson to learn from this comparison. I don’t think this comparison will end any debate, maybe it will spark more debate. But it is a helpful discovery, I would think. 



#14 OldManSky

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Posted 21 September 2021 - 03:19 PM

But the image from the SCT is much nicer, more resolution given a particular field of view, and actually for the same field of view have a greater contrast. 

 

Hmm.  In the OP, you said:

"Exactly the same processing on AstroPixel Processor. Simple integrate (with dark, flat, and darkflat calibration), and then combine RGB. That's all."

 

Except, that can't be.  Neither of these are linear, un-stretched images right after a "combine RGB."  They're stretched.  

So the contrast visible is due to the stretch -- whether it was APP doing an auto-stretch, or the operator doing it manually.  And that stretch could account for the star size/shape differences.

I'm not disparaging the comparison, it was a very good one.  And more than anything else I think it shows how little difference there is between the two.

I'm also wondering why the AT115 image shows stars with what appears to be a bit of diffraction, which I wouldn't expect from an APO refractor.

 

Nice post, and way to get us all guessing and thinking! :)



#15 aa6ww

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Posted 21 September 2021 - 03:23 PM

^5's for Aperture and the Edge HD 8.

 

 ..Ralph



#16 rgsalinger

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Posted 21 September 2021 - 03:28 PM

When I see a full resolution frame or set of stacked frames then I can tell which is better. There's too much else going on once you start processing - stretching, adjusting colors, etc. You'll never get the full the resolution that the seeing provides with a short APO, it's just not possible. 

Rgrds-Ross



#17 imtl

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Posted 21 September 2021 - 03:37 PM

Am I the only one that does not understand what is the big surprise here? It's almost double the aperture and the same speed. Of course that the better image is going to be the larger scope. The image scale is different and FOV is different. Maybe I'm not getting it...scratchhead2.gif

 

Try and compare something a bit closer like a 140-150mm refractor with the Edge 8. That would be a nicer comparison. 


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#18 aa6ww

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Posted 21 September 2021 - 04:44 PM

The AT-115 and the Edge 8 HD are similar in price so it seems like a fair comparison.

 

If the refractor got bumped up to a 140 to 150mm triplet, that would probably be similar in price to an Edge HD11 or even Edge HD14

 

That probably again wouldn't turn out so well for the refractor, though its a fun subject to think and talk about.

 

..Ralph



#19 ant-man

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Posted 21 September 2021 - 05:18 PM

Not to mention the weights of these scopes are almost the same, probably within 1 lb (the 8” edge is a bit heavier) so the comparison is very nice to my mind. Similar price, weight, speed, camera, etc etc. May help someone later with buying decisions.  
 

yes, by the way, they’re auto stretched by AstroPixel Processor. I have it on auto, as you can see in the processing window, they’re stretched to the same specifications. Which actually I think is important in these kinds of comparisons. At the end of the day, we process these images but the signal to noise ratio limits us. There’s more noise in the AT115EDT image because there are fewer pixels in that area, whereas in the SCT FOV that’s almost the whole sensor worth of pixels! 



#20 imtl

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Posted 21 September 2021 - 05:33 PM

There’s more noise in the AT115EDT image because there are fewer pixels in that area, whereas in the SCT FOV that’s almost the whole sensor worth of pixels! 

Huh?! scratchhead2.gif The SNR is the same since it is the same f ratio hence same exposure time. What changes here is the spatial resolution between the two systems.


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#21 ks__observer

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Posted 21 September 2021 - 06:01 PM

Try and compare something a bit closer like a 140-150mm refractor with the Edge 8. That would be a nicer comparison. 

+1 




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