26 replies to this topic

[ant-man]

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). 

 

 

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.

[eyeoftexas]

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.

[deepwoods1]

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....

[David Boulanger]

#1 is the refractor.

[KungFood]

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.

[idclimber]

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

[OldManSky]

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.

 

[Professor2112]

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.  

[idclimber]

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.

[Sparks-M16]

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.

[idclimber]

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. 

[aa6ww]

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

[ant-man]

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. 

[OldManSky]

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!

[aa6ww]

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

 

 ..Ralph

[rgsalinger]

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

[imtl]

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...

 

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

[aa6ww]

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

[ant-man]

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! 

[imtl]

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?!  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.

[ks__observer]

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

+1 

[Mark Lovik]

Sorry being late to this thread.  I have both an AT115 and old 8" SCT, so this comparison is fun to look at.

 

There is a subtle concept at work here.  Most of the comments have centered on the difference of star size (star bloat) between the 2 images.  This appears to the be the most obvious difference between the 2 images.  There also appears to be good focusing of both systems to keep the comparison fair.

 

Star Bloat and Aperture

 

1. Limiting resolution - assuming we have a typical 2" seeing, both scopes exceed seeing by a reasonable margin.  So the resolution for both systems (assuming the pixel resolution is not a problem) should be about the same.

 

2. Star diffraction diameters between the 2 systems are very different, where the star diffraction rings of the SCT should be almost half the angular size of the 115mm APO.  Without an obstruction the refractor should have a bit more light within the first ring, but it's twice the size.

 

SO -- if seeing is limiting why is the 8" SCT better (smaller star bloat)?

 

We tend to do histogram stretches to support the large dynamic ranges in our images.  The brighter the star - the more we have adjusted the brightness.

 

This means we are not looking at the full width half height of the stars diffraction pattern.  We are looking at light from the dimmer outer diffraction rings of these brighter stars (bigger effect for brighter stars).  So the bigger aperture almost always wins - the diffraction spread of light is tighter.

 

This is also why brighter stars bloat more.  So when this effect is small - seeing limits star size.  For bright stars - bigger apertures can hide the bloat better under the seeing limit.

[HowardSD]

Very interesting thread as I've been contemplating moving up from my Zenithstar 103 to the AT 115 triplet and funnily enough I just finished processing my Z103 image of this same target last week.

 

I may as well include here as another comparison for a smaller doublet image although heavily processed in APP & PS with the SHO palette and also stars removed and added back. Imaging time of 8 hours from a OSC, ASI533MC / L-Extreme.

 

I thought like many, the more contrast image would be from the APO, bit shocking actually! Now it's left me wondering, although think i'm still onboard with the APO as I do prefer a wider FOV and especially as my ASI533 has a smaller FOV than say a 294 or 071 besides not planning on changing my camera anytime soon as I'm very happy with it.

 

I love my WO Z103, but I do wonder about the difference in imaging between an FPL53 doublet & a good triplet, thought about the ES 102 FCD100 however if I make a move figure a little more aperture wouldn't hurt!

 

I am running this on an AVX which handles my Z103 very well, I believe the AT115 is pushing it a little but from what i've read is still within decent parameters for AP.

[KungFood]

I have the AT115, and used to have an AVX. From what I've heard there are good AVX's and not so good ones. If you have a good one and are very careful in balancing, and don't have any extra weight on the RA axis (dew heater controller, power box etc) it's a doable combo. I'd be careful though, even if it all goes right, it's pushing it's limits and you might lose more subs than you want, this was the case for me. I used to lose 25-40% of my subs on any given night. Since upgrading to an EQ6R, I lose hardly any. When I shoot at the dark site, I rarely lose any subs to eccentricity. When I shoot from my tiny patio, I lose 4ish out of a 60-70 sub run, and that's being very picky about star shape. I know people have made the AVX sing sweet songs with "heavy" payloads, but being able to reliably harvest subs with a beefier mount makes all the difference for me.

[Jared]

I wouldn't certainly hope that the SCT would have better SNR at any given sp

 

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. 

People often think that refractors are simply better for astrophotography. That's simply not true. Refractors are almost always of good optical quality, which is helpful. Refractors tend to have robust focusers, which is helpful. Refractors often have dedicated (or well matched) flatteners and reducers, which is helpful. But they aren't magic, and they aren't intrinsically better for astrophotography any more than they are intrinsically better for visual use. If one compares a refractor to an equal aperture reflector I would expect the refractor to come out on top, but not with a large aperture difference like this one.

 

Once you account for light throughput, the SCT still conservatively collects more than twice the light from the target. That means, for any given spatial resolution, better signal-to-noise ratio from the SCT. In addition, while seeing might be the single largest factor in resolution, it's not the ONLY factor when you are discussing modest apertures like these, and a well collimated 8" reflector/SCT is starting with a smaller Airy disk than a 4.5" refractor as well as tighter diffraction rings, so I would expect less bloat from the reflector as well as a deeper image for any given integration time. Where the refractor would win, obviously, is if you needed the larger field of view. 

 

Refractors are great. I own several and have loved using them, both visually and photographically. But other scopes are capable of high quality imaging, and expecting a refractor to make up for this large a gap in aperture through excellent quality is not realistic.

 

As to the test itself, it's a good thought experiment and it's great to show people that there is more than one way to reach a given level of image quality, but I wouldn't read too much into the fact that the reflector appears to have tighter stars... These are "auto stretched" which means it is essentially an uncontrolled result. Measurements of FWHM of the individual images would tell you more about how "sharp" a result you can expect from each, but that wouldn't have been very compelling to look at, so I get why you presented your findings this way. And even FWHM measurements assume the seeing was the same for both, that both were well collimated (which, admittedly, applies primarily to the SCT), and that both systems were focused to the same accuracy and both were fully equilibrated. In other words, there are too many variables outside the scope of this experiment for your results to be seen as conclusive, but they are certainly illustrative. Nice work.