M51 SCT at f10 vs triplet at f7.5

[Magnum]

Borrowed my friends Meade 8" SCT to compare to my Meade 127mm f7.5 Triplet on galaxies paired with my ASI533MC cmos, I was expecting the oversampling with uk seeing to prevent the SCT to get any more fine detail than the Triplet which is ideally sampled, but turns out the SCT won .

Here are both images processed as good as I could get them, the set image was only 22x 5 mins at gain 100, while the Triplet image was 90 x 5 mins so is a bit deeper showing the tails a bit smoother, but the actual detail in the galaxy is clearer better resolved with the SCT at 2000mm FL vs only 950mmFL on the triplet.

Details:

Meade 8" sct @f10 200mmFL, 0.39 "/pixel

Meade series 5000 127mm f7.5 triplet, 0.8"/pixel heavy crop to show detail Better , also added the full frame

Lee

 

 

 

 

Edited April 5, 2022 by Magnum
added the full Fram image

[Victor Boesen]

I almost don't care which scope won, because both images are absolutely gorgeous!! Well done

[Craney]

I agree, would be happy with both.   Super detail on SCT frame.

[mikeDnight]

I'm not surprised the SCT went deeper, but whether it won is a matter of perspective. Both images are awesome, and each has something the other does not. Truly excellent!!! 

[Padraic M]

Stunned with the detail in the SCT shot! I've struggled with a C8 XLT for imaging but from Google just now, I believe the Meade is a match for an EdgeHD and that looks correct from this image.

[skybadger]

I notice more stars in the background of the triplet but better detail in the arms of the act image. Is this processing or a  contrast effect. I expect better higher contrast stars on the triplet and lower contrast on the sct so to have more detail in the arms is interesting. Both v nice images though.

[Magnum]

59 minutes ago, skybadger said:

I notice more stars in the background of the triplet but better detail in the arms of the act image. Is this processing or a  contrast effect. I expect better higher contrast stars on the triplet and lower contrast on the sct so to have more detail in the arms is interesting. Both v nice images though.

As i said in the post, the triplet image was 90x 5 mins vs only 22 x 5 mins for the sct image plus the triplet is faster so, so more subs and faster has picked up more fainter stars and some tiny background galaxies that didnt seem to show up in the SCT data no matter how much i stretched it, im sure if id of got more data and longer subs on the sct then they would have been more similar in that regard. 

the test as presented here was purely to see which scope resolved more detail in the spiral arms, i could do a more scientific test but even the raw individual subs showed more detail.

thanks

lee

[ollypenrice]

I've done a similar comparison between a 14 inch ODK and a TEC140 on targets which included M51. My findings were similar, the take-away being that there was precious little to choose between them. Sometimes the bigger scope out-resolved the smaller but sometimes the refractor data was 'nicer,' notably with better stars. The reflector couldn't image the the Horse, Flame and Alnitak at all. It was blown out by internal reflections, though I could crop out a decent horse from between the patches of glare.

Regarding speed, the SCT has to be potentially faster simply because it collects more light. What matters isn't F ratio but flux per pixel. Bin up the pixel size and the SCT will be faster. The question is, how to do that wth a CMOS camera? This all comes down to how effective resampling is and how and when it's done in the processing.  I've no experioence of that. You present the images at the same screen size but how did you get them there? I think that's the key question.

Great images either way.

Olly

[Magnum]

23 minutes ago, ollypenrice said:

I've done a similar comparison between a 14 inch ODK and a TEC140 on targets which included M51. My findings were similar, the take-away being that there was precious little to choose between them. Sometimes the bigger scope out-resolved the smaller but sometimes the refractor data was 'nicer,' notably with better stars. The reflector couldn't image the the Horse, Flame and Alnitak at all. It was blown out by internal reflections, though I could crop out a decent horse from between the patches of glare.

Regarding speed, the SCT has to be potentially faster simply because it collects more light. What matters isn't F ratio but flux per pixel. Bin up the pixel size and the SCT will be faster. The question is, how to do that wth a CMOS camera? This all comes down to how effective resampling is and how and when it's done in the processing.  I've no experioence of that. You present the images at the same screen size but how did you get them there? I think that's the key question.

Great images either way.

Olly

thanks Olly,

Ive been doing a lot of research on this and my thinking now is that I need a combo thats somewhere between these too, on a night of good seeing I think the triplet image at 127mm aperture at 950mm FL and 0.8"/pixel was scope limited,  and the  200mm aperture SCT at  2000mm and 0.39 "/pixel is definitely seeing limited . I think going below 0.5"/pixel has no further gain unless I was in Chile, so my ideal Galaxy scope would be around 1200mm-1500mm, I think that would still get the absolute best detail the seeing allows on the best nights while still being easy to guide and not horrendously oversampled. A 6" 1200mm triplet would fit the bill but not my wallet, I think ive seen you say the same thing in the past hahaha 😛 .

so my only affordable alternative is a 6" RC at its native f9 1370mm FL ( collimation 😞 ) or an 8" sct reduced to about 1260mm, Meade also make a 6" f10 ACF which is 1500mm FL. 

I know its only going to be a small gain, but I would like to get a scope that can get that extra 10% on galaxies on the best nights over what I have. my EQ8 mount seems happy no matter how many scopes I attach to it LOL but I would probably just swap with the 127 for galaxy season then put it back for other targets.

ive now added the un cropped original of the triplet image and the wider FOV is pleasing in its own way even though its not getting the finest details.

Lee

[R1k]

Fantastic images! Which Meade 8 inch SCT was used?

How dark are your skies?

Edited April 6, 2022 by R1k

[catburglar]

I think you’ve got a few different (almost ‘off the shelf’) options to get 1200mm FL:

8inch F10 SCT with 0.63 reducer

8inch F8 RC with 0.75 reducer

10inch F5 newt plus coma corrector

12inch F4 imaging newt plus coma corrector

The latter will be the fastest of the bunch at any given image scale, but it might also take the most fettling to get it working well. If you’re happy collimating it the RC might be the easiest to live with.

[ollypenrice]

10 hours ago, Magnum said:

thanks Olly,

Ive been doing a lot of research on this and my thinking now is that I need a combo thats somewhere between these too, on a night of good seeing I think the triplet image at 127mm aperture at 950mm FL and 0.8"/pixel was scope limited,  and the  200mm aperture SCT at  2000mm and 0.39 "/pixel is definitely seeing limited . I think going below 0.5"/pixel has no further gain unless I was in Chile, so my ideal Galaxy scope would be around 1200mm-1500mm, I think that would still get the absolute best detail the seeing allows on the best nights while still being easy to guide and not horrendously oversampled. A 6" 1200mm triplet would fit the bill but not my wallet, I think ive seen you say the same thing in the past hahaha 😛 .

so my only affordable alternative is a 6" RC at its native f9 1370mm FL ( collimation 😞 ) or an 8" sct reduced to about 1260mm, Meade also make a 6" f10 ACF which is 1500mm FL. 

I know its only going to be a small gain, but I would like to get a scope that can get that extra 10% on galaxies on the best nights over what I have. my EQ8 mount seems happy no matter how many scopes I attach to it LOL but I would probably just swap with the 127 for galaxy season then put it back for other targets.

ive now added the un cropped original of the triplet image and the wider FOV is pleasing in its own way even though its not getting the finest details.

Lee

I've now come to the conclusion that, even at 0.9"PP, I'm oversampled.

We also need to remember that we don't post unprocessed images, we post processed ones. The more signal you have, whether from time or from aperture, the harder you can process and the better the result will be. A better real resolution with inadequate signal will not give a better image than a slightly inferior real resolution with a lot more signal.

Olly

[Magnum]

16 minutes ago, ollypenrice said:

I've now come to the conclusion that, even at 0.9"PP, I'm oversampled.

We also need to remember that we don't post unprocessed images, we post processed ones. The more signal you have, whether from time or from aperture, the harder you can process and the better the result will be. A better real resolution with inadequate signal will not give a better image than a slightly inferior real resolution with a lot more signal.

Olly

well my raw unprocessed subs clearly showed more detail in the sct data than the refractor so im convinced im not oversampled at 0.8”/pixel, but definitely am at 0.39”/pixel

surprised you feel that you are already passed the limit at 0.9”/ pixel Olly as from your location the seeing must be a lot better than i have in the uk? 

A popular rule of thumb ive read lately is you can sample at upto 1/3rd of your local  seeing allows  so i would imagine you regularly get at least 2 arc second seeing so 1/3rd of that would be 0.66”/ pixel.any more than that then there will be no improvement.

my triplet image had a FWHM value of 1.9 so that seems to suggest i can push it to around 0.6 on the best night using the above.

i believe the reason for this is if we have a double star separated by say 9”, if we then imaged that at 9”/pixel the stars would not be separated and would appear as a single pixel, we would have to sample at 1/3rd of that to split them. Of course if the seeing was worse than 9” then we couldn't split them anyway.

i could be totally wrong but it makes sense in my head 😜

lee

 

[vlaiv]

13 minutes ago, Magnum said:

i could be totally wrong but it makes sense in my head 😜

It simply does not work like that.

Good sampling value is:  measured FWHM / 1.6.

If you measured FWHM of 1.9" - then good sampling rate for such image is 1.9 / 1.6 = 1.1875"/px

Don't think in terms of "how much pixels I need to record a star". Optimum sampling is determined so that you have minimum number of pixels to reconstruct image "function".

Here is an example showing you what happens with two stars:

I made two artificial stars that are separated by 2.4" and image is blurred by FWHM of 1.6" - here is image zoomed to 0.05"/px (so hugely over sampled - but very "smooth" in terms of sampling):

So you see two stars "kissing" or making 8.

If I make cross section and measure light intensity - I'll get graph like this:

We can make out that it has two stars there - there are two peaks.

Ok, so I maintain that we don't actually need 0.05"/px - but instead we only need 1"/px for this case as FWHM is 1.6" (remember FWHM / 1.6 and in this case it is 1.6" FWHM / 1.6 = 1"/px).

So here is it:

Yes, this image contains almost all the information in above image. It might not look like much - but data is there. Look what happens when I enlarge this small image to be the same size of original image (mind you - proper resampling method needs to be used here when enlarging / reconstructing original image):

Also we can inspect again profile:

It is almost the same as function we already plotted. There is some small ringing - but that is due to the fact that I'm using Gaussian blur here and star profiles are not quite Gaussian in shape (Gaussian goes off to infinity mathematically and telescope PSF does not - there is actual frequency cutoff at some point) and resampling introduces some ringing as well.

Using 1/3 of FWHM will over sample by almost factor of x2 - which will lower your SNR by x2 for same imaging time. We often want "fast" systems - and purchase expensive "fast" scopes to minimize imaging time, and then go out and vastly over sample and make our setups x4 slower than it needs to be.

 

[Magnum]

The calculations make sense to me except for where the /1.6 figure is coming from in the first place, ive never come across that before. Nyquest therom modified for digital signals quotes 2.5x-3x sampling.

And how are my 0.39”/pixel subs showing more detail than my 0.8”/pixels subs,  looking at the raw subs and the unprocessed stacks there is a definate improvement .

ive just been watching a video by Gary Inn on imaging the Arp galaxies from Texas in the middle of a lake with great seeing, he owns 2 systems a 130mm refractor running at 0.8”/pixel and a C11 running at 0.34”/pixel, hes imaged hundreds of them now and is convinced he is getting more detail in the oversampled C11 system, he says “i know this is controversial but im convinced from my tests that the lower image scales are resolving more detail. 
he says its a small increase but even 10% improvement is worth it to him.”

Im not quoting him word for word, just from memory from watching it LOL, and  i feel like my tests so far agree with what hes finding.

 

Edited April 7, 2022 by Magnum

[Magnum]

14 hours ago, R1k said:

Fantastic images! Which Meade 8 inch SCT was used?

How dark are your skies?

its an old Meade 8" ACF f10 that had been run over and repaired.

I think my sky is verging on Bortle 6 but im sure it was closer to 5 a decade ago 😞

 

[ollypenrice]

46 minutes ago, Magnum said:

its an old Meade 8" ACF f10 that had been run over and repaired.

I think my sky is verging on Bortle 6 but im sure it was closer to 5 a decade ago 😞

 

I have a 10 inch ACF which, absurdly, I've never used. I need to press it into service because I'm sure it will be good, especially now that there is star-removal software which can be used in order to produce smaller stars. (The previous owner got great images apart from slightly oversized stars.)

Olly

[Magnum]

24 minutes ago, ollypenrice said:

I have a 10 inch ACF which, absurdly, I've never used. I need to press it into service because I'm sure it will be good, especially now that there is star-removal software which can be used in order to produce smaller stars. (The previous owner got great images apart from slightly oversized stars.)

Olly

Yes i knew you had one, but you have had it so long now i think you should just give it to me 🤪 

is it an f10 or f8 version?

do you still own your old atik 11000 or 4000, their pixel size could have matched nicely with it.

Edited April 7, 2022 by Magnum

[vlaiv]

12 hours ago, Magnum said:

The calculations make sense to me except for where the /1.6 figure is coming from in the first place, ive never come across that before. Nyquest therom modified for digital signals quotes 2.5x-3x sampling.

And how are my 0.39”/pixel subs showing more detail than my 0.8”/pixels subs,  looking at the raw subs and the unprocessed stacks there is a definate improvement .

ive just been watching a video by Gary Inn on imaging the Arp galaxies from Texas in the middle of a lake with great seeing, he owns 2 systems a 130mm refractor running at 0.8”/pixel and a C11 running at 0.34”/pixel, hes imaged hundreds of them now and is convinced he is getting more detail in the oversampled C11 system, he says “i know this is controversial but im convinced from my tests that the lower image scales are resolving more detail. 
he says its a small increase but even 10% improvement is worth it to him.”

Im not quoting him word for word, just from memory from watching it LOL, and  i feel like my tests so far agree with what hes finding.

There is no such thing as Nyquist theorem modified for digital signals.

There is only one Nyquist-Shannon sampling theorem and it is very clear in what it says. 1.6 figure is derived based on it.

Nyquist-Shannon sampling theorem states that one ought to be sampling at exactly x2 (no x2.5 or x3) of the highest frequency component of the band limited signal.

Not x2 FWHM, not x3 Airy disk - nothing like that. You must understand Fourier transform and Convolution theorem in order to understand frequency part of things - how the low pass filter is formed.

In a nutshell - Telescope acts as complete cutoff filter - no higher frequencies exist than critical frequency - and this is set by aperture size alone (important bit, remember it for later). In deep sky imaging, we have two additional components - seeing and mount precision.

All three combine to give you resulting FWHM (this is important bit as well). We usually approximate PSF with Gaussian (sometimes with Moffat). If we do the following:

- calculate Fourier transform of Gaussian shape of a certain FWHM and look at its spectrum and set "artificial" cut off point (We must do that as Gaussian is an approximation since Gaussian simply does not have cut off point - it goes of to infinity getting ever so smaller in value but never being completely 0) and we select it at a place where frequencies are attenuated so that any difference is simply smaller than noise for most part in images - we get 1.6

In above simulation - you will see that restoration is not perfect - it shows two stars but there is some ringing.

This ringing is in part due to my simulation using actual Gaussian function that does shoot of to infinity and ringing is just aliasing artifact.

Very important point here:

- you can sample image at higher sampling rate and over sample it and result won't change - you will get all the detail, so over sampling is not bad in that respect. It is bad because you spread out light more and lower your SNR.

- x1.6 is approximation that is based on the fact that any difference between restored and original signal is going to be smaller than noise for SNR5

- if you want absolutely no difference between original and restored signal - you need to sample at planetary / critical sampling rate. That is going to be absolute over sample for DSO imaging - but it will also be mathematically 100% correct.

Ok - that is re sampling.

Now - why do you get better resolution with 11" telescope compared to 5" telescope - well that bit is also very straight forward. It has nothing to do with sampling - it has to do with physics of light.

Remember how I said above that few things are important and to remember it - namely that larger aperture resolves more and that complete FWHM depends not only on seeing, but on combination of 3 factors - seeing, mount performance and aperture size.

One of those works in favor of C11 and one you don't control and can be of arbitrary value on a given night (or even for each sub). No wonder you get slightly sharper image with C11 (or anyone for that matter) - but that is not due to oversampling, it is simply due to using larger aperture and not being able to control the seeing (selection bias).

 

[ollypenrice]

1 hour ago, Magnum said:

Yes i knew you had one, but you have had it so long now i think you should just give it to me 🤪 

is it an f10 or f8 version?

do you still own your old atik 11000 or 4000, their pixel size could have matched nicely with it.

It's an F10, though I have the AstroPhysics telecompressor as well. I still have the Atik 11000, yes, though I've lent it to Tom at his robotic installation here for the moment. As you say, the large pixels would be well suited and it wouldn't matter if the full frame of the chip wasn't covered. Crop and go!

Olly

[Chris]

Fabulous detail! I tend to suggest people aim for around 1-2"/pixel when matching cameras to a scopes, however I may have to tweak this advice after seeing your SCT in action at 0.39"/pixel!

Plenty depends on the mount though as there's no point smearing photons. 

[vlaiv]

4 minutes ago, Chris said:

Fabulous detail! I tend to suggest people aim for around 1-2"/pixel when matching cameras to a scopes, however I may have to tweak this advice after seeing your SCT in action at 0.39"/pixel!

Plenty depends on the mount though as there's no point smearing photons. 

Why do you think you'll tweak that advice?

You think that above image is well sampled at 0.39"/px - although it is presented at 0.5"/px and has FWHM of 1.9"?

One of two images below has been reduced to 50% in size and then resized back to match the original. Which means it was sampled at 1"/px at that point. If detail is there in the image that needs higher sampling rate than 1"/px - then that detail should suffer. Can you tell which one of two images was resized to 1"/px and then up scaled back to original size? (Actually you should be able to tell as it has less noise, but all the detail is still there).

[juno16]

Absolutely gorgeous images from both scopes. Wonderful acquisition and processing!

 

Jim

[Magnum]

1 minute ago, juno16 said:

Absolutely gorgeous images from both scopes. Wonderful acquisition and processing!

 

Jim

Thanks Jim

[Chris]

37 minutes ago, vlaiv said:

Why do you think you'll tweak that advice?

You think that above image is well sampled at 0.39"/px - although it is presented at 0.5"/px and has FWHM of 1.9"?

One of two images below has been reduced to 50% in size and then resized back to match the original. Which means it was sampled at 1"/px at that point. If detail is there in the image that needs higher sampling rate than 1"/px - then that detail should suffer. Can you tell which one of two images was resized to 1"/px and then up scaled back to original size? (Actually you should be able to tell as it has less noise, but all the detail is still there).

My answer is they both look great 😀