M51 SCT at f10 vs triplet at f7.5

[Magnum]

56 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).

You seem to be determined to prove that my sct image isnt showing more detail than the refractor image, but it was quite obvious from the moment the first sub came in that there was more detail, and this continued through to the final images. Now wether this is because of the higher sampling or maybe the refractor just doesn't have as good optics as the sct, i cant say for sure. 
as far as i can tell everything else was equal, the seeing was the same the guiding was the same, the EQ8 Mount is remarkably consistent and reliable in that regard.

I said from the start 0.39 is overkill but i had no way to get an in between sampling without buying a reducer, my feeling is it there was some improvement going below 1”/pixel pixel on these very good nights and i suspect the limit was somewhere around 0.6-0.7”/pixel, below that i would say no further gain was made. 

so im not saying 0.39 wasnt oversampled because it most defiantly is, but 0.8-09” on the refractor was not on this particular night, therefore i believe that there was some gain going down to maybe 0.7 or 0.6, after that then yes it was not gaining any more

We seem to be arguing over a very small difference here, the maths is all very well but ive seen a lot of the top galaxy imagers push below 1”/pixel and most are hovering around 0.6-0.7. and a some are going down to 0.2 in Chile.

Maybe you would like to do some real world tests of your own then post them on here?

As for your resampling comparison the bottom one clearly looks sharper to me so not sure why you think they look the same.

Lee

Edited April 7, 2022 by Magnum

[vlaiv]

13 minutes ago, Magnum said:

You seem to be determined to prove that my sct image isnt showing more detail than the refractor image

No, I never tried that - even for a moment.  I just wanted to point out that although SCT image shows more detail - it is not due to over sampling and that images are in fact over sampled.

30 minutes ago, Magnum said:

so im not saying 0.39 wasnt oversampled because it most defiantly is, but 0.8-09” on the refractor was not on this particular night, therefore i believe that there was some gain going down to maybe 0.7 or 0.6, after that then yes it was not gaining any more

You won't be able to resolve detail below 1"/px - and even 1"/px is very hard to achieve.

However, don't let me stop you from trying.

Left - fully resolved image at 1"/px - right, your current image at 1"/px

40 minutes ago, Magnum said:

As for your resampling comparison the bottom one clearly looks sharper to me so not sure why you think they look the same.

Don't confuse noise and level of processing for detail, here is another comparison:

This is original bottom image that you deemed as sharper in first example.

Here is resized image from that example - with simple sharpening in IrfanView (nothing fancy):

Do you still think there is significant difference? Remember, I'm doing this on 8bit data that has been processed (not on linear 32bit data) - and even so, I can demonstrate that there is no detail down to 1"/px in your 8" SCT image.

Bottom line - don't let me discourage you in any way - if you feel your images are ok to be sampled at those rates, then fine by me. Sorry for all the confusion.

[Magnum]

2 hours ago, vlaiv said:

No, I never tried that - even for a moment.  I just wanted to point out that although SCT image shows more detail - it is not due to over sampling and that images are in fact over sampled.

You won't be able to resolve detail below 1"/px - and even 1"/px is very hard to achieve.

However, don't let me stop you from trying.

Left - fully resolved image at 1"/px - right, your current image at 1"/px

Don't confuse noise and level of processing for detail, here is another comparison:

This is original bottom image that you deemed as sharper in first example.

Here is resized image from that example - with simple sharpening in IrfanView (nothing fancy):

Do you still think there is significant difference? Remember, I'm doing this on 8bit data that has been processed (not on linear 32bit data) - and even so, I can demonstrate that there is no detail down to 1"/px in your 8" SCT image.

Bottom line - don't let me discourage you in any way - if you feel your images are ok to be sampled at those rates, then fine by me. Sorry for all the confusion.

Im sorry but you i find it quite annoying that ignore some of my findings and statements like they dont matter as much as yours.  As I said earlier the raw subs clearly showed more detail, this was before any processing not sure how you are ignoring that point LOL?. you try and prove your point by downsampling my image then further sharpening it to try and make it look as sharp as the original, this doesnt prove anything except how good or bad your software is performing the resampling and sharpening, software resampling is not the same as optical sampling with the cameras pixels, and its ability to sample that in regard to what the seeing allows. 

I also find a few of your statements condescending like “dont confusing noise for sharpness” ive been an astrophotographer  for 30 years, and im well aware all about adding noise can make us perceive  more detail in the same way my 1080p plasma often looks sharper than my 4k LCD.I think you need to be careful when assuming the person you are talking to doesn't understand  his or  her subject. 

I refrained from saying this earlier as i dont like to do that myself,  but I believe you  MAYBE wrong in your understanding of nyquest Therom when applied to digital signals,  ( as i believe there is is a certainly a difference, when applied to pixels we do indeed need to sample by 3X not 1.6) , 3x is now a widely established method for getting a well sampled image.
i have no idea of your background and you have no idea of mine so we shouldn't assume. 

You also sate that i wont be able to resolve detail below 1”/pixel, again this shows me the 1.6x figure isnt correct as their are hundreds of galaxy images posted every day well below this figure, are you actually trying to tell us that no ground based image can get below 1”/pixel even in long exposures, so I cant agree with that at all.

then what about if i add an adaptive optics unit do you still think we cant record detail below 1” arc second? 
also you seem to be only considering  stellar images,  while thats important its not the only measure of detail, The smallest features are not Individual stars but in fact the low contrast fine details in the spiral arms. 

Now if we turn to lucky imaging then Nyquest goes out of the window as we  know that with lucky imaging we can pretty much eliminate the effects of seeing and easily get to 11 x smaller than the daws limit of a scope.  Damien Peach achieves this on a regular basis routinely resolving the contrast of tiny features,  the Enke gap for instance in Saturn's rings has an angular separation of only 0.05” arc seconds. Ive done this myself easily with an 8” scope.

Probably best if we leave it there as we clearly dont agree and i only posted the comparison as i believe it shows an improvement below 1” arsc second per pixel.

if you dont agree with this then fine.

lee

 

 

 

 

Edited April 7, 2022 by Magnum

[The Lazy Astronomer]

I probably shouldn't get involved in this discussion, as it's getting slightly towards the more heated end of the debate spectrum, but I like to follow things like this when they come along (theory vs experiment) - I am a scientist by day, so right up my street!

Anyway, from following along with the thread thus far, I see no evidence that anyone is suggesting the SCT image does not contain more detail than the refracter one, because it plainly and obviously does (and both are fantastic images, by the way!!). From my understanding, @vlaiv is suggesting the reason for this additional resolved detail is a function of the significant increase in aperture with the SCT, not the sampling rate (and please vlad, correct me if that's not the right understanding) - the most compelling evidence I see that shows detail at 1"/px is not fully resolved is the comparison between the SCT image, and what I assume is a downsampled Hubble image, both displayed at that resolution.

Again, taking nothing away from the quality of these images; I hope I can one day get up to this level (and the fact that you manage it under UK skies gives me hope! 😁)

[Magnum]

6 minutes ago, The Lazy Astronomer said:

I probably shouldn't get involved in this discussion, as it's getting slightly towards the more heated end of the debate spectrum, but I like to follow things like this when they come along (theory vs experiment) - I am a scientist by day, so right up my street!

Anyway, from following along with the thread thus far, I see no evidence that anyone is suggesting the SCT image does not contain more detail than the refracter one, because it plainly and obviously does (and both are fantastic images, by the way!!). From my understanding, @vlaiv is suggesting the reason for this additional resolved detail is a function of the significant increase in aperture with the SCT, not the sampling rate (and please vlad, correct me if that's not the right understanding) - the most compelling evidence I see that shows detail at 1"/px is not fully resolved is the comparison between the SCT image, and what I assume is a downsampled Hubble image, both displayed at that resolution.

Again, taking nothing away from the quality of these images; I hope I can one day get up to this level (and the fact that you manage it under UK skies gives me hope! 😁)

Yes will try not to get any more heated LOL.

Ive been thinking about the effect of aperture but that still brings us back to the same argument, 

the larger the aperture the smaller details that can be resolved, but  if like Vliave says I cant get below 1"/pixel anyway then my 5" refractor is already capable of resolving more detail than the seeing will allow, so increasing the aperture to 8" wouldn't help any more than increasing the focal length. this is why I believe my image shows my seeing isn't limiting me to 1"/pixel sampling but a bit less than that, how much less I dont know without trying intermediate sampling rates, but from everything ive read on the subject 0.6-.07 seems quite commonly achievable on nights of the best seeing

as the 5" refractors Dawes limit is 0.92"/pixel and with this camera the sampling is 0.8"/pixel so in perfect seeing the scope is the limiting factor,  but that's already below Vlaivs theoretical seeing limit of 1"/pixel in which case he seems to be stating the seeing can never be good enough to reach the the small scopes limit.

Now the 8"sct has a Dawes limit of 0.57"/pixel  and the sampling with my camera is 0.39"/pixel so again the camera is sampling more than the scope can provide and  the Dawes limit of this scope is now around half of Vlaivs theoretical seeing limit.

so using Vlaivs 1"/pixel limit the increase in aperture wouldn't gain anything as even the smaller scope has enough aperture to resolve smaller details than that 1"/pixel limit, hence I believe my image passed that limit and it wasnt the limiting factor at all

So we come back to the fact that my SCT image showed better detail than the refractor, wether that was because of larger aperture or longer focal length in either case the 1"/pixel limit must have been passed or one wouldn't show finer detail than the other.

Now in either case I maintain this is because when using the nyquest theorem with digital cameras I believe we need to sample at 3x the seeing to resolve the finest details. im not picking this 3x figure out of the air its not hocus locus, its what has been tested to be be the case. if the seeing is 3" then we can sample at 1"/pixel, If the seeing is  2" then we can sample at 0.66"/pixel. go any more than 3x then we are wasting time. 

Plus everyone keeps ignoring the fact that top imagers have been imaging way below 0.5"/pixel for many years, are we saying they are all wrong and aren't gaining anything. Surely they wouldn't be imaging at those small pixel scales if all they were doing was oversampling for nothing and consequently slowing down their captures. This whole discussion has already been tried and tested in the real world.

Im all for theory but it must be tested with experiment, I already knew the answers to these experiments but wanted to see if I could take advantage of even the smallest % improvement by buying a larger aperture/ longer focal length scope. after this test I reckon on the best night I would be able to outperform my current refractor by purchasing a 1200- 1400mm FL and 6-8" aperture scope.

Im never as good at explain things as I am at understanding them in my head, so I have repeated myself here aproaching it from different angles, it makes sense to me LOL

 

Lee

 

 

[Mr Spock]

Take you images how you want to do them  Both your images look great so you are doing it right 👍

[ollypenrice]

Another variable is the quality of the optics. It's not enough just to think in terms of the raw numbers involved in sampling rate when making the comparison. I haven't really imaged with an SCT but I have owned and used the Meade 127 triplet along with a TEC 140 and Takahashi FSQ106. (I've also used the TEC with and without its 0x flield flattener, which throws another curved ball.) The much shorter FL of the 106 means it's not a good comparator but it is worth comparing the TEC and the Meade.  The TEC produces sharper images. And so it should, of course, given its aperture advantage and vastly higher price - but it does. The Meade's colour correction is very good for the price, very good indeed, but the premium refractors can beat it, meaning, of course, that more of the spectrum is in perfect focus. The curved ball from the TEC flattener is this: according to TEC, the flattener has no effect on colour correction and only flattens the field. According to most owners, including me, it does improve the colour correction, producing tighter stars and signifcantly reducing blue bloat. I'm in no doubt about this despite my respect for Yuri Petrunin.

So my point is that we're not just comparing numbers, we're comparing specific instruments. Bearing that in mind, I'm keen to get that Meade ACF out onto a mount!

Olly

[Magnum]

33 minutes ago, ollypenrice said:

Another variable is the quality of the optics. It's not enough just to think in terms of the raw numbers involved in sampling rate when making the comparison. I haven't really imaged with an SCT but I have owned and used the Meade 127 triplet along with a TEC 140 and Takahashi FSQ106. (I've also used the TEC with and without its 0x flield flattener, which throws another curved ball.) The much shorter FL of the 106 means it's not a good comparator but it is worth comparing the TEC and the Meade.  The TEC produces sharper images. And so it should, of course, given its aperture advantage and vastly higher price - but it does. The Meade's colour correction is very good for the price, very good indeed, but the premium refractors can beat it, meaning, of course, that more of the spectrum is in perfect focus. The curved ball from the TEC flattener is this: according to TEC, the flattener has no effect on colour correction and only flattens the field. According to most owners, including me, it does improve the colour correction, producing tighter stars and signifcantly reducing blue bloat. I'm in no doubt about this despite my respect for Yuri Petrunin.

So my point is that we're not just comparing numbers, we're comparing specific instruments. Bearing that in mind, I'm keen to get that Meade ACF out onto a mount!

Olly

Yes, now I have to give the ACF back so will be putting the Meade 127 back on the mount until I find another scope for sale that I can try. Ive been offered a 6" RC at 1370mm FL but even the seller saw this thread and thinks unless I 100% nail collimation it will struggle to beat my 127 image. Part of me wants to try it anyway but with the limited number of clear nights I get , I dont really want to waist any messing with collimation. The ACF did have to be tweaked but that only took a few minutes on a star.

Lee

[Same old newbie alert]

8 hours ago, Magnum said:

Yes will try not to get any more heated LOL.

Ive been thinking about the effect of aperture but that still brings us back to the same argument, 

the larger the aperture the smaller details that can be resolved, but  if like Vliave says I cant get below 1"/pixel anyway then my 5" refractor is already capable of resolving more detail than the seeing will allow, so increasing the aperture to 8" wouldn't help any more than increasing the focal length. this is why I believe my image shows my seeing isn't limiting me to 1"/pixel sampling but a bit less than that, how much less I dont know without trying intermediate sampling rates, but from everything ive read on the subject 0.6-.07 seems quite commonly achievable on nights of the best seeing

as the 5" refractors Dawes limit is 0.92"/pixel and with this camera the sampling is 0.8"/pixel so in perfect seeing the scope is the limiting factor,  but that's already below Vlaivs theoretical seeing limit of 1"/pixel in which case he seems to be stating the seeing can never be good enough to reach the the small scopes limit.

Now the 8"sct has a Dawes limit of 0.57"/pixel  and the sampling with my camera is 0.39"/pixel so again the camera is sampling more than the scope can provide and  the Dawes limit of this scope is now around half of Vlaivs theoretical seeing limit.

so using Vlaivs 1"/pixel limit the increase in aperture wouldn't gain anything as even the smaller scope has enough aperture to resolve smaller details than that 1"/pixel limit, hence I believe my image passed that limit and it wasnt the limiting factor at all

So we come back to the fact that my SCT image showed better detail than the refractor, wether that was because of larger aperture or longer focal length in either case the 1"/pixel limit must have been passed or one wouldn't show finer detail than the other.

Now in either case I maintain this is because when using the nyquest theorem with digital cameras I believe we need to sample at 3x the seeing to resolve the finest details. im not picking this 3x figure out of the air its not hocus locus, its what has been tested to be be the case. if the seeing is 3" then we can sample at 1"/pixel, If the seeing is  2" then we can sample at 0.66"/pixel. go any more than 3x then we are wasting time. 

Plus everyone keeps ignoring the fact that top imagers have been imaging way below 0.5"/pixel for many years, are we saying they are all wrong and aren't gaining anything. Surely they wouldn't be imaging at those small pixel scales if all they were doing was oversampling for nothing and consequently slowing down their captures. This whole discussion has already been tried and tested in the real world.

Im all for theory but it must be tested with experiment, I already knew the answers to these experiments but wanted to see if I could take advantage of even the smallest % improvement by buying a larger aperture/ longer focal length scope. after this test I reckon on the best night I would be able to outperform my current refractor by purchasing a 1200- 1400mm FL and 6-8" aperture scope.

Im never as good at explain things as I am at understanding them in my head, so I have repeated myself here aproaching it from different angles, it makes sense to me LOL

 

Lee

 

 

Does this mean the big frac is up for sale Lee?😁

[Toltec]

NIce images, the SCT image does look like it has more detail, particularly the rust coloured object in the lower right spiral arm. I don't image, yet, however I have a background in optics and metrology so I appreciate the discussion.

 

 

[vlaiv]

12 hours ago, Magnum said:

if you dont agree with this then fine.

I agree with you that we don't need to discuss that part, however, I do have a problem with statements like this:

12 hours ago, Magnum said:

Now if we turn to lucky imaging then Nyquest goes out of the window as we  know that with lucky imaging we can pretty much eliminate the effects of seeing and easily get to 11 x smaller than the daws limit of a scope.  Damien Peach achieves this on a regular basis routinely resolving the contrast of tiny features,  the Enke gap for instance in Saturn's rings has an angular separation of only 0.05” arc seconds. Ive done this myself easily with an 8” scope.

Simply because you are perpetuating some myths and leading astray people reading this.

Nyquist does not go out of the window - it is always there, it is not some assumption, it is mathematically (and experimentally) proven fact.

Encke's gap is not resolved. It is indeed imaged - and that is absolutely fine but it is not resolved. There is very important difference between being resolved and being imaged.

We image stars all the time - but stars are much much much less in angular diameter than Encke's gap. Does it mean that our telescopes have infinite precision simply because we are able to see the stars and image them? No.

To resolve - means to split, to differentiate. Encke's gap is singular feature - a dip in brightness, much like star is peak in brightness. We can image that as long as there is enough contrast (we can't image very faint star either - or rather, we need longer exposure to achieve needed SNR/contrast). Try measuring Encke's width to any precision from ground based images.

I know that there are number of images that are shot below 1"/px - but show me one, made with amateur equipment that resolves a double star that is 1.6" apart.

[mrflib]

8 hours ago, Magnum said:

Yes, now I have to give the ACF back so will be putting the Meade 127 back on the mount until I find another scope for sale that I can try. Ive been offered a 6" RC at 1370mm FL but even the seller saw this thread and thinks unless I 100% nail collimation it will struggle to beat my 127 image. Part of me wants to try it anyway but with the limited number of clear nights I get , I dont really want to waist any messing with collimation. The ACF did have to be tweaked but that only took a few minutes on a star.

Lee

Lee I don't know if this is of any use to you, but I have an Edge HD 8 and the Celestron 0.7x reducer which I'm thinking of selling? Should get you to 1400mm focal length or so. Let me know if this is of any use to you.

 

Bloody brilliant images though. Does kind of make me want to keep it but I need to pay for my TEC somehow!

Edited April 8, 2022 by mrflib

[Magnum]

6 hours ago, vlaiv said:

I agree with you that we don't need to discuss that part, however, I do have a problem with statements like this:

Simply because you are perpetuating some myths and leading astray people reading this.

Nyquist does not go out of the window - it is always there, it is not some assumption, it is mathematically (and experimentally) proven fact.

Encke's gap is not resolved. It is indeed imaged - and that is absolutely fine but it is not resolved. There is very important difference between being resolved and being imaged.

We image stars all the time - but stars are much much much less in angular diameter than Encke's gap. Does it mean that our telescopes have infinite precision simply because we are able to see the stars and image them? No.

To resolve - means to split, to differentiate. Encke's gap is singular feature - a dip in brightness, much like star is peak in brightness. We can image that as long as there is enough contrast (we can't image very faint star either - or rather, we need longer exposure to achieve needed SNR/contrast). Try measuring Encke's width to any precision from ground based images.

I know that there are number of images that are shot below 1"/px - but show me one, made with amateur equipment that resolves a double star that is 1.6" apart.

You say Im perpetuating Myths, I say they aren't myths but well established findings, ok no Nyquest doesn't go out of the window with lucky imaging but the seeing limit can certainly be broken or whatever word you wish to use to describe that.

As for the term RESOLVE Well maybe resolve isn't quite the right term but just saying IMAGED seems even less suitable. As you we could IMAGE Saturn with a smart phone but we certainly wouldn't be able to distinguish Enkes gap, so maybe RECORD or DISTINGUISH Enkes gap would be more accurate. but we are now quibbling over wording. whichever term I use Enkes gap is a smaller feature than 1" in fact its has an apparent angular width of just 0.05" 

I have pasted in the following from Damien Peaches website,

Understanding Resolution and Contrast

Two points it is important to understand is the resolution a telescope can provide, and how the contrast of the objects we are imaging affects is related to what can be recorded. Its often seen quoted in the Dawes or Rayleigh criterion for a given aperture. Dawes criterion refers to the separation of double stars of equal brightness in unobstructed apertures. The value can given given by the following simple formula:

115/Aperture (mm.) For example, a 254mm aperture telescope has a dawes limit of 0.45" arc seconds. The dawes limit is really of little use the Planetary observer, as it applies to stellar images. Planetary detail behaves quite differently, and the resolution that can be achieved is directly related to the contrast of the objects we are looking at. A great example that can be used from modern images is Saturn's very fine Encke division in ring A. The narrow gap has an actual width of just 325km - which converts to an apparent angular width at the ring ansae of just 0.05" arc seconds - well below the Dawes criterion of even at 50cm telescope. In `fact, the division can be recorded in a 20cm telescope under excellent seeing, exceeding the Dawes limit by a factor of 11 times!. How is this possible?.

As mentioned above, contrast of the features we are looking at is critical to how fine the detail is that we can record. The Planets are extended objects, and the Dawes or Rayleigh criterion does not apply here as these limits refers to point sources of equal brightness on a black background. In fact it is possible for the limit to be exceeded anywhere up to around ten times on the Moon and Planets depending on the contrast of the detail being observed/imaged.

As he explains, planets are extended objects, well so are the features in the spiral arms of galaxies and other deep sky objects, this is where your 1.6" figure is not really appropriate, I still maintain that in long exposure deep sky images 3x the seeing is an achievable figure at least when trying to distinguish those tiny extended features and contrast differences. and with lucky imaging then we are down into the hundredths of arc secs.

Anyway lets agree to disagree but I cant spend any more time on this as I need to go watch the latest episode of Star Trek Picard 😛 

Peace.

Lee

 

[knobby]

58 minutes ago, Magnum said:

cant spend any more time on this as I need to go watch the latest episode of Star Trek Picard 😛 

Fantastic end to a fantastic thread 😂

[ollypenrice]

7 hours ago, vlaiv said:

 

I know that there are number of images that are shot below 1"/px - but show me one, made with amateur equipment that resolves a double star that is 1.6" apart.

Does testing on a double star define what can be distinguished in details resolvable in fainter, more extended objects? Stars are exceptional things, optically.

Olly

[vlaiv]

1 hour ago, ollypenrice said:

Does testing on a double star define what can be distinguished in details resolvable in fainter, more extended objects? Stars are exceptional things, optically.

Olly

Biggest problem is trying to convey what max spatial frequency looks like and what is resolving power of optical system in "layman's" terms.

Both star separation and details resolvable in extended fainter objects have one thing in common - that is PSF.

We are in fact lucky because we have stars and we can treat them for all intents and purposes - as point sources. As such - their image by optical system is in fact - point spread function - as it describes how single point of light spreads under blur.

PSF is all we need to know to know everything about the blur as blur is direct consequence of PSF.

PSF acts on a single star, it acts on two stars and it act on extended object in the same way - as if it were (and it often is) composed out of countless stars next to each other - PSF acts on all of them in the same way (it "spreads" each "point" in the same manner - hence point spread function).

Having said that - I strongly object to relating any sort of visual standard of resolution that is based on star separation - to pixel size. Like mentioning Dawes limit in context of imaging.

But I don't mind the other direction at all. If you already know characteristics of PSF and you know that is its frequency response / what is optimum sampling rate - then it is easy to say if two stars will or will not be resolved.

This is simply due to the way PSF and convolution work.

[Stardust]

Both images look great  I'd  be pleased to get either. I see more detail in the sct image. If it works I'm not sure I'd get bogged down in theory that says it shouldn't. Just carry on with what you are doing, enjoy the hobby.

[AstroAdam]

Oi! It’s my ACF - if I knew it’d cause this much trouble I’d have kept it away from you 😂😂😂🤪. 
 

Seriously though, it’s been great to see what results you’ve got from it, whatever the reason for it in the long run. I’d agree that no matter what the theory says, the real-world testing in the end has produced some interesting output!  I’ll definitely be spending a bit more time with it myself in the near future…  my bigger pixels will probably help to diffuse the matter somewhat 😉.  

 

13 hours ago, Magnum said:

Yes, now I have to give the ACF back so will be putting the Meade 127 back on the mount until I find another scope for sale that I can try. Ive been offered a 6" RC at 1370mm FL but even the seller saw this thread and thinks unless I 100% nail collimation it will struggle to beat my 127 image. Part of me wants to try it anyway but with the limited number of clear nights I get , I dont really want to waist any messing with collimation. The ACF did have to be tweaked but that only took a few minutes on a star.

Lee

[Magnum]

8 minutes ago, Stardust said:

Both images look great  I'd  be pleased to get either. I see more detail in the sct image. If it works I'm not sure I'd get bogged down in theory that says it shouldn't. Just carry on with what you are doing, enjoy the hobby.

Thanks Dave, im looking forward to seeing your next test with your reduced C9.25

[Magnum]

1 minute ago, AstroAdam said:

Oi! It’s my ACF - if I knew it’d cause this much trouble I’d have kept it away from you 😂😂😂🤪. 
 

Seriously though, it’s been great to see what results you’ve got from it, whatever the reason for it in the long run. I’d agree that no matter what the theory says, the real-world testing in the end has produced some interesting output!  I’ll definitely be spending a bit more time with it myself in the near future…  my bigger pixels will probably help to diffuse the matter somewhat 😉.  

 

Hhahahahha im keeping it now,  🤪🤪🤪🤪

seriously thanks for lending it to me, its been interesting, and yes your bigger pixels will bring it somewhere between my 2 tests.

Lee

[Bibabutzemann]

Wow this topic is really fascinating, but im also confused now. I dont want to "heat up" the discussion though, im just curious. There is science behind it, this is nothing philosophical.

I think its clear to say, that are so many variables involved, that we cant deduce from this comparison, that the Apo image would benefit from smaller pixels.  I think it would be better to try out two Cams with different pixels during the same night and same scope.

 

@vlaiv showed with a downsampled Hubble image, that with a resolution of 1"/pixel,  much more details can be achieved.

But is it that simple? 

What would happen, if we downsample the hubble image to 2"/pixel, would it still look sharper than Lees images?

Also according to Lees source, Damien Peach, a scope can display details far below Dawes limit. But that doesnt mean, that those details are displayed proportionaly correct in the image, right? After all, even the image itself certainly doesnt have such small pixels to show 0,05" fluctuations. But certain details have such a big contrast, that it the detail is still visible, but in a blown up way?

But anyway, this isnt AS seeing limited, because of the shorter exposures in planetary imaging.

 

So now regarding long exposures i have another question:

Does "seeing limited" automatically mean, that you wont get a sharper image, when you increase aperture?

Lets say say seeing is 2". Does that mean, every scope that has an airy disc <2" can display exactly 2" resolution? I would think no, because both effects (airy disc and seeing) will combine to total resolution of >2".

And bigger apperture means, that you will come closer to the 2" seeing limitation.

Does that make sense?

As you can see, i have not much clue, so maybe someone can recommend a scientific paper or book, where i can read more about this stuff?

 

Anyway, @Magnum i think both your images look excellent and the processing was done quite tastefully (Pixinsight?)

Edited April 9, 2022 by Bibabutzemann

[The Lazy Astronomer]

Being lazy rather than looking for myself (it is my namesake after all): does anyone have a reference image from a professional ground-based observatory?

[vlaiv]

3 hours ago, Bibabutzemann said:

But is it that simple? 

What would happen, if we downsample the hubble image to 2"/pixel, would it still look sharper than Lees images?

At some point - level of detail will start to match.

There are two things here at play:

1. sampling rate needed to capture some level of detail

2. level of detail that can be recorder at some sampling rate

I know this will sound funny - but those are two distinct things. I'll try to make a diagram that will explain how and why this is so and what is really going on. Diagram will be in frequency domain, so maybe it is better if we consider it in 1D and think of it as sound (that will be more familiar to most?).

Above is diagram of signal in frequency domain. On X axis we have frequency while Y represents "strength" of signal at that frequency. There is some highest frequency - I labeled it above Sampling rate - which is error really as according to Nyquist sampling theorem - sampling rate needs to be twice that frequency (that is x2 in Nyquist). Bur for simplicity let's call it just sampling rate / cut off frequency (although keep in mind that two are related by factor of x2).

You need to sample at said sampling rate if you want to capture all there is, but all there is - might not be that "full" square I labeled. There could be more energy in those high frequencies.

This is the reason Hubble reference image is that more detailed - it has energy in those higher frequencies compared to other image. It is so because Hubble has much larger aperture and its cutoff frequency is much more to the right. We artificially cut of that data when we resampled it to this sampling rate.

Thing is - no matter what telescope, or sky conditions - above curve always looks roughly like that - it starts at 1 and then smoothly (above is not very smooth) falls to 0. It is just the place where it hits the zero that counts.

It looks a bit like this. We have two curves above. One is from Hubble, and other is from 8" SCT. If we constrain them to the same region that I outlined. Two things happen:

Given sampling rate will capture all the data from SCT, but at the same time - in that region, Hubble image will have much more energy at high frequencies - and will show sharper image.

In the end, it is very important to understand one thing:

When we sample at critical sampling rate - we will not have sharp image to start with, but mathematics say that you captured all the data in the box and you can fill those higher frequencies. How do we do that? This is what sharpening does to our images. Sharpening is restoring energy at those higher frequencies.

You won't be able to sharpen above cut off frequency - even if you over sample - as energy at those frequencies is 0 - you can't restore something from zero (restoring in this context is multiplication / division - actual high frequencies are multiplied with value less then 1 and if we want to restore them we need to divide with same value - value depends on shape of the filter - or PSF).

You need high SNR to be able to restore frequencies as noise is also spread over frequencies and boosting particular frequency also boosts noise at that frequency. Sharpening brings out the noise as well.

This is how we can in planetary imaging sharpen things to more than can be seen at the eyepiece.

3 hours ago, Bibabutzemann said:

Also according to Lees source, Damien Peach, a scope can display details far below Dawes limit. But that doesnt mean, that those details are displayed proportionaly correct in the image, right? After all, even the image itself certainly doesnt have such small pixels to show 0,05" fluctuations. But certain details have such a big contrast, that it the detail is still visible, but in a blown up way?

Displaying a thing is one thing - telescopes display so small things without any issues. Think about it - even closest largest stars are fraction of mas (milliarcsecond) - yet we see them in telescope.

What telescope and aperture does is simply "blows up" any small feature. You can zoom in to see airy disk of a star in telescope - does this means that star is the size of Airy disk? No - it just means that you can't resolve past that size.

With imaging it is about how many samples do you need to capture the image. Not how tiny in reality is the thing you are looking at.

In order to understand how many samples you need - you need to understand cut off frequency. Simple as that.

3 hours ago, Bibabutzemann said:

So now regarding long exposures i have another question:

Does "seeing limited" automatically mean, that you wont get a sharper image, when you increase aperture?

Lets say say seeing is 2". Does that mean, every scope that has an airy disc <2" can display exactly 2" resolution? I would think no, because both effects (airy disc and seeing) will combine to total resolution of >2".

And bigger apperture means, that you will come closer to the 2" seeing limitation.

Does that make sense?

As you can see, i have not much clue, so maybe someone can recommend a scientific paper or book, where i can read more about this stuff?

All things that you mentioned (and some that you did not) add in particular way.

1. Airy disk

2. Seeing blur

3. Mount tracking performance (sort of motion blur created by mount "shaking" as it is tracking - because it is not tracking perfectly - in essence guide RMS error if you will)

Those are three types of blur that are added successively to the image and resulting blur is combination of the three.

Make any one of those smaller and resulting total blur will be smaller. But they don't add "normally" - they add "in quadrature" - which has interesting property (this is how linearly independent vectors add or how noise adds).

If you add two values that are very different - result tends to be very close to larger component.

Say you add 10+1 normally - you would get 11 as you expect, but if you add like linearly independent vectors (square root of sum of squares) - look what happens sqrt(10*10 +1*1) = sqrt(101) = 10.05

Only 0.05 difference between 10 and result!

If one of the three components is very high - other two won't make much difference.

If seeing is very poor - you don't really care if your guiding is poor or what is the size of your aperture, but if you are shooting in good seeing with good mount then aperture size actually makes a difference

That is in part what we are seeing above - yes 8" will out resolve 5" in good seeing in long exposure astrophotography.

[MarsG76]

Great experiment... although, as said previously, I agree that both images are great.

 

[Magnum]

17 hours ago, Bibabutzemann said:

Also according to Lees source, Damien Peach, a scope can display details far below Dawes limit. But that doesnt mean, that those details are displayed proportionaly correct in the image, right? After all, even the image itself certainly doesnt have such small pixels to show 0,05" fluctuations. But certain details have such a big contrast, that it the detail is still visible, but in a blown up way?

But anyway, this isnt AS seeing limited, because of the shorter exposures in planetary imaging.

Anyway, @Magnum i think both your images look excellent and the processing was done quite tastefully (Pixinsight?)

Hi, lots of questions there but I will just answer 2 for now as going to bed shortly LOL.

With regard to the what sampling Damien is using, his latest images are taken from Chile with either a 50CM F15 cassegrain which has a focal length of 7500mm  paired with the ASI 462MC camera which has 2.9micron pixels resulting in a Pixel sampling of 0.08 Arc seconds per pixel 

Or the 106cm F17 Cassegrain which has a focal length of 18,000mm Paired with the ASI 174MC which has  larger 5.87 micron pixels resulting in a similar pixel sampling of 0.07"/pixel,  so yes very close to the 0.05" figure. 

I use a very similar sampling for my own planetary imaging using my Meade 12" LX200 + 2.5x Barlow lens getting me to 7500mm FL paired with my ASI224C  giving 0.10"/pixel and of course no where near Damiens results from Chile but I still need to cover the planet with enough Pixels, if I take the barlow out the image will not scale up to the same level and show the same detail, which is contrary to what was mentioned previously in this thread.

I definitely wouldn't call these blown up versions of lower resolution, they are clearly recording exceedingly fine resolution features that would not be seen using lower sampling. Damien told me many years ago that to show the smallest details on Jupiter and Saturn  we need to cover Jupiters disk by several hundred pixels, so we need that level sampling to achieve that. And as Damien Peach has been considered the very best Planetery imager in the world for the last 20 years  I would say he knows what hes talking about.

This is only achievable with the sub second short exposures that lucky imaging allows and at sites of the very best seeing on Earth.

Of course for long exposure deep sky imaging we cant hope to come close to that resolution , but this shows with short enough exposures the limits mentioned in this thread can be smashed through by a huge amount.

Damien is producing ground based  images that rival space telescope and planetary flybys

 As for my processing thank you very much for you hi praise , but no I don't use Pixinsight at all, I find that a rather blunt tool, I do all my stacking and stretching and colour calibration in MaximDL, but the real processing I do in Photoshop as it gives me a level of pixel level selectiveness using multiple layers that is just not possible in Pix insight. 

ive attached one of my best images of Jupiter below Damiens that I took in 2017 and 2109 with my previous C9.25 at a more modest  0.13"/pixel, ive had to grab them from my FB so not sure they are the original size , looks pathetic next to Damiens but just to show that even in uk we can record very small features using similar sampling.

, Lee

 

Edited April 10, 2022 by Magnum