Magnum

Yes as with my experience I haven't found CMOS to be an improvement really, my ASI533mc is a fine camera and looking at the the data ive taken with it is about on par with my previous Atik 428osc, ive stuck with the 533 as its a decent size sensor and I like the 16 mm square format, I only sold the 428 as it was too small. Comparing to my mono cams they are still better though as you say the worsening weather in the uk means I hardly ever manage to complete a colour image and usually just stick to Ha monochrome images, which is fine as Im a fan of monochrome astro images. ive recently had some mono ASI294 data to play with on m63 and its pretty much identical to my own data using the the Atik 383. so for me cmos are now about even with CCD but I cant say ive noticed any improvement. Ive gone down the hundreds of short subs route a couple of times with the 533 but came swiftly back to using long subs asI find they need far less stretching, and saves so much storage space and stacking time. Also I dont find that the cmos data smoothes up as quick as CCD data. the 383 can produce a very smooth noise free image in just 12 x 5 min exposures, while the 533 still needs noise reduction even with 100 x 5 min subs, though thats comparing OSC to mono again, but considering the Kodak 8300 chip in the 383 is supposedly one of the slowest sensors It stacks up remarkably fast. People seem to get hung up on the lower cmos read noise , but if you live in an area with any light pollution at all, Ive found that read noise is not a factor at all, even the highest read noise ccd cameras are swamped by light pollution and shot noise, o to me I dont care about read noise at all, even in narrowband LP noise is greater than the read noise. im going to keep using both for now as I can produce nice images with either, but the OSC cmos takes longer. Lee

Nice fine detail there Chris. lee

Nice i like that, i keep forgetting to try this one lee

So your interpretation is correct and everyone else's interpretation is wrong? All I can say is that Niquest - Shannon theorem pre dates digital cameras with square pixels so reading that alone is not enough to make a conclusion, the section I pasted above from Astronomytools website explains exactly why it needs a tweak. It seems totally logical to me and makes more sense to me than anything else ive read in this thread. I think we've done this to death now, people will read through the thread and come to their own conclusions, then maybe test it with their own experiments which is a crucial step in scientific method.

Thanks for that link, that page supports what ive been saying all along. If you remember I said we need to modify nyquest for digital, you replied "There is no such thing as Nyquist theorem modified for digital signals" Well the text below supports exactly what I said, that nyquest is a starting point but it needs a slight tweak when dealing with square pixels. ie "modified"! We dont have to agree on this, but I still dont appreciate that you said I was spreading myths, when highly regarded sources are saying the same thing. No course you wouldn't be able to achieve that with a red cat as it doesn't have the optical resolution to support that, thats the point of using larger aperture longer focal length scopes, also they dont make astro cameras with pixels small enough to even get down to that sampling with the red cat. Why do you think people buy large RC scopes in the first place. if I was buying a 12" RC then I would go for camera with larger pixels. the camera I have seems ideal fro both my 950mm triplet and my new RC6, it was a bit overkill with the 8" f10 sct. Lee

Out of pure chance I just noticed these charts on highpoint Scientific's website on choosing the optimal Camera scope combination vs your local best seeing, and looking at the Above average to great seeing column, it does indeed agree with the figures my research and experiment had been pointing too. With my 3.75um pixel camera in above average seeing it suggests a pixel sampling resolution of 0.75"/pixel, and in great seeing it suggests 0.5"/pixel. Then in the next chart in above average seeing its suggesting a scope of 1000mm FL and in Great seeing its suggesting a scope of 1600mm with the same 3.75"/pixel camera to reach the above sampling. And my original conclusion from my research and experience was to look for a scope between 1200 & 1400mm FL to edge out my current scope sits right in the middle of their suggested range. I purchased a used RC6 this morning with FL of 1370mm so Im hopefully I will be able to improve on my 127mm triplet resolution but only on the very best nights, the rest of the time it should be no worse just a bit slower at f9 vs 7.5, and I can always use a reducer to bring it back down to around 1000mm if it doesn't .

Yes and you taught me a lot of good photoshop techniques when we visited in 2012. Ive always applied every tool selectively since then. lee

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

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

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

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

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

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

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

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

Thanks Jim

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.

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 😞

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.

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

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

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

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

good test Dave,

M51 Whirpool Galaxy, Ive been working on this over 3 nights, Meade 127mm f7.5 triplet refractor, 90 x 5 mins at gain100 with my ZWO ASI533MC cmos camera an Baader Lum filter. Guided, captured stacked, strteched and corrected in MaximDL5, final processing in Photoshop. Cropped version for detail and full frame for surrounding galaxies.