Thanks, Chuck for a very useful / helpful video. Working the formula backwards, if you have an image scale in mind that you'd like to use, you can work out what size sensor you would need by: Say you wanted to achieve an image scale of 1.5, you would: Multiply focal length of telescope (RASA 11 = 620mm) by 1.5 = 930 Divide 930 by 206 = 4.5 So one would need a sensor size of 4.5 µm to achieve an image scale of 1.5.
You can’t use a fixed number to understand over/undersampling. It is dependent on the diffraction limited resolution of the telescope (or the astronomical seeing if that is larger). If you use the Reyleigh Criterion or Dawes limit to find your telescopes diffraction limited resolution you also need to use the Sampling Theorem. Optical resolution limits are measured as the distance between two stars (or lines) that can be resolved as separate objects. The sampling theorem says that you have to sample a twice this rate, i.e. sample the objects and the space between them. If your place scale is the resolution/2, your image is critically sampled. There is no more detail resolvable by oversampling. Undersampling throws away detail that cannot be recovered in processing. I show an example calculation at the link below. The short version is that the optimal plate scale is approximately: PixelPitch or PlateScale = focalRatio / 3.6 in microns
I appreciate your candidness and all of the information in this video. Thanks Chuck. My understanding of under/oversampling is that severe undersampling can be an issue, but oversampling is only an issue if you mind the fact that you're not imaging as efficiently as you could. Resolution wise, it's going to be as good as you can get, in an oversampled situation.
With oversampled you can bin the pixels. The real only problem with oversampling is that you might be wasting photons. Reason is you can sample closer to the seeing which is usually above 1 but less than 3 most locations, and not lose any fine details. But that just means you are taking longer photos than necessary. If you bin 2x2 for example with pixels that are 0.6, then you will sample at 1.2. But a 2x2 bin will detect enough signal to stand out from the noise in 1/4 the time.
Thanks Trevor, good to hear from you. I seriously only learned about this topic around 5 hours before creating the video, so I figured I better put a disclaimer on it, lol. But that's not a bad idea, maybe a disclaimer belongs on more of our videos :-)
Please correct me if I miss something, but I think you are confusing to things here!? You are saying that the pixel scale, in arc seconds/pixel (which is what you are calculating with that formula), should basically be the same for every setup, no matter what. That would mean nobody could ever "zoom in", every picture would have the same magnification, and wider fields could only be (correctly) pictured by increasing the total pixel count of your chip. There is, however, a link between pixel size and _focal ratio_, or actually aperture size! Since aperture can be seen as fixed with any given telescope, you can only change the focal length (with barlows or reducers), thereby changing the focal ratio. This is what determines whether you get bloated stars ("empty magnification") or you lose image detail (when the smallest details your optics can resolve are smaller than you pixels.) It is also not a right/wrong kind of thing, but more of a trade-off between two bottlenecks in search of the absolute finest detail. For most setups, these thoughts aren't even necessary since seeing and/or imperfect optics have a much bigger impact on these small details.
The 1-2 asec "rule of thumb" for pixel scale essentially assumes an effective aperture due to seeing of 5-9 cm, which is pretty poor. (This puts 3 pixels across a star in such conditions.) If you have better seeing, you would want a smaller image scale to avoid "chunky" stars. A scale of 0.5 asec/pix would be a better match for better seeing conditions (18-20 cm), assuming your scope is at least that large.
Hi Chuck. Trying to figure out my SG Pro settings for Bias frames with my ZWO 1600mm-Cool Pro.... with over 3 feet of snow here in Quebec, Canada.... extremely difficult to get my learning curve to Formula One speeds.... I was wondering if you can remember your Gain settings and your exposure settings to get your Bias Frames.... Please help ... and in layman’s terms for exposure settings what did you use ? 0.1 sec, or 0.01 secs , or 0.001 secs ? and what gain..... Thanks a bunch !
Whatever gain/offset I capture my lights in, I do for bias. So I have 3 sets at gain 0/offset 10, gain 75/offset 15, gain 139/offset 21. Just set your exposure time to zero.
Thanks Chuck.... Business man running his company , 2 teenagers, wife , dog , 3 feet of snow and 5 degrees fahrenheit .......... Thanks a lot for your help and advice!
I did all of this when choosing a camera for my esprit 80 - the 1600 was perfect (from memory, around 1.9). this combination also had the wider FOVs that I wanted in an imaging setup.
@@ChucksAstrophotography yeah me too. When I had the chance to buy a new imaging scope and dedicated imaging camera, I did a lot of research and understood things a lot better. When I first tried 10 years ago, I did no real research or failed to understand many of the imaging basics. Laziness on my part. I just decided to investigate what would give a good sample rate for an Astrotech 10" truss RC...the zwo 1600 is out, at 0.347 or so, the KAF 8300 chips aren't much butter, as are the KAF 16200 chips...looks like the kAF 11000 chips would be best at just under 1.0, but the price...of course, that's all dream stuff if I can manage to get another job lol!!!! So, unlikely to happen.
@@davepastern Yeah, 0.347 - trying to get autoguiding that can keep up with that would be an issue too. 10 years? You've been at this a lot longer than I have - I'm just a rookie.
@@ChucksAstrophotography Ah you misunderstood me...I bought some gear 10 years ago with the AIM of getting into imaging with my DSLR (Canon EOS 1D Mark IIn)...sadly, the scope's focuser couldn't handle the weight of the camera and had issues...I couldn't wrap my head around polar alignment too...and no guide setup...I quit in frustration and my gear has been in storage ever since. I still ned to check my EQ6 Pro and prolly do a strip and rebuild on it (a bit intimidated by that lol!!!). When mum passed and left me some money, I decided that I wanted to get back into imaging, a friend suggested getting a good triplet (hence the esprit 80 after much research). I decided on a zwo 1600 mono camera too after much research and a qhy polemaster. Sadly, I haven't used any of the gear yet as I still need to get my mount working and figure out a tonne of stuff like polar alignment with the polemaster, get a guide setup (I have an old orion guide camera and orion 80 shorty that I bought for guiding, but the mounting rings and how to mount it to the m ain scope at the time were beyond me). It's hard to learn when you're on your own (I don't play nice with other people as a rule, so clubs are sorta out of the equation). I have issues with motivation and procrastination, and now with my lower back issues, I can't just drag a working EQ6 Pro out every night (much too heavy for my frail back). Sadly, my current place sucks - I can't even see any stars to the South cos of 2 frigging ultra bright street lights that glare out the entire southern sky...I can't see the entire sky from one location on my property cos of trees and house...I'm hoping my sister will buy me out of my half of the house, but that's looking increasingly unlikely, which means I"m stuck in this filthy hot and humid location, which I HATE with a passion, and stuck without the chance of setting up a permanent observatory and having to deal with shitty light pollution that's beyond a joke... The frustration is real. I may even just call it quits and sell all the gear...
Hey Chuck. Great video. I was wondering just off the topic.... with your ASI 1600mm Cool do you do Bias frames or not ? I understand that yes we need our Darks library and of course Flats but some say with this camera we don’t have to do Bias frames ..... Need your experience and help on this one..... Do you process your beautiful images with Bias frames when using your ASI 1600mm cooled ? Thanks Chuck !
I heard the same thing about Bias, but I still capture them - and so do my friends Doug and Jason and they have the same camera. But you could always try your stacks with and without them and see if it really makes a difference. It would be an interesting test.
Hey Chuck ! Thank you so much for answering me so quickly... I perhaps like you should do Bias frames.... Your results speak for themselves Chuck ! Cheers.
I am unsampling on my setup with 3.17 arc secs per pixel. I use drizzling at x2 to reduce this which effectively divides it by 2. My stars look nice and sharp after this. Would not be too worried about exactly between 1-2. Have to factor in other areas like seeing conditions etc.
Another great video Chuck... just when I think I might be heading in the right direction there’s another thing to consider... I never even knew about oversampling and undersampling .... maybe not as significant as sensor size...
Bloated stars... that's me! No wonder, according to this forumla : Celestron 8se with ZWO ASI 1600, here then is the math. (3.8/(2032x.67 reducer))x206 = 0.5749! Bloated stars. Time to change telescopes? Thinking about an Orion 8" f3.9 newtonian 800mm focal length. That would get me close to 1.0 in this formula. Helpful as always Chuck.
Although my friend Jason is somewhere in the same Image Scale as you and his galaxy pictures are amazing. That's why he thinks the oversampling and undersampling issue may be a little overrated. But his processing skills are far beyond mine, so I probably would not go that low if it were me.
James Tickle had this to say: "With oversampled you can bin the pixels. The real only problem with oversampling is that you might be wasting photons. Reason is you can sample closer to the seeing which is usually above 1 but less than 3 most locations, and not lose any fine details. But that just means you are taking longer photos than necessary. If you bin 2x2 for example with pixels that are 0.6, then you will sample at 1.2. But a 2x2 bin will detect enough signal to stand out from the noise in 1/4 the time."
I need a 9 micron camera to pair with my Edge11 with .7x reducer. I am way oversampled (3.8 / 1960 * 206 = 0.4 image scale) but have no money to buy a larger pixel size camera at this point :(
I'm sure you can still take great images at .4. There is a lot more to this video than what I have presented in this video. I wouldn't worry about it. Read other comments here, you may find some good info.
Hi Chuck ,There is still one more thing you have to consider , the Rayleigh limit of you telescope which stands for the minimal resolvable detail your telescope can tell , IMO. Your 127mm will give you 1.1” of that , and I still don’t know how image scale affect the star , you might as well refer to someone else
Hi Chuck, this is the rest of the story. Under and over sampling comes from the Nyquist Theorem. Essentially if your image scale is the same as your Dawes or Rayleigh Limit, then you will miss much of the available detail. If you have an image scale lower than half of the Dawes Limit your sampling rate will be more than two and you will be over sampling ant trying to get something from the optics that it can’t give..the sweet spot is 2, but only if you are in space where seeing is not an issue.
@@ChucksAstrophotography Thanks Chuck. I will save up for a reducer anyway to get a wider view when required. Have you done a star bloat reduction processing tutorial?
Great explanation. Do you not take into account the crop factor when inputting the focal length? In your example the Rasa 8 inch is effectively a 600 mm effective focal length with your QHY10 crop sensor camera attached … Does this crop factor have to be factored in?
I shoot with an image scale of about 0.65 which, admittedly, does give me more bloated stars than I would like. However, this is often due to mount problems and or garbage seeing conditions. I have a very small set of data that has great guiding + seeing, and the results are really great as you might expect. I would say if you are comfortable with the results you get at your current focal length and mount, you may be fine sticking closer to the 1.0 or less image scale.
Chuck, I’m confused. I have a EdgeHD SCT 11” with 0.7 Focal Reducer resulting in a focal length of 1960mm”. According to your math, I need a camera with 14.27µm pixel size to achieve a 1.5 image scale. Not possible, so shall I trash the SCT? 😟 Math: (P/1960)*206=1.5 | P=14.27
Very interesting. I'm oversampled by 32% and I guess when I look now the smallest stars do seem a little square, but processing and bad seeing usually rounds those out. It doesn't really seem to affect the DSO or larger stars. But now at least I know the camera I just got will pair well with telescopes from 480 past 900mm focal length.
@@ChucksAstrophotography I've learned a ton from your channel. One of the things I love about this hobby is no matter how much you know, there's always something new.
Very interesting, but don't have it completely clear, I have a Celestron C8 with focal length 2033mm and asi1600 with 3.8, that means I'm very oversampled. Even with the focal reducer I get 0.64. Will have to do more research and see how I can get over with all of that.
Being oversampled is better than being undersampled. I think my friend Jason's image scale is even lower than yours and his galaxy pictures are amazing. That's why he thinks the oversampling and undersampling issue may be a little overrated. But his processing skills are far beyond mine.
Guess you have to multiply the focal length by the effect of your reducers / barlows as well. For example, the 4.7X barlow in the Quark, would have a significant effect.
Hi Chuck! Very well presented, as always :) I think the formula should also contain the telescope's resolving power in a form of telescope aperture. For example 400mm f/8 reflector would give much more bloated star than 400mm F/2 RASA. I think more precise representation would be the ratio of telescope resolving power (which is Dawes' limit) and camera's pixel size sky sampling. Dawes' limit is calculated as R = 4.56/D where D is in inches, Result is in arcseconds. This is in perfect world using perfect optics and for visible light. When taking longer Ha light wavelength with atmosphere seeing, using real life telescope, you should probably multiply the result by at least 1.5 My telescope + camera setup gets 1.76arcsec per pixel sky sampling and my telescope's "perfect world" Dawes' limit is 0.76 arcsec. So I'm a bit on undersampling side, but still within reasonable limits. Hope it helps :) Clear skies to everyone!
Thanks Denis, I knew I had only scratched the surface here, that's why I put that disclaimer at the end that it can go far deeper. Thanks for the info, when I was reviewing the specs of the ES AR102 on OptCorp, they also listed Dawes' limit. I need to turn on my thinking cap now and digest this.
Great Video Chuck, like you stated in the video description it does depend on seeing conditions-how Dark your skies are. You should be fine anywhere between .6 to 1.25
Hello Chuck, Thanks for the video. Now I understand better how important it is to match the right camera to the optical system. Since I am in the process of acquiring a camera, image scale will not be an issue anymore. Respectfully, Henri-Julien, Montreal, QC Canada.
Wa - most useful for SGPro / PHD anytime you change camera and focal. Great refresher, nice to know about the sweet spot between 1 and 2. So what's next for ya?
Kudos to Jason - I'll have to re-evaluate my new camera / ota. Between 1 and 2, I need m pixel @4.63 camera for my focal lengths otas of 750mm to 1350mm to be in the sweet spot. Just re-checked Im good with the G10 at higher focals, I'll just tile to compensate.
So the only thing that's weird is when I did this for my 11" EdgeHD is that no camera is good. With a focal length of 2800mm seems like every camera's pixels are too small.
That seems like a great focal length for solar system objects. But I've seen people take great pictures of deep sky objects with it using a reducer and having and image scale in the .4 or .5 range.
@@ChucksAstrophotography Very good point actually! I punched in the numbers for my ASI 120MC (my planetary camera) and it did come out to 1.28 for my scope with a focal reducer. Fun fact: California where I live has had like no clouds at all until I bought that stupid thing a few months back and since then I have not seen the sky on the weekend.
I’ve been running the combination of my telescopes and different cameras through the spreadsheet and now understand why some photo sessions just plain suck and others are pretty good. One more piece to the puzzle. So, I need a new camera! Thanks for turning today’s freezing rain into a very enjoyable evening!
I got a ZWO ASI294 MC with a Celsestron Edge HD 8" and my argument was that with the 8" I can do 2x2 binning, giving me a better signal ratio and perfect pixel size and when using a 0.5 reducer I get a perfect resolution removing the binning but getting a very good signal ratio due to reducing F10 to F2.5
Oh well I would say under- or oversampling is a lesser issue... there is so much more important than pixelscale. 2600mc with Redcat51, its awesome even without drizzling.
Exactly, so I would only use it for broadband - where the exposures would have to be so short anyway - it probably would not make a difference. But really, I only bought the ASI183MM for solar and lunar. If anything, I would use the ASI1600MM or QHY10 with a RASA.
#1 thing to understand sampling is to know your seeing. Before you look at your camera or your telescope, you need to know your seeing. For example, I imaged "under 1.0" and if I were to just listen to this video, I should have bloated stars. (Example here at 0.56"/px: www.astrobin.com/full/388591/0/) which was not the case. Your local guru was right to a certain point. The whole sampling thing, is more than just looking at a number and calling it good or bad. I imaged this, at 2.11"/px (www.astrobin.com/full/380925/F/) and you will not find blocky stars. Pairing a telescope to a camera needs the following, IMHO: 1. Understanding of the seeing, on average, in the locale. 2. Understanding of the telescope focal ratio. The lower this is, the more #1 (if its bad) will cause you problems. No matter the length. 3. Understanding of the pixel size of the camera. Smaller pixels are more likely to show the warts of optical or other imaging train/gear error moreso than larger pixels will. 4. Understanding of the goals you have for imaging. 5. The focal length of the telescope.
Exactly. Eliminate as much as possible BEFORE processing. Otherwise, it's like making a crappy movie and trying to fix it in editing. Better to make a good movie first.
@@ChucksAstrophotography Can you make this spreadsheet available Chuck? Long time since I set up spreadsheets. I think you have open a rabbit hole which I just fell into. It seems the more you study this subject, you have to realise that your end image is an assimulation of what is there. When we stack images, we are changing the pixels anyway.
@@fusion-music This link does the the same thing. If I had known about it, I probably would not have created my spreadsheet. astronomy.tools/calculators/ccd_suitability
Please subscribe to this UA-cam Channel for all things related to deep sky exploration and imaging, thanks!
Thanks, Chuck for a very useful / helpful video.
Working the formula backwards, if you have an image scale in mind that you'd like to use, you can work out what size sensor you would need by:
Say you wanted to achieve an image scale of 1.5, you would:
Multiply focal length of telescope (RASA 11 = 620mm) by 1.5
= 930
Divide 930 by 206
= 4.5
So one would need a sensor size of 4.5 µm to achieve an image scale of 1.5.
You can’t use a fixed number to understand over/undersampling. It is dependent on the diffraction limited resolution of the telescope (or the astronomical seeing if that is larger). If you use the Reyleigh Criterion or Dawes limit to find your telescopes diffraction limited resolution you also need to use the Sampling Theorem. Optical resolution limits are measured as the distance between two stars (or lines) that can be resolved as separate objects. The sampling theorem says that you have to sample a twice this rate, i.e. sample the objects and the space between them. If your place scale is the resolution/2, your image is critically sampled. There is no more detail resolvable by oversampling. Undersampling throws away detail that cannot be recovered in processing.
I show an example calculation at the link below. The short version is that the optimal plate scale is approximately:
PixelPitch or PlateScale = focalRatio / 3.6 in microns
Excellent info, thanks for taking the time to explain this.
I appreciate your candidness and all of the information in this video. Thanks Chuck.
My understanding of under/oversampling is that severe undersampling can be an issue, but oversampling is only an issue if you mind the fact that you're not imaging as efficiently as you could. Resolution wise, it's going to be as good as you can get, in an oversampled situation.
Yes, I wish I could have made it more clear about over sampling, I think I have caused some alarm needlessly.
With oversampled you can bin the pixels. The real only problem with oversampling is that you might be wasting photons. Reason is you can sample closer to the seeing which is usually above 1 but less than 3 most locations, and not lose any fine details. But that just means you are taking longer photos than necessary. If you bin 2x2 for example with pixels that are 0.6, then you will sample at 1.2. But a 2x2 bin will detect enough signal to stand out from the noise in 1/4 the time.
Excellent comment, James.
Great stuff, Chuck. Thank you! I feel like I should include your "this topic can go far far deeper..." disclaimer on every one of my videos. lol
Thanks Trevor, good to hear from you. I seriously only learned about this topic around 5 hours before creating the video, so I figured I better put a disclaimer on it, lol. But that's not a bad idea, maybe a disclaimer belongs on more of our videos :-)
Please correct me if I miss something, but I think you are confusing to things here!? You are saying that the pixel scale, in arc seconds/pixel (which is what you are calculating with that formula), should basically be the same for every setup, no matter what. That would mean nobody could ever "zoom in", every picture would have the same magnification, and wider fields could only be (correctly) pictured by increasing the total pixel count of your chip. There is, however, a link between pixel size and _focal ratio_, or actually aperture size! Since aperture can be seen as fixed with any given telescope, you can only change the focal length (with barlows or reducers), thereby changing the focal ratio. This is what determines whether you get bloated stars ("empty magnification") or you lose image detail (when the smallest details your optics can resolve are smaller than you pixels.) It is also not a right/wrong kind of thing, but more of a trade-off between two bottlenecks in search of the absolute finest detail. For most setups, these thoughts aren't even necessary since seeing and/or imperfect optics have a much bigger impact on these small details.
The 1-2 asec "rule of thumb" for pixel scale essentially assumes an effective aperture due to seeing of 5-9 cm, which is pretty poor. (This puts 3 pixels across a star in such conditions.) If you have better seeing, you would want a smaller image scale to avoid "chunky" stars. A scale of 0.5 asec/pix would be a better match for better seeing conditions (18-20 cm), assuming your scope is at least that large.
Excellent information, thanks Paul.
Hi Chuck. Trying to figure out my SG Pro settings for Bias frames with my ZWO 1600mm-Cool Pro.... with over 3 feet of snow here in Quebec, Canada.... extremely difficult to get my learning curve to Formula One speeds.... I was wondering if you can remember your Gain settings and your exposure settings to get your Bias Frames....
Please help ... and in layman’s terms for exposure settings what did you use ?
0.1 sec, or 0.01 secs , or 0.001 secs ? and what gain..... Thanks a bunch !
Whatever gain/offset I capture my lights in, I do for bias. So I have 3 sets at gain 0/offset 10, gain 75/offset 15, gain 139/offset 21. Just set your exposure time to zero.
Thanks Chuck....
Business man running his company , 2 teenagers, wife , dog , 3 feet of snow and 5 degrees fahrenheit ..........
Thanks a lot for your help and advice!
@@rosevan5485 You got your hands full, lol.
Chuck's Astrophotography . Yep ! Yes ! Oui !
Cheers and thanks a lot Chuck !
Glad we also got Stig into astrophotography.
Your secret stays with us :)))
I did all of this when choosing a camera for my esprit 80 - the 1600 was perfect (from memory, around 1.9).
this combination also had the wider FOVs that I wanted in an imaging setup.
That's good, mrtheoden
. I would always check the image scale too, but because of how it related to my field of view, not when it came to sampling.
@@ChucksAstrophotography yeah me too. When I had the chance to buy a new imaging scope and dedicated imaging camera, I did a lot of research and understood things a lot better. When I first tried 10 years ago, I did no real research or failed to understand many of the imaging basics. Laziness on my part.
I just decided to investigate what would give a good sample rate for an Astrotech 10" truss RC...the zwo 1600 is out, at 0.347 or so, the KAF 8300 chips aren't much butter, as are the KAF 16200 chips...looks like the kAF 11000 chips would be best at just under 1.0, but the price...of course, that's all dream stuff if I can manage to get another job lol!!!! So, unlikely to happen.
@@davepastern Yeah, 0.347 - trying to get autoguiding that can keep up with that would be an issue too. 10 years? You've been at this a lot longer than I have - I'm just a rookie.
@@ChucksAstrophotography Ah you misunderstood me...I bought some gear 10 years ago with the AIM of getting into imaging with my DSLR (Canon EOS 1D Mark IIn)...sadly, the scope's focuser couldn't handle the weight of the camera and had issues...I couldn't wrap my head around polar alignment too...and no guide setup...I quit in frustration and my gear has been in storage ever since. I still ned to check my EQ6 Pro and prolly do a strip and rebuild on it (a bit intimidated by that lol!!!). When mum passed and left me some money, I decided that I wanted to get back into imaging, a friend suggested getting a good triplet (hence the esprit 80 after much research). I decided on a zwo 1600 mono camera too after much research and a qhy polemaster. Sadly, I haven't used any of the gear yet as I still need to get my mount working and figure out a tonne of stuff like polar alignment with the polemaster, get a guide setup (I have an old orion guide camera and orion 80 shorty that I bought for guiding, but the mounting rings and how to mount it to the m ain scope at the time were beyond me). It's hard to learn when you're on your own (I don't play nice with other people as a rule, so clubs are sorta out of the equation). I have issues with motivation and procrastination, and now with my lower back issues, I can't just drag a working EQ6 Pro out every night (much too heavy for my frail back). Sadly, my current place sucks - I can't even see any stars to the South cos of 2 frigging ultra bright street lights that glare out the entire southern sky...I can't see the entire sky from one location on my property cos of trees and house...I'm hoping my sister will buy me out of my half of the house, but that's looking increasingly unlikely, which means I"m stuck in this filthy hot and humid location, which I HATE with a passion, and stuck without the chance of setting up a permanent observatory and having to deal with shitty light pollution that's beyond a joke...
The frustration is real.
I may even just call it quits and sell all the gear...
@@davepastern Wow, sorry to hear all this.
Hey Chuck. Great video. I was wondering just off the topic.... with your ASI 1600mm Cool do you do Bias frames or not ? I understand that yes we need our Darks library and of course Flats but some say with this camera we don’t have to do Bias frames ..... Need your experience and help on this one.....
Do you process your beautiful images with Bias frames when using your ASI 1600mm cooled ? Thanks Chuck !
I heard the same thing about Bias, but I still capture them - and so do my friends Doug and Jason and they have the same camera. But you could always try your stacks with and without them and see if it really makes a difference. It would be an interesting test.
Hey Chuck ! Thank you so much for answering me so quickly... I perhaps like you should do Bias frames.... Your results speak for themselves Chuck ! Cheers.
I am unsampling on my setup with 3.17 arc secs per pixel. I use drizzling at x2 to reduce this which effectively divides it by 2. My stars look nice and sharp after this. Would not be too worried about exactly between 1-2. Have to factor in other areas like seeing conditions etc.
Smart plan, Mike. That's what Jason said. Undersampling can be resolved with drizzling.
Another great video Chuck... just when I think I might be heading in the right direction there’s another thing to consider... I never even knew about oversampling and undersampling .... maybe not as significant as sensor size...
Thanks
canonguy1. Yes, like Jason says - this whole topic might be a little overrated, but certainly good to know about.
Bloated stars... that's me! No wonder, according to this forumla : Celestron 8se with ZWO ASI 1600, here then is the math. (3.8/(2032x.67 reducer))x206 = 0.5749! Bloated stars. Time to change telescopes? Thinking about an Orion 8" f3.9 newtonian 800mm focal length. That would get me close to 1.0 in this formula. Helpful as always Chuck.
Although my friend Jason is somewhere in the same Image Scale as you and his galaxy pictures are amazing. That's why he thinks the oversampling and undersampling issue may be a little overrated. But his processing skills are far beyond mine, so I probably would not go that low if it were me.
If anything, an image scale that low would concern me as to whether my guiding could keep up with.
@@ChucksAstrophotography : just wondering if the binning would factor into this equation? I usually image at 1x1.
James Tickle had this to say: "With oversampled you can bin the pixels. The real only problem with oversampling is that you might be wasting photons. Reason is you can sample closer to the seeing which is usually above 1 but less than 3 most locations, and not lose any fine details. But that just means you are taking longer photos than necessary. If you bin 2x2 for example with pixels that are 0.6, then you will sample at 1.2. But a 2x2 bin will detect enough signal to stand out from the noise in 1/4 the time."
@@ChucksAstrophotography Cool : I'll try it and let you know. I'm still after my 2nd scope, however and your calc makes sense.
I need a 9 micron camera to pair with my Edge11 with .7x reducer. I am way oversampled (3.8 / 1960 * 206 = 0.4 image scale) but have no money to buy a larger pixel size camera at this point :(
I'm sure you can still take great images at .4. There is a lot more to this video than what I have presented in this video. I wouldn't worry about it. Read other comments here, you may find some good info.
Hi Chuck ,There is still one more thing you have to consider , the Rayleigh limit of you telescope which stands for the minimal resolvable detail your telescope can tell , IMO. Your 127mm will give you 1.1” of that , and I still don’t know how image scale affect the star , you might as well refer to someone else
Thanks Xinyang, I never heard of Rayleigh limit, I'll look into this.
Hi Chuck, this is the rest of the story. Under and over sampling comes from the Nyquist Theorem. Essentially if your image scale is the same as your Dawes or Rayleigh Limit, then you will miss much of the available detail. If you have an image scale lower than half of the Dawes Limit your sampling rate will be more than two and you will be over sampling ant trying to get something from the optics that it can’t give..the sweet spot is 2, but only if you are in space where seeing is not an issue.
@@MrAlpacabreeder Excellent info, thanks
Generator
Interesting video. My 2350mm focal length and 2.9 micron pixels might need a focal reducer to bring them together then?!!!
Remember though, over and under sampling issues can be remedied through processing.
@@ChucksAstrophotography Thanks Chuck. I will save up for a reducer anyway to get a wider view when required. Have you done a star bloat reduction processing tutorial?
No, in fact I would need help in that area too, lol.
Great explanation. Do you not take into account the crop factor when inputting the focal length? In your example the Rasa 8 inch is effectively a 600 mm effective focal length with your QHY10 crop sensor camera attached … Does this crop factor have to be factored in?
Thanks, but no crop factor needs to be weighed in on this.
So when you say "Pixel Size", what are you referring to?
My D850:
Pixel Pitch - 4.43 um
Pixel Density - 5.32 mp/cm2
Pixel Area - 18.84 um2
4.43um...but I am not an expert with DSLRs.
@@ChucksAstrophotography No worries, thanks for the quick response. I was assuming its pixel pitch.
Cheers chuck for bringing this to our attention.i to did not know this either.cheers
Thanks Janes
Just wanted to clarify applying the NPF with crop sensor cameras. Since we are looking at arcsec/pixel, crop factor does not matter, correct?
I shoot with an image scale of about 0.65 which, admittedly, does give me more bloated stars than I would like. However, this is often due to mount problems and or garbage seeing conditions. I have a very small set of data that has great guiding + seeing, and the results are really great as you might expect. I would say if you are comfortable with the results you get at your current focal length and mount, you may be fine sticking closer to the 1.0 or less image scale.
Agreed Andypro
. As I mentioned Jason in the video, he captures with an image scale close to yours and his galaxy pics are amazing.
Chuck, I’m confused. I have a EdgeHD SCT 11” with 0.7 Focal Reducer resulting in a focal length of 1960mm”. According to your math, I need a camera with 14.27µm pixel size to achieve a 1.5 image scale. Not possible, so shall I trash the SCT? 😟 Math: (P/1960)*206=1.5 | P=14.27
Over and under sampling is a bit overrated as an issue. If you're happy with your pictures, then you're fine.
True that. I’m having fun too. 😛
Very interesting. I'm oversampled by 32% and I guess when I look now the smallest stars do seem a little square, but processing and bad seeing usually rounds those out. It doesn't really seem to affect the DSO or larger stars. But now at least I know the camera I just got will pair well with telescopes from 480 past 900mm focal length.
Thanks d0qtrx
, glad you found the video useful.
@@ChucksAstrophotography I've learned a ton from your channel. One of the things I love about this hobby is no matter how much you know, there's always something new.
@@d0qtrx Agreed, and I think I've still only scratched the tip of the iceberg.
Very interesting, but don't have it completely clear, I have a Celestron C8 with focal length 2033mm and asi1600 with 3.8, that means I'm very oversampled. Even with the focal reducer I get 0.64. Will have to do more research and see how I can get over with all of that.
Being oversampled is better than being undersampled. I think my friend Jason's image scale is even lower than yours and his galaxy pictures are amazing. That's why he thinks the oversampling and undersampling issue may be a little overrated. But his processing skills are far beyond mine.
Guess you have to multiply the focal length by the effect of your reducers / barlows as well. For example, the 4.7X barlow in the Quark, would have a significant effect.
Right.
Hi Chuck! Very well presented, as always :)
I think the formula should also contain the telescope's resolving power in a form of telescope aperture.
For example 400mm f/8 reflector would give much more bloated star than 400mm F/2 RASA.
I think more precise representation would be the ratio of telescope resolving power (which is Dawes' limit) and camera's pixel size sky sampling.
Dawes' limit is calculated as R = 4.56/D where D is in inches, Result is in arcseconds.
This is in perfect world using perfect optics and for visible light.
When taking longer Ha light wavelength with atmosphere seeing, using real life telescope, you should probably multiply the result by at least 1.5
My telescope + camera setup gets 1.76arcsec per pixel sky sampling and my telescope's "perfect world" Dawes' limit is 0.76 arcsec.
So I'm a bit on undersampling side, but still within reasonable limits.
Hope it helps :)
Clear skies to everyone!
Thanks Denis, I knew I had only scratched the surface here, that's why I put that disclaimer at the end that it can go far deeper. Thanks for the info, when I was reviewing the specs of the ES AR102 on OptCorp, they also listed Dawes' limit. I need to turn on my thinking cap now and digest this.
Great Video Chuck, like you stated in the video description it does depend on seeing conditions-how Dark your skies are. You should be fine anywhere between .6 to 1.25
Agreed, probably even higher - .6 to 2.0
Hello Chuck,
Thanks for the video. Now I understand better how important it is to match the right camera to the optical system. Since I am in the process of acquiring a camera, image scale will not be an issue anymore.
Respectfully,
Henri-Julien, Montreal, QC Canada.
Wa - most useful for SGPro / PHD anytime you change camera and focal. Great refresher, nice to know about the sweet spot between 1 and 2. So what's next for ya?
Thanks Jeff. I don't know what's next. I'm trying to do my homework before I make any rash decisions.
Kudos to Jason - I'll have to re-evaluate my new camera / ota. Between 1 and 2, I need m pixel @4.63 camera for my focal lengths otas of 750mm to 1350mm to be in the sweet spot. Just re-checked Im good with the G10 at higher focals, I'll just tile to compensate.
I am considering a camera for my Celestron Nextar Evolution 8 HD Edge any thoughts ? Thank you
You're welcome!
So the only thing that's weird is when I did this for my 11" EdgeHD is that no camera is good. With a focal length of 2800mm seems like every camera's pixels are too small.
That seems like a great focal length for solar system objects. But I've seen people take great pictures of deep sky objects with it using a reducer and having and image scale in the .4 or .5 range.
@@ChucksAstrophotography Very good point actually! I punched in the numbers for my ASI 120MC (my planetary camera) and it did come out to 1.28 for my scope with a focal reducer. Fun fact: California where I live has had like no clouds at all until I bought that stupid thing a few months back and since then I have not seen the sky on the weekend.
@@moeyassine lol, welcome to my world - I don't even know what a clear sky looks like any more.
But I think oversampling planets would be ok, as people usually attach a Barlow.
Okay! I’m creating a spreadsheet now. Never even thought about sampling rate. But, now it all makes sense. Thanks! Oh, and I LOVE Astronomy Tool!!
Ok cool, thanks Gary. Yeah - I thought it made sense to present this with a spreadsheet.
I’ve been running the combination of my telescopes and different cameras through the spreadsheet and now understand why some photo sessions just plain suck and others are pretty good. One more piece to the puzzle. So, I need a new camera! Thanks for turning today’s freezing rain into a very enjoyable evening!
@@gptravel531 Excellent, I'm glad I could be of service, but sorry it turns out that you need a new camera.
Buying a new camera is good thing!
@@gptravel531 Oh good! I have so many cameras, it would be a bad thing for me if I needed another.
Very informative video chuck
Thanks for posting
Thanks Utkarsh
Your one of hero's, especially after the video for the future. Please keep going forever.
Bintel Astronomy calculator is a great tool for this problem. @Dylan O'donnell
I didn't hear of it, thanks Sam
@@ChucksAstrophotography Also, this one Chuck: astronomy.tools/calculators/ccd_suitability
Oh cool, someone else told me of this but I didn't know where to find it, thanks.
I've been having the issue with bloated stars switching from my DSLR to a 290c, which is a lot
I got a ZWO ASI294 MC with a Celsestron Edge HD 8" and my argument was that with the 8" I can do 2x2 binning, giving me a better signal ratio and perfect pixel size and when using a 0.5 reducer I get a perfect resolution removing the binning but getting a very good signal ratio due to reducing F10 to F2.5
Thx for making a video on this Chuck, it going to help me on my next purchase... thx again
Glad you found it useful.
Thanks for the informative video. I never knew about the whole sampling thing.
Glad I'm not the only one, thanks Rob.
Oh well I would say under- or oversampling is a lesser issue... there is so much more important than pixelscale. 2600mc with Redcat51, its awesome even without drizzling.
Just one thing to consider as well, your 183MM is an uncooled camera.
Exactly, so I would only use it for broadband - where the exposures would have to be so short anyway - it probably would not make a difference. But really, I only bought the ASI183MM for solar and lunar. If anything, I would use the ASI1600MM or QHY10 with a RASA.
Great info! You definitely taught me something new and very interesting today. Thank you
Great, glad to hear it Michael.
Very much appreciate your informative video and comments to it Thanks
Very informative video !.. Thank you...
Nice, I didn't know about this topic
Yes, I think it's overlooked by a lot of people.
My Ghosh.. Chuck University!! Absolutely great learning. I have to put headphones to listen carefully while my kids are making too much noise.
Yup got the kids too
@@keithkorthals6183 LOL. Yup.
Thanks Ray!
Very cool! I'm all good at 1.39 😄 with my 72 APO dub and Canon T6. Educational! Thanks chuck!
Good points with video😀
Thanks Keith.
WO Red Cat 51 @ 250 FL. Paired with ZWO ASI183MM for best image. Optimistic. Like your spread sheet.
#1 thing to understand sampling is to know your seeing. Before you look at your camera or your telescope, you need to know your seeing. For example, I imaged "under 1.0" and if I were to just listen to this video, I should have bloated stars. (Example here at 0.56"/px: www.astrobin.com/full/388591/0/) which was not the case. Your local guru was right to a certain point. The whole sampling thing, is more than just looking at a number and calling it good or bad. I imaged this, at 2.11"/px (www.astrobin.com/full/380925/F/) and you will not find blocky stars.
Pairing a telescope to a camera needs the following, IMHO:
1. Understanding of the seeing, on average, in the locale.
2. Understanding of the telescope focal ratio. The lower this is, the more #1 (if its bad) will cause you problems. No matter the length.
3. Understanding of the pixel size of the camera. Smaller pixels are more likely to show the warts of optical or other imaging train/gear error moreso than larger pixels will.
4. Understanding of the goals you have for imaging.
5. The focal length of the telescope.
That’s for the info!
No prob
How about investing in a bad ass microphone .
I need to, thanks.
Chuck, check this out for undersampling. Maybe you could experiment with binning 2x2
ua-cam.com/video/EEfHKYtIRsU/v-deo.html
Exactly. Eliminate as much as possible BEFORE processing.
Otherwise, it's like making a crappy movie and trying to fix it in editing.
Better to make a good movie first.
Astronomy tools has this calculator to help with this as well: astronomy.tools/calculators/ccd_suitability
Oh boy. It seems most of us are doomed then. Very interesting.
lol, we're all going into the abyss. Thanks Steve.
@@ChucksAstrophotography Can you make this spreadsheet available Chuck? Long time since I set up spreadsheets. I think you have open a rabbit hole which I just fell into. It seems the more you study this subject, you have to realise that your end image is an assimulation of what is there. When we stack images, we are changing the pixels anyway.
lol, sure, do you know my email address?
@@ChucksAstrophotography Yes, I do have it Chuck. I'll send you a line later
@@fusion-music This link does the the same thing. If I had known about it, I probably would not have created my spreadsheet. astronomy.tools/calculators/ccd_suitability
💚💚💚👍
Thanks GW
1st 😂😂
2nd, lol.
Chuck's Astrophotography I normally get notifications 12 hours after people’s posts. Great video chuck you’re a real inspiration to people 👍
@@LAshotts Cool, thanks LAshots!