Going from 1.34" to 0.84" sample - what should I expect?

Equipment CMOS Acquisition Deep Sky
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Rafael Amarins avatar
Rafael Amarins avatar
Hi
I have a 102mm F7 Triplet Apo
Dawes Limit for this aperture is 1.14"
I'm currently using the IMX294 which gives me 1.34"/pix which for the aperture should be an appropriate sampling for the setup.
The thing is I just bought a Player One Uranus C Pro which gives me 0.84"/pix so I know I'm oversampled when using it
I know seeing is a limiting factor but apart from that I have some questions and I would like to hear from other folks who went through this situation. 

Should I notice this change in detail? 
Should the new techniques envolving deconvolution give me advantage post processing?

I understand focus, seeing and guiding are the main concerns but considering I'm having ideal conditions for all three what should I expect?
Dark Matters Astrophotography avatar
Dark Matters Astrophotography avatar
Anytime you increase resolution of the system you will put more tax on the criticality of the following:

1. Tracking
2. Focus
3. Tilt / Spacing
4. Proper Thermal Management
5. Collimation
6. Proper Seeing Conditions
7. Proper Moon Up / Down management

As for the change in detail, your stars will become larger in terms of how many pixels represent a star, and thus deconvolution routines have better data to work with. That is probably going to be the least of your concerns however. Look at the list I provided, and cover those bases. If you do all of those things well then the change should be very enjoyable.

-Bill
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Rafael Amarins avatar
Rafael Amarins avatar
Anytime you increase resolution of the system you will put more tax on the criticality of the following:

1. Tracking
2. Focus
3. Tilt / Spacing
4. Proper Thermal Management
5. Collimation
6. Proper Seeing Conditions
7. Proper Moon Up / Down management

As for the change in detail, your stars will become larger in terms of how many pixels represent a star, and thus deconvolution routines have better data to work with. That is probably going to be the least of your concerns however. Look at the list I provided, and cover those bases. If you do all of those things well then the change should be very enjoyable.

-Bill

Hi Bill
Nice point. Of all the things you've listed the one thing that I under my control and yet I might fail is the focus. I'm not using an EAF and I'll rely on a bahtinov mask. Fortunetly my location doest not have a great temperature variation during the night yet I should be double checking it every 1.5 hours or so

CS
Anderl avatar
Anderl avatar
i am nowhere understanding all the things that one needs to understand if wanting to make an valuable assumption here but to me it seems that most people that are into deep sky use the "Rayleigh limit" and not "Dawes Limit". 
As Rayleigh suggests that your scope has an resolution ability of 1.3 I guess you would win close to nothing with that new camera.

i was thinking about it myself. 
Esprit 120 reduced vs unreduced with an imx571 but if I believe that Rayleigh is more or less right I am just  waisting my time photographing with my esprit at nativ focal length.

cs
Andreas
Kevin Morefield avatar
Kevin Morefield avatar
With a 1.34" image scale, I would imagine the typical lowest FWHMs you see currently is around 2.6" with occasional subs getting close to 2.1".  This presumes seeing is at or below 2" and your guiding, focusing, and optical train are optimized.  The reason for this limitation is that you can't resolve a detail in an image with a single pixel - it takes at least two samples of a wave to define the wave in any way.  I've not seen my under-sampled FSQ106 every calculate a FWHM of less than 1.5 pixels.  A 3 pixel sample will do a better job of defining the wave but with two you are still resolving much of the detail.

Here's slide a used to try and explain why a single pixel doesn't resolve anything.  
 


Imagine that your seeing and optics present an image to your sensor that is resolved to 2".  You will have 2" / .84" = 2.38 pixels to define the smallest details.   

So I think  at 1.34" you are under-sampled and at .84" probably appropriately sampled.   Assuming your seeing is typically 2" or less and your optics, guiding and focus are optimized you should see your typical FWHM drop to the lower 2.X" range and occasionally see below 2".  

All of that said, I've imaged the same object at the same time from same location as a friend.  He was using an 10 inch scope at 1250mm FL that did not have optimized optics.   His FWHM's were 1" larger than what I was getting from my 4" refractor at 530mm.  So none of this matters if you haven't optimized everything else.
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Christian Großmann avatar
Christian Großmann avatar
Hi Rafael,

before I bought my 294MM, I only had a 183MM. I guess, you are using your 294MM with bin 2 which is the standard resolution. This camera is kind of optimal for scopes with a 700 to 800 mm focal length range. But before I had it, I was forced to use my 183MM with 2.4 micron pixels even with my 8" f/4 Newton. I never binned it. The images were oversampled and the exposure times were a bit longer. But I didn't realize it at that point. As I switched to the 294MM, the images became much better. I saw an improvement especially in my narrowband images.

I think, that it is possible to get good images with an oversampled setup. But in case of going from a well suited camera to an oversampled setup it might be a bit difficult. Personally, I would always prefer the well suited one, because there are some advantages, as you know. The technical explanations above are true, of course, and Kevins exapmle is too, although it is quite drastic. The oversampling will in your case be visible, but you can work with it. Nevertheless, I would not suggest to exchange the cameras. At least I would not be satisfied for long.

CS

Christian
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Jared Willson avatar
Jared Willson avatar
Rafael Amarins:
Hi
I have a 102mm F7 Triplet Apo
Dawes Limit for this aperture is 1.14"
I'm currently using the IMX294 which gives me 1.34"/pix which for the aperture should be an appropriate sampling for the setup.
The thing is I just bought a Player One Uranus C Pro which gives me 0.84"/pix so I know I'm oversampled when using it
I know seeing is a limiting factor but apart from that I have some questions and I would like to hear from other folks who went through this situation. 

Should I notice this change in detail? 
Should the new techniques envolving deconvolution give me advantage post processing?

I understand focus, seeing and guiding are the main concerns but considering I'm having ideal conditions for all three what should I expect?

Whether you will see a change in resolution will depend a lot on your seeing conditions. For example, I have a 12" scope at a remote site that is sampled at 0.69 arc seconds per pixel. Right now, the scope is imaging... The seeing monitor reports "2.4 arc second" conditions which is perhaps a bit below average for the site and probably about average for mid norther latitudes in North America as a whole, though there is obviously a lot of variability. The object I am imaging is fairly low at the moment, just 33º above the horizon. That will erode resolution pretty noticeably since the scope is looking through a lot of air compared to something at zenith. The scope is fully equilibrated, well collimated, well focused, and guiding is hovering around 0.4" RMS which is pretty good considering altitude and seeing conditions. The mount is perfectly capable of <0.3" RMS with better seeing and nearer zenith. 

I just checked a five minute sub exposure to see what the FWHM numbers are... Eccentricity (as reported by PixInsight) is just under 0.4 which indicates guiding is, in fact, reasonably good. FWHM is running 3.2 arc seconds.  That means each star is more than 4.5 pixels across. Your 102mm scope with the IMX294 would probably be a little oversampled for these conditions, let alone my 12" scope at 0.69"/pixel. 

Is there something wrong with my scope? Or my guiding? Or my focus? Nope. On a night of better seeing (or even later tonight when my subject is higher in the sky) it will do very well. It is perfectly capable of five minute sub exposures with resolutions better than 2" FWHM. But on an average night? With a poorly placed object? Not so much.  

The new deconvolution utilities such as BlurXterminator do a really nice job. They especially do a nice job of using different PSF profiles in different parts of the image (if you have a little tilt or some off-axis astigmatism). They can draw out existing details in nebulosity making them much easier to see. And they seem to do this all without introducing annoying halos and without significantly impacting noise levels. Pretty cool. But will they do a better job with your higher sampling rate? Well, if everything else is good, sure. But if you have a night of mediocre seeing? Or if your subject is low on the horizon? Or if your guiding is not quite as good as you hoped? I doubt you will notice a difference.

Moving from 1.3" to 0.8" per pixel will allow the possibility of more resolution in your images (both before and after deconvolution), but it won't guarantee anything. Everything in your imaging train has to be working well for you to realize any gains. As you yourself mentioned, focus, seeing, and guiding will still have a bigger impact. As you probably already know, improving resolution in deep sky imaging (without resorting to lucky imaging) is all about incremental gains. Higher sampling rate? Maybe you'll get an extra tenth of an arc second or two. Better guiding? Another tenth. Improve your collimation? Or your flattener backfocus spacing? Maybe another tenth. An extra couple inches of aperture? Maybe another tenth/maybe just better SNR. All the little improvements add up. Moving from 1.34" per pixel to 0.84" per pixel is definitely good--if everything else is optimized. I would think of it more as a prerequisite to <2" FWHM subs, though, not as a guarantee.
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Pistachio_Enjoyer avatar
Pistachio_Enjoyer avatar
To add onto the contributions already made, I found that with a smaller image scale, focus, tracking and most importantly, seeing matter much more than with a larger image scale. I currently image at .39"/px, so I have to make sure that my equipment is well tuned and the seeing conditions are good for me to image in (for me, any sub with a FWHM greater than 3.2" is wasted). Lastly, deconvolution tools, such as BXT work better for smaller image scales, especially for systems that are oversampled. Hope this helps.
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Rafael Amarins avatar
Rafael Amarins avatar
i am nowhere understanding all the things that one needs to understand if wanting to make an valuable assumption here but to me it seems that most people that are into deep sky use the "Rayleigh limit" and not "Dawes Limit". 
As Rayleigh suggests that your scope has an resolution ability of 1.3 I guess you would win close to nothing with that new camera.

i was thinking about it myself. 
Esprit 120 reduced vs unreduced with an imx571 but if I believe that Rayleigh is more or less right I am just  waisting my time photographing with my esprit at nativ focal length.

cs
Andreas

Actually my scope should resolve down to 1.14 and the IMX533 or IMX571 would take me to a sweet spot of 1"
I made a good deal on the Player One Uranus C Pro (about half what I would spend on a 533) and I'll take my chances and see what heppens. I've seen some interesting images with modest aperture OTAs
https://www.astrobin.com/uk47w9/
https://www.astrobin.com/uhqxfx/
Rafael Amarins avatar
Rafael Amarins avatar
Kevin Morefield:
With a 1.34" image scale, I would imagine the typical lowest FWHMs you see currently is around 2.6" with occasional subs getting close to 2.1".  This presumes seeing is at or below 2" and your guiding, focusing, and optical train are optimized.  The reason for this limitation is that you can't resolve a detail in an image with a single pixel - it takes at least two samples of a wave to define the wave in any way.  I've not seen my under-sampled FSQ106 every calculate a FWHM of less than 1.5 pixels.  A 3 pixel sample will do a better job of defining the wave but with two you are still resolving much of the detail.

Here's slide a used to try and explain why a single pixel doesn't resolve anything.  
 


Imagine that your seeing and optics present an image to your sensor that is resolved to 2".  You will have 2" / .84" = 2.38 pixels to define the smallest details.   

So I think  at 1.34" you are under-sampled and at .84" probably appropriately sampled.   Assuming your seeing is typically 2" or less and your optics, guiding and focus are optimized you should see your typical FWHM drop to the lower 2.X" range and occasionally see below 2".  

All of that said, I've imaged the same object at the same time from same location as a friend.  He was using an 10 inch scope at 1250mm FL that did not have optimized optics.   His FWHM's were 1" larger than what I was getting from my 4" refractor at 530mm.  So none of this matters if you haven't optimized everything else.

To be honest I've watched Jon Hayes's presentation (on youtube) of his setup too many times and learned what he explains about sampling. The concept/example you posted above is in line with what I understood of it. I appreciate your example - very didactic and simple. Thank you for the clarification. 
As for optimized I'm considering best case scenarios: near zenith target, exceptional guiding (for the gear I have) and good seeing conditions. I know that won't happen very often but I want to be well equipped if I get lucky.
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Rafael Amarins avatar
Rafael Amarins avatar
Christian Großmann:
Hi Rafael,

before I bought my 294MM, I only had a 183MM. I guess, you are using your 294MM with bin 2 which is the standard resolution. This camera is kind of optimal for scopes with a 700 to 800 mm focal length range. But before I had it, I was forced to use my 183MM with 2.4 micron pixels even with my 8" f/4 Newton. I never binned it. The images were oversampled and the exposure times were a bit longer. But I didn't realize it at that point. As I switched to the 294MM, the images became much better. I saw an improvement especially in my narrowband images.

I think, that it is possible to get good images with an oversampled setup. But in case of going from a well suited camera to an oversampled setup it might be a bit difficult. Personally, I would always prefer the well suited one, because there are some advantages, as you know. The technical explanations above are true, of course, and Kevins exapmle is too, although it is quite drastic. The oversampling will in your case be visible, but you can work with it. Nevertheless, I would not suggest to exchange the cameras. At least I would not be satisfied for long.

CS

Christian

I made a good deal on the camera so I'll take my chances. Smaller pixels indeed will decrease my efficiency specially considering it is an OSC. I have some strong emission nebula at a decent altitude some of them near the zenith so I'll have this compensated by their surface brightness. 
I'll come back in a few months and update this post with the results on the same targets. 

CS
Rafael Amarins avatar
Rafael Amarins avatar
Jared Willson:
Rafael Amarins:
Hi
I have a 102mm F7 Triplet Apo
Dawes Limit for this aperture is 1.14"
I'm currently using the IMX294 which gives me 1.34"/pix which for the aperture should be an appropriate sampling for the setup.
The thing is I just bought a Player One Uranus C Pro which gives me 0.84"/pix so I know I'm oversampled when using it
I know seeing is a limiting factor but apart from that I have some questions and I would like to hear from other folks who went through this situation. 

Should I notice this change in detail? 
Should the new techniques envolving deconvolution give me advantage post processing?

I understand focus, seeing and guiding are the main concerns but considering I'm having ideal conditions for all three what should I expect?

Whether you will see a change in resolution will depend a lot on your seeing conditions. For example, I have a 12" scope at a remote site that is sampled at 0.69 arc seconds per pixel. Right now, the scope is imaging... The seeing monitor reports "2.4 arc second" conditions which is perhaps a bit below average for the site and probably about average for mid norther latitudes in North America as a whole, though there is obviously a lot of variability. The object I am imaging is fairly low at the moment, just 33º above the horizon. That will erode resolution pretty noticeably since the scope is looking through a lot of air compared to something at zenith. The scope is fully equilibrated, well collimated, well focused, and guiding is hovering around 0.4" RMS which is pretty good considering altitude and seeing conditions. The mount is perfectly capable of <0.3" RMS with better seeing and nearer zenith. 

I just checked a five minute sub exposure to see what the FWHM numbers are... Eccentricity (as reported by PixInsight) is just under 0.4 which indicates guiding is, in fact, reasonably good. FWHM is running 3.2 arc seconds.  That means each star is more than 4.5 pixels across. Your 102mm scope with the IMX294 would probably be a little oversampled for these conditions, let alone my 12" scope at 0.69"/pixel. 

Is there something wrong with my scope? Or my guiding? Or my focus? Nope. On a night of better seeing (or even later tonight when my subject is higher in the sky) it will do very well. It is perfectly capable of five minute sub exposures with resolutions better than 2" FWHM. But on an average night? With a poorly placed object? Not so much.  

The new deconvolution utilities such as BlurXterminator do a really nice job. They especially do a nice job of using different PSF profiles in different parts of the image (if you have a little tilt or some off-axis astigmatism). They can draw out existing details in nebulosity making them much easier to see. And they seem to do this all without introducing annoying halos and without significantly impacting noise levels. Pretty cool. But will they do a better job with your higher sampling rate? Well, if everything else is good, sure. But if you have a night of mediocre seeing? Or if your subject is low on the horizon? Or if your guiding is not quite as good as you hoped? I doubt you will notice a difference.

Moving from 1.3" to 0.8" per pixel will allow the possibility of more resolution in your images (both before and after deconvolution), but it won't guarantee anything. Everything in your imaging train has to be working well for you to realize any gains. As you yourself mentioned, focus, seeing, and guiding will still have a bigger impact. As you probably already know, improving resolution in deep sky imaging (without resorting to lucky imaging) is all about incremental gains. Higher sampling rate? Maybe you'll get an extra tenth of an arc second or two. Better guiding? Another tenth. Improve your collimation? Or your flattener backfocus spacing? Maybe another tenth. An extra couple inches of aperture? Maybe another tenth/maybe just better SNR. All the little improvements add up. Moving from 1.34" per pixel to 0.84" per pixel is definitely good--if everything else is optimized. I would think of it more as a prerequisite to <2" FWHM subs, though, not as a guarantee.

Hey Jared thanks for the tips. I have a strategy or sort of. 
1- Chose high surface brightness objects. Some planetary nebula and strong emission nebulas should help me with that. As for galaxies I could start with NGC1365 and Sombrero for instance. 
2- The pixel is small but the chip too which means the file size is smaller. if I'm suffering with guiding can shorten the frame length and stack up to 1k subs 
3- Image near the zenith. Some very interesting targets comes near the zenith so that should give me some marginal but valuable gains in therms of seeing
Jared Willson avatar
Jared Willson avatar
Rafael Amarins:
Hey Jared thanks for the tips. I have a strategy or sort of. 
1- Chose high surface brightness objects. Some planetary nebula and strong emission nebulas should help me with that. As for galaxies I could start with NGC1365 and Sombrero for instance. 
2- The pixel is small but the chip too which means the file size is smaller. if I'm suffering with guiding can shorten the frame length and stack up to 1k subs 
3- Image near the zenith. Some very interesting targets comes near the zenith so that should give me some marginal but valuable gains in therms of seeing

High surface brightness objects will allow you to get decent SNR even with quite short subexposures where read noise would otherwise be hard to overwhelm, so that will definitely help with any guiding issues. If nothing else, if your guiding is mediocre you could just throw out the bottom 30% of your images based on FWHM and see what you get. You won't start to see benefits in terms of "freezing seeing" unless you are using really short subs--on the order of 2s or, ideally, a fraction of a second--but even 10s or 20s subs will let you avoid the worst of the tracking/guiding errors by culling or even just appropriately weighting your images. If the goal is to get the most out of your system that you can in terms of resolution, it seems like a reasonable approach.  Good luck!
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Brian Puhl avatar
Brian Puhl avatar
One critical element that has been spoken, but not driven home here is your mount.   Looking at your gallery, it seems you're running a CEM25.    Now I'm not experienced with this mount, but it seems that you might already be pushing its capacity as is.    Having an image scale of 1.34 might be a blessing since it will allow you some room for error.  Moving down to 0.84" image scale is likely going to push the limits of your mount substantially.   Even with my EQ6's, which I used to image at your image scale with…. some nights I was lucky to keep my guiding performance inside of 70% image scale.    I would expect for a zero reject rate, you'd need your guiding performance to stay under 0.6 arc seconds at all times.    Don't just look at RMS, pay attention to the spikes.  If the spikes are outside your image scale, your eccentricity will pay, degrading overall image quality.

Also, putting mount performance aside, I'd maybe stick with your 294 over the Uranus C.   Uranus is a planetary camera, and while it CAN be used for DSO's… personally I'd use the 294 over the 585 sensor.   DSO images from the 585 just always seem to fall short.

Kevin and others have hit the nail on the head with the rest of the comments in here.
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Rafael Amarins avatar
Rafael Amarins avatar
That's true. I do expect to reject a good amount of subs. That's why I'll do multiple tests to to see how long I can push the subs and use the ones with round stars. 
I still have my IMX294 (svbony) so this is not a permanent configuration. I understand I'll be giving dynamic range. Well depth and bit depth are compatible with commonly used cameras that until recently gives good images like 183mc and 1600mc so I see no problem with that.
Rafael Amarins avatar
Rafael Amarins avatar
Just an update
As if 0.84 wasn't enough I've switched from the 102mm F7 to a 122mm F7 refractor which gave me 0.7" per pixel
Somehow my CEM25 managed to handle it with 60s exposures and it could handle more but I wasn't inclined to waste many frames.

I'll get them properly processed and post it here but I can tell it was woth it. Now I can see NGC253 with good details and NGC1365 and its structure.
Rafael Amarins avatar
Rafael Amarins avatar
Update: 
I managed to image a couple of galaxies to have a taste of the new image scale. I went from 1.34 (IMX294 + F7 102mm apo) to 0.7 (IMX585 + F7 122mm apo/semiapo). 
These are not calibrated. No flats, no darks. I just stacked the lights on DSS and stretched a bit on pixinsight and gimp. I'm not very fond of processing. No blur x I did use noise X tho. Not impressive with no darks or flats and less than 3hrs of data on an F7 system however it shows that it is possible. 

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