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How my CDK is 13x faster than my f4.5 APO

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#26 Rouzbeh

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Posted 15 January 2022 - 10:55 PM

OK, it's the title of your thread compares two different scopes saying one is faster than other was confusing.

 

At same image scale, yes, aperture matters and not the focal ratio. In most cases, it's easier to rely on focal ratio than image scale for imaging speed.

 

Peter

You can look at it both ways. f ratio isn't all that critical.

 

Say I want to image a relatively large target like M101.

 

The "fast" f/5 Tak FSQ130 could be used to image it at a nice 1.2" / pixel

 

Alternatively, a CDK14 at f/7.2 could be binned 4x to capture at the exact scale and result in the same size image of the target.

 

The "slow" reflector would actually need 1/5 the time to reach the same SNR of the Tak.


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#27 Rouzbeh

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Posted 15 January 2022 - 10:58 PM

Also, given the large reflector has resolution to "burn", a reducer could be added to make the FOV larger to capture something as large as M33 and still

bin 2x or 3x and out perform the Tak130 in both resolution and speed.

 

 

The fast refractor can use a reducer too, but even with the tiny CMOS pixels, its lacks the focal length to get the full potential resolution the sky will allow. at closer to 2" / pixel its quite under sampled.

 

 

The CDK on the other hand even with the extreme 0.66x reducer and bin2x will still be in the correct range for critical sampling for most sky conditions at 0.9" / pixel.


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#28 freestar8n

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Posted 15 January 2022 - 11:42 PM

"Fast" has a particular meaning and value in optics.  It always has and it always will.  But that doesn't mean it is a determining factor in specific comparisons of different systems.  The fact that scenarios can be created where a slower system outperforms has no impact on the value of speed in optics.  Optical speed is important, in its own way, by itself.  Similarly aperture has value by itself.

 

But all these threads where people image with different setups and blow up an object to the same size and crop - are completely missing the point of optical speed and its value in imaging.

 

If you take a given sensor and image an empty region of sky and collect skyglow on that sensor - a faster system will collect more photons in a given time.  The aperture could be 1mm or 1km and it wouldn't matter.  Only the speed would matter.  That shows how important it is in terms of optical throughput and etendue.  It says nothing about resolution, cropping - etc.

 

Frank


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#29 Rouzbeh

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Posted 16 January 2022 - 12:05 AM

"Fast" has a particular meaning and value in optics.  It always has and it always will.  But that doesn't mean it is a determining factor in specific comparisons of different systems.  The fact that scenarios can be created where a slower system outperforms has no impact on the value of speed in optics.  Optical speed is important, in its own way, by itself.  Similarly aperture has value by itself.

 

But all these threads where people image with different setups and blow up an object to the same size and crop - are completely missing the point of optical speed and its value in imaging.

 

If you take a given sensor and image an empty region of sky and collect skyglow on that sensor - a faster system will collect more photons in a given time.  The aperture could be 1mm or 1km and it wouldn't matter.  Only the speed would matter.  That shows how important it is in terms of optical throughput and etendue.  It says nothing about resolution, cropping - etc.

 

Frank

True but I am more concerned with the object and its scale and less so with Sky glow.

 

My point is one can image the same object with the same scale and have vastly more SNR with a larger system no matter how fast the smaller system is.

 

My only constraint is FOV. I also give high priority to pixel signal than etendue.

 

But then again, there are many criteria and priorities, that are different form one imager to another.

 

My goal is getting close to the maximum resolution my sky conditions will allow in the shortest possible time. This has led me to large apertures, high QE , low noise and correct sampling (binning). If there is a way around it, I'd be very interested to know.



#30 freestar8n

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Posted 16 January 2022 - 12:28 AM

My use of skyglow is to demonstrate that f/ratio and sensor size tell you the etendue, which corresponds to how well the system sucks photons down from the sky.  That's a useful thing and that's what the terms f/ratio, speed and etendue refer to.

 

It's fine if you what you care about is getting a good image of a particular galaxy at decent scale and quickly - in which case the main thing you would care about is aperture.  You can ignore f/ratio as long as the galaxy fits on your sensor and the pixels are about the right size with or without binning.

 

There's just no need to use the term "fast" in this regard.  In your scenario aperture is the dominant factor - so you are just saying that the cdk has more aperture than the refractor - and yes that is indeed the case.

 

But if you placed the same sensor in each scope and aimed them at a field of galaxies, and then put that image on the wall - the faster system would end up making a nice overall image that looked good in less time.  The images would be very different scales and scenes, but the time to nice and uncropped picture on the wall would be less with the faster system.

 

Frank


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#31 Peter in Reno

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Posted 16 January 2022 - 10:37 AM

I fully understand your goal as I explained earlier in post #25.

 

Go for it!!!! As long as you don't mind making many sets of calibration files of different binnings, go for it!!!!!

 

Peter


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#32 Borodog

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Posted 16 January 2022 - 04:34 PM

So, I'm trying to wrap my head around the implications of all this.

 

I have been imaging mostly with a reduced C8, ~f/6.3, ~1300mm, ASI183MC, native image scale 0.38"/pixel, FOV 0.58x0.39 degrees. This setup is primarily used for lunar imaging, while it excels at, at least in my opinion. It's also pretty good for planetary nebulae, globulars, galaxies, small stuff. But I obviously can't fit large extended nebulae, M31, etc.

 

I've been advised to switch to a short focal length "fast" refractor. I've done some practice with a cheap 60mm guide scope while I mull over buying an APO.

 

What you're telling me is that, by binning, I could produce an equivalent SNR "tile" at the same image scale as the refractor in a much shorter time, and could regain the FOV by building a mosaic, which I am very familiar with from lunar imaging.

 

Say, for example, I had a 60mm refractor at f/4.8, 288mm. With my camera, 1.72"/pixel, 2.625x1.75 degree FOV.

 

To recover this FOV would require 25 tiles. To achieve the same image scale would require resampling 4.5x4.5.

 

So, if my math is correct, the ratio of the pixel signals between the two systems (after binning to achieve the same pixel scale), neglecting any potential difference in transmittance between the two, simply goes as the ratio of the two areas, yes?

 

The C8 has a central obstruction of 35%, so an effective area of 0.65 x pi x (203.2mm / 2)^2, while the refractor has an area of pi x (60mm / 2)^2, so the ratio of the two is 0.65 x (203.2 / 60)^2 = 7.45.

 

So for the same pixel scale, the C8 is 7.45 times "faster" than the 60mm refractor. And that's true regardless of camera, as long as you are comparing the same camera in both scopes. But it would take ~25 tiles to achieve the same field of view. Also independent of camera (just the ratio of the fields of view rounded up, squared).


Edited by Borodog, 16 January 2022 - 05:47 PM.

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#33 Rouzbeh

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Posted 16 January 2022 - 07:07 PM

My use of skyglow is to demonstrate that f/ratio and sensor size tell you the etendue, which corresponds to how well the system sucks photons down from the sky.  That's a useful thing and that's what the terms f/ratio, speed and etendue refer to.

 

It's fine if you what you care about is getting a good image of a particular galaxy at decent scale and quickly - in which case the main thing you would care about is aperture.  You can ignore f/ratio as long as the galaxy fits on your sensor and the pixels are about the right size with or without binning.

 

There's just no need to use the term "fast" in this regard.  In your scenario aperture is the dominant factor - so you are just saying that the cdk has more aperture than the refractor - and yes that is indeed the case.

 

But if you placed the same sensor in each scope and aimed them at a field of galaxies, and then put that image on the wall - the faster system would end up making a nice overall image that looked good in less time.  The images would be very different scales and scenes, but the time to nice and uncropped picture on the wall would be less with the faster system.

 

Frank

 

I have no doubt lower a f/ratio is faster holding everything else equal. But parameters can be manipulated to make up for f/ratio in many cases, as we saw with the example above. There still is no replacement for displacement (aperture). 

 

Aperture (for me) is proving very beneficial even though I can't use the full potential of resolution due to typical seeing, my point was its being traded for speed. I started this thread because someone asked why bother with a big slow CDK and not just use a mid sized APO at the scale I want rather then bin the CDK.

 

 

 

As for Etendue,  it doesn't really translate to results all the time. For example, when I switch from the APS-C to FF sensor the etendue of the system is doubled. However, that doesn't really result in much difference in a scenario where the object fits in the FOV of the APS-C.

 

Yes 2 x etendue but I'm merely capturing the surrounding area around the object.  

 

That's not to say its not important, I did get a FF camera for that very reason. However, a 2 x increase in SNR is more noticeable and dramatic.



#34 Rouzbeh

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Posted 16 January 2022 - 07:21 PM

So, I'm trying to wrap my head around the implications of all this.

 

I have been imaging mostly with a reduced C8, ~f/6.3, ~1300mm, ASI183MC, native image scale 0.38"/pixel, FOV 0.58x0.39 degrees. This setup is primarily used for lunar imaging, while it excels at, at least in my opinion. It's also pretty good for planetary nebulae, globulars, galaxies, small stuff. But I obviously can't fit large extended nebulae, M31, etc.

 

I've been advised to switch to a short focal length "fast" refractor. I've done some practice with a cheap 60mm guide scope while I mull over buying an APO.

 

What you're telling me is that, by binning, I could produce an equivalent SNR "tile" at the same image scale as the refractor in a much shorter time, and could regain the FOV by building a mosaic, which I am very familiar with from lunar imaging.

 

Say, for example, I had a 60mm refractor at f/4.8, 288mm. With my camera, 1.72"/pixel, 2.625x1.75 degree FOV.

 

To recover this FOV would require 25 tiles. To achieve the same image scale would require resampling 4.5x4.5.

 

So, if my math is correct, the ratio of the pixel signals between the two systems (after binning to achieve the same pixel scale), neglecting any potential difference in transmittance between the two, simply goes as the ratio of the two areas, yes?

 

The C8 has a central obstruction of 35%, so an effective area of 0.65 x pi x (203.2mm / 2)^2, while the refractor has an area of pi x (60mm / 2)^2, so the ratio of the two is 0.65 x (203.2 / 60)^2 = 7.45.

 

So for the same pixel scale, the C8 is 7.45 times "faster" than the 60mm refractor. And that's true regardless of camera, as long as you are comparing the same camera in both scopes. But it would take ~25 tiles to achieve the same field of view. Also independent of camera (just the ratio of the fields of view rounded up, squared).

 

Given I had a long FL setup similar to yours, I was looking into something like an fsq as a secondary widefield setup.

 

The FSQ at native f/5 will have a scale of 1.48. What I'm considering now is using the CDK14 and say a 3x3 mosaic to get the same FOV.

 

You mat think its 9 x more exposure but when you bin the CDK x 3 its about 7x faster at a higher scale of 1.35"/pixel. I suspect the resolution will be higher even though the CDK image is down sampled. 

 

I haven't yet tried mosaics that large, but it shouldn't too bad. If too much work with PI, I am even considering trying APP (meant to be great for mosaics).

 

You have to work out the number with the C8, the only thing I say is that the 183 is a pretty small chip and to match the huge FOV of a 60mm would require a lot of panels as you worked out.

 

Each image with CDK setup is capturing 4x panels worth of the C8 and 183 chip which makes me more hopeful.


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#35 Rouzbeh

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Posted 16 January 2022 - 07:22 PM

I fully understand your goal as I explained earlier in post #25.

 

Go for it!!!! As long as you don't mind making many sets of calibration files of different binnings, go for it!!!!!

 

Peter

 

I capture everything at native bin1 and downsample after calibration.

 

I don't think it will require different calibration frames.   


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#36 Rouzbeh

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Posted 16 January 2022 - 09:59 PM

My use of skyglow is to demonstrate that f/ratio and sensor size tell you the etendue, which corresponds to how well the system sucks photons down from the sky.  That's a useful thing and that's what the terms f/ratio, speed and etendue refer to.

 

It's fine if you what you care about is getting a good image of a particular galaxy at decent scale and quickly - in which case the main thing you would care about is aperture.  You can ignore f/ratio as long as the galaxy fits on your sensor and the pixels are about the right size with or without binning.

 

There's just no need to use the term "fast" in this regard.  In your scenario aperture is the dominant factor - so you are just saying that the cdk has more aperture than the refractor - and yes that is indeed the case.

 

But if you placed the same sensor in each scope and aimed them at a field of galaxies, and then put that image on the wall - the faster system would end up making a nice overall image that looked good in less time.  The images would be very different scales and scenes, but the time to nice and uncropped picture on the wall would be less with the faster system.

 

Frank

 

Also wanted to say, I enjoy your comments and find them very informative. Always good to discuss ideas and hope the sometimes long threads don't tire readers.


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#37 Borodog

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Posted 16 January 2022 - 11:14 PM

Given I had a long FL setup similar to yours, I was looking into something like an fsq as a secondary widefield setup.

The FSQ at native f/5 will have a scale of 1.48. What I'm considering now is using the CDK14 and say a 3x3 mosaic to get the same FOV.

You mat think its 9 x more exposure but when you bin the CDK x 3 its about 7x faster at a higher scale of 1.35"/pixel. I suspect the resolution will be higher even though the CDK image is down sampled.

I haven't yet tried mosaics that large, but it shouldn't too bad. If too much work with PI, I am even considering trying APP (meant to be great for mosaics).

You have to work out the number with the C8, the only thing I say is that the 183 is a pretty small chip and to match the huge FOV of a 60mm would require a lot of panels as you worked out.

Each image with CDK setup is capturing 4x panels worth of the C8 and 183 chip which makes me more hopeful.


That’s what I’m mulling here. Do I buy an APO first and stick with my (uncooled) camera for a while? Or do I buy a cooled APS-C sized IMX571 based camera and stick with the C8 for a while? Will an APS-C sized sensor even work well enough with a reduced C8? I understand that even an IMX294 sensor has pretty severe vignetting in such a scope.

Then there is the overhead involved in building a DSO mosaic. I have a lot of trouble with residual gradients as it is in single panels. Seems like it would be hard to get seamless mosaics.


Edited by Borodog, Yesterday, 11:12 AM.


#38 dan_hm

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Posted Yesterday, 09:29 AM

Interesting thread. To me it showcases why we have different systems for different applications. If you have a permanent observatory and lots of money to spend, something like a CDK and a camera with a big chip is a great choice. If you want something you can pick up and carry around easily, and that can capture a wider field more quickly, something smaller and faster makes more sense. Or, get both!


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#39 Rouzbeh

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Posted Yesterday, 12:05 PM

That’s what I’m mulling here. Do I buy an APO first and stick with my (uncooled) camera for a while? Or do I buy a cooled APS-C sized IMX571 based camera and stick with the C8 for a while? Will an APS-C sized sensor even work well enough with a reduced C8? I understand that even an IMX294 sensor has pretty severe vignetting in such a scope.

Then there is the overhead involved in building a DSO mosaic. I have a lot of trouble with residual gradients as it is in single panels. Seems like it would be hard to get seamless mosaics.

 

I'm not sure of the C8 image circle, I believe the Edge should cover an APS-C.

I'd go for the IMX571 in any case, its a far better camera. You could change the OTA in the future.

 

Here's a review of the QHY268 with that chip:

https://astrogeartod...qhyccd-qhy268m/

 

I believe there are several software that do a great job at making mosaics and handling the gradients. I dot have personal experience with them yet.


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#40 Rouzbeh

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Posted Yesterday, 12:07 PM

Interesting thread. To me it showcases why we have different systems for different applications. If you have a permanent observatory and lots of money to spend, something like a CDK and a camera with a big chip is a great choice. If you want something you can pick up and carry around easily, and that can capture a wider field more quickly, something smaller and faster makes more sense. Or, get both!

 

MY setup is permanent so taking everything off and switching to a refractor might end up being more work. That plus there isn't much left after getting all the parts for that CDK and camera!



#41 Jared

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Posted Yesterday, 12:41 PM

That’s what I’m mulling here. Do I buy an APO first and stick with my (uncooled) camera for a while? Or do I buy a cooled APS-C sized IMX571 based camera and stick with the C8 for a while? Will an APS-C sized sensor even work well enough with a reduced C8? I understand that even an IMX294 sensor has pretty severe vignetting in such a scope.

Then there is the overhead involved in building a DSO mosaic. I have a lot of trouble with residual gradients as it is in single panels. Seems like it would be hard to get seamless mosaics.

Building a 25 panel deep sky mosaic is actually a LOT of work. Mosaics are fairly easy on the Moon and not that bad in narrow band (as long as you have a good set of flats) since the contrast is reasonably high, so you don't have insane amounts of stretching in the shadows. For LRGB or RGB, though, it gets really hard to get a nice even look over a large number of panels. It's certainly not impossible--plenty of people have done it--but I would not consider it a viable substitute for a wider field scope. Personally, I'm comfortable with two panel or four panel mosaic with my 12" scope, but as soon as I get to nine or twelve it starts to be a whole lot of work. Even simple things like registering images becomes time consuming and error prone. Doable? Yes. Fun and easy? Definitely not.


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#42 Rouzbeh

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Posted Yesterday, 01:25 PM

Building a 25 panel deep sky mosaic is actually a LOT of work. Mosaics are fairly easy on the Moon and not that bad in narrow band (as long as you have a good set of flats) since the contrast is reasonably high, so you don't have insane amounts of stretching in the shadows. For LRGB or RGB, though, it gets really hard to get a nice even look over a large number of panels. It's certainly not impossible--plenty of people have done it--but I would not consider it a viable substitute for a wider field scope. Personally, I'm comfortable with two panel or four panel mosaic with my 12" scope, but as soon as I get to nine or twelve it starts to be a whole lot of work. Even simple things like registering images becomes time consuming and error prone. Doable? Yes. Fun and easy? Definitely not.

Good point. The combination of 0.66x and FF does offer a generous FOV.

 

I doubt Ill ever really need a very large LRGB mosaic, I worked out something like M31 would fit in 4-5 panels.

 

6 panels should cover a large narrowband target like the Heart nebula.

 

I will pass on Bernard's Loop smile.gif.

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