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Help me understand pixel size and choices?

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#1 mayhem13

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Posted 10 June 2021 - 06:03 AM

So I’m in the market for a new camera for planetary and of course stumbled across again the venerable ASI224mc........and then noticed the pixel size of 3.75..........my Canon T8i is 3.72. Considering such natively small images with planets, is there any benefit to the 224 over my Canon other than the wasted larger sensor pixels that go unused? Maybe a mono camera or smaller pixel size would be the better upgrade?

 

As a disclaimer, I’m currently using a 12” Dob and capturing with a 3x Barlow drifting. I don’t have any plans for a large SCT and tracking mount but a AT130edt is in the works for DSOs.



#2 james7ca

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Posted 10 June 2021 - 06:46 AM

What frame rate can you get with your Canon? Also, what frame size do you use with the Canon for planetary work?

 

One potential advantage with the 224MC is that it might offer higher frame rates (probably over 100 fps). For planetary imaging you probably want to capture several thousand frames for each sequence which is possible with a dedicated, CMOS astro camera. But, if you don't have a tracking mount then you will be limited by how quickly the planet/moon will drift out of your field of view. Roughly speaking, stars and planets move at about 15 arc seconds per second so it you calculate your field of view (with your 224MC) then you will be able to determine the longest single sequence that you will be able to capture before you have to reposition your Dob.


Edited by james7ca, 10 June 2021 - 06:48 AM.

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#3 Tulloch

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Posted 10 June 2021 - 07:04 AM

Don't worry about pixel size, as long as you are imaging at a focal ratio around 5x the pixel size of the camera, pixel size doesn't matter.

 

My own experience in imaging the planets has been with the Canon 700D in 5x zoom LiveView mode and the ASI224MC. On a small 6" SCT, the difference between the two was minor,

https://www.cloudyni...6-sct-test-two/

 

...however on my larger 9.25" SCT the difference was larger (although the seeing may also have had something to do with this difference).

https://www.cloudyni...-sct-test-four/

 

However, if you plan to use drift mode on the planets with either of these cameras at the focal length you are planning (12" @ f/15 = 4500mm), the planets will race across both the DSLR sensor at 5x zoom and the ASI224MC, making it difficult to accurately focus and grab enough frames for a good final result.

 

You don't need an SCT for high quality planetary images, some of the best images posted here are from Newt/Dob designs, but you do need to be able to track the planets for best results. I know you don't want to hear this, but a new camera is not what you need, what you need is a Goto mount for your 12" OTA to track the planets. You will then be able to use your Canon in 5x LiveView mode and get a good result.

 

Andrew


Edited by Tulloch, 10 June 2021 - 09:00 PM.


#4 RedLionNJ

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Posted 10 June 2021 - 09:40 AM

So I’m in the market for a new camera for planetary and of course stumbled across again the venerable ASI224mc........and then noticed the pixel size of 3.75..........my Canon T8i is 3.72. Considering such natively small images with planets, is there any benefit to the 224 over my Canon other than the wasted larger sensor pixels that go unused? Maybe a mono camera or smaller pixel size would be the better upgrade?

 

As a disclaimer, I’m currently using a 12” Dob and capturing with a 3x Barlow drifting. I don’t have any plans for a large SCT and tracking mount but a AT130edt is in the works for DSOs.

Best case, let's assume you can get something like 25fps out of the T8i.  And it maybe takes a target 10 seconds to cross the field at f/15.

 

With the 224MC, you'll still have color but will be able to get at least four times the fps - let's say 100fps. But you also have a sensor which is about a third of the APS-C width. So you'll only be able to capture 1/3 more frames with the 224MC without moving the 'scope.

 

As Andrew indicates, you're limited more by lack of tracking ability than anything else. Could you maybe consider a home-made Poncet table, or some such device?  My goal would be to be able to capture a few thousand frames before the target leaves the field.

 

Sorry!

 

Grant



#5 james7ca

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Posted 10 June 2021 - 10:55 AM

Best case, let's assume you can get something like 25fps out of the T8i.  And it maybe takes a target 10 seconds to cross the field at f/15.

 

With the 224MC, you'll still have color but will be able to get at least four times the fps - let's say 100fps. But you also have a sensor which is about a third of the APS-C width. So you'll only be able to capture 1/3 more frames with the 224MC without moving the 'scope....

...Grant

I don't think that the OP will get 25fps on the Canon over the full APS-C frame and pixel resolution. That camera's maximum burst is 7fps and I doubt that it could sustain that for more than a few of seconds. So, that means either capturing in video at a resampled, lower resolution (not a good idea) or restricting the frame size to something much smaller than APS-C.

 

I suspect the OP is using some form of crop-mode, liveview capture that should give better than 25fps but over a much smaller field. This is one of the reason's I asked about the OP's current frame rate and frame size. Knowing the later will make it easier to determine whether there would be any real advantage to going to the 224MC. Plus, knowing the OP's current capture technique will tell us whether there is still room for improvement when using the Canon (let's hope the OP is not using burst mode still capture or trying to capture at full frame or using video compression).

 

I suspect that the 224MC will give somewhat better results, but the limiting factor (as in how much better, if much at all) will be determined by how long the target will stay within the field of view. But, yes, the lack of tracking is going to be a major complication.


Edited by james7ca, 10 June 2021 - 11:43 AM.


#6 Tulloch

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Posted 10 June 2021 - 04:28 PM

I don't think that the OP will get 25fps on the Canon over the full APS-C frame and pixel resolution. That camera's maximum burst is 7fps and I doubt that it could sustain that for more than a few of seconds. So, that means either capturing in video at a resampled, lower resolution (not a good idea) or restricting the frame size to something much smaller than APS-C.

 

I suspect the OP is using some form of crop-mode, liveview capture that should give better than 25fps but over a much smaller field. This is one of the reason's I asked about the OP's current frame rate and frame size. Knowing the later will make it easier to determine whether there would be any real advantage to going to the 224MC. Plus, knowing the OP's current capture technique will tell us whether there is still room for improvement when using the Canon (let's hope the OP is not using burst mode still capture or trying to capture at full frame or using video compression).

 

I suspect that the 224MC will give somewhat better results, but the limiting factor (as in how much better, if much at all) will be determined by how long the target will stay within the field of view. But, yes, the lack of tracking is going to be a major complication.

You cannot get 25 fps from the Canon over the full frame, but you might be able to get it that high by capturing the LiveView stream at 5x zoom which is the best way to use a Canon DSLR for the planets. More details here.

https://www.astropix...resolution.html

 

I was only able to transfer the stream to my computer at around 20 fps with my Canon 700D at 5x Live View, even though Live View captures at 30 fps.


Edited by Tulloch, 10 June 2021 - 08:59 PM.

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#7 kevinbreen

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Posted 10 June 2021 - 05:18 PM

So that's it then, eh, it's the frame rate? I've an EOS 600d (t3i in US). Are you saying that, if the framerate on the Canon were comparable with the 224MC, there would be no difference?
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#8 Tulloch

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Posted 10 June 2021 - 07:28 PM

So that's it then, eh, it's the frame rate? I've an EOS 600d (t3i in US). Are you saying that, if the framerate on the Canon were comparable with the 224MC, there would be no difference?

No, there are other differences such the QE value (80% for the ASI224MC and around 40% for the Canons), and the fast shutter speed available to the ASI allows you to "freeze" some of the seeing caused by wind causing vibrations on the OTA (1/200 vs 1/30), however the DSLR has a "proper" white balance and better colour discrimination to reduce colour bleed across pixels.

 

Here is a comparison of the noise levels for the 700D vs the ASI224MC and how many frames you need to stack. 

https://www.cloudyni...sct-test-three/



#9 mayhem13

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Posted 11 June 2021 - 06:18 AM

Thanks for all the very helpful replies!......given my current method and lack of tracking abilities, it’s nice to know there’s not much advantage to a new camera at this time. I’ll keep doing what I’m doing until we move later next year.......that solution can include a dedicated pier and whatever Mount is required. With my current backyard situation, my neighbors trees present an ever growing problem! Lol

 

I do have a Celestron C5 though and never considered that for planetary.....with an EQ mount, could I produce better images than with the 12” dob? Focal length on that is 1250mm......I just thought the aperture was too small?



#10 Tulloch

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Posted 11 June 2021 - 07:45 AM

Thanks for all the very helpful replies!......given my current method and lack of tracking abilities, it’s nice to know there’s not much advantage to a new camera at this time. I’ll keep doing what I’m doing until we move later next year.......that solution can include a dedicated pier and whatever Mount is required. With my current backyard situation, my neighbors trees present an ever growing problem! Lol

 

I do have a Celestron C5 though and never considered that for planetary.....with an EQ mount, could I produce better images than with the 12” dob? Focal length on that is 1250mm......I just thought the aperture was too small?

Hmmm, good question. In my earlier post, I gave links to what I was able to achieve using a 6" and 9.25" with tracking on both. The 5" could produce an image similar to my 6", the 12" should give better resolution than the 9.25", assuming the seeing and elevation are similar. The difference between a 5" and a 12" would be even greater, however would an untracked 12" produce better images than a tracked 5"? Most probably, but the process would be so difficult it might not be worth it. 

 

However, I would recommend starting with the tracking 5" - you will find getting focus much easier (as the planet stays on the screen), capturing 3 - 5 minutes of continuous planetary imaging and stacking them will be so much easier, and getting a good result might be enough to learn the process. Getting a good result with the 12" will be better than the 5", but the experience in using the 5 will assist you with the larger aperture.

 

One possible solution might actually be a new camera, something like the ZWO183MC. This is a large sensor camera (similar to the Canon) but with smaller pixels (2.4 microns) and would allow you to get a lot more frames as the planet drifted across the sensor than the 224. It's twice the price of the 224, and you will still have the problem of not being able to track the planet, but it might be a solution. 

https://astronomy-im.../asi183mc-color

 

I'd probably still recommend a tracking mount (try the classifieds area) over the 183MC, but it's an option. By the way, how are you currently capturing your images (full frame video?), and can you share some images you have made?

 

Andrew



#11 Tulloch

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Posted 11 June 2021 - 08:04 AM

P.S. If you are handy and like a project, here's someone who converted a 12" Zhumell Dob to a Goto mount...

https://www.youtube....h?v=Cd4J3zLSb9E


Edited by Tulloch, 11 June 2021 - 08:04 AM.


#12 mayhem13

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Posted 11 June 2021 - 12:38 PM

Hmmm, good question. In my earlier post, I gave links to what I was able to achieve using a 6" and 9.25" with tracking on both. The 5" could produce an image similar to my 6", the 12" should give better resolution than the 9.25", assuming the seeing and elevation are similar. The difference between a 5" and a 12" would be even greater, however would an untracked 12" produce better images than a tracked 5"? Most probably, but the process would be so difficult it might not be worth it. 

 

However, I would recommend starting with the tracking 5" - you will find getting focus much easier (as the planet stays on the screen), capturing 3 - 5 minutes of continuous planetary imaging and stacking them will be so much easier, and getting a good result might be enough to learn the process. Getting a good result with the 12" will be better than the 5", but the experience in using the 5 will assist you with the larger aperture.

 

One possible solution might actually be a new camera, something like the ZWO183MC. This is a large sensor camera (similar to the Canon) but with smaller pixels (2.4 microns) and would allow you to get a lot more frames as the planet drifted across the sensor than the 224. It's twice the price of the 224, and you will still have the problem of not being able to track the planet, but it might be a solution. 

https://astronomy-im.../asi183mc-color

 

I'd probably still recommend a tracking mount (try the classifieds area) over the 183MC, but it's an option. By the way, how are you currently capturing your images (full frame video?), and can you share some images you have made?

 

Andrew

 

Thanks Andew. I use APT and capture livestream as JPEGs using the planetary tool. When the EQ6R comes in, I’ll give it a go with the C5. I have to admit that even though the aperture is limited, the optics on it are fantastic.....moon pic clarity at native 1250 and reduced to 780 are outstanding......better than my AT72ed2 I dare say.

 

I am very handy BTW, and have the aluminum stock ready to build an EQ platform for the Dob.....just gotta find the time. There’s also plans for over the winter to build my own 8” F5 imaging newtonian. My thinking there is lightweight for the EQ6. I’m pretty happy with reflectors as I’ve gotten pretty good with my collimation skills although at some point in time, a Tak140 to rule them all is where I hope to land.



#13 Tulloch

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Posted 11 June 2021 - 07:00 PM

Thanks Andew. I use APT and capture livestream as JPEGs using the planetary tool. When the EQ6R comes in, I’ll give it a go with the C5. I have to admit that even though the aperture is limited, the optics on it are fantastic.....moon pic clarity at native 1250 and reduced to 780 are outstanding......better than my AT72ed2 I dare say.

Oh - I just realised that your C5 isn't on a goto mount, sorry about that, I assumed it was.

 

I am very handy BTW, and have the aluminum stock ready to build an EQ platform for the Dob.....just gotta find the time.

Don't we all ... lol.gif. If you can, try to get tracking onto the Dob mount, it would become a real planet killer. Chistophe Pellier has a 12" tracking Dob which he uses on the planets, have a look here.

https://www.planetar...azimuth-dobson/

 

Good luck!


Edited by Tulloch, 11 June 2021 - 07:01 PM.



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