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QHY 290M or 178M

Planet CMOS Imaging
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#1 hallzhuu

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Posted 02 December 2021 - 06:25 AM

Hello guys, 
 

I am thinking about buying a black-and-white planetary camera to use on my university's telescopes. I will mainly use a 300mmF20 refractor (a historical refrator) and perhaps from time to time on an 8 inch F10 Meade Schmidt Cassegrain one.

 

It turns out that with either of these two cameras the imaging resolution is a bit below 1 arcsecond per pixel on the 300mmF20. Would this be oversampling and cause a problem for planetary cameras? 

 

Which of these 2 cameras would you recommend? I am now a bit inclined towards the 178M as it's larger so easier to find targets and would be better if I want to do some lunar surface photography. But I heard the 290M is a very classic and good planetary camera, and ZWO has given it a higher score for planetary imaging, what are some advantages of that? 290M has higher frame rate, but would that matter? I can just shoot a bit longer for the same number of frames on the 178M? 

Screenshot-2021-12-02-113958.png

 

Many thanks for any insight! 


Edited by hallzhuu, 02 December 2021 - 06:45 AM.


#2 PiotrM

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Posted 02 December 2021 - 06:48 AM

2,4 pixels reach optimal sampling at around f/12 so 178M isn't the best choice for a f/20 scope. ASI290 is small pixels as well.


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#3 RedLionNJ

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Posted 02 December 2021 - 10:48 AM

The potential for higher frame rates lets you capture extremely brief moments of superlative seeing (if they exist).

 

If you have access to a twelve inch refractor, I would sincerely hope it's on a mount capable of putting a planet on the sensor at long focal length. So sensor size shouldn't be an issue.

 

"Resolution" as you're using it isn't terribly relevant in planetary imaging. With 12 inches of aperture and a sensible focal length, you can expect to record features as small as 0.2 arcsec.

 

Your one stumbling block is going to be the f/20 focal ratio. The 178's pixels are far too small for that, and even the 290's are marginal.  Ideally you want the focal ratio to be between 5 and 7 times the pixel size (in microns).

 

So the 178 would lead to a maximum focal ration of around 17, while the 290 leads to a maximum focal ratio of around 20. 

 

So go with the 290MM.


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#4 JMP

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Posted 02 December 2021 - 12:19 PM

I honestly think you'd be happiest with the 174, especially for lunar. With a focal length of 6,000 mm it's going to be a challenge just getting the target on the chip!
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#5 hallzhuu

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Posted 02 December 2021 - 04:25 PM

I honestly think you'd be happiest with the 174, especially for lunar. With a focal length of 6,000 mm it's going to be a challenge just getting the target on the chip!

 

I would sincerely hope it's on a mount capable of putting a planet on the sensor at long focal length. So sensor size shouldn't be an issue.

Thank for your replies! That's exactly my biggest concern so far.

 

The telescope is quite historical (200 years old now) and does not have a go-to function, so we have to manually push it to the right place and then start the tracking motor. The RA axis has an encoder telling the current RA to the nearest 0.01'' that is reasonablly accurate and for declination we can only use a graduation by eye. For visual observations these are definitely accurate enough as I have been using these days. When I move the telescope to almost exactly the right coordinate, the object I pointed it at looks to be in the middle of the field of view from my eye. But I am not so sure for imaging as I have had some slight problems of getting the planet onto the CMOS in the past with a much smaller telescope. 

 

If that does happen, is there any clever way of finding the planet or it's just luck? For example, can I do something like using my eye to centre the planet first and gradually move to shorter and shorter eyepieces and then the camera? And I guess this is a challenge to anyone using a telescope with a large aperture for planetary imaging since large aperture usually means long focal length? 

 

That being said, I am probably more inclined towards the 290 since their sensor sizes are not too different. I am not really prepared to spend on a camera that has a much larger sensor size. I guess I will probably need to try hard to get the planet onto the screen even with the 178...


Edited by hallzhuu, 02 December 2021 - 04:50 PM.



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