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To CMOS or not???


petevasey
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Hi, all,

For almost 12 years now I've been imaging with my QSI683 wsg camera.  8-way filter wheel, built in off axis guider, good sized CCD chip.  And of course getting good results.  But friends of mine are using the ASI533 one-shot colour CMOS camera, and rave about it!  Very sensitive and with the appropriate multi band filter appears to take good narrow band type images.  So I've been browsing similar cameras, and believe the ASI 294MC Pro will nicely fit the bill - almost identical chip size so no problems with flat fields, very sensitive and not a huge pixel count.  I believe there may be some amp glow, but that's easily taken care of with darks.  I'll be using Windows XP, but tests with one of the 533 cameras were successful, so hopefully the 294 will also run ok.  I'd probably try and persuade a supplier to do me a sale or return so I could confirm that all is well.

BUT...  Although a one-shot colour is very tempting - it would seem that with the high sensitivity these cameras can considerably reduce total imaging time even though binning is not possible, nevertheless will a change really be worth it?  Any comments?

Cheers,

Peter

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In the CCD era I was always an arch advocate of mono, Peter, though I did try a good quality OSC CCD.  I'm now using (though it's not mine) an ASI2600 OSC. It is very, very impressive, though it's working at F2 in a RASA. I think CMOS has changed the game (along with dual band/tri-band filters.)

Olly

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I have a camera with the aps-c sized Sony IMX571 chip in OSC version and it is top notch. Read noise of 0.86 electrons (you could cut your current read noise by 90%!), no amp glow, for all intents and purposes no thermal noise as a 600s dark at -10c has under 4 ADUs of mean and 2 ADUs of median signal over offset. Very low maintenance camera to work with in terms of calibration. As time goes on i would assume interest in CCDs will drop dramatically, so if you are planning on selling your current camera for a decent price then maybe the time is right.

16 minutes ago, petevasey said:

But friends of mine are using the ASI533 one-shot colour CMOS camera, and rave about it! 

I was definitely shocked to see the performance on mine, so don't doubt this at all. Most of the raving is well deserved IMO.

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This is something that's been exercising my brain, but  in this case the camera I've been thinking about is the QHY 410C as it is "full frame" with pixels that are a close match for my current G3 16200. Whether the 14 bit ADC will be a problem is something to think about. Had it been 16 bit I might have jumped already.

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Maybe another thing to consider is the huge file sizes that cmos produce,  can your xp cope?  Friend of mine had a Gigabyte of data on one target.. lots of people I know use computers with terabytes of storage... ccd don't use no where near that amount ... 

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The new CMOS sensors are excellent. As Olly says, the ASI2600MC in particular is ace, especially combined with a dual-band filter like the Optolong L-eXtreme. Lots of examples on my website here. newbie alert does raise a good point though,  that you need a lot of harddrive space. Fortunately that's not too expensive these days.

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true storage is cheap, but you would be courageous to put all that time and effort onto a single hard disk... mirroring help, but its not a backup, so you can be looking at maintaining 2-3 copies of that data... which does start to add up.

That cmos is optimised for many short exposures vs ccd, is a step change almost as significant as the move from film to ccd. all those techniques used by the planetary astrophotographers start to be come very practical for deep space, loose an image to a shortcoming in your mount, a satellite, a patch of particularly poor seeing and you can toss the image and just loose a few minutes of exposure.... rather than the 10's of minutes of ccd. And this is going to feed into the data storage requirements, just store the best% of your images and toss the rest.

Speaking personally, as some one just getting into the subject, I would not consider getting a ccd new or secondhand at any price, simply because of the cost of purchasing the necessary mount to get the best out of it. And for that dream setup I am thinking cmos lets me take money out of the mount and put it into improving field of view/acquisition times...

if i already had a ccd, if i could sell it for decent money i would do and move to cmos, failing that I would keep with ccd until there was a compelling cmos sensor on the market or i got annoyed with satellites, or short weather  windows making complete image collection a painfully slow process... 

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To be honest, I use an ASI Air Pro, with a ZWO 071 Pro camera, and each fits image is around 30 Mb, so if I take 30 sec subs for 4 hours that would be around 15 GB, and that is stored on a thumb drive, which you can easily transfer to a computer. You can always back the files up online if you are worried about hard drives failing although there may be a small monthly fee for that with Google or OneDrive.

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It’s a valid comment on the mount with the latest CMOS cameras but I wouldn’t go to the extreme and throw all your budget at the camera and scope at the expense of the mount. If your mount isn’t rated for the load you are putting on it, you will be throwing a lot of your subs away, even with short exposure CMOS imaging.

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3 minutes ago, tomato said:

It’s a valid comment on the mount with the latest CMOS cameras but I wouldn’t go to the extreme and throw all your budget at the camera and scope at the expense of the mount. If your mount isn’t rated for the load you are putting on it, you will be throwing a lot of your subs away, even with short exposure CMOS imaging.

Took the words right out of my mouth. I am using the short exposures route with a weak mount: Skywatcher EQM35-PRO with 9kg payload. Depending on weather conditions of course but usually i reject around 25% of my frames taken with guided 30s exposures. Thing is, with CCD and a requirement for several minutes of exposure i would never get a single usable sub. So while its a bad idea, its not an impossible idea.

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2 hours ago, dmki said:

true storage is cheap, but you would be courageous to put all that time and effort onto a single hard disk... mirroring help, but its not a backup, so you can be looking at maintaining 2-3 copies of that data... which does start to add up.

That cmos is optimised for many short exposures vs ccd, is a step change almost as significant as the move from film to ccd. all those techniques used by the planetary astrophotographers start to be come very practical for deep space, loose an image to a shortcoming in your mount, a satellite, a patch of particularly poor seeing and you can toss the image and just loose a few minutes of exposure.... rather than the 10's of minutes of ccd. And this is going to feed into the data storage requirements, just store the best% of your images and toss the rest.

Speaking personally, as some one just getting into the subject, I would not consider getting a ccd new or secondhand at any price, simply because of the cost of purchasing the necessary mount to get the best out of it. And for that dream setup I am thinking cmos lets me take money out of the mount and put it into improving field of view/acquisition times...

if i already had a ccd, if i could sell it for decent money i would do and move to cmos, failing that I would keep with ccd until there was a compelling cmos sensor on the market or i got annoyed with satellites, or short weather  windows making complete image collection a painfully slow process... 

Laughable...This is nonsense^

You can make any mount work for a ccd, if it doesn't then it's not very good in the first place or you're too lazy to get PA or phd working for you or against you 

Stacking programs take satellite or planes going through a exposure out, again if you have them set up right..

I can quite happily do 1 min  unguided subs and produce a decent image with a ccd

 

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I certainly think @dmki has a point, not laughable at all. With CMOS sensors favoring more, shorter subs, clearly demands on the mount are less. Of course you can get a good picture with 1 minute subs with a modest mount and any camera, but when you are in order of 5 or 10 minute subs, the mount requirements are unforgiving.

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Money spent on a premium mount is never wasted.

You still need to give enough exposure to get your sky background above the noise floor, and for those of us living in reasonably dark (SQI 21.5 or better) areas that still means longish exposures.

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2 hours ago, Ags said:

I certainly think @dmki has a point, not laughable at all. With CMOS sensors favoring more, shorter subs, clearly demands on the mount are less. Of course you can get a good picture with 1 minute subs with a modest mount and any camera, but when you are in order of 5 or 10 minute subs, the mount requirements are unforgiving.

Depends what you're used to.. a 5-10 min sub isn't long .. 1-3min is short.. and regardless of your setup I think you should be able to do that with any mount.. many people using 100lb class mounts aren't putting massive scopes on them and then doing 1 min subs.. now that's a way to waste a few grand..

Logic says they haven't sold them to buy the cheapest mount when they switched to cmos

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On 30/10/2021 at 18:48, petevasey said:

So I've been browsing similar cameras, and believe the ASI 294MC Pro will nicely fit the bill - almost identical chip size so no problems with flat fields, very sensitive and not a huge pixel count.  I believe there may be some amp glow, but that's easily taken care of with darks.

You might want to do more homework before deciding on this camera. I've seen discussions where people have had problems with image calibration. Especially flats in combination with NB filters seem tricky for this camera. Even I have had problems with flats and the ASI294MM, although that may have been due to edge reflections from my 31 mm RGB filters. Nevertheless, I find my stacked images never as clean as they were when I used the ASI174MM, and that is not just because the ASI174MM has a smaller sensor. The images are good and I can go deep, but the ASI174MM produced cleaner images, imo.

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3 hours ago, DaveS said:

Money spent on a premium mount is never wasted.

You still need to give enough exposure to get your sky background above the noise floor, and for those of us living in reasonably dark (SQI 21.5 or better) areas that still means longish exposures.

 

sky background,  is that the stuff that lets me walk around my local park,  a mile or more from the nearest lamp, any time of night unaided? :)

thanks for the heads up about the 294, its on my list of sensors to consider.... when i feel ive learnt enough to do AP properly and what the implications of moving from a bortel8 can see where i am going by skylight day or night to a bortel10+ city are going to be.

 

 

@newbie alertThis is yes you can take loads of short exposures with ccd, the question is should you?
ccd's favour long exposures because the cells accumulate very little noise vs cmos.... but  make up for that with much higher read noise.
by taking those short exposures with the ccd you are accumulating an awful lot of read noise in the stacked image.

and as for satellites; those businessmen, as forum rules prohibit the language would normally use, have only just started deploying their low earth orbit satellite comms constellations and i fear that soon, a great deal of astronomy is going to be impacted in an attempt to make a few pennies for a very small number of people.

Edited by dmki
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4 hours ago, tooth_dr said:

I sometimes feel like I wasted my money on a great mount as now all I need are 2 - 5 minute subs since moving to CMOS.

 

If you decide to put your Mesu up for sale to buy a couple more  CMOS cameras and a cheap and cheerful mount to go with them,  I don’t think it will be on the classifieds for long…

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40 minutes ago, tomato said:

If you decide to put your Mesu up for sale to buy a couple more  CMOS cameras and a cheap and cheerful mount to go with them,  I don’t think it will be on the classifieds for long…

I would take the best of two worlds any time: CMOS and Mesu😆 Happy to have sold my CCDs before the market crashed.

Edited by gorann
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2 hours ago, wimvb said:

You might want to do more homework before deciding on this camera. I've seen discussions where people have had problems with image calibration. Especially flats in combination with NB filters seem tricky for this camera. Even I have had problems with flats and the ASI294MM, although that may have been due to edge reflections from my 31 mm RGB filters. Nevertheless, I find my stacked images never as clean as they were when I used the ASI174MM, and that is not just because the ASI174MM has a smaller sensor. The images are good and I can go deep, but the ASI174MM produced cleaner images, imo.

I think I may have confused the issue slightly with my wording when I said 'no problem with flat fields'.  I probably should have said 'no problem with flat fields of view'  i.e. at the moment with my existing optical setups I get round stars right into the corners with the 8300 chip.  So with the 294 chip being almost identical in size, it's a no brainer.    I suspect some of your problems may be with using 31 mm filters.  Although my QSI uses that size, the built in filter wheel puts them very close to the sensor.  If I get a 294 and want to use a multi band filter it will be a 2" one.

Because I have decently dark skies at home and a good mount, I will probably still go for reasonably long exposures - perhaps 5 minutes or longer, so I won't be getting multi gigabytes of data, and anyway the 294 chip pixel count isn't excessively large at 11.7 MB.  And of course being one-shot colour, I don't have to store RGB subs as well 😉

Thanks to everyone for responding to this thread - lots of food for thought!

Cheers,

Peter

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1 hour ago, tomato said:

If you decide to put your Mesu up for sale to buy a couple more  CMOS cameras and a cheap and cheerful mount to go with them,  I don’t think it will be on the classifieds for long…

Tongue in cheek :hello2: short subs might work but not the 30 odd kg might be an issue.  Another happy mesu (and CMOS) user here

Edited by tooth_dr
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9 hours ago, dmki said:

 

sky background,  is that the stuff that lets me walk around my local park,  a mile or more from the nearest lamp, any time of night unaided? :)

thanks for the heads up about the 294, its on my list of sensors to consider.... when i feel ive learnt enough to do AP properly and what the implications of moving from a bortel8 can see where i am going by skylight day or night to a bortel10+ city are going to be.

 

 

@newbie alertThis is yes you can take loads of short exposures with ccd, the question is should you?
ccd's favour long exposures because the cells accumulate very little noise vs cmos.... but  make up for that with much higher read noise.
by taking those short exposures with the ccd you are accumulating an awful lot of read noise in the stacked image.

and as for satellites; those businessmen, as forum rules prohibit the language would normally use, have only just started deploying their low earth orbit satellite comms constellations and i fear that soon, a great deal of astronomy is going to be impacted in an attempt to make a few pennies for a very small number of people.

Sorry but the major selling point of a cmos is the read noise thing.. if I start to see noise in my images then maybe I switch,  but as it stands I don't,clean as a whistle and I don't use any calibration frames,  just lights( signal)

I agree that Mr musk and friends are complete idiots and put profit over anything else.. but I'm talking about here and now, not in years to come,who knows what's going to happen.. They don't make much impact on me atm, the only thing that makes me throw subs away is cloud and wind.. in a few years possibly but if it affects me in my backyard then it's going to affect those that are doing the science on the mountain tops.. if they can't do their job then it will have major problems.. 

So carry on wondering around your park at night, thinking about selling your mount for a low cost version and panicking about Mr musk and friends...

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Hi Peter,

When I got back into imaging this time last year I bought the ASI533 and I think it is a very good camera compared to my 583WSG from years ago. If you want, I can upload a bunch of files for you to look over? I actually did a little test last night using pixinsight where I combined 80min of exposures on M31 of 30s, 60s and 90s to see if I could see any noticeable difference. There wasn't any other than slight differences in colour as I'd imaged over different nights and LP was varying due to neighbours' lights etc. CMOS really are awesome sernsors that are only getting better and better.

Sure you end up with lots and lots of data and it takes longer to crunch that data and more space to store it. However, I'd rather be sitting inside letting my PC crunch all those numbers knowing I didn't miss a single sub because they were all 30s long, than binning a number of my 20min subs from the 583 due to wind or guiding issues etc.

Phil

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