gregbradley
20-03-2021, 01:12 PM
I have a little bit of experience now with CMOS cameras and I know a lot are using them now. They are cheap compared to their performance and we have all seen some great example images.
But are they really the end of CCD cameras?
Given Kodak and OnSemi plus Sony have ceased CCD production that would seem inevitable. But CMOS are not without their issues, much more so than CCDs.
So far it seems to me that CCDs evolved into a mature state. Firstly they are all 16 bit whereas only a small % of CMOS are 16 bit.
Secondly CMOS can have their own peculiar noise problems CCDs don't have like rain noise (also called walking noise) where slight shifts between images end up with ripple like bands in the image.
CMOS are more prone to poor star colours and oversaturation.
Full well depth tends to be a lot weaker in CMOS although QHY offers some readout modes that will increase well depth (at the expense of read noise).
CMOS cameras tend to be a lot lighter and smaller which is easier on focusers and mounts.
Narrowband QE can be very high in some CMOS sensors.
The 1600 sensors has issues with microlense flaring on bright stars.
Drivers can be weak to poor in performance and often still being developed on cameras that are over a year old.
Reliability seems poorer judging by the number of people complaining about camera failures in short period of use.
CCDs can develop vertical bright lines (full frame CCD type), CMOS don't seem to.
CMOS suffer amp glow on almost all models except a few of the higher models. This calibrates out pretty well but not 100% perfectly and is a problem to be watched.
CMOS 2x2 binning is not as effective as CCD.
CMOS cameras are depending on Sony sensors mostly. The trend from Sony is for more megapixels and this smaller pixels along with smaller wells although this can be compensated with QHY cameras allowing different read modes offering deeper wells at the cost of higher read noise).
So the future of CMOS would appear to be smaller pixels that are less suitable for an ever larger group of telescopes and could eventually be confined to small focal length imaging. Imagine trying to image with a camera with 1.3 micron pixels? You'd have to bin it 3x3.
Looking though quite a few images now from CMOS I see plenty of nice high res CMOS images but you can immediately pick the CCD images by their usually much nicer stars with better colour and better shapes.
These small pixel large full frame sensors are very much harder to adjust for tilt and that becomes a big problem to overcome especially for those not well experienced in handling tilt.
The smaller sensors don't really give you a hard time with tilt though.
Some CMOS sensors don't calibrate well and they have extra settings to match that CCDs don't so they are more complex and harder to calibrate.
More things to match. These extra settings are not well understood and most fumble about trial and error or ask on the internet of people who also probably don't know for advice. Each setting change requires a whole new set of darks, flats, flat darks. So the calibration library can get very large.
Sensitivity, low noise and low cost are really where CMOS cameras shine.
Amp glow, white stars, complicated settings, poor drivers, extra types of noise are where they are problematic compared to CCDs.
QHY is kind of funny. They post an "all in one firmware updater" that is in fact at least 2 in 1 as it does not do the job by itself and needs a 2nd update to work. Haha, they don't say that on their site so that's a little trap for you.
The other issue is horizontal random banding which most seem to suffer from, some worse than others. It mostly shows itself in dimmer narrowband images not brighter objects. It does not calibrate out. So these lower gain settings sound good because of low read noise but you have to use higher gain settings to reduce/overcome the horizontal banding.
Out of the current crop of CMOS the no amp glow 16 bit backside illuminated Sony sensored APSc and Full frame seem the pick of the lot.
But at 3.76 microns they are already too small for 2 metres or beyond focal lengths unless you bin 2x2 or have exceptional seeing. Still good though.
CMOS seem like a longer learning curve because of the extra settings and getting to know them (the difference between some settings is almost undetectable in a single download).
Despite all this they seem to be the way of the future but need more maturing but the long term outlook is not good with future smaller and smaller pixels.
But I won't be selling my CCDs just yet. All those extra settings and weak software means less bullet proof and moreover another thing that can go wrong along with cloud, tracking and gear issues, wind etc.
The above is my opinion only from handling 3 of these CMOS cameras so far - a ZWO183mm (easy to use), QHY294m (also easy, banding is something to watch and poor drivers from QHY need to be kept updated, a QHY600m not very problematic but lots of choices and very sensitive to tilt).
Greg.
But are they really the end of CCD cameras?
Given Kodak and OnSemi plus Sony have ceased CCD production that would seem inevitable. But CMOS are not without their issues, much more so than CCDs.
So far it seems to me that CCDs evolved into a mature state. Firstly they are all 16 bit whereas only a small % of CMOS are 16 bit.
Secondly CMOS can have their own peculiar noise problems CCDs don't have like rain noise (also called walking noise) where slight shifts between images end up with ripple like bands in the image.
CMOS are more prone to poor star colours and oversaturation.
Full well depth tends to be a lot weaker in CMOS although QHY offers some readout modes that will increase well depth (at the expense of read noise).
CMOS cameras tend to be a lot lighter and smaller which is easier on focusers and mounts.
Narrowband QE can be very high in some CMOS sensors.
The 1600 sensors has issues with microlense flaring on bright stars.
Drivers can be weak to poor in performance and often still being developed on cameras that are over a year old.
Reliability seems poorer judging by the number of people complaining about camera failures in short period of use.
CCDs can develop vertical bright lines (full frame CCD type), CMOS don't seem to.
CMOS suffer amp glow on almost all models except a few of the higher models. This calibrates out pretty well but not 100% perfectly and is a problem to be watched.
CMOS 2x2 binning is not as effective as CCD.
CMOS cameras are depending on Sony sensors mostly. The trend from Sony is for more megapixels and this smaller pixels along with smaller wells although this can be compensated with QHY cameras allowing different read modes offering deeper wells at the cost of higher read noise).
So the future of CMOS would appear to be smaller pixels that are less suitable for an ever larger group of telescopes and could eventually be confined to small focal length imaging. Imagine trying to image with a camera with 1.3 micron pixels? You'd have to bin it 3x3.
Looking though quite a few images now from CMOS I see plenty of nice high res CMOS images but you can immediately pick the CCD images by their usually much nicer stars with better colour and better shapes.
These small pixel large full frame sensors are very much harder to adjust for tilt and that becomes a big problem to overcome especially for those not well experienced in handling tilt.
The smaller sensors don't really give you a hard time with tilt though.
Some CMOS sensors don't calibrate well and they have extra settings to match that CCDs don't so they are more complex and harder to calibrate.
More things to match. These extra settings are not well understood and most fumble about trial and error or ask on the internet of people who also probably don't know for advice. Each setting change requires a whole new set of darks, flats, flat darks. So the calibration library can get very large.
Sensitivity, low noise and low cost are really where CMOS cameras shine.
Amp glow, white stars, complicated settings, poor drivers, extra types of noise are where they are problematic compared to CCDs.
QHY is kind of funny. They post an "all in one firmware updater" that is in fact at least 2 in 1 as it does not do the job by itself and needs a 2nd update to work. Haha, they don't say that on their site so that's a little trap for you.
The other issue is horizontal random banding which most seem to suffer from, some worse than others. It mostly shows itself in dimmer narrowband images not brighter objects. It does not calibrate out. So these lower gain settings sound good because of low read noise but you have to use higher gain settings to reduce/overcome the horizontal banding.
Out of the current crop of CMOS the no amp glow 16 bit backside illuminated Sony sensored APSc and Full frame seem the pick of the lot.
But at 3.76 microns they are already too small for 2 metres or beyond focal lengths unless you bin 2x2 or have exceptional seeing. Still good though.
CMOS seem like a longer learning curve because of the extra settings and getting to know them (the difference between some settings is almost undetectable in a single download).
Despite all this they seem to be the way of the future but need more maturing but the long term outlook is not good with future smaller and smaller pixels.
But I won't be selling my CCDs just yet. All those extra settings and weak software means less bullet proof and moreover another thing that can go wrong along with cloud, tracking and gear issues, wind etc.
The above is my opinion only from handling 3 of these CMOS cameras so far - a ZWO183mm (easy to use), QHY294m (also easy, banding is something to watch and poor drivers from QHY need to be kept updated, a QHY600m not very problematic but lots of choices and very sensitive to tilt).
Greg.