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CMOS v CCD for photometry?

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Tim Haymes's picture
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CMOS v CCD for photometry?

Im using a CMOS camera (QHY174m-GPS) for photometry (or would like to..).  This is 12 bit scaled to 16bit.  I did some time series photometry on PQ And and used this data in the HOPS software for exoplanet transits. (Great software i might add, since i dont have any data on a transit yet). Now the software does not accept many of the stars and reports they are saturated.  I did check for saturation before imaging. Maybe i was mislead.

So my question is a general one:  Is anyone using CMOS for successful photometric work?   Any experiences welcome

- Tim

nickjames's picture
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I don't see any reason why

I don't see any reason why CMOS should be any different to CCD if you are careful not to saturate either the individual subs or the final stack and the images are calibrated accurately. Due to much lower read noise and much faster readout times CMOS tends to be used with shorter exposures so the smaller well depth and reduced bit depth (12 or 14 bits rather than 16 bits) are not really relevant. If the standard deviation of the noise in each image exceeds a few LSBs than the quantization is not relevant anyway whichever technology you are using. A key thing if you stack images before measuring them is to use mean rather than sum and ouput the resulting FITS as float rather than int to avoid saturation and quantization effects.

CMOS and CCD have the same problems as far as calibration is concerned (flats, darks etc) and in terms of aligning wavelength sensitivity with standards (filters etc) but otherwise they are pretty similar. CMOS has many other advantages so it won't be long before CCD detectors are only available for very specialist applications.

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I've used both a CCD

I've used both a CCD (Atik414ex) and CMOS (ASI1600MM) with equal success. The Atik is a little more sensitive due to it's larger pixels but the quality of the photometry was similar.

andrew.j.smith1905's picture
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Scaled darks

It is best not to use scaled darks with CMOS cameras as there is some evidence that amp glow is non linear although consistent. 

Regards Andrew 

David Boyd's picture
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Scaled darks

This could be an issue in spectroscopy as the widely used ISIS software uses the scaled darks technique. 

Do you have a reference for the evidence of non linear amp glow?

David

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I will see if I can find it

I will see if I can find it again.

Regards Andrew 

Tim Haymes's picture
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Thankyou

Thanks Nick/All - this is encouraging. I will continue investigations with CMOS.  My test images were not taken under best conditions. I hope the image above of PQ And (one of many) illustrates the image profile im using to learn HOPS.  Not ideal but it works. It isnt saturated according to AstoArt

Tim

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I use a ZWO ASI 1600 pro Mono

I use a ZWO ASI 1600 pro Mono CMOS camera for photometry this is a 12 bit sensor.

I've observed 7 exo-planets - as part of EXO-Clock without any problems - I tend to use darks that are the length of the exposure but the HOPS software will scale them anyway to create darks for the flats.

The sensor I'm using is very linear - as far as I can tell - this is looking at the peak pixel in a 100 pixel radius near the centre of the field.

I get similar results on star tests.

I've also noticed if you look at integrated counts - as photometry software does - and use the right apature then the integrated counts remain linear even when some of the pixels are saturated - however I'm told to stay within the linearity of the chip - so I do - I'm a beginner when it comes to photometry so perhaps someone can tell me why this is so...

Andy Wilson's picture
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Linearity and Saturation

Hi Simon,

The important point is once a pixel is saturated it won't count any more photons. It is possible that if a tiny number pixels are just above saturation or in the non-linear region then that non-linearity won't be obvious. There will be lots of pixels contributing. However, it is a risk and you would not be able to justify your result if you were trying to extract a reliable magnitude.

Looking at your graphs, they show a slightly different behaviour in the linear region. It appears to be a steeper slop, followed by a little bit of a wiggle.

If saturation is a problem as you want longer exposures then you can slightly defocus the star. This will spread the photons over more pixels while keeping them within the linear region.

Cheers,

Andy

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RE? Looking at your graphs,

RE? Looking at your graphs, they show a slightly different behaviour in the linear region. It appears to be a steeper slop, followed by a little bit of a wiggle.

I think this is just environmental conditions rather than something inherent in the sensor - I may be wrong -  it was a quick test with pre-existing data, little thought was taken into what star to choose - I don't think they were even calibrated.…

Andy Wilson's picture
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Linearity in graphs

We might be talking about the same thing, but in case not.

You would expect the graph to change once you are outside the linearly region and then into saturation. Once a star becomes saturated you are not detecting all the photons that land on the chip.

Cheers,

Andy

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So my point was that the blue

So my point was that the blue dots represent the peak pixel value, these you can see start to saturate and longer exposures don't increase the peak value linearly - the orange dots are the sum of all the values in the central apature, when you sum these up, the result appears to continue to be linear. 

My point really is that whilst the advice is 'stay within the linear range of the sensor' - which I do do - for my sensor at least - the actual practical process of counting the electrons is done not on single pixels but on a defined area of pixels and when this is taken into account the linear range seems to extend well beyond that of the linear range of individual pixels, and if this is the case it might have some practical application.

BTW I'm not really disagreeing just pointing out that it seems interesting... :-)

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Amp glow in CMOS

Here is a comment from ZWO https://astronomy-imaging-camera.com/tutorials/what-is-amp-glow.html

"Glows may not “grow” with time the same way dark current itself does, and may accelerate over time becoming brighter faster than dark current itself as exposures become longer."

Although I could not find my initial source I found this analysis which is similar https://jonrista.com/the-astrophotographers-guide/the-zwo-asi1600-guide/...

Just to be clear I am not saying the ASI1600MM is not linear. Like Simon Dawes I find mine is very linear but I use darks with the same exposure as the lights and I don't use bias frames.

Regards Andrew

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ISIS and scaled darks

ISIS only scales the darks if the exposures are mismatched so as long as you used matched exposures it's not a problem.

Regards Andrew

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Scaling darks

Is there something special about spectroscopy that would mean that scaling darks was a normal thing to do? Unlike flats, darks are really easy to obtain so I don't really understand why you wouldn't just take a set of darks corresponding to the exposure/temperature that you use for images. 

Regarding amp glow I have a ZWO ASI294MC which uses the Sony IMX294. The amp glow in darks is a bit spectacular (see attached for a 120s example at -10C) but it calibrates out completely. I rarely go beyond 120s since stacking algorithms work much better if they have lots of frames to stack and the read-out noise and dead time of these cameras is negligible.

Robin Leadbeater's picture
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CCD v CMOS for spectroscopy

Christian Buil has an interesting  CCD/CMOS comparison. Though specifically concentrating on spectroscopy,he covers camera noise, amp glow  and linearity 

http://www.astrosurf.com/buil/CMOSvsCCD/index.html

With high resolution spectroscopy of faint objects at the limit of detectability the sky signal is insignificant so camera noise becomes the most significant source of noise. To minimise read noise contribution, exposure lengths used with CCD are typically  >1200sec and it takes many hours to get a sufficiently noise free stack of darks, though  cloudy nights can be used of course!

Although low, the read noise in CMOS is not insignificant and once the typical slit width relative to pixel size is considered, is not that different from that of good CCD because of the ability to bin pixels with CCD  so there is not much latitude for combining shorter exposures with CMOS (though an spectrograph optimised specifically for the small pixels found in common CMOS sensors could perhaps avoid this).  The thermal noise is also typically higher for CMOS cameras which need much lower temperatures to match that of a CCD so although the performance gap is not large these days, CCD still appears to have the edge in performance (Though not in cost per area) when used with currently available commercial spectrographs.

Cheers

Robin

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Scaled darks in spectroscopy

Thanks Andrew, these are useful references.

When Christian Buil wrote ISIS he recognised that once you subtracted the bias or offset signal, the remaining dark current in CCD cameras scaled linearly with exposure time. A dark frame taken with the longest exposure time could therefore be scaled to any other exposure time required. Hot pixels behave differently and are dealt with separately in ISIS. Exposures in spectroscopy can vary from 10 sec on a bright reference star to 600 sec or more on a faint target, so a set of darks at every exposure time with sufficient images per set to reduce statistical noise could potentially take hours to record. The approach taken in ISIS was a pragmatic solution. With temperature regulated cameras such a dark frame could have a relatively long useful lifetime.

With CMOS cameras having low readout noise per exposure and nonlinear dark frames, this approach may need to change.

David

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CMOS and spectroscopy

David, I would propose with CMOS the best strategy is to pick a range of exposures and stick to them. Then given the low read noise just add more exposures as needed. 3 or 4 should be than enough say 1, 10, 100, .... They also tend to be USB 3 with fast download so you don't  lose much time between frames.

Regards Andrew 

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I just looked at my master

I just looked at my master darks from the ZWO ASI 1600MM Pro all cooled to -10C. I took measurements from each corner and the centre of the sensor for each master dark in my library and the amp glow looks linear with exposure time to me...

This is a very new field for me so there might be something wrong with my approach or analysis.

 

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Non-linearity

I'd actually say that (certainly for the blue curve) that the dark current is not linear in that plot - a proper analysis would show how close to a straight line it is. A few hundred ADU out may introduce a reasonable systematic error. 

Anecdotally, using my QHY163M (same chip as the 1600) for deep sky imaging I've found that scaling (and even allowing Pixinsight to try "optimizing" darks) just doesn't work properly. If you create a dark library for a few different exposures  (10/20/30/45/60/90/120/180/300) then they can be reused for quite a while before needing rebuilding - just make sure the offset and gain are the same as for the lights.

A useful activity for a cloudy night or two...

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ZWO ASI 1600 MM

This camera has low amp glow by CMOS standard so it looks like your ok. I have had discussions on Stargazers Lounge with other users of this camera where we got differing results so there may be camera to camera variations. Always best to check the performance of your camera as it is not difficult. 

Regards Andrew