Speaking as a volunteer who has taught both CCD courses as part of CHOICE: I think it is time for those who are experienced CMOS photometrists to start sharing informed information among themselves about “best practices” for using CMOS cameras for photometry. Perhaps Headquarters knows who these individuals are and can call for a committee of the willing to share information.
A manual or at least a “facts sheet” should be generated with very specific protocols directly related to CMOS photometry similar to those the information now existing for CCD and DSLR sensors. It should be authoritative regarding the kinds of CMOS cameras that are suitable (mono, obviously: pixel size, minimum bit depth, etc.), specifics on topics such as gain setting, well depth and etc., calibration and other topics that might set CMOS cameras apart from CCD cameras. Having Googled the topic, I know there is a lot of information out there, but how much of it is informed? Not knowing much, if anything, about critical use of CMOS cameras for photometry, I cannot judge.
With this information AAVSO can either (1) contemplate a separate CHOICE course for CMOS photometry (restricted to suitable CMOS cameras) or (2) give the CHOICE CCD instructors the tools to help any CMOS photometrist to get through the CCD courses.
This call for action is somewhat self-serving. I was not able to provide the kind of advice to a recent CMOS-camera participant in CCDI that I would have liked to provide, simply because I don’t use a CMOS camera.
IMO NABG CCD cameras are optimal for photometry at our level. But I know that ABG CCD, DSLR, and CMOS cameras have been demonstrated to be effective photometry cameras. We cover NABG and ABG CCDs in the CCD manual, we have a DSLR manual. It's time for a CMOS manual or at least an appendix to the CCD manual covering whatever differences there are between CCD and CMOS cameras.
Ed Wiley (WEY)
My impression is there is some confusion in this subject. CMOS is just a general micro-electronics technology used in most of electronics devices at time being. CCD is another much older electronics technology used in the 70's and that has remained only in image sensors years later. The principle of light sensing is the same for both, same physics, same basic principles. CMOS have been adopted in 100% of DSLR, DSLR are also CMOS cameras... And now there are science cameras that are CMOS equipped. The reason was their new functionalities, impossible with CCD, and fast improving performance for usual photography, the barrier being the huge capital investment needed to develop and industrialize such large silicon chips.
What do you mean by CMOS camera ? being so specific that new practices are needed ?
The new functions possible in CMOS seems to me not very of use for us: fast reading, selective reading... that's autofocus, face recognition, fast 4K video, any... not very interesting for us, in DSLR photometry we don't use it.
What is new and interesting in CMOS ? First the massive parallel reading, it permits fast reading with low read noise and (important) without much power dissipation resulting in low temperature increase and low dark currents. Then the reading mode of the photodiode is very linear. The well depth is in fact high if you calculate it per micron-square of sensor: about three times larger than CCD ! But ok this just a difference of some parameters level, high or low, that doesn't change the principles.
What you call "CMOS cameras" are probably a specific class of products often based on ICX or IMX sensors (designed for survey, industrial, security... ) They first existed based on CCD-like sensors (ICX) and were used in astronomy for planetary imaging applying the "lucky imaging" technique. Now they are moving to CMOS, the fast reading of such sensor at low power (low temperature) enabling new lucky imaging performance. This is at the point the technique is now extended to bright deep sky objects.
What to do in photometry with such small cameras ? I don't think the lucky imaging (stack of many very short exposures) is of interest to us, the loss of resolution due to seeing being not critical (we defocus ! ). The sensor is usually very small, FOV not ideal to me. The good points are they are inexpensive, monochrome, have low NJ noise (not so critical to photometry), have fast reading (more time to observe), low power dissipation and in certain cases (not all ! ) very low dark currents. If it's the case the cooling is no more needed and make that solution VERY inexpensive.
What is unclear with such cameras ? This is the fact they are optimized for lucky imaging: very short exposures with very high gain. The gain setting is the issue. In DSLR the calibration factor (e-/ADU) is well fixed by the ISO standard. We know it being about 1 e-/ADU at 200 ISO for an APS-C 14 bits camera. Those cameras have no such calibration factor reference. They have a large range of gain adjustemnt from said 0 to 100, units being sometime dB, sometime not. But the e-/ADU at "0" is unknown and very different between types and manufacturers. That results often in poor photometry settings with regards to SNR and dynamics/saturation. This is even made more critical by the fact the ADC is usually only 12 bits.
Maybe the solution is to make an assessment of those cameras, but I don't remember having seen this for DSLR or classical astro-cam in the AAVSO ? Maybe a problem ?
Clear Skies !
I'll be happy to hear more about it. About stacking short exposures - as a person working with this technique, I see more potential than defocusing, but maybe it's just a personal need. Someone on another topic said that single exposures have potential to objects only fainter than 1.0-1.5 magnitude below the saturation limit. As a person looking for NEBs (near eclipsing binaries) for bright transiting candidates, stacked short exposures allow not only to avoid saturation of bright target, but also go deep. Just with 300mm f/2.8 lens, having 1.65"/px scale with IMX178 and R filter, also with expected depth at least the limit (attachment) of ~10.5 mag star, it can easily find a NEB down to ~16.5 mag using 2-minute stacks (12x10s), or 18-19 mag for 15-minute ones, still having main target not desaturated, just in case if that's the variability source. The fainter main target, the less we stack, the smaller delta value will be.
There are no tutorials (at least I haven't seen) how can we filter short exposures from seeing affected frames. The idea I have, is to compare ADU levels of main source to comp stars. If the main target is somewhere between 160000-190000 ADU on 95% of all frames, and the next one shows ~200000, while comparison stars stick to their average values, there's no need to keep that frame. Conditions change, so let's say we compare only 100 previous 3s frames and 100 more forwards. That's probably only for people interested in low budget high precision photometry, as such amount of frames can take A LOT of gigabytes on hard drive. And there's lack of software of that. AIJ fails to work with such large amount of frames and that's the only one program I know, where I can extract those data. Depending what is the resolution of frames, I have to split for every 1000 frames. But if there are 100K of 100ms frames, it can take ages, doing it manually.
I think there is another confusion here, I am not speaking about single exposure, most of us stack images, either adding images or adding photometry results. You do 10 seconds exposure, this not what is done in lucky imaging where exposures can be as short as 1/100 of a second and the number of images 10000 or more !
I personally often use 5 groups of 5 images exposed 30 seconds each, adding the photometry results of each group, the 5 groups making one observing point. This allows to make a deep statistic analysis of various factors. It's also possible to stack the images of each group but with less statistical analysis. In the past I also used exposures of 10 seconds and 10 images groups from a smaller sensor. You are perfectly right, stacking (image or results) increases the dynamics if addition or floatting average is used (not using median). But what is important is the total at end: if you can get the same count of electrons with less images of longer exposures without saturating it's as good or somewhat better if the NJ noise of the sensor is not very low. With sensors having 10 e- of NJ noise it's even mandatory.
I am not familiar with NEBs processing and I am not sure to understand what you are looking for, I suppose this is a small drop of luminosity ? But if the objects are not eclipsing (near eclipsing) what makes it ? Ellipsoidal oscillation ? In any case you would get the same with less images of longer exposure if the end electron count is the same.
" how can we filter short exposures from seeing affected frames"
What do you mean here with "seeing" ? is it the blur due to the atmosphere turbulence or the related scintillation ? Photometry is affected by the scintillation, not much the blur. The only way to eliminate the scintillation effect is to integrate the flux long enough. I have never heard about other technique except using a much larger aperture. This is depending the aperture of the scope and also the sky condition. In general the overall added flux (multiple or single exposure) shall result from at least one minute of exposure, two being better for small apertures and poor seeing (there are papers on the subject).
The scintillation between target and comps is not correlated given the angle separation we usually have. Other problems like faint cloud/haze could be more or less compensated by the target/comp flux ratio.
Going to very short exposure (less than 10 s) is not a good idea I think. At a certain point the NJ noise of the sensor will get dominant and the SNR decreases. The only advantage of going short exposure in lucky imaging is to decrease the blur due to seeing and then improving the image sharpness. This has no effect on scintillation. I don't see in what it's useful in photometry, except loosing on SNR.
Clear Skies !
I freely admit that I am not familiar with CMOS cameras as used for photometry. Obviously, you know much more than I. I am only a simple volunteer trying to teach CCD1 for AAVSO when asked and having done this several times I am now faced with students signing up for the course that have CMOS-chip based cameras. I suspect you know what I mean by CMOS cameras as used in astrophotography; ZWO and QHY cameras come to my mind. Being a good teacher (in my mind at least), I am reaching out to the AAVSO community with the suggestion that those who know something about these cameras relative to photometry might be lead to help develop some "best -practice" protocols that would allow me to understand enough to help these students.
I am fully aware of my ignorance relative to these cameras and their use in photometry. Please let us focus on the goal of my original post; seeking some informed AAVSO members to help develop a document that outlines best practices for CMOS photometry similar to the existing protocols for DSLR and CCD.
I truth, I do not find any comments that are not directly relevant to this goal productive.
There are now available CMOS cameras with quite large chips that are very suitable for long exposures and don't rely on the "lucky imaging" technique that you mentioned.
An example is ASI183MM. I have started using this camera admittedly for spectroscopy but it would also be very suitable for photometry.
There is a very nice review of it here http://www.astrosurf.com/buil/asi183mm/ descibing it's use as a scientific camera. The review is mostly in french but it translates very well with google translate.
Another comparison this time in english is here http://www.astrosurf.com/buil/CMOSvsCCD/index.html
These cameras have very low thermal noise. My 300sec dark frame ar -20degC has just 3 pixels that reaches 1000 . For comparison, my SBIGST10XME has 600 pixels that reach that level.
According to Christian's tests the cameras also have a very linear instrument response.
I also feel that a document explaining how to use these cameras for photometry would be very useful. I'm not up to doing this yet as mine is on my spectrograph but surely there is someone out there that is using one for photometry.
Thanks for your support. The two links were very informative and bring up some of the questions that should be addressed by a committee of the informed. I have saved them to share if needed.
I would be very interested in this. I just acquired a color Atik Horizon which is cmos based for an EAA setup, this camera is pretty amazing on my hyperstar rig. I would be interested in figuring out using it or the monochrome version for photometry. I'm just learning how to use it for EAA right now but it is so incredibly fast with a large FOV I can't help but think it would be great for variable star observing. I'm just unsure about filters and such. Is it more like CCD vs DSLR Filtering like Johnson V Filter vs Trig-G Green and the like. I am new enough at Variable Star Observing that it gets confusing and the filters are costly to make a mistake on.
The sentence about single exposures was adresssed to another user from another topic. I have seen several opinions from astrophotographers about using long exposures with CMOS cameras, about that shorter ones (but more frames, which are stacked to the same integration time) give better results, which is opposite to CCDs. I have no idea why it would be opposite for photometry. Generally I prefer comparison results showing magnitude scatter than mathematic words about noise levels etc. This is what I expect from CHOICE. I do this myself by comparing results on ETD, as I said before.
Finding NEBs, which are near eclipsing binaries, is just detecting false positives of exoplanet candidates, which are a little futher (but still inside the aperture).
I don't have any experience about long exposure photometry due to mount/light pollution limits. Never went above 30s for single ones. But if I keep doing short exposures, for example for this 11.7 mag target with only 4" lens, having satisfying results, something is going on. http://var2.astro.cz/tresca/transit-detail.php?id=1525619181
Also about short exposures (<10s) and small SNR. I guess you meant about targets barely visible, just above noise? No, I meant about bright targets, like this 3s exposure as an example. It is actually just below the saturation limit, for this 10 mag object.
Astrophoto and photometry are not looking for the same result. Lucky imaging shows better results for bright deep sky object on sharpeness view point. Sharpness is not the goal of photometry, the goal is the luminosity accuracy.
What's the total exposure, number of stacked 3 s images, for the blue dots of your curve ?
Apparently the sigma of your blue curve is about 2.5 mmag, this is ok in a clear sky, scintillation is over. Then I don't know the other parameters of your observation but your e- count per image should be approximately 15000 e- (based on a similar observation I have). That means in a single image the shot noise is 122 e-. If the RON is 2 e- and you inner circle is about 5 pixels wide the camera noise should be about 10 e-, you are well under the condition the camera noise is negligible. At the end with 100 images the SNR should be about 1200 and the resulting sigma contribution 0.8 mmag. This is well compatible with your curve. As you can see the math works !
Hello Terry, Ed,
Terry you are right, this camera is very interesting, having good general specifications and a gain setting enabling 0.5 to 2 e-/ADU, the right range for photometry. But not all such new cameras would be a good choice, some have much too high gain, are small, dark current... depending type and brand.
But the true question is: in what the photometry process with CMOS cameras should be different than CCD ?
Apparently Christian used it the same way he used the CCD from Atik, there are only some difference in RON and other parameters, some better, some worst, but this doesn't change the spectroscopy process he applied.
In what should the photometry process be different between CCD and such ASI183 MM ? I don't see it. By the way I don't see in what we need a new manual. Just use the CCD one !
The issue is the recent excitement about the lucky imaging new fashion, stacking many very short exposure images. Someone think it should offer new possibilities to photometry, that's not obvious to me ! (The post from Gabriel is typical of such thinking, to see my answer). I think we should not encourage this.
To summarize, I think there is no need for a new manual, the CCD one should apply. The need is to advise on camera selection and the way to set the e-/ADU calibration factor. On camera choice it's unclear to me if the AAVSO could publish something, that has never been the case for CCD or DSLR. For the calibration factor determination there are number of documents on the subject or product datasheet. Christian Buil describes it in several of his posts. Maybe we could make our own, including some more practical details on the way to proceed.
Clear Skies !
Roger said:"The need is to advise on camera selection and the way to set the e-/ADU calibration factor. On camera choice it's unclear to me if the AAVSO could publish something, that has never been the case for CCD or DSLR. For the calibration factor determination there are number of documents on the subject or product datasheet. Christian Buil describes it in several of his posts. Maybe we could make our own, including some more practical details on the way to proceed. "
but , how to best determine the famous calibration factor for a CMOS sensor?
based on what technical data?
I agree with Roger, when we do photometry with material that we already have, no need for information, just use the advice that exists on AAVSO with CCD manuals or in DSLR . and adapt them to their configuration.
But when the question just bought a dedicated material for this practice, if you are not a specialist and connoisseur, it is difficult to make the right choice.
CCD wide pixel? CCD small pixels? DSLR? CMOS? CMOS like DSLR (like ASI294 MC) ...
the choices / advice will not be the same either in the budget.
(less than 2000 € or such type of sensor will be recommended, lower than 4000 € or it will rather be another sensor, etc ...)
and at that moment help would be welcome to explain the advantages and faults of each sensor.
sensors or technique to avoid and for what reasons. (ABG?)
the specificities of the sensors and their effects matrix of Bayer for example, etc ...
that would facilitate new vocations.
Personally, here is soon 6 months that I seek without success, which sensor I can buy to do the photometry, I would have much prefer to spend this time to make data for AAVSO rather than pass it to seek information that I find nowhere.
After reading all of your responses and other forum posts on CMOS cameras, I conclude that CMOS needs its own manual if AAVSO wanted to encourage CMOS photometry. Further, the best education for the beginning CMOS photometrist would be a course lead by an experienced CMOS photometrist.
Would I agree to teach CCDI with CMOS beginners included? Sure, I already have tried that. The student was great and I got him some help from an excellent CMOS photometrist. But he had to figure out a lot of things for himself. I would certainly try again, given that I have some information about these cameras (e-/ADU calibration factor, etc.). But would this serve the participant in a manner that is up to AAVSO standards?
Thank you all for your responses,
I'm glad that Ed started this thread! I've done some preliminary tests with Gary Walker's new FLI Kepler sCMOS camera, along with the ZWO ASI178mm-cool and ASI183mm-cool CMOS cameras. The former is a true science-grade camera with some flaws; the two latter cameras are consumer-grade items (the price can't be beat!). I have a lot more work to do on the ZWO cameras, but they look very promising.
The current set of commercial-grade CMOS cameras do have several defects that make processing their images, and observing, different from CCDs, and so I second the motion to consider a separate CHOICE course for their use. Christian Buil's ASI183 article highlights some of these differences: amplifier glow, 12- or 14-bit ADC, funny dark current images, tiny pixels, etc. At the same time, these CMOS cameras have multiple advantages: low cost, nearly perfect cosmetics over many MPix, high QE, fast readout. The fast readout part is what interests me right now, as it means you can do either high cadence work, or take sub-second images to work brighter without saturation. Understanding their limitations, and finding/writing software to take advantage of their unique capabilities, are my short-term goals.
Toward those ends, does anyone know of public-domain software that will do real-time stacking (not later off-line stacking) of ZWO camera images as they are taken? I hate to save thousands of subframes if it can be avoided. A plugin for MaximDL would be ideal, but I'm willing to try other approaches.
Stella has asked me to give one of the Keynote talks at Lowell. I plan to give a talk on sCMOS. I have a couple of cameras and have been experimenting for several months. I also will update the talk given at NEAIC/NEAF on the subject of sCMOS.
Right now I think we have several pioneers/early adoptors. I don;t think any of us know enough to write the manual on the subject, but we are working on it.
I wonder whether one could use "Deep Sky Stacker (DSS) Live" for it. I'm a bit reluctant to point towards this software, it's excellent in general, but it was developed with "pretty pictures" in mind and many default settings will be tuned towards "making it look good", so one has to be extra careful to configure it to "just add/average pixels, nothing else!!". But the functionality is great, e.g. it can generate a stacked image for every N (configurable) frames, and I understand it also does alignment and calibration for every sub-frame.
EDIT: DSS Live cannot calibrate sub-frames!
If it's just the number of files that is inconvenient (and not also the size), generating a single SER file from frames is something I have done with FireCapture for my ZWO camera.
FireCapture also has a "live stacking" option in the "preprocessing" panel which is simpler but might be good enough if the tracking is guided (no alignment of sub-frames needed). Anybody tried it? That might do what you are looking for .
We could suggest a feature for SharpCap about recording multiple FOV areas at once. The camera shows the whole frame, we only click on stars of interest (100x100px is enough) and the program would save them all in sequence, in one file. This would reduce the amount data a lot. For example, if we have only three reference stars in 3096x2080 file (IMX178 example) and we need only 100x100 areas around them, we capture only those four objects (including variable). Total area of interest is equal to 0.6%, making the file size of ~70 kb instead of 11 megabytes only (the final frame is 400x100 pixels). The rest 99.4% area/data size, which is background, is useless. The program shouldn't guide the star and all FOV areas must be static - that would make calibration frames usable.
Then I'm pretty sure all photometric programs would work well with such large amount of frames, even if there would be ten of thousands. Would be very useful for bright targets if comparison stars are really far.
EDIT: Attaching a concept idea from a few months ago.
Also about live stacking, SharpCap has this function, but it isn't automatic (for example, we can't choose how many frames per stack do we need, it makes only one). With my request, developer added a function that makes it now scriptable. I have never done that yet due to lack of time and knowledge about Python. To run a script, we need a pro version, though.
I think what Ed was looking for when he started this thread was some basic recipies for doing photometry with the CMOS cameras. The new CMOS cameras should be useable for basic photometry which is the goal of the CCD CHOICE course.
I am working to develop instructions for this basic use case as I have an ASI178 which will soon be mounted on my scope.
The other comments in this thread focused on some of the extraordinary capabilities of these cameras. High cadence, near video frame rates and stacking strategies to match these capabilities; these use cases should be best addressed in another forum thread. Exciting stuff but a distraction to the goal of supporting the basic CHOICE course material.
If anyone has experience with calibrating and collecting filter photometry with CMOS cameras, please contribute here!
You wrote about ASI178 which you planned to install to your telescope. Have you made any tests with it? I'm very interested in the topic because I'm planning to buy it (probably cooled version) (now I'm doing DSLR pfotometry with my Canon EOS 600D). Is it suitable? (I'm aware that additional filters needed)
I have not yet finished my testing of the 178; been real busy and will be traveling for a fortnight. But when I get back I will get back to work!
My preliminary results / recommendations center about the calibration steps. I do not believe the darks can be scaled, so be prepared to collect darks with the same exposure as your science images. For now, treat this camera like a CMOS DSLR.
The pricepoint and small pixels make it an attractive camera. I hope to be able to do grating spectroscopy with it as well as photometry.
Arne and Gary are also playing around with this camera and they may have more information to share.
I share 99 stacked frames with flat calibration (no darks were used), 12x12.5s each from WASP-58 b transit. Observed with ASI178MM-c and Canon FD 300mm f/2.8 L. Some issues with focusing, also collimation (which is normal in that piece). Works fine, because stars in left side have good shape. Sounds stupid, but I got this lens 15x cheaper than for modern Canon EF-S one. Hope you find this data useful. I can't upload single frames, because of limits.
If you don't see all 99 frames, check again soon. They are probably still uploading.
Dear Ed, dear Fellows,
I can list three papers dealing with observations of Delta Scorpii using CMOS camera. It is SANYO CG9 commercial camera, and the second magnitude was well reachable. The problem of stability of magnitude to do reliable photometry is heavy. The camera has a fixed 0.5 s of integration, it should overcome most of scintillation, but it does not. I used a similar camera SANYO HD1010 to follow N Scorpii eclipse last June: same problems even if the aperture of the camera is larger and the limiting magnitude around 5-6. The corresponding data are on AAVSO database at SGQ code. Here the 3 papers on Del Sco.arXiv:1107.1107 arXiv:1109.5865 arXiv:1112.2356
In summary: naked eye was always more precise.. and I could not suggest using photo/videocameras for making photometry at didactic level... nor smartphone.. what do you think?
Tried to talk a seller into letting me test a CMOS for this purpose to no avail! I suspect that these will work fine once the differences and techniques are well studied. I would think the technique will be very similar especially if the numbers match up!
George, thanks for reminding all of the basic intent of the thread. You have defined the issue with clarity. I look forward to seeing the results of the work you, Gary, Arne and others are doing to bring CMOS cameras into our programs.
I finally got a chance to reduce the first 13 nights of data on my new setup. It uses the excellent sCMOS camera from FLI, called the Kepler K400. These are all time series results from the excellent SRO site, but I do have a collimation issue with the scope, as the fwhm are 8 arc seconds. Attached is a screenshot from the LCG1, and hi-lighted in blue are my observations. The object is the symbiotic campaign object J1444107-074451, and the exposures are all 180 seconds. Needless to say, I am very pleased with the results.
I am very interested in your experience with the Kepler FLI CMOS camera.
I am using FLI CCD cameras and am very happy with their performance and reliability. I look forward to a presentation on your experinec at Lowell and would appreciate if you could alreaqdy give a link to the NEAF presentation if possible.
We could compare our results on J1444... as I am also using 180 s exposure with V filter.
Last year I bought a new FLI and did not go for the CMOS due to its infancy and price tag.
I have been watching your data on NSVS J1444107-074451 go into the AID. Been posting my data and watching yours. It looks like they are about he same. I am very happy with the results from the FLI Kepler camera. All my data is from an excellent site at Sierra Remote Observatory. Things are in its infancy, but I think they will only get better. Based on ony a couple of objects, if anything I am seeing an apparent reduction in the systematics with the sCMOS.
I will be giving an update of my NEIAC talk on SCMOS at the upcoming SAS meeting in June at Ontario, California. I will send you a copy of that presentation when I update it. There was an error I discovered in the one at NEAIC and I don't want to perpetuate it. That being said, I have another of these cameras on the way. I expect sCMOS to ony get better from here.
Gary's keynote address will be on CMOS, and I hope Arne and George (and perhaps others) will also present what they have learned. I think there will be a lot interest and a lot of questions, but the time for questions after a presentation is often very limited.
Would it be possible to set aside a block of time at the meeting for questions and open discussion of CMOS? This could be during a lunch break, in the evening after formal presentations, or even Sunday morning.
I'm sure that this would happen informally at the meeting in any event, but I think it it would be good to set it up in advance so people can make their travel plans accordingly.
So now CMOS camera (12-bit ZWO ASI-183mm) is used on one telescope of BSM network.
Can anybody describe how it performs compared to classic CCD? Does it give good photometry?
I have a ZWO ASI183MM but I didn't have any intention of using it for photometry because it was a 12-bit CMOS and the filters I have with it are the standard LRGB color filters. I can buy some B and V filters, and, I can test it and see how it performs once the transforms are done on it.
I have monitored this forum for a few months, ever since I decided to buy a ZWO ASI1600MM-COOL CMOS astronomical camera, and filter wheel with V and B filters. Attached is a document with the results of testing done so far, limited to linearity and precision. Although I have acquired transformation coefficients, I have not yet tested the accuracy of transformed magnitude measurements. That's a project for the next clear nights.
To summarise (see the attached for detail):
1. The camera shows good linearity to near full well capacity (checked with regression analysis and residuals plots).
3. The camera allows high precision photometry of bright stars, to 10th magnitude, as shown by (a) time series photometry of the 8th magnitude delta Scuti star RS Gru and its check star, and (b) measurement of the non-transformed V magnitudes of 4 pairs of 10th magnitude Tycho-2 stars.
4. Precision drops off at 11th magnitude, based on testing of pairs of Landolt standard stars.
5. All testing was done on slightly defocussed images, at unity gain. I could not achieve good precision on sharply focussed images.
Note that, although this camera has 12 bit ADC, full well capacity is found to be 65,504 (see Figure 1 in the attached). That is because the camera software multiplies the native ADUs by 16 and outputs those values as the results.
I have to confess that I did not record the seeing. The RS Gru fields were significantly defocussed. My measuring aperture was 32 pixels in AstroimageJ. A zoomed-in view of RS Gru near maximum with the aperture/annulus circles superimposed is attached. At this stage of testing, I was trying to get a feel for the combination of degree of defocus and exposure. Now, I would probably defocus less and use a shorter exposure.
Re the measuring aperture - something wrong with the scale. The scale bar at the top of the image shows 96 pixels. If that's correct, the aperture would be way bigger than 32 pixels.
Need to sort this out, but won't be tonight (9:27pm here in Brisbane).
The aperture for the RS Gru photometry was 64 pixels (diameter). As noted, the images were significantly defocussed. Even so, the highest pixel counts were just above the range of linearity, and just below saturation for RS Gru at the peak of the light curve. Counts for the comp and check were a little lower.
I've recently bought a simple ZWO ASI120MM planetary camera and tested it as a photometric device.
The camera has 12bit ADC. I used 2*2 binning (software binning) to get virtual 14bit precision (Arne Henden wrote in one of his posts that BSM_NH2, equipped by ZWO ASI-183mm (12-bit) uses 2*2 binning to achieve 14-bit precision).
Here are the results of a test run against XX Cyg. This is unfiltered CV photometry. Exposure=10s.
The first plot is for 10s exposures, the second -- binned (by groups of 4 points) measures with estimated uncertainty (standard deviation by 4 points). Looks not worse that DSLR photometry which I practice now (of course, with a filter applied the noise would be bigger). However, now I'm seriously thinking to buy a somewhat better camera (say, ASI183 cooled or ASI1600 cooled) for photometry.
I have not made linearity testing yet, I will try.
*** UPDATED *** Slightly better results with lower noise can be achieved using a smaller aperture (r=5px instead of 6px, at least Muniwin shows the noise minimum here. Measurements were done with AstroImageJ yet Muniwin was used to check the result). The third figure shows the light curve measured with the aperture radius 5px, the 4th one -- binned results (averaged by 4 points)
Comp Name 106 [000-BJV-171]
Comp V 10.606
Check Name 118 [000-BJV-173]
Check V 11.757
Air Mass 1.1 - 1.3
4 x 4 binning gives a 16=2^4 fold increase in combined full well capacity and max ADU count per binned "virtual" pixel, so that one might compare this to an effective 12+4 = 16 bit resolution. That is a bit of a stretch in practice because this assumes that the light is spread evenly accross those 16 pixels. Just binning 4x4, without defocusing, will break this assumption. Also the binning will most likely happen in software, not at the hardware pixel level, so that the read-noise adds up (4-fold) for the 16 pixels you combine, but that is still not so bad for CMOS cameras.
There is also at least one very affordable 14 bit CMOS astro camera (ZWO ASI 178 and other models build around the same IMX 178 sensor by Sony). It's a bit small in diameter but the 6 MP resolution and 2.4 um pixel size and low read noise certainly encourage 2x2 binning to give you near 16bit characteristics (or alternatively stacking 4 shorter exposures). For < 400EUR (uncooled version) it's good value IMHO.
Another option is stacking, it works like pixel binning however is not affected by focusing/defocusing. It can be considered as binning in the time dimension.
Concerning ASI178, unfortunatelly cooled version is discontinued.