@Morgan_Hardwood : OK, sounds good. I’m thinking about upgrading to a new model of the Sigma (maybe Quattro H) - so perhaps we could revisit this later.
BTW: None of the Sigmas need to be modified. The internal filter for IR is removable, and it’s easy peasy to put it back into the camera. This is much better than dedicating a DSLR to IR only service IMO.
Hello Morgan,
now I am completely puzzled: What do you learn from completely overexposed images, where all the pixels have the same value?
Hermann-Josef
@Jossie you find the “white level” by examining at what point a pixel corresponding to a photosite clips. Clipping depends not only on the physical photosite (and new sensors can have various types of photosites of various sensitivity) but also on the electronics which follow it, and may change depending on e.g aperture. You can read more about it here:
https://raw.githubusercontent.com/Beep6581/RawTherapee/dev/rtengine/camconst.json
One thing I’ll say about the SD14 is that it does not seem to show me any noticeable artifacts when I’m shooting normal visible-light photos. In fact, it seems to do quite well, and in some respects is even better than my Pentax K5, and in some other respects maybe not - but the difference in age between the cameras is pretty large, so it’s not exactly a fair fight. I think the Foveon does great with visible light photography - and the newer Foveon based cameras are probably even better.
Edit → Just took a shot with ISO-200 and with the main lights turned on, and it had artifacts visible. Then took a shot at ISO-100, and the spots/artifacts were gone! So, at least for IR work, it looks like you need to stay at ISO-100 on this camera. I’ll take some more shots just to be sure, but it seems that at ISO-200 you could be getting into some sensor noise. I had figured 200 would be OK (usually is, right?).
So, sensor noise is not detected with the lens cap on? Maybe it takes adjacent photosites to energize the noise? I think I just answered my own question - but what a round-about way to do it! Thanks go to the respondents that helped.
Random but unlikely guess: Could it have something to do with how the light is penetrating the sensor? Like X-Trans rectangles and related topics?
Hi Afre,
Random but unlikely guess: Could it have something to do with how the light is penetrating the sensor? Like X-Trans rectangles and related topics?
I’m not sure I understand the X-Trans explanation, so I don’t know if that could relate to this thread. I do think I’ve been chasing a ghost or my tail, just because I didn’t take the admonitions given by others in other forums to run the camera at ISO 100.
I mean, the thing takes such darned good pictures at ISO 100. So, for my “dark room” tests, I thought I’d push it up (but only a little, to ISO-200). I didn’t really believe the admonitions about ISO, I guess. But, I believe them now. The Foveon loves the low end of the ISO scale. When it’s down there, it does great in this camera. Lesson learned. The fact that it has ISO-50 I suppose is a hint.
As mentioned a couple messages prior to this one - the spots/artifacts are gone when I use ISO-100, with near-darkness or with the lights turned on - either one.
Good morning Ron,
as far as I understand cameras with a CCD detector, changing the ISO setting just changes the conversion factor from CCD counts to the output signal. This is way higher ISO produces more noisy pictures. In CCDs there is no amplification e.g. as in image tubes. If this also holds for the detector in your camera, I do not know.
I would be interested to see one of you ISO-100 (without lens cover) shots to compare to the example above.
Hermann-Josef
Guten Tag Jossie,
This is my take on the CMOS sensor design:
With the CMOS designed sensors there is a built-in amplifier under each photosite. The properties of this amplifier might be different (amplification increased or decreased), along with the integration/sampling rate. With amplification, there is always a noise-power relationship. Increased power typically hurts the signal-to-noise ratio. So, perhaps the result of a photon hit is more likely to be read out by the sensor/amplifier setup when its values are tweaked by higher ISO, which comes with a penalty of poorer noise figure, and/or the integration/sampling rate could be changed (I’m not sure how the latter item plays out). It’s been a long while since I looked at any of this kind of stuff, so don’t write it down.
As far as the difference between CMOS and CCD sensors, I found this:
https://www.cse.wustl.edu/~jain/cse567-11/ftp/imgsens/index.html
Now I have to move into the realm of conjecture instead of just bad memory. Maybe the Foveon needs higher amplification and/or different sampling schemes from the start, due to its layering structure versus the matrix of Bayer. I don’t know any Sigma people, so this is total conjecture.
Here is another article that speaks to the Foveon sensor, but I haven’t decoded it yet, except to say that maybe it implies that the simpleton explanation for the ISO differences given in my previous couple paragraphs may fall short :
The latter link is an eye-waterer.
Sigma is thought to have much improved noise figures in the newer Quattro line. Another article I read implies that the noise is not really higher on the Foveon, but the Bayer scheme “averages it out” by interpolation. Is the better noise performance of the later Sigma models partially the result of averaging the stuff out in firmware? Or is it physical sensor change that improves the picture, or both? This may explain why Sigma people want you to use their software for post processing. The noise may not really exist.
IMO - The Foveon color is unbeatable, but maybe it needs more post processing to bring it out.
Enough already! The length of my post is maybe eye-watering. Sorry.
@Ronaldlees
thanks for these links. This is the stuff I was involved in with our CCD observations, especially if we commissioned a new camera / detector.
But I did not find anything about various ISO settings.
If you are talking about amplification, this seems to be the same in CCD and in CMOS detectors: It is the conversion factor between number of detected photons and the digital number in the output. This is not an amplification but just a multiplication, because you cannot change the number of photons detected. This is what I meant above. However, I never used a CMOS detector. They did not find their way into astronomy. IR-detectors seem to be related at least in the sense that you can address individual pixels for read-out.
Still I would be interested to take a look at an ISO-100 image without the spots (lens not covered) for comparison (if possible as the raw file).
Hermann-Josef
@Jossie : Can any of the detectors detect a single photon? I’m not an expert in this realm - I’m a long-time electronics hobbyist and a (very recent) photography hobbyist/novice. Anyway, I’ll take a no-spot raw photo and post it ASAP …
Sorry, you have the wrong Ron.
@ron sorry for this!
Can any of the detectors detect a single photon?
I am not sure what quantum mechanics says about detection of a single photon. In both, CCD and CMOS, charge is built up by illumination with photons. The lower the read-out noise is the lower photon flux can be detected (all well above 1 photon 
). In the analog-to-digital-converter (ADC) the charge is converted to a digital number. And here enters what is called gain: you can specify, how many detected photons correspond to 1 count (some call it digital unit). We called this factor EPC for electrons-per-count. I think, this is what the ISO setting in CCD-cameras does: For low levels of illumination you do not want to get low numbers. Instead you decrease the conversion factor EPC and end up with decent numbers, i.e. the image appears brighter. But the noise is also multiplied.
George Rieke (2003, Detection of light) only briefly mentions CMOS detectors. He talks about “providing gain in the amplifier for each pixel”. I do not understand enough about electronics to understand if it is the ADC he is talking about.
In reading I now learned, why CMOS-detectors did not make it into astronomy: Due to the complex circuitry required for each pixel, the area-filling-factor (i.e. the fraction of light sensitive area) is much less for CMOS than for CCDs. With the low light levels in astronomy, it is the photon collecting area where the emphasize is.
Hermann-Josef
@Jossie :
Thanks for the explanation of the detectors. I thought that one photon probably would not create enough potential at the input to the ADC to be measurable above all the noise in the detector - such as the thermal, shot, and other junk noises, or the noise in the connection to the ADC, and the ADC itself. This is why I thought there might be pixel-by-pixel amplification.
I think only one sensor maker (Sony) makes a sensor with on-die ADC. I could be wrong about that. I’ve seen some pretty nice looking astro-photography shots done with the Pentax K series (which is CMOS Bayer) - but professional astronomy is perhaps an order of magnitude more demanding?
Here’s an info link to the columnar ADC that I think Sony might be using. Interestingly, the Pentax K series that’s known for hobby-astronomy photo use employs a Sony sensor - probably with the columnar ADC, which may explain the high ISO capability of those cameras:
https://www.edn.com/design/integrated-circuit-design/4313668/Modern-ADCs-improve-CMOS-image-sensors
a single photon would be lost in the noise. Best astronomical CCD cameras have a read-out noise of a few electrons.
but professional astronomy is perhaps an order of magnitude more demanding?
Astronomical cameras are all sort of self-made, i.e. built by electronics departments of the institutes running the observatory. They are cooled to about -90°C to minimize dark current. So you cannot compare their performance to cameras from the consumer industry.
Hermann-Josef
When light shines at a certain angle, some cameras could exhibit certain artifacts. I won’t elaborate because I don’t think that is what you are facing after reading your initial post. Sorry I didn’t read it before replying. 
Random guess #2 would be static electricity. It happens. I thought of this since the spots are in different locations each frame you take. Perhaps, at lower ISOs, the signal is too low even when amplified to see.
The spots are back, but have lower intensity. I have to crank the software brightness control up to a quite high level in order to see them. Should I presume that I have light coming into the room in spite of my thinking that it’s an unlikely event? My eyes cannot see anything in this room in the middle/wee hours of the morning. Not a speck of anything is visible to me - it’s mud black.
https://filebin.net/03d65n20vi6pwo4w/SDIM8091-ISO100-90SECS-F5.6-RAW.X3F?t=0fhoa3nl
I guess I could use the K5 to see if it does the same thing. If the K5 does the same thing, then I have a light leak even though I seemingly cannot find it. I wonder if anyone else has done this kind of test. The aggregation factor of the time lapse exposure is what makes astro-photography work, so I guess it makes sense that there could be light that I cannot see in the room, and which subsequently appears in the time lapse photo.
Lest someone think otherwise, normal (visible light) shots seem fine from the SD14. Here’s one shot a few minutes ago …
https://filebin.net/e0zjivhpeozwe8cn/SDIM8134.png?t=5uj7xtjn
There are no “spots” to worry about, i.e., the photo is OK, although it’s not a good composition and the background is out of focus.
I looked at the histogram of your latest image with the spots. It is very strange. This is not what I would expect from white noise. There are also short vertical stripes to be seen in the image. Where exactly do you see the spots? Could you give an enlarged example, because I only see something of which I am not sure if it noise or not.
Hermann-Josef
