Craig Stark did an indepth analysis of what Canon cameras are doing with their shots before creating the RAW files, after processing 11GB of darks and flats with them!
http://www.cloudynights.com/item.php?item_id=2786
Conclusions
So where does this leave us? There are several take-home messages from these results:
- Canon is re-scaling your data before it hits the CR2 file. Based on the thermal signal (likely based on stats from the optical black portion), it is both shifting your histogram left (i.e., subtracting a constant from the whole image) and scaling the intensity (changing the contrast or gain).
- This makes the camera appear to have very low dark current as the background never gets appreciably brighter. But, the noise increase shows that the dark current is there and it is really the noise from the dark current that’s the trouble – far more than the constant component of the increase.
- The camera warms up for a substantial period of time.
- The above makes dark subtraction a real challenge. Not only is the current changing with time as the camera warms up (which we might easily account for), but what scaling is being applied to the data is changing as well.
- Software designed for daytime photography will also rework the data and making it stay purely linear and not apply any processing can be difficult.
- The camera’s internal gain (e-/ADU) for each ISO value points toward limiting the ISO to 400 and not using higher values. Both in theory and in practice, using higher values limits the dynamic range and does not let you pull out fainter details from the noise (even if they look brighter). The exact optimal ISO value will likely vary from model to model, but it's unlikely to be the high ISO settings.
We might ask ourselves at this point why the data get rescaled in the first place. Canon, after all, is filled with bright engineers, the cameras are very successful, they take great images, and this practice has been going on in their cameras for some time (at least since the old DIGIC II 350XT). The answer, in my opinion, is that the scaling makes perfect sense. They are compensating for inherent constraints placed on them by the sensor (which in turn gets to blame physics – good luck winning that debate!). Today’s DSLRs will do in-camera compensation for all sorts of things, now including lens distortion and chromatic aberration. These corrections get into the raw data and are far more complex than dark current.
The overwhelming majority of users would want the camera to avoid shifting the histogram far to the right as the exposure lengthens. Keep things under control as best as you can to make an image! Surely that’s better than having the image get lost by being washed out. Likewise, since many people shoot in 8-bit JPEG, ISO ratings well above what might be optimal for dynamic range are a good thing as they boost the signal into the range of intensity values that work well for the 8-bit, gamma-stretched JPEGs. It lets you get an image and see it there on the screen without going back and using image processing software to stretch the raw data. So what if a bit of dynamic range has been lost – you have an image!
These are perfect engineering arguments (as is the one to have an IR blocking filter cut off the H-alpha line). The Canon DSLRs do very well in astrophotography. But, they’re not designed from the ground up for this. They’re designed for a different market and their engineers make different choices as a result. Some of the choices impact how well the camera works for astrophotography.
Hopefully, we now know a good bit more of what we’re up against. I, now, at least have some information that will let me determine better means of doing dark-correction in software. For the times when I will grab my DSLR for astro work, I also know more about the ISO setting and how to optimize it for my images. Every tool has constraints on how best to use it. This helps us understand a bit more about those built into the Canon DSLRs.