Thought I make some random noises here while it's still fresh in my mind...
If you think comparing one DSLR against another is complicated, try figuring out astro imaging CCDs too.
I thought as a first step, I'll look at two sensors, the Atik 320E and 383L Plus compared to a Canon 18MP crop sensor body. The two sensors were picked purely as they happened to be two I randomly came across as representative of more affordable models. Affordable is used here as a relative term! The Canon 18MP crop sensor body could be the new 60Da, or one of the 550D, 600D, 60D, 7D preferably with filter modification for astro use.
The 320E is small, around 5x crop factor and weighs in at 2MP. The 383L+ is about microFourThids size at a tad over 8MP. My gut feel was that something like the 320E was just too small to be worth bothering with as an upgrade to replace a DSLR. The 383L+ is starting to get interesting, apart from the price which is above a new 5D mk2.
As the sensors are different sized, at different pixel densities, how do I compare them? I made an assumption of a 100mm objective. Further assume the focal length could be freely adjusted such that it gave the same pixel field of view. From there you can calculate the focal ratio and differences between them in exposure time. As it turned out, this wasn't too significant since their pixel sizes were in a similar ball park. The Canon and 320E were near enough the same, with the 383L+ pixels around 20% bigger per side.
Making the very big assumption that all three have the same colour filter for now, quite simply bigger area is better. Factoring in the speed differences for a normalised 1 arcsecond per pixel, the 383L+ is about 80% of the output of the Canon, and the 320E is a pitiful 12%.
But that isn't the whole story and assumes the sensors are all equally sensitive. But are they? I'll have to drop the 320E from comparison here as I can't find data on it. The 383L+ is based on a Kodak KAF-8300 sensor which has a handy datasheet. Canon don't publish their data, but you can find measurements for various models here
. Looking at the charts for both they are pretty similar. The KAF-8300 has roughly the same blue response, with slightly higher peak green and red response. Overall the colour filter characteristics aren't that different, not at least a stop different to be significant.
My verdict: for one shot colour filter imaging, a DSLR is ball park competitive with similar size CCD imaging devices, with the DSLR being at a significantly lower cost.
What if you don't want to do one shot colour imaging? If you want to do narrowband or multiple pass colour filter imaging, then the advantage swings rapidly in the direction of mono CCDs. Take what is probably the worst case scenario for a DSLR: H-alpha lines at around 650-something nm. Assuming the DSLR has been modified, we get a QE around 25%, but due to the colour filter, only 1 in 4 detectors are effective. The KAF-8300 has a mono sensitivity around 650nm of about 45% depending on which variant of the sensor is used. It's advantage is it isn't hobbled by the presence of a colour filter at this point, so it gets all the benefit. The smaller sensor of the 320E only gives a relative mono response chart, where 650nm is about 0.55x the peak response in green. In the absence of this info, I'll assume it is comparable to the KAF-8300.
So factoring in the H-alpha sensitivity and the colour filter loss of the DSLR, how does that shake things up? The 383L+ is now a whopping 6x more effective than the DSLR, and the 320E is an estimated 85% of the DSLR.
Verdict: for narrowband, a mono CCD will give a significant advantage providing it is competitive in sensor size. A small mono sensor will negate that potential advantage.
What does this mean to me? I'll stick with DSLR imaging for now, since a small CCD does not offer significant value over a DSLR for their cost. A big CCD does, but I'll be paying for it!
Big warning: I didn't take into consideration noise effects here. The CCDs are cooled so reduce thermal noise effects, so they may provide further benefit over a DSLR. However that is far too complicated for me to work out.
Bonus points if anyone actually bothers to read all the above.