A further point is that the dpreview post uses
*peak* QE, not average QE. Peak QE for a DSLR would clearly be higher than 10-15%, but what is on view with Ray's calculation is a mean sensitivity across the visible broadband spectrum.
A further confusing issue when comparing CCD with DSLR is gain. Greg's post gives the example of a D800E at ISO6400 showing plenty of picture. The complicating factor here is that due to gain, the full well capacity of a D800E pixel at ISO6400 is (according to
sensorgen) only 833 electrons. Compare that with a CCD camera!! Another factor ignored when considering the comparison is noise. The D800E at ISO6400 will have plenty of signal, but what of the noise...? Ray's initial post assumes that systems being compared at equivalent SNR.
Perhaps I muddied the waters by bringing my own anecdotal experience into the mix. When I put my own spreadsheet together using Ray's formula, i can compare my Canon 1000D and Canon 450D against my newly acquired SBIG ST10XE. Referring to the SBIG data, and given my camera doesn't have microlenses, I'm using an average QE of 55% for the ST10XE. (That is taking an average QE across 450-650 wavelengths in 25nm increments as per the SBIG graphs for my camera). Comparing these cameras based on the formula and assuming an average DSLR QE of 20%, the ST10 would have 4.05x the relative sensitivity of my old Canon 1000D and 4.88x the relative sensitivity of the Canon 450D.
This might muddy the waters further, but now for a practical test: compare the following images of NGC6164:
- Broadband image with Canon 450D @ ISO1600, FL = 650, f-ratio = 5, integration = 12.1 hours
- Narrowband 6nm Ha with ST10XE, FL =1278, f-ratio = 6.4, integration = 4.7 hours
I think the comparison is 'chalk and cheese' (especially in light of the slower scope in (2) ), and quite insightful given the discussion here in this thread. Even reducing the mean QE of the DSLR in the formula to 10% still suggests that the ST10XE is (only) 9.77x more sensitive than the 450D used here. Considering that the ST10XE image is narrowband and taken with a slower telescope, perhaps 10% is still being generous...