jhapeman
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 p.1 #2 · Re: 20da Experiences? | |
Kyle,
I think you read wrong. I have read that the ISO is about 10,000 or higher, and that's right. You can barely take a daylight exposure with those cameras without taking ridiculously short exposures through the filters--which are very narrow-band and therefore cut out most of the light.
If I take a five-minute exposure of M42, the core will be totally blown out and even the faintest of regions will be visible. With a DSLR, those faint regions won't be visible even at 10 minutes.
The quantum efficiency of the astrocameras ranges from 50% on the low end (meaning 50% of the photons that hit them are converted to signal), to over 90% on the high end. A great comparison is the STL-11000XM and STL-11000CM (with color). The QE peaks at 51% for the XM, and peaks at 42%, 37%, 34% for BGR, respectively. This means the color version peaks at an average of 38% QE, or roughly 35% less sensitive than the monochrome.
Your DSLR has a similar array on it to the STL-11000CM, but the chip also has inherently less sensitiivty--I have heard the average is about 30% QE at max. This means the QE on the STL-11000XM is about 70% greater than a DSLR--approaching twice as much QE.
Next you have huge well depth compared to DSLRs--this is why we take long exposures. We can with those cameras, and still get good data. It also maximizes the S/N ratio (the noise is higher becuase the ISO equivalency is so much higher--hence the peltier cooling as well).
Now let's talk some really sensitive cameras--like the back-thinned SITE chips, with 90+ QE. A 30s exposure with one of these easily reveals Barnard's Galaxy, or detail in the spiral arms of M51. Lots of detail. There is just no comparison to a DSLR....
Jeff
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