With all this discussion of astrophotography, long exposure, and heat build up I just have to jump in.
If you don't do astrophotography this may bore you. :ugh:
I bought my Pentax K1x0D cameras for many reasons, astro was not one of them. I use the filterless K110D exclusively for astrophotography (well, occasionally I enjoy shooting daytime IR with exposures under 1/1000 sec). I don't own a Canon DSLR and do not intend to buy one. But the advantages of a Canon are:
1. Lower noise.
2. Better Live View, they can take a 1000 shots of Jupiter without cycling the shutter (at a reduced but useful resolution).
3. Auto Dark (AKA Noise Reduction) can be disabled on their CMOS sensor cameras unlike the K20D.
4. Most all astro software supports Canon, almost none supports Pentax. Images Plus will accept our PDFs but does not provide camera control for Pentax.
5. Ability to focus from the computer. I don't mean AutoFocus which is useless for faint night sky objects. I attended a Images Plus workshop where he manually focused a Canon lens on a ceiling light from a computer. Focus is one of the hardest parts of astrophotography.
6. And so on.
With my Pentax I get fairly good performance with each exposure of up to a few minutes from my light polluted location. In Astro you want the longest exposure you can manage, and a lot of them. I don't think you will get much with the 8 second exposures mentioned. I take dozens of 2 minute exposures.
I started taking astrophotos with Pentax K1000. No way could film do what I do now.
I've measured the K110D current draw from an external power supply a few years ago. While the shutter is open (lens cap on) it consumes almost 4 watts. The current during an NR dark frame (shutter closed) is about 3 watts. As 5teve mentioned NR heats the sensor almost as much as the exposure itself. Current at idle (electronics still on) is 1 watt. That is a lot of heat to dissipate.
How much noise does this heat cause? I did the below 7 frame test a couple of years ago. Starting with the camera at room temperature each run, I took seven 1 minute exposures with the camera capped and no NR. First run my timer allowed 8 seconds between frames to save the RAW file. Second run had a 30 second delay, Third 60 second. I measured the noise in the "black" frames using Images Plus. Without an inter frame cool off period the noise rose 7:1 in 7 frames and showed no indication of tapering off! This causes havoc with astrophotos since doing post imaging dark subtractions can't track such a rapid rise in thermal noise. Also, nebulas that I could see in the first frame vanished in the noise by the last frame. This is why I have resigned myself to using NR plus a cooling off period which reduces my overall exposure time for the session. OTOH this year I've been able to image objects I could not when I maximized exposure time by taking dark frames after the session.
In April I decided to do a longer test to see when the curve does taper off. As you can see as the exposures reach about 20 the sensor heat remains fairly steady. Also, the 7:1 increase in 7 exposures repeated nicely.
http://pages.cthome.net/astroleo/HeatTest.jpg http://pages.cthome.net/astroleo/HeatTest21.jpg
Can you do astro with a Pentax DSLR? Sure, I'm processing photos from Tuesday night right now. Is a DSLR, especially a Pentax, ideal? No. I have an older SBIG ST-7E astro camera (1.4 megapixels cost me $3000 in 2000) that will run circles around a DSLR. It has sensor cooling to 30 degrees centigrade below ambient and control temperature to within 1 degree. This results in low noise and consistent dark frame subtraction. Why do I now use my Pentax almost exclusively? I wanted more pixels and easy color and could not afford a new SBIG! My SBIG still serves me, as an guide camera!
There is a lot of miss-information about astrophotography in this thread, I hope I cleared a few things up.