Quote:
Originally Posted by Cimitar
Hi Tony,
I'm also using a Canon 600D (un-modified), had it for approx 15 months. I'm using it with an 8" SCT and image at F10 + F6.3. I find ISO 800 is best for the camera and I can achieve 10min subs when my guiding is good. The sweet spots with this camera appear to be: 60sec = ISO 6400; 120sec = ISO 3200; 270sec = ISO 1600; 300-600sec = ISO 800.
If I'm imaging nebula, galaxies and galaxy clusters I aim for 10min subs at ISO 800. I tried 15min at ISO 800 however there was too much noise. If I have trouble guiding I drop to 270sec at ISO 1600 (not sure why but I've found 300sec at ISO 1600 produces more noise than 270sec).
Long story short, I created some test images using the figures above for each ISO and noticed that the end result (brightness wise) was the same for each image, except noise. The ISO 800 image at 10mins was the cleanest. Mind you, using a 10min sub each time will take a significant proportion of your night and may result in lost subs due to wind etc. Hence of late I'm generally sticking with 180-270sec subs at ISO 1600.
If the moons out you may need to reduce the exposure time and/or ISO as you'll get a washout effect and you may have trouble distinguishing your target from the background. For brighter objects like the Tarantula it's possible, however you may struggle when compared to say The Helix.
Cheers, Evan
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I find that this is largely accurate:
http://www.sensorgen.info/CanonEOS-600D.html
Following this I personally wouldn't bother going below ISO 800. The read noise does continue dropping but so does the dynamic range.