Hi Markus,
Greg's advice is very good and his wonderful results speak for themselves. I want to expand on some of his comments.
What ISO to use?
Greg is correct that you can use much higher ISO's than you are using. DxO Mark does noise tests on senors and comes up with a comparative ISO. Your 5Dii has a low light ISO value of 1815. I have two cameras, one is an APS C with a DxO mark rating of ISO1160, the other is ISO3200.
I can use the ISO1160 camera up to 6400 with noise reduction or stacking of 3-4 images
I can use the ISO3200 camera up to 12800 but I do get purple stars. At 6400, image quality is good. I would suggest that you should be able to use at least ISO3200 and probably ISO6400 with good results.
What is the maximum exposure time?
The 600 rule & its variants were derived for film a long time ago. It was derived by assuming that the viewer was looking at a 5x7 inch enlargement at arms length. Human eye can resolve about 1 arc min. 1 arc min at 800mm viewing distance is about 0.25mm on the print. Print is 5X enlarged so drift equates to about 50 microns drift on the film.
The sky moves at 15.04"/s and so(skipping some steps) it takes about 600/f seconds to cause that approximately 50 micron smear on the film at the celestial equator less as declination increases. Using the 600 rule, T(s) = 600/f will result in 50 micron streaks at the celestial equator. Fast film of yesteryear had grains that were 25-50 microns and so it gave the illusion of one star image on one grain.
But all of this calculation is pointless unless we also consider viewing distance and size.
Using the same 600 rule today, 45 microns results in 10 pixels of smear for a typical 4.5µm pixel DSLR viewing at 100%. But who views at 100% these days? My camera produces a 5000x7500 pixel image. I don't have a 7500 pixel screen. If you want to zoom into 100% to inspect details in images, you need to track and guide! But for wide field images, we typically want to enjoy the aesthetic of the whole image and so we can decide on a final display resolution and pick exposure time to suit.
A few years ago there was some discussion about this in these threads :
http://www.iceinspace.com.au/forum/s...d.php?t=105063
and this one
http://www.iceinspace.com.au/forum/s...d.php?t=102081
I derived a different formula for DSLR's that took into account, pixel size and screen viewing size. The original post is below and I've attached an Excel file that does the calculation for you. You can either do a specific calc in the top section or generate a table in the bottom. I've pre-populated both with info for your 5Dmkii.
Revised rule for DSLR's
These rules or formulae are based on a set of starting assumptions and have been kicking around for a long time.
In the old days of 35mm film where very fast films used for tripod astrophotography had very course grains and so a bigger movement could pass off as no drift. It was based on someone looking at a 5x7 inch enlargement at normal arm length reading distance perceiving trails as almost stationary. The eye can resolve about 1 arc minute so it can in fact resolve approximately 250 microns on a print which scales back to 50 microns on the film. Pixels on a modern DSLR are about 6µm. We mostly look at screen based images and images that are highly reduced from full size.
So I've derived a revised formula, modernized for DSLRs.
max exposure time(s) = [14*N*P] / [FL*cos(d)]
where
N..........Number of pixels of drift
to work this out load any full frame image from your camera and scale it to the size you want to view on screen. Look at the scale percentage. Number of pixels drift = 100 / percentage
P..........Pixel size (microns) Most DSLR's are between 3 and 8 microns. You can look it up in your manual or just split the difference and make it 6.5
FL.........Focal length(mm)
cos(d)...cos of the declination. Use the declination of the stars in the field that are closest to the celestial equator in the camera field