Tue 13-Nov-12 09:53 AM | edited Tue 13-Nov-12 10:07 AM by polardan
Here's a quick one. D700, 20mm lens, ISO 4000. F2.8. 30 second exposure.
I can't help you with the D90, sorry.
There is a lot of trial and error in getting the composition correct. However the sky doesn't tend to move away too quickly! In this shot, I was able to shine a torch on my boy to help compose (then set the timer and scramble up to join him) , but still needed several shots to get it all level. It was a absolutely pitch black otherwise (new moon, middle of the Australian desert), and stars were not really visible through the D700 viewfinder.
How much star movement depends on the time and the focal length. There are many on here far more educated than me on the subject so I won't go into it. But basically wider angle, less star movement, longer exposure possible.
The blurred stars at the top of the image are caused by the lens. Stopping down to F8 should prevent this, which I will try next time.
I have found that adjacent star images (with the same settings of course) join very well in panorama programs and I did this to get rid of the blurred stars in the corners of images.
The other thing you might look into is image stacking. Basically several photos taken consecutively, then blended by a program that realigns them.