Originally posted by ErZi I need to get back in action this winter.. so many good pictures. Why I can't get as good as those
I'm using 300mm prime and o-gps. I find that about 10s exposures are longest ones for me.
Maybe it is my tripod or the calibration of the o-gps but I'm not getting any decent pictures on longer exposures. I also get feeling that I need to do re-calibration quite often (every 1-2h or so).
I should also try some wide field astrophotos too.
I think I have some answers for you. Its not your tripod nor the calibration of the GPS unit. It is your geographic location that are the root of your problems. Let me explain. The GPS satellite constellation flies in orbits that oscillates between 60 degrees North and 60 degrees South around the Earth. You are located near Helsinki Finland, if I am not mistaken, which is at 59 degrees North. So the GPS satellites fly up from the south, and essentially turn overhead and head back south again.
What does that mean to you. Well GPS is a distance triangulated system. The GPS receiver essentially gets information from 4 satellites (latitude, longitude, elevation and time) that is converted in to a distance from the known position of each satellite. If all the satellites are clustered in a single area in reference to your location, you are going to get poor fixes. You need a good spatial dispersion of the satellites for perfect fixes. A perfect spatial dispersion would be about 120 degrees apart around you with one overhead. However all of your birds are to the south of you - i.e., at best half the optimal coverage. You will get location fixes, but their quality will be somewhat wanting.
The O-GPS unit in order to do astro tracking, will want a good fix, with a good heading (direction in which the camera is pointed), with the smallest error possible, with good consistent uniform updates. You are probably getting good updates, with a random set of bad geometry (the satellites cluster more tightly from time to time, which will reduce the quality of the fix). This is probably leading to your needing to re-calibrate more often. Also, with a location and a larger than normal error component, coupled with a long focal length which is going to have a very narrow field to track in, you are going to be subject to a perfect storm of events, which leads to abbreviated tracking opportunities.
Here is a link discussing GPS location and navigation in the polar regions.