I would like to take some pics of the sky. I wont be able to get much cos I dont have a scope and I dont have a driven mount for my camera. (I dont know whether to save up and get my dream lx200, buy a cheapo ETX 70/80 or a DS2090. Or give the double arm scotch mount a whirl) anyway, thats another topic for another day!

My question is this....
How long can an exposure be before the star/point of focus starts to trail?

I know I could do some test shots but I will struggle with focusing and that will blur my stars on top of any trail. I want to get round my exposure time head scratch first.

I know the stars will certainly move during the session as a whole but I can fix that. If my frames are smudged its game over.


Ive only ever used a 12" LX200GPS with a posh ccd and also spectrmeter. That was at OAM on Mallorca about 4 years ago..... I had my very first go last night with my camera and this is what resulted. Im quite happy it. I only took one batch of pics and had one crack at reducing the data. And to throw a spanner in the works my tripod mount slipped during the exposures. It slipped so much that the moon didnt even overlap when you compare the first frame to the last. I was stunned when it still matched up ok!

Moon
9/12/08 21:33 approx 0'c
Canon 350d w/Canon 90-300 @ 300 (£30 ebay jobby)
30 frames of 1/1000s f5.6 ISO200
5 dark frames of 1/1000s
Reduced and combined in MaxDSLR.... only one High-pass kernel sharpening filter was used. I used Planet Stretch.
Cropped and reduced to 75% of original
http://www.bautforum.com/picture.php...&pictureid=382

How long you can expose without trails depends on the lens used and where it is pointed. The sky's rotation is fastest near the the celestial equator (0 degrees declination) and least near the pole. Its rate decreases with the cosine of the declination.

Simply take a test photo and see what you get. Back in film days I'd have worried about things like this as film was expensive but in this digital age it costs nothing to do a lot of experimenting. I'd start around 15 seconds. It all depends on what you can tolerate.

I built first a simple single hinge drive for my 50mm lens with 35mm film that worked very well for film shots up to 10 with me turning the screw a quarter turn every 15 seconds. I later built a double arm version hitched to a circa 1930 wind up alarm clock. It drove lenses up to 400mm very well up to 30 minutes though I used a small spotting scope with cross hair eyepiece for a guider so the set up was complicated but all I could afford as a college student working my way through school nearly 50 years ago.

Today there are stacking programs that can stack a series of short exposures adjusting for the rotation allowing the equivalent of much longer exposures. Not as good a a single exposure of the same total time but sure a big improvement just the same. DeepSkyStacker is a free program that appears able to do this. If you use one of the reject methods it would eliminate planes and satellites at the same time.

Simple star trail shots can be quite interesting as well. If light pollution is really bad you can again stack a series of short exposures this time without alignment to create star trail shots that would be destroyed by light pollution prior to the digital age.

Rick

Thanks for you reply Rick.

"The sky's rotation is fastest near the the celestial equator (0 degrees declination) and least near the pole. Its rate decreases with the cosine of the declination. "

That is something I should remember! Im going to have to dig my OU books out.

Its good to hear how you got on with the scotch mount. 400mm for 30mins! If I can get remotely close to that success id be happy. I might have to get building this weekend. Or bidding on ebay!