Capture time with scope on tripod

[Taff]

Hi' early on i used my 80mm F400mm scope on the camera tripod.I set the camera at 800 iso aimed at M31 each frame was 15 secs long.When i had a look at the captures on the PC all i had was star trails.I thought 15 secs would have been OK.Or have i got the camera on the wrong iso settings when useing a scope on a camera tripod Thanks.Mark

[iamjulian]

I left this one because I thought someone would be able to provide a better explanation than me, however , If you photograph the stars with just your camera, then 15 seconds is probably fine - the stars don't move much in 15 seconds if you just look up with the naked eye.

However, when you start magnifying them, by shooting through a telescope, then the appear to move a lot faster. Think about it in the same way that if you look through your highest power eye piece, the stars move a lot faster than with your lowest power eye piece.

The ISO won't have any impact on star trails. It is the shutter speed that you need to increase. However, if you increase it enough to stop the stars trailing, perhaps less than a second, the amount of light that can enter your camera is probably going to be too small. You therefore need to have a way of moving your camera/telescope at the same sidereal rate as the stars.

[jgs001]

Mark, the longer the focal length, the more the movement is magnified... I think at 400mm you're looking at 2 or 3 second exposures to avoid trailing. With my Z2 at 380mm focal length equiv, I was operating at 3 seconds.

The 18-55 at 18mm, it's 25-30 seconds, 50mm is about 8-10 seconds, and that's for acceptable trailing, which means you have to look at the stars in real close to see the trailing.

Someone posted a scan of a table which gave approximations for static tripod exposure lengths.. it was about 1.5 years ago though... I've never been able to find it since...

[Taff]

Thanks for your reply.Mark

[brianb]

Someone posted a scan of a table which gave approximations for static tripod exposure lengths

Don't need a table, just divide the focal length of the lens / scope (used at prime focus) into 500, that will give you the longest possible exposure (in seconds) you can get away with without significant trailing. e.g. 50mm: 500/50 = 10 secs.

You can actually get away with a bit longer than this for stars near the pole, which appear to move more slowly. Divide by the cosine of the declination if you can be bothered.

[Taff]

Hi Brian' Thanks for your help.Mark

[Yoddha]

John,

Do you talk about table like the attached?

I saw it today in this article : GALLERY-Articles; Astrophotography... in the part "Camera and Tripod". The Focal length is in mm the next columns are in seconds.