Quote:
Originally Posted by toryglen-boy
many thanks for that, but its far too much like hard work !! 
can you post a practical example? |
Example 1:
8" scope of 64" focal length, so has a focal ratio of 8
3" scope of 15" focal length, so has a focal ratio of 5
Say you take a 2 min exposure with the 3".
Then you need a 2*(8/5)^2=2*64/25=5.12 min with the 8". Bear in mind though that you'll have an image about 4 times bigger in scale.
Example 2:
8" of 40" focal length (so it has a focal ratio of 5)
4" of focal length 28" (so a focal ratio of 7)
5 min exposure with 4" equates to 5*25/49=2.5min (approx) with 8"
Note added: Like many things you can't just take such calculations as gospel. In the first example the exposure in the 8" may be fine for the neb, but since stars are point sources, you may end up overexposing the stars and so it may be better to stack two 2.5 min exposures.