Digital Camera Question

[neill]

Hi all,

As you may or may not know, I have a Canon 350 D and am looking to learn more about the workings of it. My question deals with the combination of the f number and exposure times.

What is a good f number for taking astro pics or does it depend on the type of object you are capturing, i.e. moon, planets, deep sky, wide field shots etc.

Also what is the maximum exposure time for the different f numbers before you start getting star trails etc? What are good exposure times for different objects like the moon, planets, general sky shots etc before they get overexposed?

I appreciate that there are a few questions in there but any help would be useful.

Thanks

Neill

[Administrator]

Hi Neill,
I have the 350d also but I only have experience of widefield astrophotography, although I have dabbled in prime focus astrophotography also.

I find that the effect of star trails depends on how far away you are from Polaris, as well as exposure time. Cassiopeia for example is quite close and I can get 25-30 secs with a 50mm lens, remembering that the equivalent of a 50mm lens in a 350d is 50/1.6=31.25mm. If I take the same photo further away from Polaris I will only get about half that exposure before getting trails. At maximum telephoto which on my lens is 115mm, I can only manage 6-8 secs before getting trails.

The f number does not affect the appearance of star trails, but does affect the sharpness. I prefer to shoot at f4 as this is when the lens starts getting nice and sharp, although shooting wide open can be just as aesthetically pleasing as it makes the stars appear 'bigger'

When doing longer widefield exposures, over 30 seconds say, the main problem is light pollution. At 60secs, under most conditions, the whole frame becomes red with LP. Probably the best thing to do is take multiple 25sec exposures and 'stack' them. Although I haven't tried it myself I have seen the results!