f-stop for astro photography

[vulcan]

I have a Nikon D200 and intend using my f1.4 50mm lens for some constellation shots. Is it better to use a wide aperture (short exposure) or a longer exposure with a smaller aperture? I am using a NextStar 6se with piggy-back mount. I think the optimal aperture for such a lens (for sharpness) in probably f5.6-8. Obviously depth of field is not a problem! Also, what about ISO rating?

[FraserClarke]

Astro stuff is really really really low light. General rule is to go as fast as you can and as high ISO as you can -- and even then you'll need a decent length exposure. On a D80, I get the main stars in constellations with 30s at ISO800 at f/2.8. You should do very well with the f/1.4 I'd have thought.

[pete_l]

Photographing stars is probably the hardest test you can give a lens. Even high quality branded lenses don't always give top quality results when used wide open. Be prepared for a few surprises.

If it was me, I'd try some test exposures: give F1.4 a bash, but also try slower settings - all the way down to F4. it might break your heart to have to "waste" all that expensive glass by having it stopped down so much, but when you look at wide field views from others, you may well find that they are using similar settings in order to preserve pinpoint sharp stars.

The other thing to be aware of is that the faster the lens, the more critical focus becomes. Since the light cone from the lens to the sensor converges much more steeply for a wide open lens (since the light come is wider from a wide open lens, yet still has to converge to a single pixel) than for a stopped down one, getting the point of exact focus will be harder, too. That will only really show up when you look at the full resolution image afterwards.

[brianb]

Yeah ... f/1.4 is really rather more than the design can cope with but my Canon 50mm EF f/1.4 lens is pretty crisp at f/2 ... you will find that focussing is critical too!

[ollypenrice]

It is a trade off between speed and nice shapely stars in the corners. You just have to experiment. You won't get away with wide open, though, I bet.

Olly

[Psychobilly]

I normally use the nifty 50 (f1.8) at f4... if we ever get clear skies I am goign to try opening it up a 1 or 1 1/2 stops and correcting the dodgy stars usign the Lens Correction tools in CS...

Billy...