Silly ISO question

[JKB]

So although quite new to this AP lark I have had some quite good success so far (well I am happy!) but was wondering.....

With the lighter nights is it better to drop the ISO rating down to say 400 or 640 rather than 800 that I have used thus far?

I quite fancy a go at M31 tonight which is reasonably large and bright. If I use 400ISO will it lose data from the galaxy but help keep the noise from the lighter sky down? or should I stick to 800?

Thanks for any help

Jamie

[JoLo]

Outside of using a light pollution filter, changing the ISO or exposure times are your only ways of controlling the brown out of light pollution. It really is a local phenomenon and essentially boils down to what you will tolerate for light pollution. If your LP is objectionable at a certain exposure length, try the same picture with a shorter exposure or lower ISO.

As with all things in this hobby, it is a tradeoff.....do you want to collect more signal (and noise), or will you sacrifice this a bit to keep the light pollution down. Processing will help to an extent, but it really comes down to your personal esthetic choices.

[dph1nm]

If I use 400ISO will it lose data from the galaxy but help keep the noise from the lighter sky down? or should I stick to 800?

Changing ISO won't change the signal-to-noise, so it is irrelevant as far as the sky goes - only altering the total exposure time can do this. What it may do is prevent you from saturating the centre of M31, which I guess is a good thing.

NigelM