[rcheshire (Rowland)]

I have a question about the pros and cons of using an extender, such as a Barlow or Powermate for AP. I asked the same question rather awkwardly in another post.

Armed with a little more knowledge, I'm questioning just how far one should go with additional magnification.

At f/6 a 4x extender reduces aperture to f/24, and for an equivalent exposure (same ISO) increase exposure time by 16 - would that mean more noise and an increase in SNR - I think the maths is correct?

I guess my question is, for LRBG photography with a DSLR, what are the practical limits? Is there an optimal ratio, in practice, between increased magnification, exposure time, and ultimately image quality? I guess that auto guiding would be a factor to consider also. But for arguments sake, let's say no auto guiding.

Another way of looking at it. How far can you practically push additional magnification until it becomes a matter of questionable compromises.