Extenders for AP.

[rcheshire]

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.

[Moon]

Here is a section of a recent image I took with no reducer or extender. The image scale is about 1.1 arc seconds per pixel. If I added an extender, I would be magnifying this fuzzy image. I would question if you went too far beyond 1 arc sec/pixel. That's one problem.

Second problem - my auto guiding needs to be better.

Third problem - the field of view is reduced. Imagine if you only had one lens for your SLR and it was 500mm. It would start to get frustrating when you want to take a picture of a nice landscape. Rosette nebula / Andromeda Galaxy - these things are huge and need a short focal length. If you are imaging a tiny planetary nebula or planet then there is a case for it. In my situation, these cases are rare and widefield is the way to go. The wider the better!!

James

[rcheshire]

Thanks James, that's very helpful. Seeing is believing.