#1  
Old 11-06-2010, 09:00 AM
rcheshire's Avatar
rcheshire (Rowland)
Registered User

rcheshire is offline
 
Join Date: Apr 2010
Location: Geelong
Posts: 2,617
Extenders for AP.

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.

Last edited by rcheshire; 11-06-2010 at 10:11 AM.
Reply With Quote
  #2  
Old 11-06-2010, 02:34 PM
Moon's Avatar
Moon (James)
This sentence is false

Moon is offline
 
Join Date: Oct 2008
Location: Melbourne
Posts: 1,147
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
Attached Thumbnails
Click for full-size image (zoom.JPG)
11.7 KB13 views
Reply With Quote
  #3  
Old 11-06-2010, 03:09 PM
rcheshire's Avatar
rcheshire (Rowland)
Registered User

rcheshire is offline
 
Join Date: Apr 2010
Location: Geelong
Posts: 2,617
Thanks James, that's very helpful. Seeing is believing.
Reply With Quote
Reply

Bookmarks

Thread Tools
Rate This Thread
Rate This Thread:

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off

Forum Jump


All times are GMT +10. The time is now 06:37 AM.

Powered by vBulletin Version 3.8.7 | Copyright ©2000 - 2024, Jelsoft Enterprises Ltd.
Advertisement
Testar
Advertisement
Bintel
Advertisement