#1  
Old 25-11-2009, 09:48 AM
blueskies123_89's Avatar
blueskies123_89 (George)
Registered User

blueskies123_89 is offline
 
Join Date: Jan 2007
Location: Melbourne
Posts: 89
Unfocused stars

Hi

I've been experiencing this problem for a while - when I focus on the centre of the field, the edge becomes unfocused. I assume this is because the field isn't flat, but I don't know if it should be as severe as what I am experiencing now.

I have attached an unedited 10min sub of M42 in OIII to illustrate the point. The scope is a meade SN8, via baader MPCC and thousand oaks OIII filter. Also attached is an original size crop of the edge of the frame, at the running man region.

The same problem exists without the OIII filter. Without the MPCC I have both the unfocused stars and coma.

Thanks for the help.

EDIT: Camera is Canon 400D. The shot taken at 400ISO (doesn't really matter)
Attached Thumbnails
Click for full-size image (help.jpg)
176.1 KB32 views
Click for full-size image (help2.jpg)
171.8 KB33 views

Last edited by blueskies123_89; 25-11-2009 at 09:49 AM. Reason: camera
Reply With Quote
  #2  
Old 25-11-2009, 10:23 AM
multiweb's Avatar
multiweb (Marc)
ze frogginator

multiweb is offline
 
Join Date: Oct 2007
Location: Hinchinbrook
Posts: 18,138
That's exactly what it is: big field curvature. You need a flat(ter) field to be in focus all across the camera sensor if it's large enough. I'd check your MPCC spacing first.
Reply With Quote
  #3  
Old 25-11-2009, 10:40 AM
blueskies123_89's Avatar
blueskies123_89 (George)
Registered User

blueskies123_89 is offline
 
Join Date: Jan 2007
Location: Melbourne
Posts: 89
Quote:
Originally Posted by multiweb View Post
That's exactly what it is: big field curvature. You need a flat(ter) field to be in focus all across the camera sensor if it's large enough. I'd check your MPCC spacing first.
Thanks, a few questions:

1) I thought MPCC was a coma corrector, not a field flattener.
2) How do i change spacing on it, I only bought the MPCC, not the set. Do I need to get the set of spacers and stuff?

Thx
Reply With Quote
  #4  
Old 25-11-2009, 10:48 AM
multiweb's Avatar
multiweb (Marc)
ze frogginator

multiweb is offline
 
Join Date: Oct 2007
Location: Hinchinbrook
Posts: 18,138
The MPCC is primarily a coma corrector but it will significantly flatten your field too. In my measurments I got a 20% improvement on my newtonian in field curvature with the MPCC. Ideally the back of the MPCC (glass surface) should be 55mm +/- 1~2mm from your sensor plane. You can get different spacers to achieve the correct distance that works for you.
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 08:48 PM.

Powered by vBulletin Version 3.8.7 | Copyright ©2000 - 2019, Jelsoft Enterprises Ltd.
Advertisement
Bintel
Advertisement
NexDome Observatories
Advertisement
Meade Australia
Advertisement
OzScopes Authorised Dealer
Advertisement
Lunatico Astronomical
Advertisement
Celestron Australia
Advertisement
SkyWatcher Australia
Advertisement
Astronomy and Electronics Centre
Advertisement