#1  
Old 09-05-2013, 07:41 AM
Kev11 (Kevin)
Registered User

Kev11 is offline
 
Join Date: Jun 2011
Location: SE NSW, Australia
Posts: 68
Optical Geometry - Eyepiece with DSLR

Is there anything on-line which will let me calculate the spacers necessary to focus images onto the DSLR with an eyepiece in the train (my high school optics was 53 years ago!)? I have a standard SW 8" and have successfully used a 25MM EP (Celestron ordinary) in the standard Bintel nosepiece/T ring with my Canon. However no other EP, longer or shorter FL, will come to focus.

Reply With Quote
  #2  
Old 15-05-2013, 08:25 PM
Andrew C
Registered User

Andrew C is offline
 
Join Date: Oct 2007
Location: Adelaide
Posts: 82
possible alternative

Kevin,

I wonder whether you can find out what is going on by removing the nosepiece/T ring temporarily and focussing by eye on a reasonably bright, crisp image (e.g. the moon, street light) using each eyepiece in turn. In each case, then replace your eye with a sheet of white paper and move it back and forth until the image is clean. That might give you a rough idea of how far the focal plane is behind the EP in each case, and you can go from there.

Cheers,

Andrew
Reply With Quote
  #3  
Old 16-05-2013, 08:39 AM
Wavytone (Nick)
BigBanger

Wavytone is offline
 
Join Date: Jul 2008
Location: Killara, Sydney
Posts: 3,997
Kevin,

Diagram here:

http://www.google.com.au/imgres?imgu...9QEwAw&dur=375

If the focal length of the telescope objective at A is F and the focal length of the eyepiece is f, then:

1/f = 1/BC + 1/CD

magnification m = CD/BC

Effective focal length of the entire telescope, with eyepiece and camera = mF
Effective focal ratio of the entire telescope = mF/A, where A is the clear aperture of the objective or primary mirror. Note also that F/A is simply the focal ratio of the telescope objective when you are using it visually.

To focus the image on the camera sensor you need to shift the eyepiece out (away from the objective) so that BC will be a bit longer than f.

For a magnification m=1, the distance BC = 2f and CD = 2f, so the distance between the primary focus and the camera focal plane = BC+CD = 4f. For a 25mm eyepiece, this is 100mm. To this you must also add the physical length of the eyepiece optics from the field lens to the eye lens, which - depending on your eyepiece - could add anything from 25 to 100 mm.

If you are using a 25mm eyepiece, the distance CD from the eyepiece to the focal plane in your camera must be > 50mm and if m is say 3, it must be > 75mm, give or take a bit.

So, for a 25mm eyepiece this implies the distance between the primary focal plane at B and the camera sensor at D will be at least 120mm, and could be as much as 200mm or more.

If you are having problems focussing... my guess is that you have the eyepiece too close to the camera.

Last edited by Wavytone; 16-05-2013 at 11:16 AM.
Reply With Quote
  #4  
Old 19-05-2013, 09:27 AM
Kev11 (Kevin)
Registered User

Kev11 is offline
 
Join Date: Jun 2011
Location: SE NSW, Australia
Posts: 68
Thanks Andrew and Wavy, I will pursue those ideas when I get a chance.
Kevin
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 02:20 AM.

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