#1  
Old 05-02-2020, 07:37 PM
Martin Pugh
Registered User

Martin Pugh is offline
 
Join Date: Nov 2006
Location: Canberra, Australia
Posts: 1,339
Adding filters

Hi.

It is generally accepted that a 3mm thick filter adds 1mm to your overall backfocus (refractive index of glass). A 2mm thick filter adds 0.6mm.

If said filter (UV/IR block) causes internal reflections, and you choose to remove that, and place a UV/IR block filter on the front of your lens instead, am I right in thinking that this does not impact your backfocus or if it does, and since those kind of filters are very thin, the impact is minimal to negligible.

cheers
Martin
Reply With Quote
  #2  
Old 05-02-2020, 09:39 PM
JA
.....

JA is offline
 
Join Date: Oct 2016
Location: Melbourne, Australia
Posts: 2,945
Quote:
Originally Posted by Martin Pugh View Post
Hi.

It is generally accepted that a 3mm thick filter adds 1mm to your overall backfocus (refractive index of glass). A 2mm thick filter adds 0.6mm.

If said filter (UV/IR block) causes internal reflections, and you choose to remove that, and place a UV/IR block filter on the front of your lens instead, am I right in thinking that this does not impact your backfocus or if it does, and since those kind of filters are very thin, the impact is minimal to negligible.

cheers
Martin
Hi Martin,

I think you are right to think that FRONT mounted filters (before the converging optics) will not change the focal plane distance from the back of your telescope/lens (Backfocus), since the front mounted filter will be dealing with parallel ray bundles (night sky at infinity focus) which since they are parallel rays will all be refracted equally by the FRONT/external filter; however: if that same filter is placed anywhere within or after the converging optics, where the light ray bundle is not parallel (where it is converging) then the light rays will all be refracted by varying degrees by the filter, due to their differing angles of incidence and path length through the glass. And also, of course, the thicker the filter (and the greater its refractive index), the greater the effect on focal plane change.

I THINK

It'd be better with a diagram, but ....

Best
JA
Reply With Quote
  #3  
Old 05-02-2020, 10:05 PM
Atmos's Avatar
Atmos (Colin)
Ultimate Noob

Atmos is offline
 
Join Date: Aug 2011
Location: Melbourne
Posts: 6,980
I think JA is spot on, that’s been my understanding as well.
Reply With Quote
  #4  
Old 05-02-2020, 11:22 PM
ericwbenson (Eric)
Registered User

ericwbenson is offline
 
Join Date: Sep 2009
Location: Adelaide, Australia
Posts: 209
Hi,

See here for a diagram and some math to go with it:
https://www.faintgalaxy.com/focusshift.html

Yes changing the thickness in the collimated section does nothing, changing the thickness (or glass type) in the converging section causes the shift.

Best,
EB
Reply With Quote
  #5  
Old 06-02-2020, 09:59 AM
Martin Pugh
Registered User

Martin Pugh is offline
 
Join Date: Nov 2006
Location: Canberra, Australia
Posts: 1,339
Yes, thought as much. Needed confirmation.

thanks for the replies
Martin
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 04:19 AM.

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