BackFocus - WO GT81 - Filter Drawer - EOS DSLR

[xa-coupe]

I've finally decided to work on setting backfocus on my setup. It's seems straight forward until you add something like a filter drawer.


A WO GT81 with a reducer needs a 55mm backfocus ( according to the WO site) but a canon DSLR has a 44mm flange to sensor distance, and a filter drawer (ZWO) adds 31.5 mm ... making 75.5mm or 20.5mm more than I need.


2 questions;


1: am I adding this all up correctly?
2: If I am, other than removing the drawer and adding the filter 'permanently' is there anything I can do?


TIA.

[Harpage]

You're adding it up correctly Jeff, but remember that your T-Ring will also add about 11mm backfocus (to add up to 55mm), so you're actually at 86.5mm.

As to solutions, well really you'd have to either use a lower profile T-Ring and filter drawer (former exists, not sure about latter), or just thread the filters right onto the flattener. It has M48 threads I believe.

[xa-coupe]

Thank you Danh, I've been looking further into it and have now come to the same conclusion as you; I have a great filter drawer for when I buy a dedicated astro camera and I will screw the filter directly in to the threads on the flattener.