ZWO ASI 2600mm and 56mm Back Focus

[Fabiano]

Hello all, I just bought a ZWO ASI 2600 and attached it to my EFW 36mm filter wheel using the 4 screws. How do I get the required 56mm back focus (55mm plus 1mm for the filter) with this setup?
So far I have: telescope side - T2-M48 (16.5mm) - EFW (20mm) - ASI (17.5mm).
This adds up to 54mm.
The T2-T2 (2mm) is not used if the camera is directly attached to the EFW, so I am missing this 2mm. What am I doing wrong?

[Ouroboros]

6 hours ago, Fabiano said:

Hello all, I just bought a ZWO ASI 2600 and attached it to my EFW 36mm filter wheel using the 4 screws. How do I get the required 56mm back focus (55mm plus 1mm for the filter) with this setup?
So far I have: telescope side - T2-M48 (16.5mm) - EFW (20mm) - ASI (17.5mm).
This adds up to 54mm.
The T2-T2 (2mm) is not used if the camera is directly attached to the EFW, so I am missing this 2mm. What am I doing wrong?

The required back focus is usually 55mm. The rule of thumb is that filters add 1/3 their thickness to focus. You can get sets of thin plastic spacer rings if you need to add back focus.  Search for spacer rings on FLO (or similar supplier) and you’ll find a choice several different sizes/manufacturers.

PS anyway it might be best to assemble it and try it. Star shapes will tell you whether the back focus needs changing and whether you need more or less. 

Edited 22 hours ago by Ouroboros

[Ouroboros]

Welcome by the way, @Fabiano.  

[michael8554]

Why do you need 55mm ?

Maybe you have a Coma Corrector or Field Flattener, you haven't included one in your addition ?

Michael

 

[Fabiano]

Thank you Ouroboros and Michaell!! Well, I have a flattener, that's why I am trying to get 55mm back focus. But I'll try with my current setup (I live in a cloudy area, so I need plenty of time yo make tests).