Help with back focus

[smr]

Hi, 

I'm trying to work out how to achieve the correct back focus with my Z73 and Field Flattener and the ZWO 2600MC Pro. 

 

I have the original Z73 Field Flattener pictured here:

 

Here's the William Optics Back Focus Diagram...

 

The back focus distance required looks to be 54.8mm. The ZWO 2600 MC Pro comes with the following spacers

 

That would achieve 55mm of back focus and not 54.8mm.

Would this be ok?

[rickwayne]

Gotta try it to be sure. That's one downside of those big sensors, you've got a lot of field to deal with! You can get shims -- basically just plastic washers -- in fractional-millimeter thicknesses, so if your testing indicates problems you could  get a slightly shorter adapter tube (e.g. 16mm) and build up the required difference with those.

Here's a useful little visual aid for deciding whether you have too much or too little backfocus.

adjusting field flattener.pdf

Edited January 18, 2021 by rickwayne