Hi. What are the correct back focus values for a ST-8XME and CFW8 combination? I've always been using: ST-8XME: 0.742" = 18.8 mm (with the 'newer' 0.25" cover plate and no D block) CFW8: 1" = 25.4 mm So for the combination 1.742" = 44.2 mm. This is from the file "stbackfocus.zip" in the archive (https://diffractionlimited.com/downloads/sbig/AppNoteArchive.zip). Is this correct? I have a hard time finding any other references to this value and of course its always a critical value when determining if a certain image train will work, and for purchasing adapters etc. Thanks, Keith
Those numbers look correct for the mechanical backfocus, based on the CAD models. Please be aware that the optical backfocus is slightly less. You need to subtract 1/3 the thickness of the glass elements (based on the typical range of refractive indices). The window in the front of the camera is 0.125" thick. You also have to account for the thickness of the filters in the CFW8. So subtract 0.042" for the front window. Assuming 3 mm filters subtract 1 mm or 0.039".
