I was reading https://astronomy-im...tions-55mm.html which sets out how to achieve 55mm backfocus with common combinations of equipment using ZWO cameras. One thing that caught my eye is that when using the filter wheel (with or without OAG) on a small format sensor it actually sets out 56mm of backfocus, with the fine print saying "Add 1mm based on optical path difference of the filter". I am curious though - when looking at the filter drawer backfocus combo, it only achieves 55mm, seemingly making no allowance for the filter thickness if it's added to the optical path via a drawer as oppose to a wheel? Anyone know why?
With my current filter solution (screwing it into an extender ring in the optical path AFTER the field flattener) backfocus did not appear to change with or without the filter so I assume that any difference it made is within the margin of error for the scope backfocus requirement (which is 55mm from the end of the extender ring that screws into the field flattener.) Just curious if this would, for some reason, be different with an EFW as ZWO seems to suggest? My line of thought being currently I have 55mm of physical backfocus plus a 1.85mm thickness filter in the optical path, but if an EFW went in there instead I'd actually be at 56mm plus a 1.85mm filter, possibly pushing me outside the margin of error that I currently seem to operate within?
Hopefully this question makes sense.. Thanks