Hi, Please check this out and see if there is a problem. I need to spec the length of a "custom ring" between a field flattener and my FW8/STF-8300 I (now) have a SkyWatcher 120mm ED Esprit Refractor that requires 75mm backfocus from field flattener to the CCD sensor. My sensor is a STD-8300 with FW8 w/o adaptor; the filters are 2mm thick (Baader 36mm). The backfocus chart I have from Diffraction Limited gives 34.7mm So the "custom ring" I will spec for PreciseParts.com should have an effective length of (75 - 34.7) = 40.3mm . Make sense? Thanks and clear skies!
Hi Pete - was good to see you at NEAF ! You have calculated that correctly, assuming you don't have an adapter plate to have a different thread size on the top of the FW. If you do, I would plan for 3-4mm, shortening that 40.3 to say 37.5mm.
Colin, The picture of the FW8-8300 on the Diffraction Limited website (http://diffractionlimited.com/product/fw8-8300/) displays the FW8 with that 3.7mm adapter plate, correct? (I got he 3.7mm from the backfocus table "trjeaded adapter plate adds) Thanks!