An example. The RHA has a backfocus of 162mm. Being precisely made I assume that is accurate to +/- 1mm.
The measured distance it comes to focus at is 165mm on my scope.
The difference is the +1mm for the 3mm filters and about the same again for the CCD optical window and the remaining 1mm is probably manufacturing tolerance.
There is a note on the astrodon website explaining that 1/3rd the thickness of filters needs to be added to backfocus calculations.
The term "Optical backfocus" applies to this being accounted for. So any glass the light goes through add 1/3rd the thickness. So the CCD window is probably 3mm and your filters may be 3mm (Astrodon or Baader, Astronomik are only 1mm). So you would add 2mm to the mechanical backfocus to get the optical back focus which is the one you need to work off.
From the Astrodon Website:
- Please note that camera manufactures often specify the MECHANICAL back focus of their products. 3 mm thick filters will ADD 1 mm (0.04") to that mechanical backfocus as measured from the telescope or from the MonsterMOAG prism. The camera window, often 3 mm thick will add another 1 mm (0.04"). So, You may have to add 2 mm (0.09") to the mechanical backfocus.
- There may be some confusion as camera manufacturers measure backfocus from the focal plane of the CCD to the outer surface of the camera. When they account for the thickness of the filters, the SUBTRACT the 0.04", which is correct as measured from the CCD. However, most people measure backfocus from the back of their scope of from a corrector, and then add/subtract spacers to arrive at the correct backfocus. In this case, as measured from the scope, the 0.04" must be ADDED. A subtle point, but does get people in trouble from time to time.
- Also, please note that different filter wheels from the same manufacturer may have different backfocus, as indicated in the list above.
Greg.