I have found a discrepancy in the documentation regarding the backfocus distance for the STXL cameras. According to the original STXL camera manual, the optical backfocus distance for the camera plus FW8G-STXL filter wheel is 2.357", but according to the backfocus calculator spreadsheet on the Diffraction Ltd website, this distance is 2.347". Can you confirm which is correct?
Your focuser should be able to accommodate the 1/100 of an inch discrepancy. So I wouldn't worry about it.
In practice there is some variation in the thermal interfaces for the sensor, resulting in some variability from unit to unit. Most of the spreadsheet values were measured from sample cameras. That probably explains the small discrepancy. I agree with Colin, it is nothing to be concerned about.
Thanks Colin & Doug. I was thinking more in terms if I ever have to get another custom adapter made for my STXL camera. Which back focus value should I use to calculate how long my adapter needs to be? Mike
Kindly, I think you're obsessing over an inconsequential distance. If you really need an answer, use the shorter number. That way your focuser has to move out a tiny bit if its wrong. Or you can add shim washers or unthread things a bit. You should be designing the adapter length so that the focuser is at the mid-point of travel. e.g. if it moves a total of 1-inch, then center it at 0.5 inches, so that it can move in and out 0.5inch, which will allow for machining tolerances and filter differences. Standard machining tolerances, as used by somebody making your adapter, are normally +/- 0.005 inches, meaning when a part is made (milled by CNC machining or lathe), it could be long by 0.005 or short by 0.005. Tolerances add up; so every machined parts has a tolerance, and can be under or over by a small amount. The values in the manual and spreadsheet are typical; the manual has an old drawing that was based on the design; the spreadsheet is as measured. If you specify the 2.347, it could come out at 2.352 or 2.342; If you specify 2.357, it could come out at 2.362 or 2.353. The last point is that the optical back focus distance of the filters is based on a typical Refractive Index of 1.5, which translates to a Filter Optical Distance = thickness of the filter / refractive index. e.g. 3mm / 1.5 = 1mm = 0.039 inches. The actual glass used in your filters may have a slightly different R.I., so you'd need to know this from the filter manufacturer, as well as the tolerance on the thickness of the filter. So all this adds up to a small amount of uncertainty.
