Per the STXL manual (revision 1.3, December 2012), the optical backfocus of the camera is 24.33 mm to the front of the camera (with no adapter). The optical backfocus of the camera and standard filter wheel is 43.76 mm, which implies the filter wheel is 19.43 (43.76 - 24.33). However, when I measure my standard filter wheel from the back to where the 3" adapter mounts (the "mounting surface"), I get 24.95 mm which is 5.52 mm longer that what's in the manual. Likewise, per the manual, the optical backfocus of the camera and self-guiding filter wheel is 60.86 mm, which implies the self-guiding filter wheel is 36.53 mm (60.86 - 24.33). When I measure my self-guiding filter wheel from the back to where the 3" adapter mounts, I get 41.7 mm which is 5.17 mm longer than what's in the manual. Am I measuring incorrectly or is the manual wrong or have the filter wheels become thicker since the manual was written?
There's a window in the camera, and there are filters in the filter wheel. Did you take that into account?
Colin, I don't think that matters. Whether or not a filter wheel has a filter in it doesn't change the physical depth of the filter wheel. And I used the camera's optical back focus but get the same difference if using the camera's mechanical back focus, per the manual. Is my reasoning correct, that per the manual, if : Camera = 24.33 mm (the first diagram below) Camera + standard FW = 43.76 mm (the second diagram below) Then standard FW = 19.43 ?? (the amount between the two thick blue lines) Am I also correct in thinking the 19.43 is the physical depth of the filter wheel? If so, I'd like to understand why I get 24.95 when measured with a micrometer. I've tested my micrometer and it's pretty accurate. Camera back focus, page 30 in manual: Camera + standard FW back focus, page 31:
Eric, when you install the filter wheel on the STXL, you remove the entire red front cover of the camera, and replace it with a red spacer.
Colin, Thanks, that clarifies things. Looking more closely at the manual, with cover for the camera (first diagram above) is taller than the red spacer used with the filter wheel (second diagram), and the 24.33 mm camera back focus is with the larger cover. Since I have a FW I should ignore the 24.33 mm camera-only back focus. That was my mistake in thinking. Per the manual, the (camera + FW back focus) is 43.76 mm and I measured my standard FW at 24.95, which means the back focus of the camera with spacer is 18.81 (43.76 - 24.95), right? That implies the spacer is 5.52 mm (24.33 - 18.81) shorter than the camera-only cover. Does that sound right? I bought the camera with the filter wheel and didn't get the larger cover so have no way of verifying my numbers.
That's exactly the mistake. I agree with that. That seems off. Perhaps your measurement included the 5.1mm thickness of the adapter ring that connects FW to the focuser draw tube, and that would add up to about 24.5mm. Plus there is a raised area on that "telescope side" and also the red spacer is slightly inset. Wrong - not the "camera with spacer". Have you tried our Backfocus Calculator spreadsheet? You can get it here: https://diffractionlimited.com/downloads/BackfocusCalculator.xlsx It gives: STXL Series 24.1mm FW8S-STXL 19.4mm SubTotal: 43.5mm (which is almost identical to 43.76mm from the manual) Less 3mm thick filter 66% index of refraction = -1mm Total: 42.5mm Yes, the spacer is shorter. The idea is it is designed reduce total backfocus.
Totally agree with Colin - just use our back focus calculator spreadsheet. It's accurate to within camera tolerances (there can be some very small variability in the height of the thermoelectric cooler). https://diffractionlimited.com/downloads/BackfocusCalculator.xlsx
