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prism and the focal plane of the imaging camera (
B
). If too
close, the focal point will be inside the helical focuser on the
MMOAG. If too far, the focal point may be too far above the
focuser, and require an eyepiece extender or just be too far
away to be stable. Please see our web page for a more current
set of distances and help
The backfocus distances for some popular large format cameras
at the time of this writing are:
Finger Lakes P CFW-4-7s - 1.8”
Apogee U16M + AI-FW50-7s - 2.0”
SBIG STL/FW8 + STL-to-AP(male) adaptor - 2.2” (1.9”
with internal CFW)
Add the midpoint of the MMOAG where the prism is located to
these numbers, plus the 0.1” raised flange of the fitting (0.625 +
0.1 = 0. 725”), and the values for
B
become:
FLI P CFW-4-7s + MMOAG - 1.8+ 0.7” = 2.5”
Apogee U16M + AI-FW50-7s + MMOAG - 2.0” +
0.7” = 2.70”
SBIG STL/FW8 + STL-to-APmale adaptor + MMOAG -
2.2” (1.9” with internal CFW) + 0.7” = 2.9”
The minimum distance
A
with the helical focuser screwed all the
way in is 2.8”. Add 0.6” for the backfocus of guider cameras,
such as the SBIG ST-402 and the minimum distance
A
becomes
3.4”. The helical focuser has approximately ½” of travel. So, it
