77> Thanks!
77> Francis J. O'Reilly
Francis,
I would suggest that you compute your primary/secondary separation so that the focal length extends to a point that is in the middle of the travel of your focuser.
For instance, with an 8" f/6 (focal length of 48" as measured with a foucault test device or a reflected focused image of the sun on a wall) and the following "other" considerations:
10" od tube; focuser that has a racked in height of 2.5" and a drawtube travel of 2" (for example)
tube wall thickness of 1/4"
Calculate as follows:
1/2 the od or 5 inches Plus
2.5" for the focuser height Plus
1/2 the travel or 1"
this comes to 8.5". Subtract this from the true focal length of the primary mirror (48" - 8.5 ") and you get
39.5" separation between the primary and the secondary. This distance should be the measurement from the center of the primary to the center of the secondary. We'll not quibble over the sagitta of the primary or the offset of the secondary. That should be accounted for in the amount of travel in your focuser.
Mike
* RM 1.3 * Multitasking = screwing up several things at once.