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RE: Measuring Focal Length

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Posted by Jack McQuaide on May 27, 2000 08:38:57 UTC


One thing I can tell you from my first scope is to double/triple check all measurements before cutting the focuser hole.

I cut mine BEFORE I got my secondary mirror. I already had my homemade spider mounted and in place. I figured that I was close enough. When I got my secondary mirror (bought a new edmunds sci secondary) it was 4 times as thick as I thought it was going to be! Having never seen a seconday mirror before, I figured it couldnt be very thick, after all it was only 1.3" across the minor axis.

I had to modify the secondary mirror holder and use a shorter holder screw, to get the mirror centered in my "pre-cut" location.

The telescope works great. Other than that, my focuser now gets used in the 75-90% extended range. I figure that if I had waited to get the secondary mirror, I would have had the focuser mounted about 3/4" further out on the tube.

Just make sure you carefully measure, then just before you cut the hole, remeasure again.

Jack McQuaide

Good luck, and have fun.

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