The distance from the primary to the secondary is only part of the distance from the primary to the focus point. You also have to take into consideration the distance from the secondary to the focuser. The best thing to do is to finish up the top end of the scope, leaving the bottom end long and move the mirror and its mount up and down the tube until you get a good focus on Polaris or a light at least 3 miles away. You want to put a 25mm EP about 1/4 the way from the outermost extent of the focuser which should allow for a 6mm EP and barlow at the bottom of the focusing range.
I like putting 3 blocks to support the mirror mount at the bottom and screw the mount into those with the screws along the mirror axis. This allows for the removal of the primary without serious loss of the collimation.
BTW, is this the same Sharon that just got her mirror finished at the SDAA mirror making class?

• Hi, Bob - sassielassie - June 26, 2002 - 03:52 UTC
  • Fused mirror - bobmay - June 27, 2002 - 20:18 UTC