View Single Post
Old 12-05-2021, 09:11 PM
Stefan Buda
Registered User

Stefan Buda is offline
Join Date: Jul 2013
Location: Melbourne, VIC
Posts: 698
Originally Posted by multiweb View Post
It's a shame you couldn't source a reliable supply of glass. You end up doing things you could have done without. Although you'll have more control over the quality of the final product. No surprises.
Schott Borofloat is technical glass that is not intended to be used for telescope mirrors even though it is a high quality version of Pyrex. The internal stress comes from the manufacturing process - the glass is cooled a bit too fast on the production line, as it is a continuous casting process. People reported making good Newtonian mirrors out of it and it is available at a good price.
Maybe the shallower curve of an f/4 mirror doesn't free up as much of internal stress as my f/2.7 primary.
Originally Posted by Joshua Bunn View Post
Thanks for the update, Stefan.
How do you go with maintaining secondary to primary mirror spacing on the CDK, when they are suppose to have a Fixed exact distance? I know there is about 1mm tolerance, is this distance what you use to focus with?
When I first set up the optics, I place a Ronchi screen at the focal plane, which is at a fixed distance from the backplate, and point the scope at a bright star. Then it is just a matter of adjusting the secondary collimation screws until the focus is on the screen. One of the collimation screws has a bit of Loctite on it and requires a flat screwdriver to be adjusted. Once the spacing is close, then the collimation is adjusted with the other two remaining screws that don't require any tools. This procedure is done with the secondary focuser set at the midway position. The focuser has a range of plus minus 1mm, which equates to about plus or minus 5mm at the focal plane. Raytracing analysis shows that the focal plane position tolerance is about plus minus 1mm.
Reply With Quote