Quote:
Originally Posted by dave brock
That's a little worrying because as you focus on closer objects, the focal point moves away from the primary. That is, if the mirror is 36" focal length for infinity then the distance you need to hold the eyepice for a closer object will be, say, 37"-39" for example. Have you done a focal length test?
Dave
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Quote:
Originally Posted by Satchmo
David ,
This does sound fishy . Just project the Sun image onto a piece of paper and move until you get focus ( get someone to hold the paper just beside the Sun for you and hold the end of the tape measure) . A sanity check while grinding is to calculate the saggita and then buy a drill bit whose diameter is just shy of the required depth and lay a steel straight edge over the drill bit as it lays in the centre of the mirror and grind until you can sit the edge down on the mirror flush with the drill bit in contact. Your saggitta is about 1.56mm so a standard 1.5mm drill will do the job !
If you are still grinding just spray a light mist of water mixed with Glycerine onto the surface for quick reflectivity. If you have a polish the full Moon can also be used as a FL check. Always use the Sun or Moon - don't complicate things as a beginner by trying to focus anything else.
Mark
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OK Thanks for that. I will try the moon tonight.