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
Quote:
Originally Posted by racecar
I'd say hello fellow mirror makers, but I am yet to finish my first mirror. I am fine grinding at the moment.
I am making a 150mm F6 Newt (at least I hope it is an F6). While it was wet, I held a 40mm eyepiece up to it at arms length and tried to focus on the neighbour's roof. I figured that as it was around F6, and a sphere is close to the Raleigh Criteria, I would get a reasonable image off axis if my arms were long enough.
It surprised me that I could focus on a near object holding the objective at arm's length. I could not get it to focus at infiinity, which was a relief, because I don't know how easily i could figure an F4 or F3.
It is also amazing how much light is reflected off glass during the daytime. I am considering leaving it uncoated for use as a drag and drop lunar telescope.
Anyway, I am glad that there are still mirror makers out there and this thread has had some good tips.
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