Quote:
Originally Posted by Sunfish
Thanks for that encouragement. I have three Pyrex mirror blanks given to me to work on. A rough ground 100mm f5 for practice . An f7 200 mm which I would like to regrind to F4 . And finally , once I can work out what I am doing , a blank 12 inch which may be sometime before I can get to.
I have just received my grinding materials, 80 , 220 , 600 silicon carbide , 25 micron and 14micron aluminium oxide as that was what I could get locally.
I have made a mirror cell for an old finished 8 inch f7 so I can the tube assembly and secondary assembly process sorted out.
Any tips starting out or advice about my mirror cell?
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The 200f7 will probably be fine with simple three point support and two lower edge supports at 90 degrees, such as in dobson's original instructions. For dob mount this is ok, as the gravity load is always in the same direction. If it may be used on an equatorial mount, then this doesn't work. I had to redo my cell for my 200f6 dob as I also use the tube on my CGEM equatorial mount. I used a design by Gary Seronik , which is a backing disk of wood, three small 90 degree brackets around the mirror circumference and the mirror fixed with nice thick but not to large in area blobs of silicone to the disk, plus small blobs between the brackets and mirror edge.
https://garyseronik.com/a-simple-dou...our-reflector/
Ensuring 2-3mm of clearance (which is filled by the silicone) means dimensional changes of the wood don't result in stress/deformation of the mirror. It seems to work ok, star test is still good and symmetric. If it was stressing, the star test would probably appear 3-lobed.
The self-weight of a 200mm mirror seems to be not too critical in terms of support. Main think is to be able to collimate it.
An added benefit of the double-plate cell is that mirror distance along the tube (for example to get a bit more back focus at the focuser) can be tweaked a little by having the acorn nut stick up higher and using longer collimation bolts.