Just got some 19mm plate. Now what size to build.

Gregted (Greg Tedford) · #1 21-01-2016, 10:28 PM

Just got a great deal on some 19mm plate glass.

I've bought a circle cutter off evilbay and will pick up some cheap 3 or 6mm to practice on.

The plate is 550 by 1100 so I can go crazy but while I'm waiting for the cutter to arrive, I thought I would ask for opinions on what size and f ratio you all think I should go for.

I made 2 dobs about 15 years ago but pretty mild at 6inch/f8 and 8 inch/f6 so I would probably like to go a bit bigger this time around.

I originally thought of 12inch/f6 but I've seen some 16 inchers and think an f4 might be do-able.

My old ones were ok but I'm 15 years older and a bit more patient these days so I think I might be able to give a 16 inch some justice.

I'm in a suburban back yard in Tannum Sands with a street light right out front so not the best but I was looking at a truss dob setup.

Anyway, pretty excited about this years project so let me know what you think.

Merlin66 (Ken) · #2 22-01-2016, 07:28 AM

Gregg,
at that thickness you'll probably have to consider trepanning the disk.
(We used to cut 8" blanks from old porthole glass.)

pixelsaurus (Mike) · #3 22-01-2016, 07:59 AM

My 2c worth. 19mm is not very thick at all. If you build a 16" f/4, the sagitta (mirror depth) is nearly 7mm. Little room for error in grinding. You will probably need to build an 18 point flotation cell for the mirror or better. I'm not saying it can't be done but it would need to be done well.
https://stellafane.org/tm/atm/grind/measure.html
http://www.newportglass.com/grind1.htm

Gregted (Greg Tedford) · #4 22-01-2016, 01:51 PM

Thanks Ken and Mike for your input.
I have looked at the sagitta and might go for 12/F4 to make it easier for my first largie. 4.76mm sagitta. That will leave 14mm in the centre.
I have enough to add a 16 incher later if the 12 inch works out. I see my Scope collection getting larger.
Here is a link to a vid cutting circles from 19mm plate.
https://youtu.be/x-84Sdq7jaE
I did trepann my circles back then but I might try the circle cutter and see what happens. The bloke at the glass shop has done it and thinks it should be do-able.
What did you use to trepann your circles?
I fixed some flashing on a plywood disc on the drill press and cut off a plastic pot top for a dam and drilled under water with some grit. Worked great.