Building a 12"/304mm Dob.

dj9990 (Dave) · #1 13-05-2018, 10:19 AM

Hi people, I'm Dave. some time back, I was given 2, 12 Inch peaces of plate glass. One's 19mm and the other's 26mm.
So the obvious conclusion, was "lets build a telescope".
Anyway, to cut a short story long, before I even got all the gear together for this little project I've hit a snag, "My tool glass size", should I be using a 12" tool on a 12" mirror? I'm a bit concerned about how Figuring would go, with such a large tool.
Does anyone know if there's an optimum tool to mirror ratio?
Or do I just use my big tool for the grinding and make a separate smaller pitch-lap for the polishing and figuring.
Dave.

OzEclipse (Joe Cali) · #2 13-05-2018, 01:47 PM

Dave. People use sub diameter tools to save money. Same diameter is the preferred ratio. Grinding and polishing a 12 is certainly possible. Without guidance it’s not easy. You should consider buying a finished mirror and building the scope around that. Joe

Wavytone · #3 13-05-2018, 10:16 PM

Dave buy one - aluminising it will cost more than a reasonable 12” mirror - and save yourself a lot of sweat and grief.

Only reason to make a mirror is because you want something unusual that you can’t buy.

astro744 · #4 14-05-2018, 09:55 AM

Welcome and don't be discouraged but I do recommend a smaller mirror fist for anyone contemplating making a 10 or 12" as you will learn certain techniques along the way that are invaluable for when contemplating a larger mirror. Start with a 6" or 8" or do both and then too you can use the tools that you used on each for the larger mirror in addition to a full size tool, particularly useful if you say want to do a 16" one day. The smaller tools can be used to correct zone errors.

It is very satisfying to create an optic from a piece of glass but it requires patience (lots of it) and attention to detail and persistence to get the accuracy and surface finish that you need. Along the way you will learn how to make optical test equipment as well as how to do the required tests and interpret the results which will come useful later for evaluating any mirror. Note 26mm is considered thin for a mirror of 12" size and given that it is plate glass rather than Pyrex it will have a larger coefficient of expansion. This means that even during figuring the mirror shape will fluctuate more than say a Pyrex mirror making figuring and testing more difficult.

Unfortunately cost of producing ones own mirror will likely be much higher than buying one of the mass produced ones but you will get exactly what you want. Note you don't have to get it aluminised until it is 100% to your liking as you can star test a non-aluminised mirror and also view the Moon easily at a good magnification as it is bright enough.

Lognic04 (Logan) · #5 14-05-2018, 10:40 AM

If you do go forward with the 12, I'm looking forward to seeing the updates on the mirror!

yoda776 (Matt) · #6 14-05-2018, 07:32 PM

I too would be interested in the updates on the mirror.

dj9990 (Dave) · #7 15-05-2018, 02:35 AM

Encouragement guys, that's what I need.
Of coarse I'm gonna give it a go, I've got about 95% of the build already without spending a cent, the only crap I've had to buy so far was the grit, the cerium oxide, a dial gauge and the pitch, that I'm having a ***** of a time finding in AU.
I've read and watched a tone of stuff on telescope making over the last few months
I'll stick some pix when Ive got something to show, I'll start rough grinding tomorrow, I'm shooting for an f6 if all goes well.
Catch ya on the next lap.
Dave.............

pixelsaurus (Mike) · #8 15-05-2018, 06:50 AM

Hi Dave,
Welcome to the Dark Side. This is the place to be : http://www.iceinspace.com.au/forum/s...ad.php?t=81660

Rkonrad (Richard) · #9 22-05-2018, 02:10 AM

Good luck Dave give it a go and post updates

dj9990 (Dave) · #10 22-05-2018, 02:42 PM

OK, I've been rough grinding for about 6hrs with 60 grit, and my sagitta is down to 1.14mm. I should reach f/6 at 1.42.
I've run out of 60 grit, I can either wait till my next lot of 60 grit, turns up, (I should have checked my sagitta before I ordered it), or I could start on the 80, I think that mite be the go.

dj9990 (Dave) · #11 17-06-2018, 06:54 PM

Well all's been going great with my mirror, I've just finished the 400 carbo and I'm just about to go on to the 1000.
The curve is looking beautiful.
Now you wanna know where it all fukked up?
I went out into the shed last night to get something from under my grinding bench in the dark, any way, to cut a short story long, "I knocked my mirror on the concrete floor", taking a massive crunchy/shattered hunk of glass out of the back edge.
The front surface is fine, but #$&^%#$ it's a mess.
I could still go on, but it was already too thin and this mess will have added a mess of stresses in the glass.
Well ****t happens.

Lognic04 (Logan) · #12 17-06-2018, 07:16 PM

Bugger!

Nickm523 (Nick) · #13 24-06-2018, 09:56 PM

Good luck with it! Really cool project!