Rough Polishing the 20" f/4.5 Telescope Mirror
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- Опубліковано 28 сер 2024
- This segment shows how to cold press and prepare the surface of the pitch lap. Gordon Waite shows how to set up his fixed-post mirror polishing machine, and demonstrates spin polishing on a 20" telescope mirror.
@datzfast This video just shows rough polishing. This is really only for removing the pits, and usually produces something pretty close to a sphere. I do the parabolizing stroking by hand with smaller polishers. Thanks! Gordon
It's great to have someone who really knows their stuff be prepared to show others. How long was this one long polishing session to get rid of all the pits?
I sooo much loved to learn to do this stuff! But I never feel I have enough information to do it.
Thanks Gordan
Another excellent presentation!!!!!
Five Star +1
Bill
Hey Gordon,
Excellent posting man, very informative and easy to follow.
One question, why is there a whole in the centre of the mirror?
Regards
I’m suddenly wondering... what’s the material of the rotating part of the grinding machine? A couple disks of plywood glued together? Or is it like a plate of steel? It’d be one hell of a flywheel, it’s bigger than most manhole covers.
@hummerume sorry, i mean "hole" not whole :)
Were you using it for a Cassegrain Telescope with this hole in the middle?
No. The quartz mirror blanks just had a hole in it in the beginning. It was intended for another purpose originally.
Hi Gordon,
I'm after some advice if I may. I'm a novice mirror maker and undertaking my first mirror, a 10" f5. I've built a mirror o matic machine which has worked really well for the rough and fine grinding and I've managed a good even spherical grind with regular spherometer testing . What I am struggling to understand though is when polishing using a 7" lap using a fixed post polishing technique in the sweet spot I keep getting a hole dug into the centre of the mirror when Ronchi testing. I've tried everything to avoid this but it keeps happening unless I introduce a small amount (1" approx) of swing on the arm. My lap is dental plaster with Gugolz 64. I have faceted in a random fashion as per one of your videos. Any thoughts on why the fixed post approach isn't working for me? Oh and the lap has excellent contact with regular pressing. Hope you can advise. Many thanks. Pete.
Your polisher is perhaps a tiny bit small, but not so much as to be a problem. 7.5" would be a bit easier. How much does the edge of the polisher overhang the outside edge of the mirror? Should be slightly over an inch or so. Usually if you put that swing on the arm during fixed-post polishing you will polish in a turned-down edge. Perhaps email me a photo of your setup and a photo of the face of your polisher?
@@GordonWaite I have sent you a mail with a few images attached. I very much appreciate your help. Peter.
Great video's Gordon, do you have a set from start to finish that I could buy.
Sorry, I don't. Thanks for your generous comment, though!
Do you sell any of the stuff you make?
how much does the tool overhang?
What is pitch lap?
It's as it sounds, its a Lap (tool used to grind liquid abrasives, typically formed when the grinding is started.)
made with Hardened Pitch (A form of resin from trees.)
I'm guessing the turn table must be perfectly flat and have no wabble.
In theory, you would like it flat and wobble-free. In reality, you rotate the mirror on the turn table quite often to even out the effects.
@@GordonWaite I looked through a bunch of your other videos and googled to try to find pictures but no luck...so...is your fixed post turntable rotating on something like inline skate wheels placed underneath and near it's circumference, or, is it a circular plate supported solely by a central shaft? It seems like the central shaft option wouldn't be good since unless it and the circular plate are like 3" thick steel, there's going to be a large bending moment that will tilt the plate when the tool pushes down near the edge of the mirror.
@@perspectivex I have several small machines that are used for 12" mirrors or smaller. They have a single central shaft, but the shaft is 2.5" diameter steel. For all larger machines I use six skate wheels under the turntable for additional support. -Gordon
@@gordonwaite9073 h-o-l-y c-o-w. 2.5" shaft. That is huge, relative to what I was thinking to use. Whatever I finally build I'll test its canting under load equal to the maximum offset weight for grinding and then add wheels or not, but, probably will have to add wheels.
Why do people grind and polish mirrors when they can buy them already done?
Some people just like to make things. And a skilled person can make a mirror that is considerably better than common mirrors available commercially.
GordonWaite --- I have been an amateur astronomer for 40 years but I never had a desire to polish a mirror... Well, that is until now; especially after watching several UA-cam videos on the subject. Problem is that I am now 71 years old and in poor health. I might be able to do a 6-inch mirror, but nothing larger.
I used to help run a mirror-making class about 15 years ago. The record for the class was a 96-year-old gentleman who finished an 8" mirror. He also built the telescope to put it in!
I just had the disturbing realization that "pitch lap" rhymes with "bitch slap."
You have a future in music...