Mirror grinding

JJDOBBER79 (Jas) · #1 22-06-2014, 08:43 AM

Hi diyers, I am thinking of grinding a mirror. After reading a lot about it, I have concluded that I am a quite analytical person who gets obsessed with a project quite easily so I think I would be good at it. Everything I read tells me to start with a 6 or 8 inch. The problem is, I don't have any use for a scope of this size as I currently use a 12 inch. Why is it suggested to start small? Surely one uses the same strokes and the same amount of strokes so, theoretically, it wouldn't take any longer to do a 20". Also, surely measuring Sagitta and testing is easier, the bigger you go. Is it just the risk of screwing up an expensive large blank that makes people start small? My understanding is that there is not much chance of making a mistAke that can't be rectified by going back in grit and trying again anyway. I just figure, if I'm going to go to the effort of doing this, I might as well get an aperture upgrade out of it. What does everyone think of this. Am I crazy

JJDOBBER79 (Jas) · #2 22-06-2014, 08:53 AM

I'm thinking a 20" by 1". I know it is definitely not advisable to do a thin mirror first but seriously, it's an inch thick!!! That's a thick piece of glass. As long as it's supported correctly with wet newspapers or the like, I can't see it getting broken easily. Obviously, the focal ratio won't want to be too high. Keeping the weight down is necessary too as I don't have dark skies in my backyard. I am thinking of an obsession type scope with wheelbarrow handles.

cfranks (Charles) · #3 22-06-2014, 10:13 AM

No, you're not crazy. My first, second and an assist on a third were 15", the first and the optics of the second are still in public use after 28 years. It is a lot more effort than a smaller one and the polishing, if done manually, would possibly kill the project. I built an Hindle machine to do the grinding and polishing and it still took many, many hours. I got a lot of satisfaction too.

Charles

sopticals (Stephen) · #4 22-06-2014, 03:53 PM

Hi Jas,

definitely a doable project. 20" by 1" thick is thin, but not too thin if the right care is taken during grinding/polishing/figuring. (I have ground out, polished two 22"(f5.23,f5.36) and one 25"f4.68 usable mirrors out of 3/4"(19mm) float glass).

Currently have just finished grinding two 25"(640mm) by 1 1/2" thick mirrors. Thicker blanks are probably more foolproof, but thinner blanks to made into reasonable mirrors not impossible.

In earlier days (1960ish) when I was ground my first mirrors 6"/7"ers nobody in their right mind would have attempted anything bigger than a 6" or 8"(few were game to try the 8). One was greatly discouraged trying to go thinner than the old standard 6:1 thickness ratio. A 12" minimum thickness 2" (24" @ 4" thick etc).

Times have surely changed with some of the "experts" of today saying up to 18:1 is OK. As for a beginner, what size? Many amateurs today start at apertures well beyond the old 6"/8" threshold, with 16" and larger as a first effort. I know of two cases where a first mirror has been of the 30" size and both were only 25mm thick. The downside of pushing the "thinness" boundary is the flexture astigmatism problem.

astronobob (Bob) · #5 22-06-2014, 10:19 PM

Ive had a little experience with mirrors, not a great deal tho. Back in the early '90s I saw out 4x 6" F9 spheriods 2x 8" at F9 spheroids and a 10 at F7 & porabilizing (W stroke) they all worked well, tho then came the time where buying was cheaper.
I also agree with Stephen that starting out small was advisable and even in the 90's there were not a lot a people into Astro & in particular, who had the 'Passion to start & finish a mirror - therefore, not a great loss.

I remember being commended on my 1st as even in the early 90's approx 80% of mirrors started ended up as door stops or paper weights Lol, , That said, I wish I had started on a larger intially and see no reason why one cant start on what ever size they deem worthy.

Also, being analytically minded is great, I thought that cleanliness of operation was paramount, being marticulous with avoiding contamination of gritts, as in, having a shower, washing hair, under fingernails, clean clothes, between changing gritt grades, plus thoroughly clean work post, and Not storing courser gritts higher than finer gritts etc etc. My Gritts were sent up from Sydney, and also travelled in seperate packages and boxes and on different days - sounds a little to extreem, but better safe than sorry

All in all, your the one doing it, and I dont think your mad - follow your instincts, if you think you got what it takes - Go For It, Its a fantastic journey and awesomely rewarding

JJDOBBER79 (Jas) · #6 23-06-2014, 08:52 AM

Thanks for the encouragement guys. I think I will do it. I am moving house soon so hopefully I will have a shed to grind in. It will certainly be an interesting thing to attempt. The way I look at it is this, it will probably cost me $400-$500 to grind a 20" if I get a good deal on a blank.
Worst case scenario, I botch it beyond repair and I learn some very important lessons. So, basically, I have paid $400 to learn about the art of mirror grinding. If there was some kind of course that one could do to learn this, I would definately pay $500 to do it. Who knows, I may even get a large scope out of it.

sopticals (Stephen) · #7 23-06-2014, 01:24 PM

Hi Jas,

Some blanks available in the classified section, under accessories.

astronobob (Bob) · #8 24-06-2014, 01:30 AM

One thing I used to do was use industrial grade gritt, but only upto 800grade - then Id use optical grade gritts from 500grade upwards to polish - industrial grade gritts can be bought by the kilo for like $10-15 as you dont have to use optical grade during rough grinding, saves a few $$

I also have a good size jar, about 2 cup measures worth, of optical grade gritt here somewhere - if your interested, I can post it down to you for the cost of postage ? will have to find it to let know what grade it is ! Im not going to use it !

Stefan Buda · #9 24-06-2014, 10:17 AM

Quote:

Originally Posted by JJDOBBER79

Worst case scenario, I botch it beyond repair and I learn some very important lessons. So, basically, I have paid $400 to learn about the art of mirror grinding. If there was some kind of course that one could do to learn this, I would definately pay $500 to do it. Who knows, I may even get a large scope out of it.

The course one can do is the Small First Mirror and it doesn't cost much.
You most likely end up saving time by applying to the big one what you have learnt on the small one.