16" first time mirror grind?

[Rustysplit]

Hi all,

I feel the distinct call of a large'ish dobsonian. I would love to build a highly portable truss design.

Is it asking too much to jump straight in and try to grind a 16"er? I am pretty good with my hands, and being a cabinet maker, am not at all phased by the actual 'scope construction.

Advice from the experienced mirror makers would be very welcome.

[Beulah]

I have yet to grind my first 14" Mirror myself.

I am not phased by the concept at all as I am also experienced in the craft and fine woodworking side of things.

Although I do not offer the experience of someone who has ground many mirrors, the usual advice is to start with a smaller mirror, such as an 8". However I was told by someone who is an expert that if you already have the chops and have developed patience as you would have with fine carpentry, it is possible to start with a larger mirror.

The reason why I have not started yet is because the workspace is used for woodworking and I do have several projects on the go which obviously creates sawdust. I would like to keep the workspace spotlessly clean for the duration of the mirror grinding.

[kirkster501]

The "hogging out" or coarse grinding would take forever I'd imagine on a 16 incher? The figuring would be no walk in the park either.....

[Rustysplit]

I had considered the amount of time this may take up. I am toying with the idea of building a mirror-o-matic type machine. This way I could, on the extreme optimist end of feasability, do the hogging out whilst getting on with my paying jobs or the dreaded paperwork. I realise that I can't just leave it running, but it may help with finding the time as well as paying the bills.

I may well be getting way ahead of myself here, but I have a friend who has a large ceramics kiln. Has anyone tried the slumped blank method to reduce hogging out time?

Crikey! Once the mind starts racing, it just keeps making things more complicated. hehe!

[kirkster501]

Well, that's the fun of it I suppose and doing the actual work and making the mirror is part of the enjoyment, not just the mirror end result.

I have likewise been tinkering with the idea of making another. I made a 6" inch mirror in my mid-teens thirty odd years ago and it was straight forward enough. So like you, considering a "big 'un" !! But not so big that the joy disappears and it becomes a chore.....

[woodbob123]

Started on a 24" as my first just to make it more difficult 1" thick. First make a grinding machine. I first hogged out over a couple of weeks, went through the grits year or so later . Stripped down the now polishing machine to remove any grit that may have been left, and did some polishing about another year later. So make sure you are in it for the long haul!!!!. My workshop has a metalwork lathe and used for many things so I make a tent affair to enclose when grinding / polishing. Did promise my other half it would be finished this winter so working on a fouchat tester think this last stage will take a long time but with the help of SL figuring may go better than I fear, I,m sure you can do it

[Rustysplit]

Don't tell me things like that, starting on a 24". You will get me wondering....... I have started to look around for a blank supplier. I think as you say, this will be a longish term thing. My hope is to have a finished large Dob in time for the Autumn. I guess work load will have more influence on that than anything else.

There is something that has grabbed me about making the entire "scope. I know I could buy a 16' mirror from TS for a little over a grand, or even a Meade 16" LB in their sale at the moment for around £1350. Somehow, I can't justify it whilst knowing that I could make it.

[Glasspusher]

Well, that's the fun of it I suppose and doing the actual work and making the mirror is part of the enjoyment, not just the mirror end result.

Kirkster501, that's the important point for anyone thinking about making a mirror. Do it as a worthwhile activity in it's own right, mirror making can be a rewarding and satisfying activity.

I would say that it is not impossible to do a 16 inch mirror as a first time project if you can get help from someone who has been throught the process. I am more than willing to help anybody who wants to take up the challenge but I am located in the North East which could be a problem for some. Many would say, with some justification, that a mirror around the 8 inch size is a good starting point. I have made a video over 20 years ago detailing the making of an 8 inch mirror, it won't win any oscars but it does contain a lot of useful information. If anyone would like a copy send me a pm.

woodbob123 a 24 inch 1 inch thick mirror is a MAJOR undertaking. For this diameter the glass is VERY thin and prone to flex, working such a thin mirror by machine will present significant problems when figuring as a result of flexure. You will need a very rigid polishing table and need to rotate the mirror relative to the table on a regular basis (randomly).

Rustysplit, a ceramic kiln is not ideal for glass work, the heating elements are loacted in the wall of the kiln which tend to heat the glass more at the edges, in a glass kiln the elements are located in the walls and the lid, this allows more uniform heating of the glass helping to control stress in the glass. Attached is a picture of a 20 inch fused glass mirror I made recently.

John

[Astrobits]

Blanks can be obtained from Image optics aka Galvoptics.

www.image-optics.fsnet.co.uk/

Go to Telescope mirrors/ Dobsonian Mirrors on this site.

400mm is £85+ postage.

nigel

[Rustysplit]

Nigel, thanks for the link. I had found that one and their are on my list.

John, That fused mirror looks great. Quite a weight saving there. Unfortunately I am down in surrey, or I would love to pick your brains. I have been looking at your thread on mirror making and trying to take it all in. The slumped glass idea was just my enthusiasm running away with me. Interesting to hear about the different heating effect though.

[woodbob123]

Glasspusher thanks for offer of help. Rustysplit when I started decided that to improve on the 12" dob we already had double was the way to go, bought the blank from galvonics, have thought of a furnace (would be a nice job now with the snow) but told that cooling presented the greatest problem.

But as mentioned doing it you must enjoy the challange and be prepaired to learn by mistakes. Hopefully soon will be putting up images of where I've got to with polishing and figuring. Spent all my working life (retired early) fixing things and have found the best way for me to understand is through my fingers. Which of course means many mistakes. My wife is more the astronomer I just enjoy building things some of which I finish and some which i finish and hardly use. If you are interested I would give a list but don't wish to hijack thread. Bob

[Daniel-K]

took me and barry a few hours to hog out the center on our 16 inch mirror were down to polishing it just need to find some were clean enough to do it

[woodbob123]

Rustysplit see your in redhill if I can find a spare motor/gearbox would be pleased to let you have it ( only if) how close are you to M25 will be going to kent soon and happy to meet you if needed could explain my machine to you. Bob

[Rustysplit]

That is extremely kind of you Bob. I am 5 mins from the reigate junction.I would be very interested in going through the machine design. Which type did you build, a mirror o matic type?

[Glasspusher]

Just to avoid any confusion, the video on mirror making that I mentioned in a previous post is on CD and is playable on any computer.

John

[woodbob123]

Are you available in the day cup of coffee would be nice. Hard to explain my machine, a simple reason I used things I had available in my will be usefull store (or junk pile as wife calls it). to give you an idea main centre bearing is off a transit back axle used the wheel bearing assemble (cut off the axle) and machined it down to give minium thickness removed the oil seal etc, have a lathe so that was not so hard. That will take all the sideways force I need. Drive belt yes off a transit, drive motor was from a small hoist (type you see lift people in / out of swimming pools) so DC powered. Top and bottom were some 1" ply. Only way is when I get camera back will send a couple of photos.

Should be passing you around 1 on the 1st feb. (not that my life is planned), on the way to grandaughters party. will make arrangements via pm's. Should you be hoping that spare wheel bearing is available, used that on the base of my 12" Dob it can be pushed with little finger and will do a complete rotation now have to build a braking system. Bob

[Astrobits]

Bob, I am very interested in your 24" x 1" mirror make. I have plans to go to 25" as my next mirror for myself and have been wondering what problems I am going to meet if I try a 1" thin mirror at that size, how it is likely to perform and what mounting will be needed. I have previously made and used a grinding m/c for up to 20" mirrors and am familiar with making thin mirrors up to 18". As I live in Highbridge just across the water from you perhaps we could get together sometime for our mutual benefit?

Nigel

[woodbob123]

Nigel Happy to help/ use your ideas, I'm retired so you are welcome to pop over live near J24 M4 easy to find. If you cut a back axle up I'm sure rustbits will be happy to have the other side lol. Weekdays are best for me just pm me and we can arrange it. Bob

[Rustysplit]

Sorry Bob, a current rush at work means that I will not be around. I would be interested in seeing some pics of your machine though, if you have any?

I think I will get the galvoptics 400mm plate glass blank. This is 25mm thick. Do you guys think this will be ok?

[Glasspusher]

Rustysplit, for what it's worth my experience has shown that a 12 inch x 25mm thickness is OK, and can make a good mirror without having to worry too much about flexure. 14 inch x 25mm and things get a little more difficult and attention has to be paid to controlling astigmatism. At 16 inces (400mm) things get challenging, more difficult by machine than by hand, it is doable but be prepared for difficulties. Anything over 16 inches in 25mm plate is really for those with a few mirrors under their belt. I managed to get a good figure on a 22 inch x 25mm some years ago but it was by no means easy going. I have also attempted a 24 inch x 25mm which got the better of me, it might have been ok at low powers, certainly would have made a lovely coffee table.

Good luck with your project, looking forward to hearing about your progress.

John

[Rustysplit]

Thanks for your advice John.

The more I think about it, the more I think I will have a go by hand. The time it will take me to make the machine may be better spent just getting on with it. Do you think that by hand, it is a realistic, albeit, challenging size combination?

[Astrobits]

I can only support John's comments here. A 16" f/5 has a sagitta of 5mm so the glass at the centre of your mirror will only be 20mm max thick at the end of coarse grinding. The next grinding grade or two will take off a bit more thickness. Experience with smaller mirrors always helps which is why it is usually suggested as the best route to take.

As regards source of glass it doesn't matter where you get it from, float glass is float glass. Just avoid any that might have been tempered ( ships portholes etc. ). Image Optics/Galvoptics are fine.

John: I like the look of that fused mirror. I would be very interested in your experience with fused blanks. Do you do them yourself and if so how big can you go? and many other questions for another time.

Nigel

[fwm891]

took me and barry a few hours to hog out the center on our 16 inch mirror were down to polishing it just need to find some were clean enough to do it

I made a polythene tent inside my garage to do my polishing. 4 walls ceiling and floor covered

[Glasspusher]

Nigel, thanks for your interest in my fused blanks. They are very much in the developmental stage at the moment, but I am quite close to being able to produce a fused blank free from stress. My capacity at the moment is 24 inches in diameter, my aim is to go bigger in the future. The blanks will be slumped in the kiln so minimal grinding will be necessary. Coma correctors are becoming cheaper and we are entering the age of F3 ish mirrors. The blanks will have a convex back which will need consideration during working...nothing that cannot be overcome.

John

[Astrobits]

We'll have to keep in touch. I have plans for a big, very fast primary for a Nasmyth/Cass system to bring the focus out of the Alt axis. I have been playing with Beam 3, on and off, for some time and am now trying to get my head around another Optics design prog that I have recently downloaded.

Nigel