Mirror making.

[steelfixer]

Hi all

My project for the up and comming year for me is to build my own reflector from the ground up.

I figure if I am going to undertake this I might as well make it worth while, so I am aiming for a minimum mirror diameter of 20 inches.

My First question is on the mirror.

If the mirror is going to be coated does it have to be made of solid high grade glass.

Can it be a laminate?

As the coating (to my understanding) is Aluminium based why cannot the mirror be manufactured from a ground and polished Aluminium billet.

Where is the best place to get the calculations I will need to undertake this.

All basic questiond I know but you have to start from the begining.

Thanks

Graham

[barkis]

If I could get straight to the point Graham. Forget a laminated mirror, and also a metal one. Oxidisation of an aluminium disc would be problematical, as well as cooling difficuties.

I Think a more conventional approach to your first scope will lessen the risk of any problems.

Grinding polishing and figuring a 20" glass blank, is a challenge in itself. I admire your sense of adventure, but a bit too ambitious is as kind as I can put it.

You will attract a lot of help and advice as your project develops, and the best of luck.

Have you sourced materials yet, or is this just a germ of an Idea at present?

Jean Texereau's book on mirror making would be a great instructional aid to you. I think Amazon sell it, and not expensive either.

Ron.

[NGC 1502]

Hi Graham. It's good to aim high, but a 20" mirror is very ambitious. It's usually considered best to make smaller mirrors first, before going large.

Here is a great book for building big Dobs - The Dobsonian Telescope, A Practical Manual for Building Large Aperture Telescopes by David Kriege and Richard Berry It's full of excellent advice on every aspect of making a large scope, but warns about the pitfalls too, and includes instructions an making a much more modest 8" Dob.

Good luck with the project, Ed.

[swamp thing]

I would echo Rons comments. On starting with a 20".

I have heard it said, that if you wish to grind a 12". Then it's best to start with a 6", as it will be quicker to grind a 6" then a 12" than a 12" straight off.

I wish you every success with your build.

Regards Steve

[Peter Drew]

Sorry to also be in the "negative" camp although the comments are meant to be positive. Experimental laminate mirrors have a poor history, at best you would have to fuse them in a high temperature furnace and then carefully anneal them. Glass can be as ordinary as plate glass but you would have difficulty in sourcing a thick enough blank at 20" these days, anything else suitable would be pretty expensive. Aluminium mirrors would not be thermally stable enough for your intended use. The advice to tackle a smaller one is good advice, if you find you can make a good 8" nothing else will give you as much confidence to go for a larger one, if you can't , well, enough said.

[steelfixer]

Hi all.

Thanks for the advice.

I already have a very good 8" so making another seems pointless.

I will be doing a lot of research before embarking on this so the recomendations of the books is very helpfull. Any other links on this subject along with sorces for the glass blanks will be greatfully accepted.

I don't want to come across as arrogant or big headed ect but with my engineering back ground of 35 plus years coupled with a long term past time of cutting and polishing precious gemstones for jewellery I feel the prospect of creating a large mirror is not over ambitious and is well within my caperbilities.

I am in the extreamly fortunate position of owning an engineering company so I will be using all my facilaties to manufacture a machine to do the baulk of the grinding work.

The research for this machine is already in an advanced stage and I have managed to sorce some detailed drawings from the USA.

I also intend to manufacture a precision built Equitorial mount to carry this scope as well.

Like I said I am looking at this as a year long project so there is no rush and all steps will be researched in depth before any work is commenced.

Thanks

Graham

[Peter Drew]

That's the spirit! However, having already got a good 8" mirror is a very good reason to make another 8", it will give you a benchmark for your abilities and a feel for working glass, this can be done whilst work on the more ambitious project is underway.

[Trull]

Hey Graham

Have you got a copy of Machinerys Handbook? How is your math?

If you poke through that, most of your design will come to mechanical engineering... I've also started on another ATM adventure, but I too am starting with a little 'un and aspire to great things.

[barkis]

Making an astronomical mirror is quite removed from engineering.

Of course the hardware to support it and enclose it is where the engineering expertise will be beneficial.

Generating the curve in the glass is a simple process, although a 20" lump is more suited to a machine bed, where the physical demands are removed. I wouldn't wish to grind out an f4 sagitta in a 20" by hand.

When it comes to the fine grinding and polishing stages, it becomes more of a harmonious relationship between the maker, and the glass. The figuring process becomes even more of an amorous gentle persuasion.

Not sure how many guys who have made a mirror would describe the process that way mind you .

Ron.

[swamp thing]

Great enthusiasm Graham.

If anyone can do it straight off that's the kind of attitude that'll see you through.

Would still agree with Peter. Why not just grind another 8" whilst getting the project underway. It may just save a bit of time in the long run.

The very best of luck.

Steve

[steelfixer]

Peter - That is a fair point, compairing the two side by side would be very benificiall.

Trull - I donot have a copy of that particular book but my engineering and maths are HNC level. Well they were before the years took thier toll on the brain cell count.

I do wish you all the best with the new build.

Ron - That is the most poetic way of discribing the making of a mirror I have ever heard. I have talked to several people by email who have made large mirrors and they have never put it like that.

In fact when discribing the figuring stage a couple of them sounded more like it was a recipe for potential suicide or at the very least a commital order.

[Ericgraham]

Good luck Graham! When I made my first mirror sometime around 1960, I broke all the rules of the day. Then they said to make a 4 or 6 incher with a 6:1 diameter to thickness. So I made a 10 inch from one inch thick plate glass and it worked out fine.

You have the advantage of the Internet and groups like SGL. Get as much information as you can and then your chances of succeeding with a large first mirror will be greatly improved.

Eric

[steelfixer]

Steve - Thanks for the reply. The 8" would be benificial might even push it up to 10" so I can test the machine's caperbilities as well as mine.

Eric - you sound like my kind of guy, I love to hear about people who push the bounderies and get a successful outcome.

[fwm891]

Graham, That's some project and I wish you well. personally a 12-inch (to me) would be a good starter as its a useful size and difficult enough optically. Larger optics physically are just smaller optics re-scaled but tempramentally nothing like them.

You mention laminated glass - are you thinking of BVC (Black Vitrified Ceramic glass from Canada) I've used it. It grinds and polishes easily, it had some bad press initially but its a great material. It is similar to the borosilicate glasses regarding expansion. And you can get it with a pregenerated curve ground in (though I think the rough and subsequent grinding stages give you a 'feel' for working any material.

Good luck.

[Peter Drew]

Graham. It might be a good idea to start sourcing mirror blanks now as suitable and affordable ones can be hard to find. I don't know if the Canadian BVC blanks are still available, I have an 18" which was turned into a successful mirror. I also have a 42.5" BVC blank you might like to have a go at once you've done your 20". .

[Demonperformer]

some large mirror blanks here: low expansion dobsonian mirror, pyrex mirror blanks

[NGC 1502]

Any other links on this subject.......will be greatfully accepted.

Hi Graham, here is a good source of info -

Stellafane: Build a Dobsonian Telescope

That link, plus the Kriege & Berry book I've already mentioned would give a lot of the info you will need.

Hope you find it useful, Ed.

[Peter Drew]

Graham. The Galvoptics blanks look usefull, you will need a minimum 25mm for a tool and 40mm for a mirror for your large project. You could do worse than contact members of the TROK team c/o Liverpool Astro Society, they have considerable amateur experience in the production of thin mirrors up to 30".

[Glasspusher]

Graham, It might be worth thinking about how you are going test the 20 inch mirror, methods that can be applied successfully to smaller mirrors are not always suitable for larger mirrors. You might like to have a look at some notes that I have prepared on the various test methods.....http://www.nicholoptical.co.uk/The%20Testing%20of%20Astronomical%20Telescope%20Optics.pdf

Learning to test mirrors is not a skill easily aquired, it takes years to master the various test methods....something worth bearing in mind.

I think it would be possible to make a 20 inch mirror only if you had the guidance from an experienced mirror maker. I would be more than happy to help, but we may live too far apart to make that possible, I am in the NE of England. You could contact your local astronomical society to see if any members have the skills and are willing to share.

Hope this helps.

John

[Ericgraham]

Graham, another decision that you will have to consider is the focal length. As the fl decreases, mirrors get harder to figure and test. An 'easy' f7 or f8 would require a monster tube, but it sure would be impressive!

For testing, considerer Bath interferometer. They are inexpensive to build and provide an objective evaluation of the state of your figuring. Join the Yahoo Interferometry group for help and information.

For info on grinding and polishing machines, check out the Mirror-O-Matic Yahoo group.

Eric

[Glasspusher]

Eric, it is worth mentioning that the Bath interferometer in it's native state is only suitable for testing relatively small and long focal length mirrors. This is because it employs artificial nulling which is extremely senstive to accurate accurate definition of the mirrors edge. This means that the quality of the interferogram has to be high, with a sharp well defined edge. For larger faster mirrors some additional nulling optics are required which complicates the situation. As a result, the Bath interferometer is not such an attractive testing solution as might be thought, certainly for larger faster optics.

John

[steelfixer]

Wow I only been away working for a few hours and your response is brilliant.

Demonperformer those blanks look like a sound starting point to me.

Many thanks to all who have replied I will study them and thier links when I get home tonight.

Many thanks

Graham

[Ericgraham]

I haven't tried one yet, but reports from others on the Yahoo group that I mentioned suggest that they have had considerable success with large and fast mirrors.

One factor may be the development of sophisticated free software by Dale Eason.

You seem to be knowledgeable in this area, so you might want to question the group about your concerns and clarify the range of optics than can be tested with simple Bath interferometers

Eric

Eric, it is worth mentioning that the Bath interferometer in it's native state is only suitable for testing relatively small and long focal length mirrors. This is because it employs artificial nulling which is extremely senstive to accurate accurate definition of the mirrors edge. This means that the quality of the interferogram has to be high, with a sharp well defined edge. For larger faster mirrors some additional nulling optics are required which complicates the situation. As a result, the Bath interferometer is not such an attractive testing solution as might be thought, certainly for larger faster optics.

John

[Glasspusher]

Eric, I have made and used a Bath interferometer and a Ceravolo interferometer. This is the relevant information regarding the software nulling process...

Artificial Nulling

For anyone considering making a Bath interferometer the whole presentation is here..

Interferometry and Fringe Analysis

John

[Astrokev]

Eric, I have made and used a Bath interferometer and a Ceravolo interferometer. This is the relevant information regarding the software nulling process...

Artificial Nulling

For anyone considering making a Bath interferometer the whole presentation is here..

Interferometry and Fringe Analysis

John

30 - love !

John, having read many of your other posts regarding mirror making and testing, I would recommend anyone interested (and brave enough) to embark on such an endeavour to take good note of your advice and experience.

Watching this thread with interest. Sounds like a great project.