Mirror making.

[Trull]

Making an astronomical mirror is quite removed from engineering.

Nah - I think figuring, parabolising, testing and executing a well made mirror is all about the engineering. Of course a lot of artists and craftsmen are in denial about their engineering excellence, and it has to be said that as an engineer, I personally like to see engineering credited with achievements and offering a way to better peoples' lives. Soap box? check!

The reason I pointed out the Machinerys Handbook (I have the 16th edition) was because this is a brilliant text which details different materials, different methods and a shedload of engineering practise which will save time and expense. You can get previous editions via flea bay for as little as 30quid (which is fine if you are as happy in imperial as well as SI units).

Going back to the top, I think the reason that aluminium billet is not used for mirrors is that although thermal contractions are at approx the same rate as normal float glass (which is why aluminium windows frames are used), the metallic crystals will interfere with polishing as there are differing micro hardnesses across the structure. Aluminium is sprayed onto glass for its reflective properties and then covered in a thin layer of SiO to protect it. If you fancied a metal mirror, why not consider one of the beryllium alloys? These are used already, but have fairly eye-watering costs!

As far as I understand it (and I completely defer to John Nichol!), glass makes a good substrate because its relatively stiff, and the thermal expansions due to the cutting method are lower. On a microscopic level the abrasive grit acts like a chisel which creates tiny particles from the glass. Because the particles break off the heat created by friction is less and is carried into the water. For aluminum the action smears the metal which then accumulates at crystal boundaries. These boundaries heat up, expand and then receive extra cutting in a nasty feedback loop. There is also another issue with annealing aluminium so that it attains complete homogenity so that internal stress relief is minimal when you cut through the surface.

Hope you do well in your examination of the choices, and I wish you well - hope you will ask on the forum as you explore the possibilities.

[fwm891]

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.

Above comments go for me too...

Francis

[steelfixer]

Just grabbing 5 mins from work to catch up.

Trull you can rest assured that I will be asking many questions over the comming months so be prepared to get sick and tired of me.

I have had a look at the locals in respect of who has made what and the only large mirror build I have found in recent times has all but been abandend.

It seems they got to the figuring stage and then gave up.

I will have to make further enquiries as to why.

I think the first thing I will be undertaking will be the grinding machine.

I am looking at going down the route of a fixed post machine for the grinding and polishing.

I will also make it possible to have a recipricating post for the hogging out as I donot fancy doing it by hand.

I have also decided after considering all the advice to start with a 12" F8 newt.

Because of its sheer size I am thinking of a truss type scope.

As it will be mounted on an EQ mount I am thinking along the lines of a very lightweight tube with an Alumimium exoskeleton to keep it rigid.

Looks like I am going to be burning the candle on the cad for quite a few days and nights.

Thanks for all the interest.

Graham

[ismangil]

Excellent project. Maybe you'll end up producing high-end mirrors, who knows?

[Astrokev]

Because of its sheer size I am thinking of a truss type scope.

As it will be mounted on an EQ mount I am thinking along the lines of a very lightweight tube with an Alumimium exoskeleton to keep it rigid.

Graham

If I remember right, John (Glasspusher) made a wonderful aluminium skeleton tube not so long ago for a 12". Suggest you search this out for inspiration.

[Gina]

WOW! Great project! I salute you Even a 12" F8 scope is going to be about 8ft long - well... and 8ft focal length anyway. A real big boy! Couldn't get that in my obsy

[Peter Drew]

Graham. I've recently built a 12" F8.5 Newt with a one sided A frame aluminium skeleton tube. Initial try outs were on a EQ6, it's currently on a cross axis English mount. It's pretty large overall and needs a fair bit of space. I'll try and get some help with posting up some pics soon, I'm an experienced retired professional telescope maker but a Numpty when it comes to computery things! .

[angusb1]

I'm looking forward to seeing the mirror but even more to the mount. Keep us updated, we want to see pictures!

[Gina]

It's always nice to see pictures

[swamp thing]

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

It might be worthwhile emailing them first. As their website is a bit dated. I contacted them recently about supplying a mirror, and this is a service they no longer supply.

They may still supply blanks but when I spoke to them, was told they are finding Pyrex blanks (or equivalent ) very hard to come by.

Regards Steve

[Trull]

If you intend putting the scope on a GEM, you may wish to think about not designing a Newtonian. Why not shorten the tube with a folded design? Then you can have the eyepiece at a comfortable height, and the tuning fork effect of vibration will be much less and you'll be able to fit the beast's truss arms into your car!

I've got a design for a Chiefspiegler - which I reckon is the future for larger diameter longer f-ratio scopes.

[steelfixer]

Peter - I think you might want to brace yourself, With your background I predict many many questions coming your way.

Gina - Pictures and updates will be a regular feature as this project progresses.

Steve - Thanks for the headsup on that I will email them to find out what the situation is on the blanks.

Trull - This is going in my obs so the EP height will not be a problem as I do 90% of my observing through a camera.

[fwm891]

Graham,

Here's some images of a 10-inch f6.5 I built as a test piece for a 22-inch Dob, (photos also included). There's also an image of the 22-inch polishing machine which is little bigger than the BVC disc itself. The tool is a concrete casting made against the rough ground BVC blank.

Both scopes had 6 truss poles to reduce upper end weight and keep the CoG as low as possible. Plus a rocker box with 3-points of contact. The BVC had had about 7 hours polishing when these photos were taken. The rotating table is driven at about 12 rpm and the tool was allowed to 'spin' with quite a bit of top weight added.

I never finished the 22-inch due to a job change and sold it and the polishing machined on. The scope was completed, sadly not by me!

[Glasspusher]

Interesting design Francis, what a pity that you did not get to complete it. I am pretty sure that BVC is no longer made, the business making the blanks was sold on and I think the present owner has lost interest. You probably know that anyway. Thanks for sharing the images.

John

[fwm891]

Hi John, No hadn't realised ASM Products had finished producing BVC mirrors. Its years since I did my last mirror from a blank bought at Astrofest off the Orion Optics stand.

I will probably do another in the 10-12 inch range as an f6 - f7 imaging optic, but not for a while.

Still have to 10-inch, mirrors could do with a re-coat but still a nice scope. Featured in S&T in 2001 if I remember rightly.

[Ericgraham]

Yes, Astrokev, I do feel like a tennis beginner playing against a Wimbledon champ.

While I only suggested that Steelfixer consider using a Bath interferometer to test his mirror, should I withdraw my recommendation on the basis of Glasspusher's (John) comments and Dave Rowe's analysis?

Then I started thinking about two earlier occasions when gifted experts in their field predicted failure. The first was Lord Kelvin (and others) who 'proved' that heavier than air flight was impossible. Well not really impossible, at least impractical. In a way, they were correct, your average 1890 steam engine would never get off the ground.

The second is similar, Astronomer Royal Sir Richard Wooley had the misfortune to state that space travel was 'Utter Bilge' just a year before the Sputnik launch and a dozen years before the first man on the moon. In a way he was right too- I spent a summer working with him and he explained himself- space travel was too expensive to be feasible and he wanted the money for better telescopes!

So instead of backing off, I'll take a look at Dave's analysis. Equations don't scare me and I can eat Zernike functions for breakfast. Dave's example showed that a one percent error in the diameter of the interferogram for 12 inch f5 mirror would make a 1/20 wave error in the analyzed wavefront. And it gets worse for bigger and faster mirrors.

Actually that wouldn't be too bad. I used Foucault testing on my 10 inch f7 mirror half a century ago and determined a 1/10 wave wavefront error. The Foucault test requires a subjective comparison of the darkness of pairs of location on the mirror. Experts may be able to do it, but even then one has to wonder why so many mirrors come with certified Strehl's so close to 0.999!

There are numerous other factors disrupting mirror tests, but subjectivity shouldn't be one of them. I doubt my 1/10 wave measurement was accurate, I would have loved a better method.

Dave's analysis assumes a one percent error in diameter. Since many amateurs use a multi megapixel DSLR with a thousand or more pixels across the interference image, that one percent error in D seems unreasonably large. If Dale is smart- and I think he is- then he can easily measure to less than one pixel and cut the error even more. Now we would be down to a 1/100 wave error or less. Plenty of leeway to test bigger and faster mirrors.

John has used a Bath interferometer and found it lacking in accuracy for him. That represents an important data point. Other in the Interferometry Group have had better luck. More data points.

So I'll stand my ground for now, and least until I can add my own data point and report it to this forum.

Eric

[Glasspusher]

Eric,

I am embarrassed that this appears to some to be a points scoring contest, from my point of view nothing could be further from the truth. I don’t think those comments were intended to be taken seriously.

Making optics has been an important part of my life, I have found it rewarding and fulfilling. I want others to enjoy making their own mirrors; my mission is to help if I can. I have made hundreds of optical surfaces but I am not sure that I consider myself an expert. Almost every time I make a mirror I learn something new, that is why I keep doing it. A grand slam title will have to wait a little while!

When it comes to testing optics it is the practicality of performing the tests that interests me most, and is an area in which I have some experience. What seems straightforward in theory can turn out to be difficult in practise. As you say Eric, the Foucault/zonal mask test is largely subjective and as a result figures derived from it must be viewed with caution. In the hands of experienced users the test appears to be quite effective, personally I am not a fan of the test but it has its place in the right circumstances. Some tests methods have a steep learning curve, others require complex and expensive extras, and all are fun to experiment with!

Your comment regarding Strehl values interests me greatly. Too many people accept without question Strehl values that seem unrealistic. A reality check on what Strehl is all about is long overdue. Mechanical engineers work to and often quote tolerances. What is the tolerance of a mirror with a Strehl of .999? If anyone has the answer please start a new thread with your explanation.

Graham, best of luck with your project. I will be following the thread with interest.

Best wishes to all for 2012.

John

[Peter Drew]

I think we should remember that Steelfixer is a complete beginner as far as mirror making is concerned. After caveats expressed by others in the know did nothing to put him off, the thrust is now towards helping as much as possible. At this point, throwing into the mix, alternative designs and sophisicated test methods might not be as helpfull as intended.

[barkis]

Yep!, I'm in full agreement with J.N. and P.D.

The only aim of this thread is to assure the OP of as much help as he might request in the pursuit of his project. We can leave the trumpet blowing aside, as it will serve no purpose.

Ron.

[Astrokev]

Eric,

I am embarrassed that this appears to some to be a points scoring contest, from my point of view nothing could be further from the truth. I don’t think those comments were intended to be taken seriously.

John

Apologies to Eric and John if my "30 love" comment was taken seriously (I'd hoped the winking smilie would have indicated my tongue firmly in cheek, and no implied point scoring was seriously intended).

As someone with zero experience in mirror making (though I have been tempted to have a go in the past!), I was simply making an observation based on John's first hand experience of some of the test methods under discussion, compared to Eric's comments which seemed to me to be based more on the results of others. As a trained scientist, I considered John's primary source knowledge to outweigh Eric's secondary source information, based on the information available. I was not trying to make any subjective judgement about Eric and apologise unreservedly if this was implied.

I agree with the further comments made in that this thread (and the forum generally) is here to offer help and support to those who need it. Long may the expertise and knowledge, in all their expressed variations, continue to be freely offered.

[Ericgraham]

No apology needed Kevin. Mirror testing is like politics and religion- there is no consensus on what is true but plenty of strong opinions.

Unlike the other two arenas, the goals of amateur mirror testing are clear: an accurate, objective and inexpensive test. Of the dozen or so tests that I have tried or considered, I don't think any one is a clear winner, and they all have their proponents and detractors.

If Steelfixer hasn't been totally scared off, I encourage him to try one or more tests, and if he is puzzled by the results, post them here and watch us argue - oops, I mean discuss - them.

Eric

[FraserClarke]

I have also decided after considering all the advice to start with a 12" F8 newt.

Graham

Hi Graham,

I can only encourage you in this endeavour -- sounds like you have a great basis to build from. Do you have a strong driver to go for F/8 ?? The only reason I ask is that if your long term goal is to make a 20+ inch mirror, it may be worth trying a faster (F/5 or so) 12-inch. The difficulty of making a mirror goes roughly as the square of the f/ratio (faster ratios behind harder), so an F/5 would give you a lot more relevant experience for the larger mirror. I'm guessing it is unlikely you'll make a 20-inch F/8 !?!?

On sourcing blanks, I'm not sure if you have a 12-inch blank already, but "Vacuum Coatings" have 12-inch blanks available;

Link: Telescope making

I bought a 12 inch blank from them a few months ago, which I believe was the last "non-holed" blank they had. They still had ~30 centre bored blanks available though, and though that adds a bit of complexity to the manufacturing, it wouldn't affect the performance of the final telescope.

Fraser

[fwm891]

Hi Graham, Another source for 'bits' is Willmann-Bell in the USA. They don't list any larger blanks or kits on their site at the moment but things like ronchi screens and 'hex' tiles for tile tools are supplied.

Telescope Making Supplies

Good luck with the project

Francis

[steelfixer]

Good morning all.

Sorry I have not replied to any post I have been laid up with some wierd virus that totally screwed up my vision and balance.

Francis - That is a very impressive looking scope, its a shame you never got to finish it. Love the fixed post grinding machine as well.

Eric, Kevin and John - Discussion and debate of all methods of testing can only serve to help me in the long run.

I did not take any of the comments as a 'point scoring' exercise but found all the different view points as being very helpful.

The more opinions and ideas that are put foward and discussed in my humble opinion the better.

Many thanks to all involved and I wish you all the best for the new year.

Thank you

Graham.

[Gina]

Sorry you've been poorly Graham - hope you're feeling better now Happy New Year you too