I cobbled together a tester out of some bits and pieces but am a little unsure how to finish it on a couple of points
Regarding the head , were do I mount the lightsource in relation to a ronchi screen and knife edge , some pics around show it in front of the screen but others behind the knife edge ?
Secondly does the light source get centred directly at the centre of the test stand ?
I think it would be a great idea to have a mirror making thread section on IIS which would allow people to start threads on their mirror making progress and swap ideas rather than a thread being buried in the ATM section which tends to be mainly about builds and rebuilds using commercial mirrors.. What do you guys think ? . I would be happy to be moderator on it in the sense that I would try to keep things 'focused' and keep my eyes on the section and lend advice where I can.
Mirror making is something that is almost approaching a lost art in comparison to 40 years ago where if you wanted a `huge' 6" reflector you would probably have to grind one yourself as a 6" mirror would have been a weeks wages. I don't think any amount of the advanced work I do these days can replace the romance of the art I felt as a 14 year old , walking around an old washing machine with a 6" blank on top - the smell and sound of fresh Carborundum cutting glass and the smell of resin on a lap and the excitement of seeing my frosty glass take on a shine which I could reflect light from.
Something I definitely want to do also at some stage, nothing like the satisfaction of using something you made yourself. Prob is I'd probably bite off more than I can chew and screw it up, I don't think I could be satisfied doing a spherical mirror as I have a weakness for fast scopes.
I would love to someday make a mirror..... and telescope (when I have time ). But wouldnt knopw where to start, so having a threaded group with info would be great.
My only claim to fame was belonging to a mirror making club run by the guy who was incharge of figuring the optics of the AAT! Brings back memories; that was thirty years ago
A great chap and enthusiastic amateur astronomer. We used to make 6" mirrors in exchange for materials to make bigger scopes - I still have a 8.75" f6 in a box somewhere.
The biggest take home lesson I learned from David, was build a machine to do the hard work, and still have enough energy and enthusiasm to enjoy the figuring!!!
Hi, i made my own 8" f6. Mirror, tube, mount, stepper driver circuits, computer control, gear box -the lot. I bought the secondary and the focuser. It was very therapeutic and i learnt so much about the science and theory whilst doing it. My aim was to do astrophotography and i did some average sort of stuff, but i could not get the quality of control in my gearbox - limited budget, and have since converted it to a Dob. I now have a Celestron C8+ for photography. But my hand made mirror Dob outclasses it for general viewing. I was really proud of it, still am. I love the history of astronomy and I felt I was following a line of endeavour that went back to Galileo. Hope i don’t sound too pompous but its like being part of the continuum of history. I have three books, collectors items i believe, that i used as background research; Telescopes and Accessories, George Z Dimitroff and James G Baker 1st Ed 1948; Amateur Telescope Making, Edited by Albert G Ingalls 4th Ed. 1935 - with a forward by Harlow Shapley; Amateur Telescope Making Advanced, Edited by Albert G Ingalls 1st Ed. 1937. I also have How to Make a Telescope by Jean Texereau which I used extensively. Modern materials make it easier to obtain the quality finish but the science of optics has not changed.
Good stuff Peter. It gives great satisfaction when you turn a telescope one has made to the heavens and see the cosmos revealed in all its glory.There is that special feeling when what you are seeing is due to the work of your own hands,thats the reward for the effort and time put in.
I think it'd be fascinating to have the skill and time to make your own mirror. No doubt very educational too. Interesting to follow the exploits of you guys taking on the challenge.
Great idea!
Firstly, can you guys recommend some good books to get started on ?
I can recommend "How to Make a Telescope by Jean Texereau" if it’s still available. Its old but it is very straightforward. Lots of diagrams and a simple explanation of the how to figure and test using a knife edge tester. You can not imagine the pleasure I felt when I first used my knife edge tester, the figuring was way out of course, it was still spherical, but to see my blank light up with the pattern as predicted was a real buzz. I used to knife edge test just to see it.
For you other guys that talk about not having the skill, without wishing to insult the great mirror makers or belittling their skill, it requires patience and time. A small mirror of say 6 inches to start should be within anyone’s capabilities. I won’t lie and will admit that when figuring I probably produced all possible figures, and maybe some that aren’t in any book, I never got frustrated I just stopped for a day or two and then kept going till I got it right, and as I say knife edging was real fun.
Remember the shorter the focal length the more demanding the figuring, but if your using it just for casual observing then extend the focal length.
When I made my mirror the cost of a figured mirror was, for me at least, prohibitive - over US$600, so making them was more common. But I did it for the challenge in the end. I calculate that I worked for about US$3 per hour, and I am considering doing it all over again to make an 18" f4 Dob.
Hi all, just been googling up on Graham Loftus, (a well known New Zealand mirror/telescope maker),who was a great friend of John Dobson. Both of these men made mirrors up to 24" in diameter out of 19mm thick float glass.In fact the largest of the Loftus mirrors (please correct me if Im wrong), is 36" in diameter and only about 1" thick plate glass. Graham said that 19mm thick float is fine for up to 20" diameter mirrors. He said it was more difficult to finish larger diameter mirrors of this thickness.
Loftus made a comment: a large aperture with even a fairly mediocre figure will out perform a high quality telescope with small to medium aperture,(my paraphrase).
I would encourage anyone who has been teetering on the brink of "whether to,or whether not to" have a go at making their own "light bucket" mirror/telescope to "have a go".If your game then your in for an adventure.
I would be frightened of fracturing a 20" 19mm mirror during grinding. I guess it would be mounted on something but then you would have flexure problems.
Last edited by rainwatcher; 11-12-2011 at 01:32 PM.
Reason: typo
There was a 24" Loftus Dob appeared on TradeMe over here about a year ago. Had an angled EP/Focusser arrangment. Was up in a barn about 30 km from me but the wife said 'NO'
I think the asking was about $2500 or thereabouts. Not sure if anyone bought it or where it went if it did sell. Was a huge wooden box of a thing with steel frame base on wheels. Can't rmember if any detailed specs were given on the mirror, I don't think so.
I would be frightened of fracturing a 20" 19mm mirror during grinding. I guess it would be mounted on something but then you would have flexure problems.
Hi Peter, I have personally ground out the following sized mirrors using 19mm thick float glass: 18.5", 20.25", 22", 22.15", 25". No problems with fracturing, easy to grind and polish.The key with working with mirrors this thin is to have a solid flat surface for support to work on, plus to float the mirror on several sheets of wet newspaper, or artificial grass, or such like. Because float is softer than pyrex or quartz it polishes out very quickly, so in the figuring stage its a good idea to use rouge instead of cerium oxide, as the cutting action is much slower, this gives one much more control over developing the figure required. Also by using rouge one is more likely to avoid a rough (dog biscuit) finish, which can be image destroying.
Loftus made a comment: a large aperture with even a fairly mediocre figure will out perform a high quality telescope with small to medium aperture,(my paraphrase).
I'd certainly disagree with this idea. A smaller telescope with excellent figure will show fainter stars than a larger one with a mediochre figure - so less weight to carry around and less thermal mass. I've seen this myself with an excellent 20" against a 36". When it comes to non - stellar sources things probably get more complicated.
My copy of "How to Make a Telescope by Jean Texereau" arrived today. Second hand off Amazon.
Not sure if I've missed it in the various threads but is anyone able to comment on options for getting a mirror aluminised. I'm Brisbane based.
Ballpark prices for different sizes, who does it etc.
Still not quite decided if I'm going to try pushing some glass and that's one part of the process I've not seen any info on.
The mathematical reductions in Texeraux probably put off a lot of people - nowadays this can all be handled by software.
Another great book in the French tradition and more recent is 'A Manual for Advanced Telescope Makers' also published by Willmann Bell.
The Stellafane website is a great source of links for mirror making stuff ( probably the best there is ).
Aluminising - worth checking with Oplass in Sydney ( see older ads for CQin Co, in Australia Sky and Telescope for a # ) . They are only sending stuff for coating to and from China now. I have another contact for quality coatings just coming on line shortly . Costs depend on size- a 12" mirror with Quartz overcoat is about $350.
Hi all,
I have a question for someone who has made their own mirror. I have figured a 17" blank of 19mm plateglass, and put it into the future telescope assembly. I can view the moon sharply at 840x magnification, Jupiter to about 350x (due to bad Sydney seeing at present), and defocussed stars have fairly equal rings both sides of focus, with no sign of astigmatism. Does anyone know if this is a good enough indication that the mirror could be coated? Mel Bartels says it should magnify to between 30 and 50 times per inch of aperture.
cheers,
Frank
). But wouldnt knopw where to start, so having a threaded group with info would be great.
