The title of a 70's British ATM book?

[wurzelmike]

Your eye's are more sensitive to yellow/green area of the visible spectrum...

That's something I didn't think of but if it's just a case of brightness I think this LED is going to be TOO BRIGHT! It's huge. But I'm thinking of playing around with the tester, try different LED's. With and without a slit/pinhole etc, and just see how things work out.

Another thought, if I image the results via a web cam, what part of the specture would the web cam be most sensitive to ? This is asking far too much from my old 'O-level' physics...

all the best, Mike

[Glasspusher]

Here is a useful link showing how to wire up an led so that it's brightness can be varied...a useful asset when testing a mirror. Simply ensure that you are using an ultra bright led in the circuit. I generally use the 5mm diameter variety. I recently bought all of the components from a well known store (M......). Not the cheapest source for components, but convenient.

http://www.learningaboutelectronics.com/Articles/How-to-vary-the-brightness-of-an-led

John

[fwm891]

Another tip: Flatten the domed end of the LED and grind/polish to at least 1000 grit wet-n-dry taking the flat end quite close to the LED source point - will give you more even illumination/dispersion of the light cone.

[wurzelmike]

Thanks John, I picked up a 'Pot' at the same time as the LED, so that's been covered. The Stellaphone site has an example that helped with the maths.

Thanks for the tip fwm891, I'm planning on making the Stellaphane version (have to start somewhere) and once I understand how these things work I'll start playing around with it.

All the best

Mike

[Ajohn]

Personally I would say go to the internet archive and download and read a book called "how to make a telescope" by texereau and make the sort of tester he describes and use it. Just the principles of the tester there isn't any need to copy it slavishly. There are only 2 points really that might be varied. The couder mask he describes has rather a lot of holes. You might like to start with holes about 1in dia at the centre . 70% zone and the edge of the mirror. You can just stick with that and also check that the shadows move smoothly and evenly as the knife edge moves back and forth without the mask on. You can also use the mask as he describes it. After you have had some practice that type of mask will become easier to use. There is no need to use a screw as a micrometer as he describes either. Marks on a piece of paper would do or rigging it up to a digital calliper some how. One of those could also be used for the sliding part. You can work out how accurate the measurements need to be from the info he gives. His discussing an F6 or F8 8in mirror. The F6 version does need more accuracy than the F8 but the test converts subs light wave differences on the mirror to measurable differences at the knife edge.

He doesn't like people to polish the mirror with it face up and the tool on top as dust is more likely to scratch the mirror. Dust will too. On the other hand if a fair bit of the polishing is done with the mirror face up with strokes as he describes there is a better chance of getting a smooth curve right up to the edge of the mirror.

You can use a led to illuminate a slit in a tester. Some also use a bare led but it's a fact that the size of the illumination - the slit - has a direct bearing one how easy the test is to use and how accurate it can be. Texereau is the only person I am aware of that describes how to set a slit up. It's rather important actually and can even be used as a secondary test for a true sphere before the mirror is figured. On the other hand you can follow many methods shown on the web and at best produce a 1/4 wave or worse mirror. You may as well go out and buy a complete mirror if you are happy with that. If you want when you have a tester made up a ronchi screen can be mounted on it as well so you can compare the accuracy of the 2 methods. In real terms they are good for spheres but as the knife is as well why bother.

I think Howards book mentions easier ways of making a lap than Texereau. His book mentioned earlier is also on the internet archive.

There are also null tests for paraboloids and variations of what is called the caustic test. The latter is what the big telescope makers used to do a final test on mirrors before they switched to null tests. One version of the caustic test sometimes called the wire test can also be used on the tester. It's very very precise and just needs a piece of wire held taught instead of the knife edge and say a 5x eye cup style loupe. It very good for finding out if you have done what you think you have done.

It's also possible to use a web cam to view the tests which ever is used. Main problem is finding a suitable lens that will cause the mirror to fill the frame.

On the other hand you can say well I read it on the web so it must be true and do what ever you like.

On 6in mirror at F8 if you read Texereau's book I think you may find that there may be no need to figure it if a 1/4 wave mirror is ok. The difference is so slight - actually in some ways that can make it a more difficult job to do. He goes into that aspect as well. Unlike others this is a guy that earned a living going round correcting the work of other big telescope mirror makers and also taught the subject.

John

-

[Alfven]

If it is there is a copy on amazon, only £978 (+ £2.78p&P)

http://www.amazon.co...58974837&sr=8-1

£2.78 FOR POSTAGE!!!

[fwm891]

The usual thing if you ask 10 mirror makers how they test their mirrors you will get 11 (or more) different replies. Personally I'm not keen on Couder masks, I prefer a stick and pin method of locating the shadow pattern on the mirror, because you can see the whole of the mirror and watch the shadow progress across the mirror while you move the knife edge.

Then use a wire test to finalise things.....

[Ajohn]

My main point really was to make a tester. It's worth the effort. I tried to find some straight info on testing on the web, This page has a video which show you what to expect and the the problems associated with mirror F ratio. Not too impresses with his demo of the wire test. What happens with that one is that results vary according to the diameter of the wire and the width of the slit. He just shows the principles of it and the caustic test. The usual way of calculating the actual mirror profile is better than the one he shows as well as the errors move around. The graph he shows does clearly show the problem though. Slow download so if lots do it overnight is probably best.

http://mirrorworksho...onalVideos.html

If somebody is making a tester of any type it's worth going through the calculations for the mirror that they intend to make as that is the best way to find out how accurately distances need to be measure and may save them making one that is unsuitable. Once again Texereau explains that rather well and it's easy to knock up a spread sheet. If you haven't got a program download libreoffice. Office n000 for free. If real accuracy is needed a micrometer barrel can be used rather than a screw. They crop up on ebay regularly.

http://www.libreoffi...m-requirements/

My tester has currently got a halogen car bulb in it. But next time I use it I may try a prefocus torch bulb and or a led. No way will I try a slitless tester as some are described on the web where a led is used complete with it's domed end to reduce the intensity of shadows. The video will show why. For a led I might try a WARM white ordinary led with the end flattened off and polished. There are some 10mm types that have the leads formed into a sort of reflector in the led. Some smaller ones may be like this as well. The high intensity leds put out rather a lot of blue and believe it or not this can damage your eyes. A green led would be safer, The power leds are even worse as far as blue is concerned.

People also build interferometers. Just search Bath interferometer.

John

-

[wurzelmike]

Thanks for your suggestions Ajohn. I've downloaded pdf copies of Texereau and Howard and I've picked up a real copy of the Howards book from the Library this week. Along with the stellaphane sight I'm having a little information over load at the moment. But it's all good, I'll just need to put a crib sheet of all the best suggestions together and work from that

There's a really cool, or should I say rather warm , way of making a lap on the Stellaphane site, using something like this http://stellafane.or...51-img_3336.jpg the results look really good so I'm hoping to try that method.

And thanks for mentioning the Caustic test, it's not something I've come across so I'll being looking into it.

@fwm981, have to admit I was scared of the 'stick and pin' method, it just looked too simple. But a couple of diagrams and I'm convinced it's for me, I love diagrams

Thanks for the replies guys,

Mike

[fwm891]

....this http://stellafane.or...51-img_3336.jpg the results look really good so I'm hoping to try that method.

Mike - Beware - 1) you have to remove the melted pitch from the channel you've just created! 2) you locally change the temper of your pitch due heating the channel edges, minor but if your going to the extent of a caustic test small things will show.

Stick to a sharp knife, low pressure and plenty of washing-up liquid as a lubricant.

I cover my heated mirror blank with ali foil and cast my laps onto the ali foil when making my initial pour. The foil is left on the lap face so I can mark out the facet pattern on the foil with a felt tip pen before cutting the channels with a new stanley knife blade. I remove the foil from the facet faces once all the pitch chipings and washing up liquid have been washed away.

Mike

[Ajohn]

I've seen mention of ronchi testing. Fortunately the video shows the problem with that and that the ones with preformed lines to suit a specific mirror have a big problem really.

I think one of the books mentions forming the facets on a lap by forcing in the the sort of "mesh" mats people used to use on draining boards by the sink. It can work and did once for me but didn't another time. Pressing the mirror on the lap with a fine mesh between them can help polishing a lot. My favourite is older. Rouge etc on the lap and moving the mirror around on top of the pitch after it's gone semi solid to get good contact all over. Then cutting the channels with a saw and digging them out. The mirror and the tool need to warmed up. One of the books describes poring the pitch directly on the mirror to get an even layer. Might be Muirden's Beginners Guide to Making Telescopes though.

The next one I do will probably be done as Texereau suggests - cast a long strip on the level and cut out square and stick em on with heat. I have plenty of spare pieces of timber around this time. It seems he recycled used pitch by adding a little castor oil. I tried turps substitute once to soften it a little and it didn't really work out. He's an infuriating bloke. Once you have had a go most people will realise he is generally right. That hot V tool will raise edges and getting them off wont be fun. The plastic mat generally doesn't get right down to the tool so out comes the saw at some point. The saw always gets down.

For pressing I used a 2 gallon bucket of water.

I have fabricates a tool by using an old glass tool, plastic padding ( automotive filler) and tile squares. The plastic padding had no problem keeping the tiles in place. This time I am going to have to use a plaster tool for polishing. I will probably cast the tool on the mirror with cling film to stop it sticking. Varnish it and apply the pitch. If I even make a plaster grinding tool I will hold the tiles or bug steel nuts or what ever in place with plastic padding again.

The pro's often use the slides of a lathe for the caustic test. It can be done in other ways that just mean moving the knife edge back specific distances and measuring across. It's all discussed in Amateur Telescope Making Book III edited by Ingalls. Should still be about in libraries but is also available on Amazon new and 2nd hand. I don't rate Book I at all really and wouldn't buy it. Book II maybe. Book III I have bought - long time ago.

On the pins by the way I hope you have looked at the shadows you will be setting them too. The video does actually show them realistically. For one the pins are likely to have rather bright refractive edges around them and for 2 the shadows aren't that clear. Broaden up the source and the edges may disappear and the shadows get even harder to see. To zone it roughly I would try 5 holes as as I suggested. Ideally these should be mirror radius/100 diameter. If those are too small chances are it's a slow mirror so they can be opened up. Then all you need do is check for bumps etc as the knife is moved smoothly back and forth with the mask off. The shadows should just move smoothly across the mirror. The idea with any mask is that the adjacent holes darken evenly when the knife edge is in the correct place to measure the zone. It takes rather delicate adjustments.

John

-

[Ajohn]

I came across this site recently that might have some interest to some. He really does make scopes and shows some of his equipment etc. He put up a spread sheet I needed to save me writing one myself which is how I found the site. Some rather famous software got something very wrong and he pointed that out to me. He seems to have moved on from a more complicated tester to a bath interferometer. Most go the Ross null test but that one isn't as simple as many believe. I'm mad enough to try to make an F3 mirror for a compound scope for instance. Some one on here used a program that is used for this test and it states that I can use it to obtain 1/4 wave accuracy. What it didn't tell him is that this just isn't good enough. An F3 mirror is an extreme case but most people who use this test and make mirrors check out the accuracy they need to achieve in the set up to get what they need in terms of the accuracy of the mirror using ray tracing software.. The spacings can be alarmingly tight and difficult to achieve. With faster mirrors there are other problems as well however they are tested. Light source to knife edge etc spacing for one.

http://atm.udjat.nl/

John

-

[barkis]

The Everest Scale is not a bad method to assess the mirrors figure. You have to learn to ignore the diffraction scatter around the pins, although not as bad as the ones inside the Couder screen apertures.

Talking of Books on the subject. This old chap was no slouch. Albert Ingalls 3 Volumes hide a wealth of knowledge.

Ron.

[Ajohn]

It's interesting to note that the test had been a described as a wooden stick with pins in it long before it appeared in those books and was given a name. Not in the USA either. Also a gizmo with weighted strings that is used in the same way. Then there is the lots of pins approach.

The trouble with centre, 70% and edge zone testing is being sure that the centre has been picked up accurately. A sensibly sized hole in a mask at the centre will black out much like a full mirror will when it's accurately spherical and give a very precise knife edge reading. As the outer holes of a mask are used things can get a more difficult. It's a case of watch for a pair darkening at the same rate etc and noting the direction - bit iffy but it can be done accurately. Watching shadows move against pins can be iffy too and in my opinion needs some experience. There have been people about who churn out good 10in F6 mirrors with nothing more than a block of wood with knife on it and pencil marks. Only thing though is that they make mirrors all day and every day.

Personally I think most people making a mirror for the 1st time will try everything they can think or hear of.

John

-

[barkis]

The Leviathan of Lord Rosse's would need figuring. At 72" f10, and of speculum metal, not an easy task .

There were two identical mirrors made, one in service, and the other prepared for its turn in the telescope when the other was too tarnished to get good observations with.

They were swapped over every six months. That was a beast of a scope, and some great discoveries were made with it.

Spiral galactic arms in M51 were first seen visually in that monster. Must have been a nightmare centering an object using ropes and pulleys.

If those guys could be brought back to see what's available these days, they would swear it was witchcraft .

Ron.

[wurzelmike]

thanks for all the thoughts on testing everyone, seems like I'm on a steep learning curve. I suppose it's just a case of trying things and finding out what works. Finished reading the Howard book tonight, so I imagine thats a good start

[Ajohn]

Texereau's is a better start really. There are also some decent video's on youtube relating to testing so you can get an idea what to expect. Some on a quartz mirror in particular. Something about making a pair of them. It shows a mask in use which is hard to describe in books. That person seems to use a block that carries the knife edge slid along by hand with the position read with a dial gauge. As usual though it's a fairly fast mirror so the shadows look easy to judge. He was asked to give details of the tester he used but hasn't. Pity but the main aspect of that, slit width and hole size is covered by Texereau as he shows how to adjust it and gives suitable sizes. How you light it and move the knife edge is up to you but I would look at using a digital calliper for measurement with the knife mounted on it.

Where most people seem to go wrong with this sort of set up with a slit is how they mount the knife edge. The knife edge is rocked to cut into the light coming back from the mirror and it has to be square to the slit when it does. People make the slit and the knife edge fixed so the angles can't be adjusted. This just means mounting say a razor blade etc on another piece that extends a couple of inches that can be rocked to line the 2 up. Better still go get some pieces of metal from B&Q and make the knife edge. In fact it should be possible to get all you really need for a Texereau type tester from B&Q. Tube could be used instead of drill rod. MDF for the wooden parts and the metal can be cut up with a hacksaw and finished with a hand file. Nothing needs to be that accurate. Aluminium could replace the brass parts.

There are loads of pictures on the web of foucault testers. Many use the same sort of slides as Texereau's. Some pages also show how their tester developed when they came to actually use it. Dial guages are very popular for measurement. For some reason no one seems to base one round a digital calliper. Probably because they aren't so accurate as a gauge or a micrometer spindle. On the other hand they measure to +/- 0.05mm at least. The 0.01mm on them is a bit of a joke really. If some one wanted to make a posh set up they could use a 4in version of one of these which would save trying to drill holes in hardened stainless callipers. They do dial gauges and all sorts of other things too.

http://www.arceurotr...igital-Readouts

John

-