The 22" mapstar mirror

[Ajohn]

I think Howard's book is on the archive.

I did try drilling holes in callipers but found I needed a carbide drill and went of the idea. Lots of people like the idea of using a dti. Some move by hand some move mechanically. A lot depends on what is around. A spare focuser can be used as has been shown. Used micrometer spindles can be cheap. DTI's too. The metal parts I used don't cost much either and where metal is best used really easy to make.

John

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Edited July 28, 2015 by Ajohn

[Stub Mandrel]

It's easiest to clamp a calliper if you haven't got a drill that will touch it. For the modest forces here you could even epoxy it in place, then when you are done, peel it off.

If you want an accurate but cheap way to move anything, get a long M6 screw ('bolt') and force it into the central hole of a plastic 'fixit block'. The block can be attached to anything and the screw used to move it in and out quite accurately. The pitch of the screw is 1mm so a knob or dial attached to it with 40 divisions will move it almost exactly 1 thou per division or 50 divisions will move it 0.02mm. You can use an allen key superglued or epoxied into a cap head screw and the key acts as a 'pointer' that is easily aligned with a printed off dial 50-100mm in diameter.

The reason for using the fixit block instead of a nut is you will get zero backlash.

[mapstar]

Guys at present I use a micrometer stage with 25mm of travel.

I posted this a while ago it is smooth and works well so I will utilise this in any future development of a tester.

[Ajohn]

I wish I had one Damian but I wont spend what they usually cost plus some only have 1/2" travel. Some people who test thin mirrors test vertically to avoid mirror flex problems. That makes an easy build x-y jig difficult but it's possible to manage without the 2nd axis testing in the normal fashion. Many do. It's also ideal for that caustic test I mentioned but if your having the mirror checked some other way that's over the top. That mic spindle on mine has a 1/10000" vernier on it and that has real meaning on that test. The only woolly part is the actual mirror rad.

I tried push pull and a rule as a scale for rough figuring but found I prefer the mic spindle.  That could even be a chopped up mic. Also pin pricks on paper measured afterwards. Stub mentioned the bolt. Knobs can be found with the graduations on them. There are cheaper digital machine scales about. ArcEuro maybe. There are all sorts of options really.

Once some sort of stage is made it's pretty easy to try a number of things out including stationary source, slits, different ways of moving and measuring etc all part of the "fun" - ? if that's the right thing to call it.  My feeling is that it is worth spending some time on the stage what ever type it is. Sometimes it's just a straight piece of wood with the moving part pressed against it. I simply thought that a piece of rod and a couple of brass V's was just as easy and it would definitely slide well. The tilt bolt could also run on brass or glass even. The main problem with mine was the knife tilt bolt - the thread is too coarse so I should have added a big knob or use something finer and probably a knob as well. Texereau gets round that by making the platform a lot wider and uses a finer bolt than I did.

I'm might just soak the rusty bits in a mix of 25% molasses and water for a few days  to get rid of the rust and more or less use the stage as it is. It's time for an upgrade on it anyway to try a big  moving source  but I'll probably still make another slit.

John

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[mapstar]

Evening Mirror watchers

Over the last few days I've made another couple of hours progress and things are looking smoother.

More work tomorrow and I will update with some ronchi images of how the mirror stands.

The 10" lap I'm using is beginning to get a little thin with all the pressing so I shall make a new lap too.

Thinking the temperatures were going to be a little warmer for this time of year I had picked up some gugolz 64 pitch from John to make a new lap.

The temperatures don't look to be materialising so I shall have to test the hardness before use and soften it if need be.

I've also made a good start on the shell build which will have to progress a little quicker to catch up with the mirror.

Damian

[RAC]

My 20" I just did was all done in about 10-12degC with very hard pitch. Never once did I have to re cut the channels or scratch it up apart from scrub it with a brass brush. This was for the polishing lap and the figuring lap. When I did the 18" the channels would only last an hour or two. So there is a lot of give with what works.

[RAC]

One thing to note If you plan on using Figure XP is that apparently it doesn't use the correct math if you select and use the fixed light source option. Only the moving light source setup is correct. Not sure how true that is but the moving source is easy anyway.

[mapstar]

Here's the progress over the last few days,

I took a check of the lap today after I finished work, and after a test press with the mirror face down on the lap I could see the contact was not that good. The lap is down to about 3mm in places on the edge as the ply wasn't the best and I can see it was slightly warped. After several attempts with warm pressing with the mirror face down and the back wet I decided not to continue using it and make a new one. It just wasn't pressing out in the middle.

So I headed to the garage which turned out to be 4 hours as I got into making a new lap base this time out of quality baltic birch. Whilst the new 10" lap was gluing (4x 1/2" thick disc's) I continued with the upper tube assembly build which will hit SGL later.

Here are the images I took this afternoon  before I started 

Inside ROC

Outside ROC

It is a fair way off at present but getting nearer by the day. I should have the new lap completed by tomorrow evening so will resume work probably monday. It will also give me time to look into making a set up where I can get foucault images working with my camera although I am still going down the matched Ronchi route I would like to see how the other methods work too.

Here's a peek at the upper tube assembly ply as I made a start

More soon 

Damian

[RAC]

I've always liked the idea of plaster tools as it is very stable and will never go out of shape. What I do is take a mold of the mirror at the end of rough grinding then take a mold of the that mold so I have a plaster model of the mirror to mold any laps on top of.

[Soupy]

Ohhh! Parabolic mirrors and now we have ply and scope construction. Getting very exciting now buddy!

[barkis]

I did the same thing Raymond.

I used Dental cement, which  is a superb medium for a lap,and quite inflexible when set.

It isn't expensive, in fact when I explained to the Dental technician where I bought it , what I was going to use it for, 

he gave me a large bag full of it FOC  It made a 2" thick base for a 15" lap, and a couple of sub diameter ones too.

It does get rather hot when it's curing, and as I used the ground mirror as the mould, I used a tissue and baco foil lining over the mirror

to prevent too much heat transference. It was a heart in mouth wait, but no disaster happened .

 The pitch squares were individually made to stick on later, so not a lot of pressing 

to get a good match was needed, and little or no channel pruning  out was necessary either.

Ron.

[mapstar]

I did the same thing Raymond.

I used Dental cement, which is a superb medium for a lap,and quite inflexible when set.

It isn't expensive, in fact when I explained to the Dental technician where I bought it , what I was going to use it for,

he gave me a large bag full of it FOC It made a 2" thick base for a 15" lap, and a couple of sub diameter ones too.

It does get rather hot when it's curing, and as I used the ground mirror as the mould, I used a tissue and baco foil lining over the mirror

to prevent too much heat transference. It was a heart in mouth wait, but no disaster happened .

The pitch squares were individually made to stick on later, so not a lot of pressing

to get a good match was needed, and little or no channel pruning out was necessary either.

Ron.

Dental plaster is something I've thought about trying but with the ply at hand it's just easier for now.

What did you use to stick the squares to the plaster with? I think I read in tex that bees wax or something similar was used?

I have some more work planned today as I've said already, but probably not much on the mirror.

Damian

[barkis]

I drew the pattern of the grid onto the lap, coated with Beeswax, which allowed the pattern to stay visible, then

played a light flame over the wax to position and set in the pitch squares.

One recommendation I got from one ATM book, was to mix a little beeswax into the pitch before making the lap.

I tried it once, but found that the lap often skidded when polishing, so I quit that,

Ron.

[Physopto]

Hi Damian,

 Correct me if I'm wrong but would dental plaster not be better as it is rigid and not likely to warp. I'm thinking of when you are getting to the finish as it is less likely to cause problems when figuring the mirror. Would it not be more likely to stay the correct shape under a slight misbalance of applied pressure when doing the final work  as all wood will flex to some extent ? The amounts of glass you are shifting are very small. Even paper is an abrasive at these miniscule measurements. I'm used to working with measurements down to 10^-10 metres, so can appreciate the problems.

Derek

Edited August 2, 2015 by Physopto

[Ajohn]

I found the attached file on the web for download so no problems attaching it. It covers just about everything I have ever seen mentioned on dental stone but avoids using epoxy for sticking tiles. I suspect that is what people will have to look for in the UK not plaster. Some have used hydra stone over here, or a name very like it. It's a much stronger type of the usual modelling plaster.

Some one on the youtube uses a mixer to mix dental stone up quickly (Gordon ??). It sets pretty quickly so probably best to mix a little to see what happens rather than just diving in.

I've attached the file in pdf format.  I saved it in odt which is a Linux word format so used something else to convert to pdf. If Adobe wont accept I can post in some other format if needed.

Tex casts squares, heats them over a low flame and plonks them down on a warm mirror. From other sources I have seen this seems to be the pro way even on very large mirrors. He mentions that Ritchey ( as in Chretien telescope) painted hot bees wax on poor quality pitch.

I've used Howard's method. Let it cool somewhat, pour in a spiral on the warm mirror, cover everything with rouge and rub around to achieve full contact, then cut slots with a wet saw when it's fully set. Messy so next time I'm going to cast strips, cut squares and stick them down tex style.  Bet it sticks to what ever I cast it on.

Whoops One thing to add. For sub diameter Gordon ??? again seems to have decent method. He uses an annular ring of wood of some sort to act a former with what ever is used to form the sides running down onto the mirror. Looks like he uses the thin plastic film (fablon) that can be stuck to shelves etc in the kitchen to stop the plaster from sticking to the mirror.

John

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tiletool.pdf

Edited August 2, 2015 by Ajohn

[mapstar]

Hi Damian,

Correct me if I'm wrong but would dental plaster not be better as it is rigid and not likely to warp. I'm thinking of when you are getting to the finish as it is less likely to cause problems when figuring the mirror. Would it not be more likely to stay the correct shape under a slight misbalance of applied pressure when doing the final work as all wood will flex to some extent ? The amounts of glass you are shifting are very small. Even paper is an abrasive at these miniscule measurements. I'm used to working with measurements down to 10^-10 metres, so can appreciate the problems.

Derek

Hiya Derek.

I know John uses both dental plaster and also ply laps and doesn't have any problems so both work.

The ply is baltic birch and very dense. 2" thick over a 10" disc would be very difficult to flex.

I have cast the lap using the method I used last time which was to put the lap face up and put a dam of foil around it and then just pour the pitch in.

I have waited for it to cool to warm before putting it face down on the mirror to press with weight.

It has grease proof paper between and gap nearly formed to the shape already. By tomorrow it should be ready for use.

Damian

[Physopto]

Thanks Damian,

I did talk to you a good while ago a bit about the process. But what I'm getting at is the even smallest flex due to warping during a polishing episode. It would be impossible to detect by eye. A bit like the mirror glass warming and changing shape during the polishing, as has been said you have to wait to test it, allowing it to cool again? I suppose the pitch is softer anyway and the pressing removes any warp. Your thread brings up some interesting points and discussions.

Just hoping you have no more problems and all goes well.

Derek

[mapstar]

Yeah it's a while since we chatted and hopefully will do a lot more in November.

Fingers crossed everything will go OK from now on and constantly checking and pressing will ensure that.

Back to it this aft once I've finished work. Probably more on the wood work too.

The lap was pressing overnight and today so by the time I get home it should have formed to the mirror fully and be ready for the channels cutting. Once that's done I will cover it in cerium and do the mirror face down test to check contact and make a start when it's good.

Damian

[Stub Mandrel]

The professional mirror makers features in an old video posted recently were using polishing machines made entirely out of wood! The laps appeared to be wooden too.

One interesting bit shows them using two laps of different sizes at different diameters.

[wookie1965]

A bit of inspiration,Master optician John Brashear, seen in his optical shop in (1915) Pittsburgh during the grinding, polishing and optical figuring of the 1.82-m Plaskett Telescope mirror. Few opticians in history have equaled Brashear's skill at producing objectives for astronomical telescopes. One that does come to mind would be E. Harvey Richardson, former staff member of the Dominion Astrophysical Observatory and the scientist responsible for many innovative improvements to the telescopes here, as well as being involved in the design of many telescopes and spectrographs world-wide. The legacy of world-class optical design is continued here in Victoria by Richardson's former grad student John Pazder. This excellent mirror, now located in the Centre of the Universe, was replaced by a ceramic mirror that was affected less by temperature changes. (Photo from the DAO archives courtesy NRC / Dr. Dennis Crabtree).

[Stub Mandrel]

> This excellent mirror, now located in the Centre of the Universe, was replaced by a ceramic mirror that was affected less by temperature changes.

I'm curious as to why such a mirror would be 'archived'. I would have thought a university somewhere would snap it up - half the cost of a big observatory must be the mirror!

<edit> Ah! I see it is still (just) in use - once the second largest in the world, glass made in Germany, ground three times in Pittsburgh (partly due to the 'mystery scratch' incident!)

https://en.wikipedia.org/wiki/Dominion_Astrophysical_Observatory#Centre_of_the_Universe

Edited August 4, 2015 by Stub Mandrel

[mapstar]

That's some chunk of glass Paul and I have to admit mine has looked to me almost like that at times   Although I will admit that it is like buying a new TV. At first they look immense then seem to shrink (I don't mean by grinding and polishing bits off).

I made the new lap from a mix of Gugolz 55 and 64 with a little boiled linseed oil in to soften it slightly.

The new lap pressed overnight Sunday and then I cut the facets in it and then pressed again over night with approx 5kg on it. Today when I arrived home from work it was time to heat up the lap and do a warm press with some cerium on the mirror to stop it sticking.

Here's the lap contact after just one warm press. The lap is face up the mirror face down on top with the mirror back wet just to give a better view.

After a couple of presses it looked to be in good contact.To make sure I put the mirror on the polishing table and left it for half an hour with 5kg on, occasionally turning it and making sure it didn't stick while keeping it wet. I then made a start again with the polishing the 5kg remaining on top. I also added a threaded insert just so I could screw a bolt in just to be on the safe side so I could stop the weight accidentally going anywhere.

I had half an hour using various strokes (narrow V through the middle to about 80% dia, W strokes varying from 60% up to about 90% dia) Will update with the test images when the mirror has stood to cool.

Damian

[Glasspusher]

Stub M, Those old blanks had very poor thermal properties, the thickness of the blank resulted in a long cool down time which was a problem. Even the small mirrors in amateur telescopes can suffer from the same problem hence the need for cooling fans. The development of new types of glass/ceramics with far superior thermal properties made the 'old' mirrors redundant.  All professional observatory telescopes now have mirrors which are  either thin or have a 'honeycomb' type structure to aid in rapid equilibrium between the glass and the air.

John

[RAC]

I noticed you haven't pointed the edges of the lap as this helps with blending of the laps action on the mirror.

[mapstar]

I noticed you haven't pointed the edges of the lap as this helps with blending of the laps action on the mirror.

Not yet Raymond I will see how it goes as I'm just working in the centre section as this is still under corrected with a slight hill.

I'm also working on a set up for foucault testing so I can image with the camera I have.

Did some initial testing and found the camera needs to be quite far back from the knife edge.