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mapstar

The 22" mapstar mirror

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Spend too much time at the optics and you can end up completely parabolised, that's for sure.

James

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Amazing, such dedication !!

From what I've read, you're going for a spherical mirror rather than a parabolic mirror. Is that right Damien ?

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Producing a sphere initially is the traditional path to further work for parabolizing.  :smiley:

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Producing a sphere initially is the traditional path to further work for parabolizing.  :smiley:

ahhh right :)

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Damian, I'm sure you were under no illusions about the task you embarked upon with this large objective, and John Nichol would certainly have

highlighted the difficulties you would encounter. Frustration will always be your companion when you are attempting the figuring process,

and when it gets too bad, you should take a break. A 22" f3.7 is a very difficult adversary for anyone to tackle.

All of us following your progress are pretty much mesmerised by what you've achieved so far, and certainly willing you passed the winning post.

You have the discipline to complete the job, but don't be averse to leaving it to one side for a spell, and get back to it when you feel like it.

Have a read of Mel Bartel's Parabolising pages might inspire , they are quite interesting.

Best Wishes.

Ron.

http://www.bbastrodesigns.com/JoyOfMirrorMaking/Parabolizing.html

Thank you everyone for the encouragement and kind words.

Ron, I have looked at Mel's page a few time's it's amazing the amount of literature you read when you get into all this. A lot quote Texereau as the be all and end all of mirror making.It's not the case just another way to do it.

It's all helpful especially the encouragement and interest of others on here. I have found a few site's on mirror making and logs of people's battle's with mirrors.

I have had many a battle with enthusiasm and have my occasional low points but always end up at the same conclusion that time and effort will sort it out.I just sometimes can't see the wood for the trees

More work possibly tomorrow but definitely on Friday and the weekend.

Cath as Peter say's I'm working towards a sphere then I can start the exciting (in mirror making) bit of parabolising

Damian

Edited by mapstar
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I'm surprised you haven't made a full sized lap for producing the sphere. It works well especially with mirror on the bottom with a random mix of over centre and mild w strokes with say 1/4 dia long strokes. Plus random steps round and try and be random with the rest as well. Mirror on top tends to bring up the centre quicker though. Tool on top tends to wipe up towards the edges of the mirror which is why it's best to finish off this way to avoid any chances of a turned down edge as most of the light in an image comes from the outer parts of the mirror,  pi r^2 etc. 

Some one mentioned using the fringes ahead of a knife edge in front of the ROC earlier. This work rather well and is probably the best way to check  right up to the edge. It needs a slit source to work well though - around 0.04mm from memory over a hole say 5mm diameter.  When you get to a sphere with a source like that you will find it very difficult to cause the knife edge to grey out the mirror evenly but that too is a very sensitive test but will usually give bright diffraction rings around the rim if it really is spherical making it difficult to see what's going on there. The knife edge has to be set very precisely. Texereau did that by swinging it on an L shaped  arm with M6 bolt on the end and mounting the knife edge so that it could be tilted to square it up with the slit.  :grin: I did it the other way round and tilted the slit and as it was separate lost it. The diffraction effects can be reduced by widening the slit but that can also mess up accuracy.

There are various ways of making a lap. Very big mirror makers cast squares and stick them on. I've followed a book by Muirden and it worked for me but it assume the mirror is transparent. The mirror and tool need to be warmed up, not too hot to hold but sort of warm and cosy style. Pitch melted and then stirred while cooling, lift the stick out and let some drip back in. It needs to take a few seconds to melt back in. Pour an even spiral over the tool. Let it cool until it takes a finger print without sticking. Cover the mirror with plenty of polishing powder mix and place it over the lap while keeping it moving around - w and centre over centre stokes etc. If the mirror is transparent lack of contact can be seen. I've managed to get full contact this way. If not and the pitch gets too cold trim up and hot press again etc. I cut the channels with a wet tennon saw.

I've tried the other ideas, very coarse draining board mats when they were available, didn't really help and the channels still needed trimming up with a saw. Also a hot knife but it leaves a lip. One thing I did find which helped was pressing with a piece of fine plastic mesh over the lap. This gives a lot of very small facets and I have done this on any size of lap I've made - onions used to be sold bagged with it. A good coat of polishing solution prevents sticking.

I've never done anything as big as this and suspect I would bite the bullet and cast squares or maybe cast a sheet and cut it up. I think a sheet would be easier to maintain a constant thickness. Cutting it up pass but I suspect I would give it a try. One thing that does help with pitch - it sticks well to warm surfaces.

John

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keep pushing on buddy. We'll be observing through our dream scopes in no time! :grin:

Cheers John

I've yet to start the build of mine but it'll be on it's way soon

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That looks amazing RAC. (Not that I know what I'm looking at but I think it is telling me that the mirror is almost perfect, and only a few nanometers out from where it needs to be) :)

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Here's some motivation for you. I just finished off my 18" f3.3 mirror last night.

16859348401_9f885829c1_b.jpg

18inch f3.3 Done! by Raymond Collecutt, on Flickr

16859348251_0b69c79949_b.jpg

18inch_Surface by Raymond Collecutt, on Flickr

That look's very nice Raymond and a pleasure to behold, certainly inspirational

On a testing point did you use a matched Ronchi test for it to start with? and obviously other test's? Could you explain your testing as it's obviously produced a very nice mirror as can be seen from your image. I one day hope to be in the same position of being able to say been there done that and I have the picture's and test image's to prove it.

Well done and thank you for sharing it with us, can we also see a pic when you get it coated. 

Damian

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I'm surprised you haven't made a full sized lap for producing the sphere. It works well especially with mirror on the bottom with a random mix of over centre and mild w strokes with say 1/4 dia long strokes. Plus random steps round and try and be random with the rest as well. Mirror on top tends to bring up the centre quicker though. Tool on top tends to wipe up towards the edges of the mirror which is why it's best to finish off this way to avoid any chances of a turned down edge as most of the light in an image comes from the outer parts of the mirror,  pi r^2 etc. 

Some one mentioned using the fringes ahead of a knife edge in front of the ROC earlier. This work rather well and is probably the best way to check  right up to the edge. It needs a slit source to work well though - around 0.04mm from memory over a hole say 5mm diameter.  When you get to a sphere with a source like that you will find it very difficult to cause the knife edge to grey out the mirror evenly but that too is a very sensitive test but will usually give bright diffraction rings around the rim if it really is spherical making it difficult to see what's going on there. The knife edge has to be set very precisely. Texereau did that by swinging it on an L shaped  arm with M6 bolt on the end and mounting the knife edge so that it could be tilted to square it up with the slit.  :grin: I did it the other way round and tilted the slit and as it was separate lost it. The diffraction effects can be reduced by widening the slit but that can also mess up accuracy.

There are various ways of making a lap. Very big mirror makers cast squares and stick them on. I've followed a book by Muirden and it worked for me but it assume the mirror is transparent. The mirror and tool need to be warmed up, not too hot to hold but sort of warm and cosy style. Pitch melted and then stirred while cooling, lift the stick out and let some drip back in. It needs to take a few seconds to melt back in. Pour an even spiral over the tool. Let it cool until it takes a finger print without sticking. Cover the mirror with plenty of polishing powder mix and place it over the lap while keeping it moving around - w and centre over centre stokes etc. If the mirror is transparent lack of contact can be seen. I've managed to get full contact this way. If not and the pitch gets too cold trim up and hot press again etc. I cut the channels with a wet tennon saw.

I've tried the other ideas, very coarse draining board mats when they were available, didn't really help and the channels still needed trimming up with a saw. Also a hot knife but it leaves a lip. One thing I did find which helped was pressing with a piece of fine plastic mesh over the lap. This gives a lot of very small facets and I have done this on any size of lap I've made - onions used to be sold bagged with it. A good coat of polishing solution prevents sticking.

I've never done anything as big as this and suspect I would bite the bullet and cast squares or maybe cast a sheet and cut it up. I think a sheet would be easier to maintain a constant thickness. Cutting it up pass but I suspect I would give it a try. One thing that does help with pitch - it sticks well to warm surfaces.

John

-

Hello John,

A lot of thought's there thanks and just to elaborate on a few bit's which I have touched on in previous post's which maybe you missed???

I'd be surprised myself if I made a full size lap as I would probably need me to spend quite a bit of time in the gym to push it across the surface, the amount of drag on an 18" lap is phenomenal and I have 40kg of weight on the base of the table and the suction still tried to lift the legs. It's hard work

I will give thought to the test you mention as the diffraction around the Ronchi image's is quite severe so other method's may be needed although the Ronchi does show a lot.

Several people have already commented on lap making via the 'Tex' cutting squares method and it is something I will considered if I make another lap. I'll probably have a go at it as I have tried several other way's (in previous post's) although everything is good in theory and I suspect it will end up a very messy affair

I'll definitely agree with you on a few of your point's .

The pitch bit, that it stick's to everything mostly what you don't want it to hot or cold!

That you have never made anything as big as this by suggesting working mirror on top or making a full size lap both of which just aren't practical or feasible due to the physical effort needed to use them

The creation of micro facets is a good idea, either by the onion bag method or by placing the wet lap on greaseproof paper whilst pressing which wrinkles the paper and after several uses create's micro faceting 

Damian

Edited by mapstar

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I always used micro faceting on my laps, whether it was a 12" lap, or a 6".

The theory that it is the edges of the lap squares that facilitated the polishing action,

means that multi. facets theoretically would speed up the polishing, and also

the figuring process.  Of course we know that speed is not the factor that is important.

Good contact, and at a speed that prevents micro ripple is important.

Keep the Faith Damian, you've already got the winning post in sight,

you just have to give the mirror a quiet talking to  :smiley:.

Ron. 

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I always used micro faceting on my laps, whether it was a 12" lap, or a 6".

The theory that it is the edges of the lap squares that facilitated the polishing action,

means that multi. facets theoretically would speed up the polishing, and also

the figuring process.  Of course we know that speed is not the factor that is important.

Good contact, and at a speed that prevents micro ripple is important.

Keep the Faith Damian, you've already got the winning post in sight,

you just have to give the mirror a quiet talking to  :smiley:.

Ron. 

Great info Ron. I have read about the way the facets work before but cannot remember there is a full page on the web about it somewhere? Tex also has a bit in his book about the surface quality using various type's of lap although the picture's are quite dated now it does give a good representation of what it looks like at a microscopic level.

I have done another 2 hour's today so the mirror is just settling before I put it to the test again, I like the bit about giving it a quiet talking to although I must admit it's not responding well to the expletive's that I sometime's use  :wink:

I shall post some image's although I do believe they may not be much different to the one's above in the TDE battle

Damian

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I've just been busy with the camera again before I get ready for work  :sad:

Here's the result's of this morning's efforts

Inside ROC

post-28847-0-99647400-1426763735_thumb.j

Outside ROC

post-28847-0-69108100-1426763762_thumb.j

I have used the same W type stroke as before deepening all the curve but the last half an hour I just worked across the middle 1/3 of the mirror with approx 1/2" overhang in a attempt to push the TDE out. It's also flattened the hill a little

More thinking this aft before returning to the fray tomorrow morning with renewed vigour

Damian 

Edited by mapstar
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This really is gripping stuff, the tension is killing me, keep at it Damian

Ian

Sent from my Fone

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Some of my post was for people who might be thinking about making a mirror Damian. I mentioned the full sized lap because of an earlier comment about the centre polishing slowly. The usual way round that with big mirrors on the bottom but generally done with a machine is to  use a smaller lap in the centre and forget the initial pure spherical mirror. It's possible to measure what will loosely be 2 radii this way as it's done to make sure it isn't too deep for the eventual parabolic shape.  I have a similar problem with an F3 mirror for different reasons - the figuring depth. Trouble is I am not sure if it's suitable for hand work as the "cenral sphere" might be off centre compared with the outer one. In real terms they wont be distinct spheres but measurements can still be taken.

RAC's mirror is a nice piece of work. The software he shows is probably one of the ones on here

https://stellafane.org/tm/atm/test/software.html

I know for a fact that I can't make measurements as accurately as RAC can. I don't do it often enough.  :embarrassed: I've tried all sorts, masks and shadows touching sticks, centre, 70% and edge correct and the rest smooth - not too bad that one and etc anything else I came across. In the end I followed Schroder's caustic test in ATM III. The others were fine for rough figuring. A lot is made of the difficulty of making Schroder's test rig but really all that is needed is a Tex' one sitting on top of another so that things can be moved in an X Y fashion and the knife is replaced with a wire. I viewed it with  10x eyecup loupe as it was easier to mount than an eyepiece. Nothing ambiguous about this one at all. It's relatively easy to position of the wire to better than 0.001in.  I tried the Dall null test as well. Had problems but later found that it needs setting up pretty precisely by ray tracing it. It's probably better used to null the mirror and work back from the position the bits finish in or to null the mirror looking for zonal defects.

A lot of this area depends on how accurate the mirror needs to be. There is also need to consider the source and knife / wire spacing when high accuracy is needed. Big mirrors help with that a bit.

:embarrassed: My F3 mirror is on the back burner.  It took a long time to come up with a design for a flat field cassegrain.  :grin: Finally did it and concluded that I couldn't hope to make some of the small glass optics accurately enough for it to be a good as it should be. It will probably turn into a Dall Kirkam at some point. Not sure.

John

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Edited by Ajohn

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Damian,

     Just out of curiosity how much time to you leave the glass sit before testing? 

I usually leave mine for  8 hours before testing.

You are getting there,your edge is getting ever so close now.

Rick

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Damian,

Just out of curiosity how much time to you leave the glass sit before testing?

I usually leave mine for 8 hours before testing.

You are getting there,your edge is getting ever so close now.

Rick

Hi Rick

I normally leave the mirror after washing it off under cold water for a couple of hours but then retest just before another session so it will be tomorrow morning when I have another look.

As you say the edge is getting nearer but it's so hard to judge changes

Damian

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That look's very nice Raymond and a pleasure to behold, certainly inspirational

On a testing point did you use a matched Ronchi test for it to start with? and obviously other test's? Could you explain your testing as it's obviously produced a very nice mirror as can be seen from your image. I one day hope to be in the same position of being able to say been there done that and I have the picture's and test image's to prove it.

Well done and thank you for sharing it with us, can we also see a pic when you get it coated. 

Damian

I used the Ronchi test to a point, i would say that point was when the mirror was about 1-2 waves under corrected then i used Figure XP from there on but every time i tested the mirror i would first have a look the Ronchi grating and then measure it and enter the numbers into Figure XP. Zones and roughness shows up well with the Rronchi grating so you must always use it. It's too easy to charge away and do what Figure xp say only to find you're making a mess of things.

Also rotate the mirror in 45deg steps to check for astigmatism and make sure you have as little as possible before you start figuring!

I returned to a sphere two time before i found what figuring method worked well for me and that was using a small lap (4inch) with tight W strokes working only the middle out to say 30% then after 5min out to 40-50% then 70-80% then the full mirror and back to the middle. I found the mirror deepend very smoothly like this rather than using strokes across the mirror and offsetting them from the center to push the correcting out more.

My measuring setup is just a focuser with a dial gauge measuring its travel. I can say for sure that i can get very consistent results.

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I used the Ronchi test to a point, i would say that point was when the mirror was about 1-2 waves under corrected then i used Figure XP from there on but every time i tested the mirror i would first have a look the Ronchi grating and then measure it and enter the numbers into Figure XP. Zones and roughness shows up well with the Rronchi grating so you must always use it. It's too easy to charge away and do what Figure xp say only to find you're making a mess of things.

Also rotate the mirror in 45deg steps to check for astigmatism and make sure you have as little as possible before you start figuring!

I returned to a sphere two time before i found what figuring method worked well for me and that was using a small lap (4inch) with tight W strokes working only the middle out to say 30% then after 5min out to 40-50% then 70-80% then the full mirror and back to the middle. I found the mirror deepend very smoothly like this rather than using strokes across the mirror and offsetting them from the center to push the correcting out more.

My measuring setup is just a focuser with a dial gauge measuring its travel. I can say for sure that i can get very consistent results.

Thank you for that Raymond.

Nice to know that the Ronchi test was the main tool you used in the making of your superb mirror as some don't really like it. In my opinion it's easy to set up, use and is not over complicated like some of the other test method's mentioned which is all good for someone that is an amateur. If it produces a mirror that is close for me then the last stages will be a null test and finally an auto collimation test by experienced eye's.

Back to technique, as I work the mirror it is rotated randomly between wet's to avoid astigmatism and hopefully produce a figure of rotation 

Explaining your test methods and how you had to rework your mirror to end up with the finished item is a help as it proves it can be done several way's. Much respect for finishing a lovely looking mirror

Damian

Edited by mapstar

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I managed an hour yesterday working exclusively on the edge. Not much change but I have all today planned on it so again I will concentrate on the edge and leave the middle to deal with later.

Will update this evening

Damian

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I think I would try a large star lap before deliberately changing part of the mirror Damian.  That way you can work with some overhang without working the edges much and still keep an even figure. The other way is to press the lap over a ring round the circumference. Didn't work for me, I'd guess it depends on how soft the pitch is.

I'm guessing but with an 18in lap I would try 5 or 6 points going say 4 in into the lap and using an over hang of circa 1/2 that. More points = more work biased to the centre. You'll probably find that this deepens the centre but I'd be surprised if it removed all of the glass that has to be removed from there to form a parabola. What I found with laps was they had to be tried to find out if they were going to work out.  A bit suck it and see despite all of the info that is about. The classic small tool on top for getting to a parabola would be 1/2 diameter. Used to the edge to deepen the centre and it's centre run round the edge to correct that part.

John

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I'm now working with a 10" lap John and have been for the last few page's on the thread.

More work today and concentrating more on edge work than the centre.

post-28847-0-28478700-1426951599_thumb.j

Looking like a lovely spiral galaxy  :smiley:

These things take time and it get's very difficult to see any movement on the image's

Inside ROC

post-28847-0-74524900-1426951739_thumb.j

Outside ROC

post-28847-0-32657000-1426951765_thumb.j

More work to follow and I'll post an update later

Damian

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Hi Damian,

What are we expecting to see exactly when it looks right to you? I'm completely in the dark so to speak with that.

Looks like you will be able to arm wrestle a bit when finished :p

Derek

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