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mapstar

The 22" mapstar mirror

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Thanks for posting the video Raymond and technique as it looks to be proving very successful.

I suppose working hand wise with a spinning table makes it easier to concentrate on just the stroke width length and movement?

Moving around the table which I've been doing puts in another variable to contend with which in some cases may be a good or bad?

Looks lovely and I sincerely hope mine looks similar soon.

Damian

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I don't think having the mirror not spinning will effect it too much, just more work walking around. I don't try and make every stroke perfect as there are so many strokes it all averages out and a bit of random may be a good thing.

It sure is nice having a spinning table.

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Mirror looking good Damian, bet you can't wait to get it back and get on with the figuring!

That is a cool video Raymond, I'm looking forward to seeing Damian's rotating table now! :)

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Mirror looking good Damian, bet you can't wait to get it back and get on with the figuring!

That is a cool video Raymond, I'm looking forward to seeing Damian's rotating table now! :)

If I attempt another mirror! I will make one, but for this one the manual table will have to do.
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When you picking the mirror back up Damian?

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I don't think having the mirror not spinning will effect it too much, just more work walking around. I don't try and make every stroke perfect as there are so many strokes it all averages out and a bit of random may be a good thing.

It sure is nice having a spinning table.

Random is good, in fact it's  desirable. as long as you resist the temptation to waltz the tool around, everything will be A1 with random.

That was a dictate from a book, not my personal wording.

What I do recommend, is that you think about what you are doing with the lap, and predict as best you can what result  a particular action is going to produce.

Once you start to get that right, you are going places, and in the right direction too  :smiley:.

Ron.

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Evening all,

Today I made the 3 hour round trip to pick the mirror up from John.

It was good to see him again and the enthusiasm and passion he has for mirror making is infectious

John was working on a couple of mirrors when I arrived early, and these he tested whilst I was there, interesting to see how different they could be and what corrective methods he would employ to sort them out.

We also tested my mirror which had been stood for a few days and it looks good. As in the previous test images the middle is raised although John said it wouldn't take long to sort that out. After yet more mirror making discussion I boxed the mirror up bid him farewell and headed home.

Tomorrow is a case of making ready for the mirror work by cleaning and moving the table back into the kitchen. Slow and steady with plenty of testing every half an hour to see where I'm going. 

Fingers crossed but I'll also be juggling this with the scope build as I have all the materials massed to make a start.

Damian

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Well I finished me 20" f3.8 two nights ago. It turned out really nice with a very smooth figure and surface. No lap smaller than 8" was needed not like my 18" that needed a 1" lap just in from the edge so this one is a bit smoother.

I fully recommend the w stoke with an 8" lap for this and your sized mirror with constant checking to know what diameter to work out to most as you get deeper and closer to the end.

post-14823-0-50269000-1437630164_thumb.j

post-14823-0-43100400-1437630175.gif

post-14823-0-51414800-1437630152_thumb.j

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Well done Raymond

Looks really nice and as you say smooth which is what is needed.

I will be using a 10" lap and testing regularly so as to not overshoot.

Did you slow down with your strokes towards the end as they look quite quick in your video.

Off to get sorted around then make a start tomorrow and this weekend.

Damian

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Many congratulations Raymond.

That is a beautiful sight. It's a long time since I've a knife edge view of a perfectly smooth figure.

There's something infectious about it that stirs the soul.

All the work and effort, including the disappointments, and frustrations that can plague the mirror creator,

disappear completely under the spell of this magic vision.

I can't express it any better than that.

I can only express a wish that Damian too reaches this pinnacle too, I'm certain he will.

Ron.

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Well done Raymond

Looks really nice and as you say smooth which is what is needed.

I will be using a 10" lap and testing regularly so as to not overshoot.

Did you slow down with your strokes towards the end as they look quite quick in your video.

Off to get sorted around then make a start tomorrow and this weekend.

Damian

Yes the last few sessions were taken slow and a more watery mix of cerium used. You can go fast so long as the lap stays silky smooth to push. I never saw any roughness when rough parabolizing.

I only used 50grams of cerium to polish out and figure this mirror!

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You are going to test every half hour or so Damian but how long are you going to figure for each time between testing Damian? 15mins? Or is that too long?

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By the way Raymond, that mirror looks amazing!

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You are going to test every half hour or so Damian but how long are you going to figure for each time between testing Damian? 15mins? Or is that too long?

The time spent figuring will get shorter and shorter the closer you get. To start with you could test every hour but when you get down to within 500nm you will want to spend no more 5min figuring before retesting. Within 100nm you will want to do no more that 1-2min before checking what your next plan will be.

Rest time for the mirror when your close need to be maybe 1/2 hour or so. I always pick the mirror up with a bunch of paper towels to stop my hand heating up the edges.

With this 20" I just did my goal was to not need a super small lap for tricky corrective work as that creates roughness so as the mirror got closer I tried my best to only work what was needed but I still ran over the complete mirror at the end of each session to smooth thing up.

Number one will be to get a good testing setup and use Figure XP, make a couder mask and start measuring the mirror from about the half way point.

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Afternoon fellow star gazers

Thanks again for posting Raymond with useful advice , it's interesting to see how  people go about making a mirror and there is a bewildering amount of ways to do it. I've never really looked into the figuring software side although it does produce very good mirrors as you have quite rightly shown. 

I'll have a good look at it once I've downloaded as it will be something else to check the mirror with. Final testing will be done at John's on his auto collimation rig all being well.

I have made a start today again on progressing the mirror.

Firstly I set the test equipment up again to perform both the Ronchi test and Foucault with a start image taken outside centre of curvature

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To begin with I would be smoothing the overall shape as there is a hill in the middle. I worked in two half an hour sessions vary the W stroke width and length from tight wider W across the middle.

One thing I have learnt, and a tip for anyone attempting this, is that cerium in nigh on impossible to get off around the edge if it is allowed to dry and stain the glass. So I was mind full of this and kept the edge clean with a sponge soaked in filtered water.

After each session I tested and slow progress the lines are a little less bowed out (indicating a hill when measured outside COC) especially the central line which is smoother in the middle. I know the differences are very subtle to see but they are progress so I shall stick with that and progress slowly.

post-28847-0-85404800-1437758739_thumb.j

More work the same tomorrow, and I shall post the updates as the images progress

Onwards with pushing glass molecules 

Damian

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Looks like a quarter to a third of the 'hill' gone to my eyes?

I'm already getting nervous, don't know what you must be like!

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Looks like a quarter to a third of the 'hill' gone to my eyes?

I'm already getting nervous, don't know what you must be like!

Yes nerves are jangling so slow and steady is the way.

The hill has reduced a little not really sure how much in terms of measurement but it has moved.

I'm looking forward to tomorrow and seeing it change a lot more.

Damian

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Nice work buddy, we are with you all the way on this! :)

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I have lots of fingers crossed for you

Makes this IPad a bit tricky to use, mind

Richard

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Keep up the good work looking good at the moment [emoji106]

Mark

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Looks like it's moving along nicely.

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Morning all,

Yesterday I had five half an hour sessions with the mirror and 10" lap. I've been using much less Cerium than previously and a 5KG weight atop the lap. A lot of slow and steady varied strokes and things are progressing

Here's the results in Ronchi form outside ROC as the work progressed

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The central hill has begun to disappear so I know need to blend in the outer edge to the central region.

I intend some more work today and if I can I will try to get some Foucault images too showing the overall surface profile

Damian

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How will you know when you have finished, Damian? I'm not familiar with the tests you are using

Richard

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How will you know when you have finished, Damian? I'm not familiar with the tests you are using

Richard

There are quite a few ways, so I understand Raymond above uses a Couder mask (like the one below) and the Foucault test in which certain areas (the cut outs) grey (null) out at certain offsets from centre of curvature.The figure XP program is used to generate the correct curve and data at what distances the areas null

post-28847-0-49081400-1437907591.jpg

I will initially use the matched Ronchi tests where you generate a series of images at different offsets (measured with a micrometer stage) from the radius of curvature. Here's something like what I will be using

post-28847-0-67563300-1437907337_thumb.p

 After that maybe a Ross null test, but ultimately a test on Johns auto collimation bench and his experienced eye to tell me if it needs more work.

Hope that helps a little as I'm just an amateur at this so feeling my way like everyone else 

Damian

Edited by mapstar
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