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mapstar

The 22" mapstar mirror

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Hiya Derek

I've been struggling this week for time on it and only managed 5hours of work on it last week.

I have more work planned this aft so will update with the progress images later today for definite

Damian

I like the way you say ONLY!!  That's a lot of work and every bit gets you that bit closer. I'll keep an eye out

Derek

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Hi Guys,

Been a while since an update so your all probably wondering where I've got to. I'm still here pushing glass molecules about and at present I am trying to get rid of the hill I made in the middle whilst working on the edge.

Here's a few Ronchigrams of the mirror how it stands working from inside centre of curvature to the outside. Not sure why but my camera just didn't want to play ball today so the images are a bit blurry. The inside show the middle bands curving in and the last few outside show the bands curving out indicating a central hill. The images I took previous to this session did look better but the mirror had cooled overnight. 

post-28847-0-62018100-1432582453_thumb.jpost-28847-0-44976000-1432582457_thumb.j

post-28847-0-02876400-1432582461_thumb.jpost-28847-0-77035200-1432582464_thumb.j

post-28847-0-28843800-1432582468_thumb.jpost-28847-0-80695200-1432582470_thumb.j

post-28847-0-00337600-1432582474_thumb.jpost-28847-0-33069700-1432582477_thumb.j

post-28847-0-82494100-1432582480_thumb.j

I have been working through the centre with minimal overhang but now I've moved to just working in the centre to bring the hill down. Hard to tell whether the turned edge is still there but I suspect I will end up going back to it once I've sorted the middle out although it may help the edge with the varied central strokes

I've also tried to capture a few Focault images but can't remember how I did it last time and they turned out very poor so I've not added them to the post.

I will fingers crossed get some more time on it wednesday and will try my best to remove the central hill 

Damian

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Well to my untrained eye they look as though the edge is reduced. Good to see.

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I agree the edge is certainly looking like it's almost gone... Hill looks strange but that is an easier fix. Well done buddy :)

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Looks good. Keep going on the center. There looks to be lots of correction in the outer zones. I would start taking some measurments.

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Looking real good Damian, the edge looks good, now to get the rest of it back without losing the edge again, your really not that far awayfrom the finish line.

Rick M

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Cheers for the replies and encouragement guys.

Hopefully I'll get the centre sorted this week but as I did that much edge work it will take a lot of time to get back.

I also Must try to get my foucault test rig sorted so I can get some images of that.

Another trip to see John soon too so he can take a good look on his own test gear.

Damian

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I'm sure you'll easily do the centre Damian. Let's hope John sees the improvement on the TDE that you need buddy. :)

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Hi all

I managed four half an hour sessions working just on the centre with a slight change evident. I have some more work planned today so hopefully I'll get a meaningful update on soon.

I have recut the channels in the lap and also gave it as good a clean as possible. Also been in touch with John and I'm going to have another trip up to see him for his experienced eye to give it the once over.

Damian

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Well just take your time Damian, I would hate to see you get so close then rush it when getting too excited close to  the finish line.

All the best

Derek

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Afternoon all

I managed a couple of foucault type images which gives an indication of the hill I am dealing with just for interest.

I have tried to put it together with a couple of Ronchi images to show how the lines bend according to the actual visual. Easiest way I can describe the foucault image for those that aren't familiar is like shining a light from the left across the mirror surface which picks up the front slope and casts into shadow the rear slope of any height changes. 

post-28847-0-16018500-1432905887_thumb.j

Ronchi inside

post-28847-0-92517500-1432906037_thumb.j

Ronchi outside

post-28847-0-10803700-1432905996_thumb.j

It's a fair old lump but after the edge work it is going to take a while to work and then blend. I obviously didn't clean the mirror up well as my finger prints are all over it showing just how sensitive a test it is

Damian

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I was going to say those must be fingerprints as the edge didn't look like that last time I saw it buddy! Was a bit worried that you had managed some chips, glad you haven't!

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I was going to say those must be fingerprints as the edge didn't look like that last time I saw it buddy! Was a bit worried that you had managed some chips, glad you haven't!

I'm sure you would've heard about them before if I had.

I did manage another 2 hours today just working the centre but it's going to take a while yet till it starts to reduce

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Hi all

Well the central hill is taking some shifting and due to it being not much bigger than the 10"lap I have made a smaller lap to tackle it.

I will have it finished probably tomorrow so will get back to it Thursday with plenty of test stops to make sure I don't go too far. Then it's back to the 10" lap to smooth things and carry on.

Damian

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Damian,

When are you off to see John? Are you getting rid of the centre hill first?

Derek

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Damian,

When are you off to see John? Are you getting rid of the centre hill first?

Derek

Hiya Derek

John is away for the week at present but it's going to take me a while to remove the hill and smooth the mirror.

I don't really want to work the outer areas much more till the hill is sorted then I will be able to see where to go from there.

I will then take the trip up so John can cast his eye on it

Edited by mapstar

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How long is a while ? ;)

Don't answer it's just a rhetorical question.

It's a long job you've set yourself. I think if I had been doing it, by now ( actually a long time ago), I would have fixed up a washing machine motor and table for myself!

Hope you conquor the hill soon.

Derek

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I know Derek but I have chosen the elbow grease method and will see it through to the end eventually. If I ever take leave of my senses again ( :grin: jokingly) and attempt another it will be by the mechanical route 

Thought I'd add these as they are quite interesting. There are a lot of finger prints around the edge as it was a quick test before returning to the polishing table. Imagine the light being shone from the right :-

post-28847-0-56957600-1433282407_thumb.j

post-28847-0-11990700-1433282412_thumb.j

post-28847-0-17542900-1433282416_thumb.j

post-28847-0-32203200-1433282419_thumb.j

And after another 2 hours just working on top of the hill. The strange effect down the side is cerium which had dried on the mirror whilst I was solely working the middle. It will come off with a bit of polishing. Also the effect is opposite as if the light is being shone from the left side. Looking closely you can see where I've flattened the top of the hill. 

post-28847-0-09103800-1433282540_thumb.j

post-28847-0-80308700-1433282543_thumb.j

Here's how the Ronchi test looks for the same 

post-28847-0-09644200-1433282619_thumb.j

I'll finish the lap tomorrow more than likely, which should give a little more control although being smaller as I said before it will work faster so I need to be careful

Damian

Edited by mapstar

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Aye Damian,

You can see the correlation between the two different methods showing the same centre hill. Very interesting. I'll keep watching.

I have just spent the last month and a half digging in the garden to remove an old pond plus about 70 square Metres of grass and replace it with new rolls of turf. As always it is the preparation that takes the time. But rushing either the preparation or the finishing straights can screw things up. So best of luck and be careful you will get there.

Derek

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I've ordered a 20" blank for my second mirror. It's going to be an f3.8 and in a visual only dob. It'll be interesting the second time around.

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The Trebor mint is back again buddy! Unbelievable how much the hill stands out, but be careful with that smaller lap! :)

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The new smaller lap is finished and now pressing. I shall start work tomorrow all being well

Damian

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Hi Damian,

If you are pressing the lap now, does it not take on the shape of the hill in the middle? Or does that not matter as it is relatively small?

Derek

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The hill if you look at the Ronchi is like a mirror within a mirror. It will take on the shape but once I smooth it down I will swap back to the 10" lap to smooth everything off.

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Hi Damian,

If you are pressing the lap now, does it not take on the shape of the hill in the middle? Or does that not matter as it is relatively small?

Derek

The apparently huge mountains and valleys the shadows reveal under a knife edge/ Ronchi, are really infinitely small.

A simple test of how a mirrors surface can be affected by simply placing your thumb on the surface for a few seconds, will raise the temperature sufficiently 

so that under the knife edge, that area will appear as a huge hill, but as you watch, it will reduce in size, and eventually disappear.

You might even witness the waves of heat as they dissipate into the air. Even when air currents are present around the test rig, they can be seen swirling around

That is why testing, and in particular, final testing of the mirrors Parabaloid, should be in a stable temperature, and free from draughts.

Some Mirror makers build a test tunnel in order to make that situation more achievable.

@ Damian.  I'm pleased you are still full of determination, and winning the TDE battle. It can be a long journey, but perseverance always wins.

Ron.

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