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Graham

Figuring the mirror.

15 posts in this topic

As learning curves go this has to be the steepest one I have ever under taken.

No amount of reading and researching can fully prepare you for the joy :wallbash: of figuring a mirror.

What I really need is to find a person local to me so I can have a one to one on this subject.

 

Just taking photos is a mission in itself.

 

This is a simple view of what I am starting out with.

 

DSCF0002.JPG

 

 

 

 

 

I decided to test using a Ronchi screen.

 

First image is inside the ROC.

 

DSCF0012.JPG

 

Second image before I did some corrections.

 

Again inside the ROC but a little closer to the ROC to get fewer lines.

 

DSCF0026.JPG

Tweedledee likes this

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After a bit more correcting.

 

This image is outside the ROC.

I am getting there slowly.

Just need to work out weather the edge is high or low.

 

Mirror Test 2 020.jpg

 

Any help would be greatly appreciated.

I have a feeling the radius at the edge needs to be reduced .

I have discovered that it only takes a few minuets of correcting with the pitch lap to make a large difference.

Oh the joy :lol: 

Edited by Graham

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Hats off to you Graham for taking this on.

Lets hope it comes good in the end.

Graham likes this

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Hi Graham, you are really breaking new ground now :thumbsup:

 

A phone call to John Nichol at Nichol Optical might help, he's been making mirrors all his life and is a very friendly helpful chap. You'll find his number on this website, and lots of useful info on testing mirrors...

http://www.nicholoptical.co.uk/index.htm

 

Have a look at this Ronchi pdf first if you've not already seen it...

http://www.nicholoptical.co.uk/pdf/The Ronchi Test.pdf

 

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2 hours ago, Bino-viewer said:

Hats off to you Graham for taking this on.

Lets hope it comes good in the end.

 

Thanks Rob.

No hoping about it I will get there.

 

1 hour ago, Tweedledee said:

Hi Graham, you are really breaking new ground now :thumbsup:

 

A phone call to John Nichol at Nichol Optical might help, he's been making mirrors all his life and is a very friendly helpful chap. You'll find his number on this website, and lots of useful info on testing mirrors...

http://www.nicholoptical.co.uk/index.htm

 

Have a look at this Ronchi pdf first if you've not already seen it...

http://www.nicholoptical.co.uk/pdf/The Ronchi Test.pdf

 

 

Thats brilliant thank Pete.

I will have a look whilst I am waiting for the mirror to cool down.

Fascinating little fact I have discovered.

Even the slightest amount of work heats the glass enough to distort things.

Every time I do a correction and then clean the mirror I have to leave it for a couple of hours for it to normalise before I can test it again.

:chin_scratch2:I wonder if I have enough time left on this mortal coil to finish it :rofl:

 

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Progress report. :facepalm:

After many sessions with out any improvement to the mirror.

I spent hours and hours scouring every bit of information I could get from the web and I discovered I have the dreaded rolled down edge.

My heart sank as the consensuses of opinion says I have to regrind to get it back to a sphere and start again. :wallbash::wallbash::wallbash:

 

Today I spent my dinner time watching a video by a guy called Gordon Waite.

Now I have watched about every video this guy has made to date with regards to making mirrors. 

In fact watching his videos is one of the reasons I took on this challenge.

 

For those who do not know him he is a professional mirror maker in the states and has an incredible reputation for making bespoke mirrors to the absolute limit of perfection.

 

I don't know why I did not look through his tutorials for a solution to the edge problem from the start. :facepalm2:

 

As I said this dinner time I did just that and he has a method of repairing this very problem. :party:

After getting home from work I set about trying his solution.

I have now done three short sessions using his method and it is working at an astonishing rate of knots. 

I am anticipating at the current rate of progress to have correct the edge by the end of tomorrow.

Well here's hoping.  :)

 

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I have seen his videos as well Graham, infact I have watched everyone of them as well.

 

Good luck with the mirror.

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Up date.

Took the better half out for a meal and after getting back home had a concentrated session on the edge problem.

After two hours of careful polishing using Gordon's methods the rolled edge has gone. :party:

I now have the Ronchi test pattern over the entire surface of the mirror.

Tomorrow I can start work on smoothing out the zones to blend the surface to a smoothly contoured paraboloid.

If I ever get the chance to meet Gordon I will be buying him a pint or two. :ok:

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Update.

Well it is amazing how quickly things can change.

Had a short session to smooth and blend the surface.

When I put it back on the Foucault tester I had totally messed up what I had achieved with the figuring. :facepalm:

So I have had to take it back to a spheroid and will now start all over again. :girlycry:

 

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2 hours ago, Graham said:

Update.

Well it is amazing how quickly things can change.

Had a short session to smooth and blend the surface.

When I put it back on the Foucault tester I had totally messed up what I had achieved with the figuring. :facepalm:

So I have had to take it back to a spheroid and will now start all over again. :girlycry:

 

Two steps forward and one step back. You'll soon have sorted and will be even higher up that learning curve. :thumbsup:

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Thanks. 

It stunned me at just how fast it suddenly went pear shaped. 

Lesson learned. 

I will not be doing that again well not until the next time :lol:

Tweedledee and Astronigel like this

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Now you are working with a surface already accurate to a gazillionth of the thickness of a gnats whisker, you'll have to watch you don't even cough anywhere near it. :lol:

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2 hours ago, Tweedledee said:

Now you are working with a surface already accurate to a gazillionth of the thickness of a gnats whisker, you'll have to watch you don't even cough anywhere near it. :lol:

 

Thats what it was I sneezed :rofl:

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