The 22" mapstar mirror

[Soupy]

Great images Damian. The trebor mint has progressed nicely

Glad John, Nigel and you know how to interpret the images though, I wouldn't know where to start!

[mapstar]

Great images Damian. The trebor mint has progressed nicely

Glad John, Nigel and you know how to interpret the images though, I wouldn't know where to start!

To be fair Paul I struggle with them, as you say more experienced eye's know what they are looking at. done a couple of hour's with the smaller lap today but I can say the mirror is now looking a little less smooth and I'm going to have to really study the image's to see if the edge is getting better. One thing I do know the middle now has a hill!

[barkis]

Centre oblate is no big worry Damian, it's easir to get rid of a hill, much more difficult to cure a depression careful you stay away from TDE..

Sub. diameter tools can change the surface very quickly, so work slowly, and test regularly.

If it takes a long time, so be it, you have to become as one with the task, and if that sounds silly, it isn't

Don't be in too big a hurry to judge the surface after  a session, what you see shadow wise, will certainly change as time passes.

Reserve final appraisals long after polishing sessions.

Remember there's  no rush.

Are you using Rochi  gratings, I think you said you were, those shadows indicate such.

Perhaps a Foucault tester, although not suitable as a final tester, will give a better insight as to where you are with the surface as

it passes through the light cone At the radius of curvature.

The knife edge tests can reveal much useful info.

Ron.

[Soupy]

You sound a bit like Yoda there Ron...

Didn't you have a hill before Damian?

[mapstar]

Centre oblate is no big worry Damian, it's easir to get rid of a hill, much more difficult to cure a depression careful you stay away from TDE..

Sub. diameter tools can change the surface very quickly, so work slowly, and test regularly.

If it takes a long time, so be it, you have to become as one with the task, and if that sounds silly, it isn't

Don't be in too big a hurry to judge the surface after  a session, what you see shadow wise, will certainly change as time passes.

Reserve final appraisals long after polishing sessions.

Remember there's  no rush.

Are you using Rochi  gratings, I think you said you were, those shadows indicate such.

Perhaps a Foucault tester, although not suitable as a final tester, will give a better insight as to where you are with the surface as

it passes through the light cone At the radius of curvature.

The knife edge tests can reveal much useful info.

Ron.

Thank you for the advice Ron 

Yesterday I worked for the first time with the smaller lap for a total of 3 hour's through the day on the mirror cooling and testing between sessions. The surface to be honest hasn't changed that much at the edge but the middle now has a bump forming and overall the surface is rougher so to say

Stoke yesterday was at an approximate 5" from the edge with very little overlap. More work today so as the mirror is cooling off I thought I'd add a bit to the thread.

Damain

[Mark62]

It looks like ur doing a super job Damian ( not that I'd know [emoji4]) but keep it up

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[mapstar]

It looks like ur doing a super job Damian ( not that I'd know [emoji4]) but keep it up

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Cheers Mark although I wish that was true 

[Astrobits]

I would use the 10" lap and a W stroke centered about 3"-4" from the edge and see how that affected the "hill" near the edge.

If you do a Foucault test try to see the faint interference bands near the knife shadow when the knife edge is away from the coc. They are 1 wavelength apart, ie. 1/2 wave on the glass. This might give you some idea of how much glass will need to be removed

Nigel

[mapstar]

I would use the 10" lap and a W stroke centered about 3"-4" from the edge and see how that affected the "hill" near the edge.

If you do a Foucault test try to see the faint interference bands near the knife shadow when the knife edge is away from the coc. They are 1 wavelength apart, ie. 1/2 wave on the glass. This might give you some idea of how much glass will need to be removed

Nigel

Thank's for commenting Nigel

I have tried a slightly different stroke on the second session and this has been a tight W as you say about an inch overhang working towards the centre with the lap overhanging the mirror about an inch at the end of each stroke. As I read it the glass needs removing across the entire mirror to bring it down to the edge without creating a deepened centre which will be difficult to correct as Ron say's .

The edge seem's to be improving to me. I shall have another session before the day is out using the same stroke then leave leave it overnight to settle and take image's

Damian

Edited March 9, 2015 by mapstar

[Astrobits]

Quite right. If the centre gets too deep ( hyperbola ) then the rest of the mirror has to be taken down and that represents a LOT more work than it took to overdo the centre in the first place. That's why the old books refer to it as the "fatal hyperbola".

My stroke would have up to 2" overhang at the centre of the stroke, going to 3"-4" at the end. With the large, fast mirror you are making the parabola is very steep and significant amounts of glass need to be removed right up to the edge and there is only one way to remove that.

Nigel

[mapstar]

Testing resumed today and I took the following images

 

More work this morning although I have to go to work worst luck!

Damian

[mapstar]

Just a couple of pic's of the 10" lap in action, I apologize for the blurry shots think the camera is as dizzy as I am after all this going around in circle's

 

It's working well just an addition to the thread whilst pressing again

Damian

Edited March 10, 2015 by mapstar

[Demon Barber]

Damian,Sun not over the yard arm yet,otherwise a drink might steady your hands,           Hic!

Mike

Edited March 10, 2015 by Demon Barber

[mapstar]

Damian,Sun not over the yard arm yet,otherwise a drink might steady your hands,           Hic!

Mike

After this mike I'm gonna have a few! Make sure I bring some up that there Galloway to celebrate 

[cotterless45]

This tale is better than "The Archers" , I'm hooked !

Nick.

[Uplooker]

This tale is better than "The Archers" , I'm hooked !

Nick.

To be fair this isn't quite as Eastenders as the Archers is becoming ;-0

Ian

[barkis]

Not a brilliant example Damian.

But here's a Ronchigram of a paraboloid.

Photo by Samuel Borello from NE Howards Telescope Making Book.

Not the best photo. you'll see 

Ron.

[mapstar]

Thanks for the image Ron hopefully I will one day see the same reflected back at me.

At present it's a battle with the edge and then hopefully achieve a nice sphere. The battle is still raging and it will be quite a while until the war is won.

I will be using a matched Ronchi test but this will be backed up hopefully in the end by the Ross Null test and then the final test by John on his test equipment.

Long and winding road to travel as yet.

Damian

[barkis]

You'll get there alright, you have achieved enough already to show you have the will to 

get past the winning post.  I look forward to that day  .

Ron. 

[Physopto]

Hi Damian,

I see you are still at it. Keep it up, Mike and I are expecting to see it finished  up at Galloway next time.

We are watching with interest.

Derek

[mapstar]

Morning all,

I've steadily worked away at the mirror and here are the latest Ronchigrams. Roughly 12 hours since I started to work on the TDE

Inside ROC

Outside ROC

I had a few speck's of moisture on the mirror centre from where the lap had been pressing on top in the image's . The turned edge is improving but I'm also getting a hill forming in the centre which will be dealt with once the edge is sorted.

I was running low on Cerium Oxide (2.5uM) Wednesday so yet  again .5Kg more ordered which hopefully will see me to the end but only time will tell.

More work this afternoon 

Damian

Edited March 14, 2015 by mapstar

[RAC]

That's starting to look a little better now. I've been in the same boat with my 18" f3.3. I couldn't get the last bit out but it totaly went away when figuring it with a small 4" lap althoght I did git the edge better than yours is currently before I started deepening it.

[Soupy]

Great progress Damian the edge is definitely getting smaller.

[Mark62]

Super job Damian even I can see the difference. Keep up the hard work. Softly softly I think from now on u don't want to go back

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[barkis]

Have you been using  1/3rd centre over centre strokes Damian? 5 minute periods with cold pressing regularly?

You can shorten to 1/4 strokes if it is reluctant to cooperate.

I assume your lap is fairly hard, which is good?

That is a good way to fix TDE, but you might inherit an Oblate surface in the  centre, which

is not unusual. That is easy to remedy though.

Not trying to teach Grannie to suck eggs matey, and you are winning the battle

with that huge lump of glass. You're a braver man the I  .

Ron.