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The 22" mapstar mirror


mapstar

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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 :icon_salut: .

Ron.

Cheers Guys,

I've been trying to work the edge with the 10" lap starting at the edge and working inwards in a tight W as glass need's removing all over to bring the surface down to the bottom of the turned edge.

I'm obviously working the edge more as you say the oblate centre proves this and can be dealt with later.

It's beginning to show progress but as John said don't go mad.

Looking forward to this aft and tomorrow to see how the image changes fingers crossed it will show more improvement.

Damian

Edited by mapstar
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Result's of today's session 

Inside ROC

post-28847-0-27632400-1426363678_thumb.j

Outside ROC

post-28847-0-07089900-1426363785_thumb.j

Got to admit it's difficult to see any difference :confused:  Tomorrow is another day with the new cerium oxide stock so I'll carry on with the polishing

Damian

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Hi Damian,

 I have been looking at a chap ( Gordon Waite) over in the USA doing a 20" f 4.5 mirror. He got 1/30 wave I believe in the end. Some feat! I also watched his measurement of Ronchi and Foucault tests. Now I am beginning to appreciate how much work goes into your mirror.

Best of luck,

 Derek

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Hi Damian,

I have been looking at a chap ( Gordon Waite) over in the USA doing a 20" f 4.5 mirror. He got 1/30 wave I believe in the end. Some feat! I also watched his measurement of Ronchi and Foucault tests. Now I am beginning to appreciate how much work goes into your mirror.

Best of luck,

Derek

Hi Derek

Yes I've seen the video's of Gordon definitely good to watch and it does show the work involved in making a large mirror.

Only thing it doesn't portray is the time scale and mental strength you need to see it through to final conclusion. It's taken a lot and will take more before I'm finished.

Done another 2 hours today although was hoping for more but had my dad's car to fix so cut short today.

I'm still battling the turned edge and get to a sphere so I can actually start the figuring process of creating the parabola.

Long way to go yet with the edge problem though.

Damian

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Gordons videos are what helped me the most. One thing that maks life easy is a powered turn table. I was' t going to make a mirror without one.

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I looked up turned down edge and the effects. Sounds like an awful lot of effort to get rid of it!

Still you got this far so I know you will do it. Just remember a bit done here and that is just that bit less to do in future. Every bit counts and soon adds up! :grin:

Derek

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Gordons videos are what helped me the most. One thing that maks life easy is a powered turn table. I was' t going to make a mirror without one.

I've done it all by hand as I wanted a true reflection of what can be done with basic tooling. It's not gone too bad up to now

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Today has been another 2 hours of edge work and I'm sure things are improving albeit at snail pace.

More image's after the last session 

Inside ROC

post-28847-0-39238100-1426446373_thumb.j

Outside ROC

post-28847-0-39080900-1426446337_thumb.j

Was hoping for a bit more work than I managed today but as usual side tracked onto dad's car repairs.

More work tomorrow all being well but I have my own car to sort out and wait in for the delivery of a lovely focuser for the scope build  :grin:

Damian

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 Looking much better Damian but you gotta love the

way diffraction makes the edge look worse then it

really is.

maybe I missed it but what do you have coming for

a focuser if I may, have to find me one of those as

well.

Rick

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Looking much better Damian but you gotta love the

way diffraction makes the edge look worse then it

really is.

maybe I missed it but what do you have coming for

a focuser if I may, have to find me one of those as

well.

Rick

Thanks Rick it's been a battle of I think 17 hours up to now.

I agree the diffraction make's it look far worse than it is and when you move closer to ROC with less lines it amplifies it even more.

I'll put the focuser on the build thread when I finally get there so it all ties up with that. Making a few changes which I've been thinking about on the next build it all depend's how long the mirror take's.

Damian

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Coming along nicely. There is always more work needed at the edge than at the centre and big, fast mirrors accentuate this.

It just goes to emphasise how easy it is to overdo the centre which then leads to an awfull lot of work.

Nigel

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Coming along nicely. There is always more work needed at the edge than at the centre and big, fast mirrors accentuate this.

It just goes to emphasise how easy it is to overdo the centre which then leads to an awfull lot of work.

Nigel

Thanks Nigel,

I'm going to carry on with the stroke I'm doing at present and hopefully the edge will behave itself soon? although that maybe another 10 hour's work or so? I shall then make a start on the central hill and hopefully onto a sphere. I will consult John on the way to go about the next stage although a sphere is my first goal. 

Damian  :smiley:

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Managed an hour through the day but it's time for work.

I'll carry on tomorrow with a test first thing to see if things have changed then probably another couple of hours of going round in circles as that's what mirror making is all about

Damian

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I'll be looking forward to the figuring stage to see how you get on. These big fast mirrors are fun untill it gets frustrating when the glass dosn't do what you want.

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I'll be looking forward to the figuring stage to see how you get on. These big fast mirrors are fun untill it gets frustrating when the glass dosn't do what you want.

I'm looking forward to that too and like the rest it will probably be a fight all the way!

More work on the turned edge today

Damian

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Battle has continued over yesterday and today so another 3 hour's of the turned edge.

Here's the outcome

Inside ROC

post-28847-0-59407900-1426618095_thumb.j

 Silly me I had a cup of coffee stood not too far away in the image so there's a little thermal distortion on the lower side of the middle right hand band

Outside ROC

post-28847-0-18783500-1426618131_thumb.j

I'm not too sure there has been much change so may have to think about different tactics? I plan on another 3 hour's and see what things are like then? 

Damian

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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

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