continuing the giant binocular

[savarin]

At last I am ready to polish the mirrors. First job is make the lap.
I used plaster that took over a week to thoroughly dry then soaked it in tile grout sealant to prevent it from adsorbing water from the polishing.
Used some firm plastic sheet to make a dam around the tool and poured a hot mix of pine resin and caster oil in.
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Just before it was poured I tested the consistency with a 30 second thumbnail press to see it it raised a small lip, it did so it should be soft enough to conform to the mirror (He hopes)
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then cut the facets and randomly score the lap then brush with a stainless wire brush to remove all particles.
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coat the mirror in a very thick cerium oxide and water polishing compound layer
Lay the lap on top with a little weight and leave to cold press and hopefully conform to the mirror
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Panic stations arise when you cant separate the mirror and lap.
I will find out tomorrow.

[chips and more]

Totally fantastic! You are doing things way beyond my pay grade!

[savarin]

Well, that was a bit of a fiasco.

1, the resin was too hard and didnt conform to the mirror surface after 12 hours.

2, the interface between the resin and the plaster tool started weeping and dribbling down the side.

Making a new ply tool base. I wont seal it with anything as I think (but dont know) that the sealant on the plaster may have still had sufficient volatiles left to soften the resin at the bond.

Ive melted it off the tool and will re-pour with more caster oil in the hope I can get it soft enough to conform.

Fingers crossed.

In the mean time I'm reading up on a Bath Interferometer  http://gr5.org/bath/  and may make one, I only need 1 lens as I have all the other items on hand.

More work.

[savarin]

The rabbit hole is open.
I remade the lap with softer compound and all seems well so far.
Polishing has started, 2 hours and a reflection is seen.
I'm using a high grade cerium oxide.
I feel it is a little shinier at the edge that the centre but I haven't measured it yet.
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the sharpie marks are on the back of the glass

[Adasha_Machining]

I'm still finding all this optics work fascinating. Keep posting about it, Charles.

[TerryWerm]

To be honest the entire project has been amazing to me, and it has kept my interest for quite some time now. Thank you for continuing to post about it, Charles!

[savarin]

Thanks Adasha and Terry.

Now at 4 hours polishing and the reflection is much sharper.
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But there are now a few scratches showing up.
I'm really hoping they are left over from the grinding, they are very fine and dont show up in the photo.
If the polishing lap has 1 teenie weanie bit of grinding grit I will have to scrap it.
My fingers are crossed they will polish out and not get worse.

[TerryWerm]

How many hours of polishing do you anticipate for this project?

[savarin]

Its difficult to say Terry, it usually works out to around one to two hours per inch of dia.
I'm hoping it will be under 24 hours per mirror but once I have the surface as smooth as I can measure I will switch to black rouge to get even smoother. (I hope)
I believe Carl Zambuto who is considered to be the best mirror maker takes even longer that that.
I find a 2 hour session of polishing at a time to be around my limit at present, my mind starts to wander or I want to do something else.

[savarin]

Some of the scratches at 10x magnification, they look very severe.
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In the grand scheme of things they actually dont cause much of a problem with the finished mirror but personally they hurt like hell because I know they are there.
Theres three possible causes and remedies
1, left over from fine grinding, I stopped at 1200 grit, maybe I should have gone to 1600 or finer.
2, contaminated polishing lap, ditch it and pour another one.
3, contaminated polishing compound, mix a litre with de-minerilised water, allow to stand, decant the top and use that.
I think I will follow 2 and 3 and see what happens.
I did a quick test for a check, tester in the foreground, mirror on the test stand in the background, 100 line Ronchi screen.
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It was virtually impossible to capture the image with the phone camera.
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Still not polished out, the lines should be straight and sharper.
Some zones and possible a bit of turned edge.
I dont know what the bright streak just off centre is.

[Piggiron]

In the mean time I'm reading up on a Bath Interferometer  http://gr5.org/bath/  and may make one, I only need 1 lens as I have all the other items on hand.

Hey Savarin,
Not sure if this will work like a Bath Interferometer, but would be much easier and cheaper to make.
Here's the link to the "Applied Science" video about self-mixing laser diode: https://www.youtube.com/watch?v=MUdro-6u2Zg

I ran into this video a few years back and thought "That would make a cool and very accurate DRO".
Turns out I may well have been correct since I recently saw RLS makes and sells them.
According to the video, it's capable of measuring sub-micron distances.

[savarin]

Unfortunately I dont believe so but my knowledge is not sufficient to work it out.
The standard Bath is easy and simple to build and use.

[savarin]

Quick update.
I havnt started the Bath Interferometer yet.
Got the majority of the scratches polished out but a couple of obstinate ones were left so went back to 1000 grit.
That was going very well when the lap stuck and flung the mirror off the machine.
DONT PANIC 
I actually caught it just before it hit the ground I dont know how but I did PHEW!
But I didnt catch the grinding lap it caused a very deep scratch about 30mm long and smashed so I had to make a new one.
Back to grinding at 800 to remove that scratch then 1000 grit then 1200 followed by 1600.
Then polishing again.
I wonder when I will be able to start on the second mirror.

[TerryWerm]

OUCH!

[34_40]

      The patience of Jobe!