The Giant Binocular

[savarin]

cheers, I think I will need it

 

[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.

 

[Downunder Bob]

Hi Charles, the trials and tribulations of your project. I'm not familiar with the process of using resin and castor oil, but I'm guessing that it sets after it has conformed to the profile of the lens. Would you be able to spread a sheet of cling film over the lens and then pour the plaster onto that and it would harden with the correct profile.

 

[savarin]

Its a mystical art form Bob, the resin or pitch remains a liquid albeit very very stiff.
If left alone it should flow, again, very very slowly like over months.
The plaster was to be the substrate for the resin, this time I'm using plywood.
I didnt have sufficient caster oil to allow the cold resin to flow so I'm adding more with a more vigorous testing regime before I pour it.
The channels and scratches in the face of the resin was supposed to allow it to flow and conform easier but it was too stiff.
I could have warmed it to let it flow easier but then it wouldnt re-conform as it polished.
This stuff sticks to cling film like the proverbial to a blanket.

 

[twhite]

I found pouring the laps to be a hit or miss situation. A small window for optimal forming then putting the grooves in and re-setting the lap to the mirror. I have used various sub straights. Plaster, the grinding tool most of the time. Have even used 40 lb modeling board for the 17” mirror. I did that to save weight as I did all work by hand.

Patients is required for this step for sure.

 

[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.

the sharpie marks are on the back of the glass

 

[savarin]

Now at 4 hours polishing and the reflection is much sharper

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.

 

[savarin]

Some of the scratches at 10x magnification, they look very severe.

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.

It was virtually impossible to capture the image with the phone camera.

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.

 

[savarin]

Quick update.
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.
I wonder when I will be able to start on the second mirror.

 

[Eyerelief]

Quick update.
Got the majority of the scratches polished out but a couple of obstinate ones were left so went back to 1000

TOO LATE! I went straight to panic! Glad you caught it! Those cat like reflexes of yours paid of huge this time.