The Giant Binocular

[rwm]

Very impressive work. I can't wait to see how this measures out. How do you know when you have the curvature you want? Can you use a Foucault test if it is not a parabola? My dad used to grind mirrors for reflectors. I think the largest he ground was 10" and it had some defects. This requires more than knowledge and science. There is definitely some black magic at work here!

 

[savarin]

Hi Robert, I can use Foucault to check, if its spherical it will blank out all over or use Rhonchi and look for straight lines.
The grinding is straight forward but polishing is where the black magic comes in.
I think thats a pun because I use black rouge to polish.
The hardest part will I think be getting the second mirror to the exact same RoC

 

[twhite]

Hi Robert, I can use Foucault to check, if its spherical it will blank out all over or use Rhonchi and look for straight lines.
The grinding is straight forward but polishing is where the black magic comes in.
I think thats a pun because I use black rouge to polish.
The hardest part will I think be getting the second mirror to the exact same RoC

I use the same thing. They are easy enough to make. I use a green laser on mine.

There is an app for Rhonchi that I used in the past.


Cutting oil is my blood.

 

[twhite]

Similar to this one.

Ronchi Test Calculator

Ronchi Test Calculator: match, tape band, unwrap and grid tests

www.bbastrodesigns.com

Cutting oil is my blood.

 

[savarin]

the surface at 800 grit, cant see much difference but it feels really smooth already.

A peek into the magic drawer where all the mystical components reside.

 

[savarin]

At last, fine grinding the first mirror is done.
This is 1200 grit, dry mirror, and it gives a reflection at an oblique angle.
There has been no change in the RoC over the last 5 grits.
I'm hoping by keeping the machine settings as they are it will produce the same RoC in the second mirror.
I wont start polishing this one till the second is ready.

 

[savarin]

Started on the second mirror, then had to strip the machine to adjust the bearings, got that done then worked through 80, 120, 240, 400 grits and its sitting on the same depth of curve of the first mirror then started on 600 grit when this happened.

a huge chip and three deep scratches (only 2 in this part of the image)
I have no idea what caused the chip. Now to 45' bevel deeper and back to 240 grit or larger to remove the scratches.
So far 2024 has been a total crap year.

 

[twhite]

Started on the second mirror, then had to strip the machine to adjust the bearings, got that done then worked through 80, 120, 240, 400 grits and its sitting on the same depth of curve of the first mirror then started on 600 grit when this happened.
View attachment 476864
a huge chip and three deep scratches (only 2 in this part of the image)
I have no idea what caused the chip. Now to 45' bevel deeper and back to 240 grit or larger to remove the scratches.
So far 2024 has been a total crap year.

That sucks. I have had that happen also. Just 1 piece of coarser grit is all that happened.

Stick with it. That all can be corrected.


Cutting oil is my blood.

 

[extropic]

leaving a 'like' didn't seem appropriate.
Hang in there.

 

[savarin]

I'm so glad I dont have to do this manually

It takes longer to set up than do the job.
10 mins grinding with the diamond disk and this is the result.

the darker ring just above the chip is the bevel.
I'm hoping thats enough to allow me to finish the grinding.