Mirror making problems

[nightstang]

I found a post by Glasspusher here that has helped me out a little.

I currently have a post on cloudynights where I am trying to figure this mirror out.

It is my first one and I have a ton of work into it and really want to finish it.

Here is my situation.

I have a 10" pitch lap.

My mirror is 12.5" X .75" and currently an f/4.7

These pictures are all INSIDE focus.

1st I had this

Then I stopped using my turntable because I wonder if it has something to do with my astigmatism look here.

I then Did about 30 min of MOT 1/3 COC stroke and ended up here.

I am still concerned and a little baffled on what to do.

Any ideas?

Thanks,

Dave

[fwm891]

Hi Dave, You seem from the photos to have two problems:

A central zone and an edge zone.

I suspect (without numerical data to check) that your MOT working has caused a central depression - the edge doesn't look serious so:

My inclination would be to work two short sessions TOT 5 min 1/3rd D, then MOT 5 mins 1/4 D both these should have a 'W' pattern to the strokes to smooth out the edges of your zones and take you back towards a sphere. Working much more MOT only will just deepen the central zone into a hole.

You're really not far away so short periods of working, resting and testing. francis

[Glasspusher]

Dave,

Your mirror is a little on the thin side for a 12.5 inch however, it should be possible to fix it. The mirror is showing astigmatism which is probably a result of it being a little on the thin side. Before you do any thing else you need to get rid of the astigmatism. This can be done by working the mirror face up with the lap on top. You need to use a longish stroke (about 1/2 D) making sure that the centre of the tool passes through the centre of the mirror. Make sure that you have good contact between the lap and mirror. The mirror should be on a level surface with a layer of carpet (or similar) under it, now rotate the mirror every 5 mins of so during working to help prevent astigmatism. Work for 1 hour and retest the mirror. It may take several hours to remove the astigmatism. Look for symetrical patterns in the ronchi bands to show that astigmatism is not present. There is a good chance that this will smooth out the zones that are present at the moment. If not we can look at the best course of action to take when the mirror is free from stig. It best to work one step at a time resolving one problem before dealing with the next.

Hope this helps.

John

[nightstang]

Thanks guys.

Since the last post I already had done some work I dont know if it has helped much but what I did was 30 min of TOT with 1/3 COC.

So I am here now

Inside Focus

and this is OUTside focus

I have a plan and I am wondering if you think it would help. The reason is I think my problem is all caused from my turntable not spinning true. What that has caused is the edges of the table my mirror sits on to go up and down about 1/8" as it goes around and around.

I think if I would have made sure and rotated the glass every so often this would have been canceled out and I wouldn't be in the situation im in.

Does this make sense?

If so I am thinking of just making a full size tool.

Going back to the spin method on my turntable. BUT

this time making sure I do short duration polishing sessions. Say 20-30 min AND then making sure to rotate the mirror on the tool about 45deg after every session.

Dave

[Glasspusher]

Dave, A full sized tool is best for dealing with astigmatism, rotating the mirror every few minutes is an absolute must, I prefer random rotations rather than say 45 degress evrey time. OK, with a full sized tool your stroke length needs to be at least 1/3 D or even longer. You have one objective an the moment; to achieve a figure of rotation, ie no stig. I am sure that a full sized tool on top will also reduce the zones, you may need a stroke a little longer than 1/3 to achieve this, starting with a 1/3 stroke makes sense. Looking forward to hearing how you get on.

John

[barkis]

I'm not sure the variation you speak of would result in your mirror looking the way it does.

polishing strokes initally should be variable, and random to prevent lapsing into a repeatable pattern, as that is when anomalies can be produced on the surface.

The reason it is the way it is, and the cure for it has been explained in the posts preceding mine. The liklihood is, too much pressure during polishing has cause the mirror to flex, and you have polished the defects into the surface as a result of that flexure.

Remember, the surface defects are in the order of microns, but look mountainous under the knife edge. Too much generated heat in polishing can also be damaging, so keep the stroke frequency way down to negate that possibility. You can rectify the problem by following the advice in the prededing posts.

Good luck.

Ron.

[nightstang]

Firstly I want to say thank you guys so much for your help. I may never be able to finish this without it. So for that I wanna let you know I very much appreciate you helping me.

Today I may not be able to get much work done on the project but I will certainly be keeping this thread updated.

I know I have a long road ahead of me but I feel invigorated and more optimistic when I have someone with experience keeping me outta too much trouble =).

I am already in the process of making my turntable more smooth running. I will make the full size tool and do my polishing with no weight. I am thinking of running it with about 4 inches of overhang and making sure I rotate the mirror to different spots every time I do a session.

Thank you,

Dave

[Glasspusher]

I am thinking of running it with about 4 inches of overhang and making sure I rotate the mirror to different spots every time I do a session.

Dave, I am a little confused by this comment, I take it you are using the spin polishing technique. You will not be able to remove the astigmatism unless you polish through the centre as explained previously.

John

[nightstang]

Aha!

I will just do sessions by hand of 1/3 D and see how it comes along.

Thanks John =)

[nightstang]

Wow! My 1st pitch lap was easy. Today I had to redo it 3 times! Finally I got it done. Grooved with micro slits and hot pressed. Tomorrow I plan on starting my journey of fixing this astigmatism =).

Before I start though I want to make sure I am doing as you say.

So here is my plan.

1. I will still use the turntable but wont be doing spin polishing.

2. I will make sure that either tool or the mirror never are put in the same spot as previously. I done this by marking the sides of the turntable so I can keep track.

3. I should start with MIRROR on top so the tool is actually going to be spinning.

4. I will do at least 1/3 COC strokes for about 10 minutes and rotate the tool to a different mark on the table.

5. I will go through all 13 marked spots which means a little over an hour of work with MIRROR on top.

6. I will retest the mirror and hopefully be happier and show post my new ronchigram!

This sound about right?

Dave

[Glasspusher]

Dave, I would work with the mirror face up, ie tool on top so that the back of the mirror is supported on the turntable, stroke a little longer that 1/3D will do no harm. Would it be possible for you to post a picture of the turntable?

John

[nightstang]

Yeah I can post a pic but dont laugh ok =)

Dave

[nightstang]

[nightstang]

What I have done is rig up my drill press to be a turntable. Ive done every step so far on this badboy. I remember using my sphereometer on it while in the grinding stages and was amazed at how sphereical it was. At 30 micron grit my dial indicator wouldnt move at all no matter where I put it. That shows me how awesomely precise a mirror can be with a ronchi/foucault test.

Dave

[nightstang]

WOO HAPPY 4TH OF JULY FOR ME!

I know this is not done but I think I have the astigmatism gone =).

Here is INSIDE FOCUS. This was after doing 6 sessions each 10 minutes long. Did it all by hand without using the powered turntable. 1/3 or a little more COC stroke with Tool on top.

Here is OUTSIDE FOCUS.

So now I am thinking I need to do the same method as before but do MIRROR on top now to get rid of the TDE and undercorrection.

Ill wait for advice =).

BTW Thank you John. Your advice and encouragement is really helpfull!

Dave

[Moonshane]

I know nothing about mirror making but that looks a million times better to me.

love the turntable idea.

[Glasspusher]

Dave, as you say you still have a turned down edge. Work with short strokes with the mirror on top and you should reduce it, you can then think about parabolising.

John

[barkis]

MOT and short strokes centre over centre will sort the TDE, but you could generate an Oblate Spheroid, but that is easily corrected.

Instructions how, if and when it happens.

You're doing good though, it's beginning to respond to the guidance.

[nightstang]

I got it cold pressing right now. I am not sure how many sessions I want to do. I was thinking 6 ten minute ones just like last time. The thing is I am very very low on Cerium Oxide =(

I have about 2 ounces of dry stuff left and about 2 ounces of liquid left.

Dave

[nightstang]

Ok I just finished my 6 sessions of MIRROR on top 1/4 COC.

Hrmm I am not sure if I am going in the right direction.

Here is Inside focus.

MID focus (if there is such thing )

and Outside.

I am kinda afraid to continue doing what I am doing it looks like I am headed back to where I started.

Dave

[Glasspusher]

Dave, the middle of the mirror is starting to come up as would be expected from working short strokes MOT, unfortunately the edge problem is still present. I would go back to working with the mirror face up and a longish stroke at least 1/3D or a little more. This will smooth out the mirror and should help with the edge problem. I use one level teaspoon of cerium oxide with about 300cc of water. At least one hours work will be needed with random rotation. If after this this edge is still turned down, I would continue for another hour with the same stroke. Good luck.

John

[nightstang]

Ok sorry John I didnt give up on ya I been hard at it just had some problems. I ran out of CEO then my pitch lap broke when pulling it off. Then when I did get the CEO in my power went out for 2 days then I had to remake the pitch lap 3 times before I got it right. Then after several hours of polishing and testing and polishing and testing and polishing and testing ........ I finally used my thumb and got rid of the TDE heh.

Anyways!

I wanted to show you my progress and get some input from here.

Here is INSIDE focus:

And OUTSIDE focus:

I also wanted to add that I probably haven't beveled the edge of the mirror like I should have. Right now I only have about 1/32" of beveling I think by taking that out to 1/8" I should have this TDE taken care of.

Thanks,

Dave

[nightstang]

Ok since my last post I have gotten it spherical then started parabolising it.

Here is where I am at and I feel kind of stuck on where I am at here and what I should be doing next.

Help me John!! hahah!

Thanks, Dave

Inside Focus:

Mid Focus:

Outside Focus:

[UTMonkey]

Sorry to butt in, but I would like to say how much I take my hat off to people who attempt this kind of stuff.

I am pretty sure I would get nowhere near as far as the OP as I would need help at every step.

Good luck with it all.

Mark

[nightstang]

Update:

Trying to smooth out the hole I did some short W strokes with MOT but only overhanging the tool about 3 inches on the sides.

Here is where I am at.

INSIDE:

MID:

OUT:

I think maybe a little more work. According to the ronchi software I should be seeing my outter lines start to look more like a "C".

This makes me really anxious. I feel like I am very close but know I could easily screw it up.

Thank you Mark. I see why these mirrors cost so much. You could do it though it just takes patience.

Dave