394 mm diameter ultrathin mirror project

vorblesnak · #1 19-10-2005, 12:52 PM

I am not sure, (exposing my ignorance here), if Australia uses the metric or the the other system of measurement. So I will convert my notes to metric and if I am wrong someone let me know. I assue the US is the only hold out these days.

I am working on slumped ultrathin mirrors. How thin can we go?

The first project was a 406 mm f 3.5 that is 8 mm thick. That is the mirror in the photos from OSP. I have now begun a thinner project. It was a 457 mm disk but errors in the slump caused me to reduce it to a 394 mm disk. I try and post to the atm list but as there was interest here I thought I might post here also, if that is OK with every one. I have included my first two postings to ATM. There is some history on the 406 mm at ATM also.

www.atmlist.net

October 15, 2005

Something there is that loves a fool and a mirror grinder.

I had slumped a piece of 3/16ths, (5mm) glass over a bad form. What I got was an outer ring with a nasty case of ripply droop. I ground on it a bit and realized I was going to grind through before I got the ripple and the droop out. So, having never tried to cut glass before, and having this piece of now worthless glass, I decided to cut the outer 1.25 inches, (32mm) off.I will post pictures later, but I made a beam compass out of a scrap piece of fir stock. I did this by nailing a wheel cutter to the end of the scrap and then running a screw through it at the turn point. The screw acted as my point for the compass. I put a couple of layers of masking tape near the middle of the glass and then using a ruler I marked the middle of the circle on the masking tape. I etched/scribed a circle on the back of the curve and then using the ball on the end of the cutter I tapped from the inside of the curve following the etch line through the glass. It worked! I cannot believe I got away with that.

So now it is a 15.5 inch instead of an 18 inch.

The specs on this one:

15.5 inch diameter, 3/16th inches thick
3/16 inch of sagitta
80 inch +/- focal length based on r squared / sagitta
f5 more or less

394 mm diameter, 5 mm thick
5 mm sagitta
2032 mm +/- focal length based on r squared / sagitta
f5 more or less

Should be a fun project.

October 17, 2005

I finally got the camera out and took a shot of the project. This is the mirror with the pieces I cut off in front of it and the cutter I cobbled together to do it. In the middle of the mirror blank is an odd shaped white blob. That is the masking tape on the back that provided a place to hold the point of the cutter. On the lower left is a series of grind marks. Those were caused by trying to grind the ripple and droop, off of the edge of the original blank, using a bottle bottom. It didn't work.

If you look closely at the pieces I cut off you will see the ripple pattern that formed. I had not fully understood the softness of the glass as it slumped and so did not dress the edge of the form well enough. I have not made that mistake again.

David Davis
Toledo, Oregon

http://www.atmlist.net/contrib/vorblesnak-at-peak-dot-org/15%20inch%20ul tra%20thin/15.5%20mirror.jpg

[1ponders] (Paul) · #2 19-10-2005, 01:15 PM

to the forum David. You'll soon be inundated with all the other Daves, Davos, David and (I haven't heard this one since school days) Dabbas welcoming you.

Plus I think you'll find there will be quite a bit of interest in your mirror "construction". Plenty of ATMers here.

NOt having a clue on how to grind a mirror I'll leave the comments to the more informed, but good on you for giving it a go.

[1ponders] (Paul) · #3 19-10-2005, 01:18 PM

Oh and I'm sure someone will do it so I better get in first. "Holy Toledo, Dave, nice Mirror"

I cut me sometimes.

"cut" - glass - get it. Sorry I'll go away now

Welcome anyway mate.

Oh and we do use metric. Except when trying to get parts for American made mounts and telescopes. Most of us are pretty well multi-numerical

h0ughy (David) · #4 19-10-2005, 01:22 PM

Please pardon my ignorance, but how can you control the shape of the mirror being that thin?

Looks like you have a lot of skill in this area, In Newcastle NSW there is a nice bloke (George Livanos) who can virtually make a scope out of anything. he would be interested in your technique, its just a pity he has no computer to send this thread to him?

asimov (John) · #5 19-10-2005, 01:22 PM

"HOLY T.........I !?"

Too late, Paul beat me to it!?....oh well...Welcome David.

h0ughy (David) · #6 19-10-2005, 01:26 PM

Quote:

Originally Posted by [1ponders]

Oh and I'm sure someone will do it so I better get in first. "Holy Toledo, Dave, nice Mirror"

I cut me sometimes.

"cut" - glass - get it. Sorry I'll go away now

Welcome anyway mate.

Oh and we do use metric. Except when trying to get parts for American made mounts and telescopes. Most of us are pretty well multi-numerical

Paul, I think the long lunch may have started a bit early

Welcome Dave (please vote on the how many dave's thread). If you have had a look around you can see we don't bight, just nibble

acropolite (Phil) · #7 19-10-2005, 09:25 PM

Hey David, Glad you could come. Thanks for the info and welcome to Iceinspace.....
BTW h0ughy, David has already been mentioned in your thread (post#27) http://www.iceinspace.com.au/forum/showthread.php?t=4584&page=2&pp=26 ...
really you should have known...

h0ughy (David) · #8 19-10-2005, 09:30 PM

Quote:

Originally Posted by acropolite

Hey David, Glad you could come. Thanks for the info and welcome to Iceinspace.....
BTW h0ughy, David has already been mentioned in your thread (post#27) http://www.iceinspace.com.au/forum/showthread.php?t=4584&page=2&pp=26 ...
really you should have known...

Can't see the Davids for the Daves

ballaratdragons (Ken) · #9 19-10-2005, 11:00 PM

The thinnest mirror I have ever heard of was microscopically thin.

It was a 12" diameter piece of solar film held around the entire circumfrence by a circular wooden frame, suspended 2" above the ground. A tripod was constructed to hold a 2ndry mirror and a focuser, then placed so the 2ndry and focuser were directly above the solar film.

A wind break of Garden plastic was placed around the whole lot, and then they placed a drop of water into the centre of the solar film. They kept adding water a drop at a time until the correct f ratio was achieved.

It worked!!!! But only at Zenith!

Now that's a thin mirror!!!!!!!!!!!!!!!!!!!

Good luck with your mirror David, at least you'll be able to see the rest of the sky, not just Zenith!

h0ughy (David) · #10 19-10-2005, 11:08 PM

Quote:

Originally Posted by ballaratdragons

The thinnest mirror I have ever heard of was microscopically thin.

It was a 12" diameter piece of solar film held around the entire circumfrence by a circular wooden frame, suspended 2" above the ground. A tripod was constructed to hold a 2ndry mirror and a focuser, then placed so the 2ndry and focuser were directly above the solar film.

A wind break of Garden plastic was placed around the whole lot, and then they placed a drop of water into the centre of the solar film. They kept adding water a drop at a time until the correct f ratio was achieved.

It worked!!!! But only at Zenith!

Now that's a thin mirror!!!!!!!!!!!!!!!!!!!

Good luck with your mirror David, at least you'll be able to see the rest of the sky, not just Zenith!

Wow, thats a great story Ken, how long ago was this? Was it someone on this site?

ballaratdragons (Ken) · #11 19-10-2005, 11:15 PM

No Houghy.

2 members of our Ballarat Astro society tried it about 16 years ago after seeing it done at an Astro meeting in Melbourne.

It was a comedy of errors. A few of us stood around laughing at them as it kept falling over and the wind blew the wind-break away!

Our 2 members never got ot to work

But, 10 points for trying!

and 20 points for the comedy show!!

Merlin66 (Ken) · #12 20-10-2005, 08:28 AM

Great minds think alike??!!

On a similar vein, I've been dabbling with vacuum forming of thin ( 3mm) glass mirrors, you don't need a lot of vacuum.... (The original Mylar trial NBG, the surface just doesn't have the accuracy for an astro mirror)....

Couldn't get below about 2- 2.5m sagitta before " BANG", so soon figured out that the blank would have to be pre-slumped to say 5mm then held on a vacuum frame. The vacuum pulls the shape into a perfect parabola. Peerless Glass here in Melbourne are keen to help ( Can't find thin Pryrex sheet yet, any ideas?) So I'm meeting with then today to look at the manufacture of stainless steel moulds. Working with nominal 18" disks.

Merlin66 (Ken) · #13 21-10-2005, 08:28 AM

Just to give feedback... the guys at Peerless Glass Plenty Rd, Melbourne ( ask for Adrian) can cut and slump a 5mm thick, 450 diam glass blank to a 6mm sag ( about f4.5) for A$ 30.00. I've ordered a couple to continue the vacuum trials.

Keep you posted.

vorblesnak · #14 21-10-2005, 03:35 PM

Gentlemen, thank you for a warm welcome. Thanks also for being fearless enough to try thin mirrors. Gonna have to try that vacume pan idea.

The big question seems to be how do I support the mirror. I grind these things mirror on top of a full size tool. That way as the surface comes into a good match I get better and better support. Unlike a full thickness mirror the thin slumped glass is very light weight, very easy to push. I 'hog it out' or course grind with 220 or 320 grit, then finish it off. No pressure, it warps the glass. No stiffeners, they cause astigmatism. Lay the glass on the tool, spread my fingers out and push in a nice gentle rythm.

Here is my latest post to the atmlist www.atmlist.net

I have a couple of more pictures uploaded. Here is the mirror on the
tool, on my favorite grinding table.

http://www.atmlist.net/contrib/vorbl...and%20Tool.jpg

Here is a composit image with three check points on it. The left most is
about 1 hour, just after I cut the outer ring off. The middle is a little
over two hours. The right hand one is about four hours.

http://www.atmlist.net/contrib/vorbl...cs%20Small.jpg

I do these check point shots to prove to myself I am making progress.
That spot on the right hand edge is very slow to respond, but I am making
headway.

394mm diameter
5mm thick
f5 more or less

David Davis
Toledo, Oregon 97391

cristian abarca · #15 22-10-2005, 07:45 AM

Hi David and welcome. What type of glass is it? and how does it go in temperature change?

Regards Cristian

Orion · #16 22-10-2005, 10:51 PM

Welcome David. I am curious to know how you will establish a parabola on such a thin mirror? I am in the process of figuring a 24" mirror 39mm thick and thought I had a thin mirror!
Good luck with your project and I hope it turns out the way you want.

vorblesnak · #17 27-10-2005, 12:49 PM

Quote:

Originally Posted by Orion

Welcome David. I am curious to know how you will establish a parabola on such a thin mirror? I am in the process of figuring a 24" mirror 39mm thick and thought I had a thin mirror!
Good luck with your project and I hope it turns out the way you want.

I seem to have a hard time getting here. Many times the browser times out and cannot see the forum. I suspect there is a problem tween here and there.

I'm here now!

On the 16 inch, (406mm), I ultimately used a sub diameter tool with the mirror on the bottom. I am not sure I have that luxury this time so I am exploring using a petal lap. This leaves most of the lap in the middle but reduces the outer edge area so less work takes place out there.

Alternately, a parabola is a flattened sphere. I could leave the middle alone and using short strokes on a hard lap, flatten the outer 30 percent or so of the curve and have my parabola.

David Davis
Toledo, Oregon 97391

Merlin66 (Ken) · #18 27-10-2005, 03:10 PM

Just picked up the 5mm thick slumped 18" blanks. I'll post a couple of photos over the weekend. The lack of proper moulds has given them a sag of 8mm.
Good enough to continue the vacuum trials.

Merlin66 (Ken) · #19 07-11-2005, 06:48 AM

Checking out the slumped blanks, there appears to be about 1mm of astigmatism across the disk with the edges less (?!) slumped and a deep centre. The depth was even greater than I thought, and they are about f1.8.

With the uneven edge difficult to get them to seal properly for vacuum trials, but I'm sure they "could" work.

The trick seems to lie in the successful design of the steel slump mould, Adrian is only using a thin spun steel shell which I think is not sitting perfectly flat nor is 100% symetrical. Will continue......