Making a 310mm Zerodur planetary mirror

[Stefan Buda]

I took on the job of making an f/5 paraboloid mirror from a supplied Zerodur blank.
The problem is that the blank is 60mm thick and I never worked Zerodur before.
The blank weighs 11.3 kg and needs to be reduced to a thickness of no more than 40 mm to have any chance of becoming a good planetary mirror.

Instead of converting one third of it into ceramic powder, the plan is to salvage some of the surplus material. This is expensive stuff and would be nice to have some off cuts that I can use for other projects.
My largest diamond disc is 9" diameter with a hub diameter of 40 mm, and that allows me to cut to a depth of about 94 mm. Any suggestions?

[DavidU]

Try talking to a granite supplier, they seem to be able to slice it for bench tops etc.

[bojan]

Maybe this could help finish the job?
https://www.diamondwiretec.com/home

[Sunfish]

Some stone and tile saws have a cut depth of 150mm . Maybe you can hire one.
Or the stone cutters can water jet cut.

[Stefan Buda]

Thanks for the suggestions.

About 20 years ago I had the same problem with a 16" Pyrex blank and I made a wooden support structure for it so that the stonemasons could cut it for me. After a few day I got a call that their machine did not have enough clearance to be able to do the job. At that time I decided that I wouldn't waste any more time with granite cutters and do it myself.
I still have the improvised machine and the tools I used for the job, so I will do the slicing myself, although the abrasive wire cutter idea is nice, I don't know if anyone in Melbourne has one.

As a first move I ground the side of the blank cylindrical because it was very rough, a bit oval and a bit barrel shaped. I also ground a 3mm chamfer on what will be the back edge.

This thing is hard, same as fused quartz. It took hour to get this far even though I removed less than a millimetre from the diameter.

[JA]

Hi Stefan,

There are some 300-500mm diamond blades available that you could try, usually at reduced rotational speed compared to their smaller brothers. With glass (or similar) I suppose you want a continuous blade that may be difficult to find above 300mm diameter ( I found 1 at 300mm), but there are plenty of segmented blades (better material clearance for aggressive cutting) that are available in the 300 to 500mm diameter range. I suppose it goes without saying to be careful since you only have 1 piece of the Zerodur and these types of blades may induce some micro vibration which may not be good. On the other hand it may be just what you need. On problem is that as the size goes up so does the thickness of the blade (an hence heat generated).

Anyway here are a host of possible options. There is even a 300mm Husqvarna continuous edge diamond wheel listed in amongst the many at this link (also a 350mm EHWA with tiny gaps) ....
https://sydneytools.com.au/search?p=5&q=Diamond%20Blade

300mm Husqvarna continuous edge diamond wheel
https://sydneytools.com.au/product/husqvarna-585421708-300mm-12-continuous-rim-superior-trade-series-diamond-blade

350mm EHWA with tiny gaps
https://sydneytools.com.au/product/ehwa-emsoc350-350mm-14-j-slot-ceramic-tile-diamond-blade


PFERD is a quality manufacturer I would check with them (1300 073373) about using their large diamaeter segmented wheels to carefully cut glass. (I'm fairly sure they'll say not to), but .... here's an example of a 350mm wheel for info....https://www.pferd.com.au/diamond-cut-off-wheels-general-purpose-segmented-t

Best
JA

[Top_oz]

Even if you found the right blade, I haven't seen any consumer level tile, brick, masonry cutting machines that would cope with something so tall and thin. At the least I'd think you're up for a workholding exercise.

My suggestion is to go with whatever worked last time. How did you do it?

[Stefan Buda]

Thanks JA,
That 12" disc looks good and I may buy one if I can figure what they mean by "10mm segment height".

Yes Tim, I did go ahead with the old way of doing it.

After I turned the blank over, I used various diamond tools to make a, 20mm deep, 80mm diameter pocket and then I used a small diamond coated disc to make a 20mm deep undercut a the bottom. After that my old 9" diamond disc was able to complete the cut.

[Top_oz]

It just means the diamond edge is 10mm across (radially).

[Stefan Buda]

It also means that the picture shows a different product.

[JA]

Yes I agree. They appear to be using the image of their 230mm disc also for their 250mm and 300mm wheels if you look at the Sydney Tools link I referenced above and just changing the text. That being the case, the diamond distance between the inner and outer radius of the diamond annular section is "close" to 10mm, well more like 14-15mm if you scale off the dimensions taking the 230mm as a reference. Of course the best would be to go in to a store and ask, hopefully see one.

Have a look here/read the text > there are several blades specifically rated for glass /hard ceramic uses in the 300-400mm range as well as a 350mm variant of the same style of Husqvarna blade as earlier (last image bottom of page)....
https://www.udt.com.au/view/our-diamond-blades/alumina-ceramic-silicon-carbide-blades

Best
JA

[Stefan Buda]

Thanks for the link. I bookmarked it because they seem to have some tools that I may need in the future. However I remembered that my machine's spindle has barely enough torque to drive the 9" blade, so I would be in trouble with a 12" one.

[sharpiel]

Hello Stefan. Are you making the whole scope as well as the mirror?

[Stefan Buda]

Only the primary mirror.

[sharpiel]

That's a shame for those of us who enjoy your builds.

[Stefan Buda]

Not to worry, there are more projects to come, but at the moment I'm still making improvements to the manufacturing methods for my CDK250. Once that is finalized I'll start another phlog. Perhaps a spectrograph or a solid Schmidt camera.

But to get back to the Zerodur, I had to generate the curve in two cuts as my cup wheel is not large enough for this job. After that I used a large bronze ring to blend in the curve. The required sagitta is almost 4mm deep and at this stage it is within 0.1 of a mm.
I also gave a bit of a grind to the back face to make the original machining marks disappear.
The weight of the blank is down to 7kg, but it took me four days of work to get this far.

[sharpiel]

Can’t wait. “Phlog” tho? So bad phonetically...

[Stefan Buda]

Yes, that's why I like it. Sorry.

[Peter Ward]

Again I am in awe of your ATM skills....the nice thing about Zerodur is: you don't get polishing/figuring "blisters" due thermal expansion of the substrate (or so I am told)

I expect the end result will be awesome :thumbsup:

[sharpiel]

+1 to that!

[Stefan Buda]

Thanks Peter,

Yes, some people may not appreciate that the heat generated during the polishing/figuring process adds to the unpredictability of the result.

I also expect to end up with fewer scratches on the finished surface because of the harder material. My office, where the polishing machine is located, is not exactly a NASA clean room. I even stop polishing on windy days.

[Stefan Buda]

I did the casting for the grinding tool substrate out of Ultracal 30. The top is flat - only the lens distortion is making it look curved.
It will be a tile tool.

[Stefan Buda]

The casting initially weighed 6.3kg and went down to 5kg as it dried out.
The tiles came from Bunnings.
I glued them on with the shiny face toward the casting, so that the required epoxy layer would be thinner.

[Stefan Buda]

I used an improvised angle grinder with a diamond tool to trim the tiles and then used the ring tool to grind off the high spots before I starting on the mirror blank.
I started with 120 carbo and after six wets the two surfaces were well matched and most of the ridges on the tiles were gone. From this stage I started using the spherometer and found that I needed to deepen the curve by about 0.06mm. I made the correction with the help of the ring tool on the blank and by having the tile tool spend more time at the bottom over the next few wets.

[TrevorW]

This is what the hobby is all about, great effort and its a joy to follow your progress ;)

[sfarndell]

I go away for a few weeks and this exciting project pops up. Enjoying your build progress and look forward to the next installments.

[Stefan Buda]

Thanks Trevor and Scott for the encouragement.

The grinding is done. I finished it with 9 micron aluminium oxide.
All hand grinding as there is nothing to gain from machine grinding and I prefer to keep abrasives away from the polishing machine.

Next is to make a couple of polishing laps.

[Stefan Buda]

I had to stop work on this job for a couple of weeks, but I'm back at it now.

Usually I polish my mirrors with a full diameter lap but my current polishing machine would struggle with a 12" lap, so I'm doing the polishing with an 8".

I'm at 3 hours now with minimum loading and when I get to about 5 hours, I will set up the Ronchi tester to measure the radius of curvature and check the figure.

[multiweb]

Good day Stefan, how does a machine compare to manual strokes? Do both discs rotate automatically?

[Stefan Buda]

Hi Marc,

The machine I'm using has a single motor driving the platform and the two eccentric mechanism that push and pull the top disc, and because of that, it can't generate real random motion like a human. Three motors with individually adjustable speed controllers would be much better.
Only the bottom disc is driven. The top one rotates automatically.

[multiweb]

Do you do final figuring by hand then? Or do you use the machine to simulate shorter or longer strokes with the tool offset?

[Stefan Buda]

I do most of the figuring with the machine but the finishing touches I do by holding the lap by hand while the mirror is rotating.

After 5 hours I measured the RC and I found it within expectations.
The figure was nice and spherical, with a bit of a dip towards the centre.

I did another test at 8 hours and the mirror is drifting towards an oblate spheroid.

It will take a long time to get full polish as at 8 hours it looked like not even half the way there.

[Stefan Buda]

I got to 19 hours with the polishing and I can still see a hint of grey near the the edge, so I'll give it a few more hours because once I start the figuring, the edge will get very little further polishing.
On the other hand, the figure is quite nice and spherical, with a bit of raised edge. I tried to capture a ronchigram, outside of focus, with a handheld camera.

Also I improvised a pitch hardness tester today. Seems to work ok.

[Stefan Buda]

I had to stop polishing for about a week because of the low temperatures we were having.
I'm at 22 hours now and started deepening the centre.
Soon I'll have to switch from the Ronchi tester to a Ross Null setup.

[Stefan Buda]

I attached a Ross Null lens to my Ronchi tester and carried on the parabolization with the 8" lap until the outer 2 inches or so looked flat. After that I started using a 4" figuring lap so that I don't over deepen the 70% zone, which would be a pain to correct.
After a few hours the lines were straight enough to make test results difficult to interpret. See attached Ronchigram.
So it came time for setting up the Bath interferometer. The first noisy interferogram is attached and the processing shows that my conic constant at the moment is -0.74, and the figure exactly where I expected it to be.
After a few more iterations I should be approaching the diffraction limit.

[multiweb]

Good day Stefan, did you make your lens or purchase it?

[Stefan Buda]

Hi Marc,

I made two test lenses one 60mm diameter and one 80mm.
In my experience the Ross Null test is only good enough to get you to about half a wave to your target before you lose sensitivity. Also it is very good at cancelling astigmatism, so you can't test for that.

[Stefan Buda]

Well, it took me 7 iterations with the interferometer, each comprising about 10 minutes of polishing, before I reached the diffraction limit at about 0.84 Strehl.
Another half a dozen shorter iterations got me to 0.958 Strehl ratio.

The attached graph shows only the spherical component, without other residuals. If there were no other residuals, the Strehl ratio would be 0.994, which is not a real world number unfortunately.

I'll work a bit more on the residuals and see if I can improve if further.
The problem is that a full evaluation of the surface takes about half a day's hard work because many interferograms need to be captured and processed in order to cancel out air currents, vibrations and instrumental effects.

[Joshua Bunn]

Fascinating Stefan, such fine work and a pleasure to watch.

[JA]

If I read the results correctly that would be something like Lambda/25 wave-front error even counting up to the extreme edges and less than Lambda/50 for something like 90% of the total area. That's a wonderful and amazing result.:thumbsup:

Best
JA

[Atmos]

That’s quite phenomenal Stefan, always in awe of your work!

[Stefan Buda]

Thanks Joshua and Colin for the encouragement.


It would be, except the graph I posted only shows the spherical components of the Zernike polynomial series.

[Stefan Buda]

For people addicted to Zernike polynomials, here is the final results as generated with DFTF software from 40 interferograms.

I also went back to the Ross Null test to check the smoothness of the surface and to measure the final focal length. The smoothness looked very good, with no visible zones or other anomalies. The focal length is 1mm shorter than intended, but within expectation.

[toc]

I have to ask - do you sell these mirrors, or is it just a labour of love?

[Stefan Buda]

Hi Tim,

The owner of the Zerodur blanc approached me to turn it into a planetary mirror.
It may be a labour of love, but I have to charge as I am self employed with no other income.

I'm still working towards developing my CDK250 as a commercial product and not too far to go now.

[Peter Ward]

Stephan! .....seriously?

Sorry for asking, but I suspect you have professional optical skills here that you have modestly not mentioned.
Did you get bored on Mt Olympus?
How did you acquire them?

The results you have posted are absolutely awesome.

:prey2:

[Stefan Buda]

Sorry Peter, I've only just seen your reply.

I don't know how professional optical skills are defined, so all I can say that I've been pushing glass since 1985 when I made my first Newtonian, ready to view Halley's comet.
However I did not practice the craft for commercial reasons until recently.
The other thing is that making an f/5 mirror is not too difficult if you can measure it with an interferometer and you already have committed most of the learning curve mistakes on other pieces of glass.

Thanks for the appreciation.