This is a photo of a 12.5 inch diameter, 1.5 inch thick, telescope mirror blank that has been lightened by casting the mirror in a mold that made lots of hexagon-shaped pockets in the back of the mirror. This is the story of how I did it.
Why light-weight mirror blanks? Well, telescope mirrors are made from thick glass. Thick glass is heavy, no, really heavy.
Most people don't realize how heavy glass really is because they usually only deal with small pieces of thin plate glass. Handing a large telescope mirror
to someone who has never seen one before, is a revelation to them about just how heavy glass really is. I like to tell them that it is as heavy as granite.
In a large telescope, the primary mirror can constitute 1/3 or more of the weight of the entire instrument. Supporting the massive weight of the mirror requires that
the rest of the telescope be sturdily built. The result is that large telescopes can be shockingly heavy. I know this first hand because I have thrown out my back
several times moving my relatively "easily portable" 17.5 inch Dobsonian Telescope. So I decided to try making
some-light-weight mirror blanks, just to see if I could do it.
Let's start this story at the beginning. About two years ago I bought a kiln and began experimenting with making my own
telescope mirror blanks
by fusing together disks of thin glass in my kiln to make the thick glass needed for telescope mirrors. The process works great, and I have produced wonderful
solid blanks up to 14.5 inches in diameter and 1 1/2 inches thick in my new, bigger kiln. Once I had the process of making solid blanks pretty much perfected,
I wanted to try making light-weight mirror blanks.
So, how to make a light weight mirror? There are two schools of thought on that. One way is to make the mirror thinner. Telescope mirrors are generally made of thick glass because thick glass is very stiff and won't distort, even under its considerable weight. Thin glass is not very stiff and distorts easily even under its own weight. So mirrors are made thick to keep them stiff. Making a mirror thinner leads to problems keeping it properly supported so it doesn't sag out of shape. The mirror cells get really complicated, and sometimes the mirrors sag anyway.
The other way to make a light-weight telescope mirror is to keep the mirror thick so it will be stiff, but remove pockets of glass from the back in such a way as to lighten the mirror without compromising its stiffness. Since the mirror is now lighter, but still thick, it has even less tendency to sag and distort under its own weight than a solid mirror. The mirror cell can be very simple and low-tech. The problem is, to go this route, you need to cast the mirror yourself. A daunting task for most people. I am not afraid of complex and time-consuming projects with steep learning curves, (basically I was too dumb to realize I couldn't do this, so I went ahead and did it anyway) so I dove in and started experimenting.
One of my first attempts at making a light-weight mirror blank resulted in my Waffle-Back design.
This mirror has a lot of small circular pockets in the back of the mirror to lighten it. This design works, and it sure is pretty, if I do
say so myself, but it doesn't lighten the mirror blank as much as it could. The amount of glass left between the circular pockets is excessive.
I played around with different sizes of pockets and different ways of arranging them, looking for improvements. I could improve the design, but
not as much as I wanted to. I stuck with circular pockets for a long time, because circles are easy to make. Eventually though, I realized that
hexagons would work much better, and abandoned the circles.
Why hexagons? Hexagons can be used to tile a plane with no gaps between them. Circles though, no matter how you arrange them, will always have gaps between them. Those gaps represent excess glass. Naturally I do need gaps between the hexagons to make the supporting ridges that keep the mirror thick and stiff. But those ridges are uniform in width, and I can control the width of those ridges easily by adjusting the spacing between the hexagons. I could make the gaps just big enough to do the job with no waste. The big problem I had was how to make the hexagons I needed? I tried tots of things, and none of them really worked. I tried using hexagon cookie cutters. I tried making wood hexagon molds. I tried hexagonal chocolate molds. It was all an utter failure.
Then I had the eureka moment. I was wracking my brain trying to figure out where I could find hexagons in lots of different sizes for my experiments.
Suddenly I noticed a hex nut on a piece of equipment. The lightbulb went off. I slapped my forehead and said Duh! Hex nuts are hexagons. They come in
an amazing variety of sizes. They are easy to find and relatively cheap. I went online and found a table showing the dimensions of standard hex nuts.
I decided that 1 1/4 inch ID nuts were the perfect size for the pockets I wanted to make in the back of the mirrors. They are just a little
under 2 inches across the flats. So I popped down to Pinellas Bolt and Screw and bought a box of big nuts.
I can't use the nuts themselves to mold the pockets, but I can use the nuts as molds to make plaster hexagons the same size as the nuts. These plaster
hexagons then become the molds for the pockets in the back of the mirror blank. I arrange the nuts in a pattern that leaves 8 nut-shaped voids I can pour
The recipe for this plaster is 50% Hydrocal plaster and 50% 200 mesh silica flour (by volume) with enough water to make it soupy so it can
be poured. In just a few minutes the plaster hexagons are hard enough to break out of the nut molds. I clean up any rough edges or flashing with a rasp.
It may seem like a convoluted way to do things, but making the plaster hexagons this way actually goes fairly quickly and easily. In a short time I had a big
pile of hexagons made. The hexagons are 1 inch tall and 1 7/8 inches across the flats. I was now ready to make the mold for the mirror blank.
I had drawn up the pattern I wanted to make. I wanted to actually see the pattern and make sure there were no problems before continuing.
So I mocked up the pattern of the hexagons on a paper circle 12.5 inches in diameter. I cut some of the hexagons with a diamond tile
saw and shaped them with a belt sander to make the partial hexagons for around the edge. I used square wooden dowels 3/8 inch in size as
spacers to get the spacing between the hexagons the way I wanted it and keep it uniform.
I start building the mold with a kiln shelf that has been well coated in kiln wash. I drew a 12.5 inch diameter circle on the
kiln shelf. This is the footprint of the finished mirror blank. The hexagons and partial hexagons will need to be arranged within this circle.
Again I am using the wood dowels to get the spacing right. To keep the hexagons from shifting around, I glue them down with a slurry of kiln wash.
To glue down the hexagons I mix up some soupy kiln wash and use a spray bottle
to wet down both the hexagon, and the position on the shelf where it will go. I then dip the bottom of the hexagon in the soupy kiln wash
and quickly put it in place. The plaster hexagon and the porous kiln shelf wick the water out of the kiln wash slurry in a matter of seconds,
and the hexagon locks in place. If I don't wet the pieces first, the kiln wash slurry hardens instantly. I have only a very short period
of time to maneuver the hexagon into place after it touches town on the kiln shelf before the kiln wash seizes up. If it isn't in the right
place, I have to break it loose, scrape off the hardened kiln wash, and try again.
Once all the hexagons and partial hexagons are in place, I put the outer ring of the mold in place. The ring is cut from soft fire brick
with a band saw. The pieces are glued together with furnace cement. This particular mold has been used several times before. It is beat up
and has multiple repairs. Once the mold is finished, I place the shelf, with the mold on it, into the kiln.
Here I am cleaning 3 disks of 1/2 inch thick glass in my dishwasher. These particular disks are 14.5 inches in diameter. I didn't have any
photos of the 12.5 inch diameter disks I used in this blank. After thoroughly cleaning 2 disks of glass 12.5 inches in diameter and 1/2
inch thick, I place them in the mold. and fire up the kiln.
Here is a shot of the blank cooking in the kiln. At this point, the two disks have fused together, and slumped down to fill the spaces
between the hexagons, and the gap between the hexagons and the outer ring. Now comes three days of annealing and slow cool-down to room temp.
I have had several people write to me and ask what firing schedule I use for making these blanks. Here it is below.
I have a ramp-soak controller for my kiln. It took a lot of
experimenting to find the proper times and temperatures for annealing. I started with the Bullseye Glass annealing tables, but got
terrible results with the float glass. After ruining a lot of glass I discovered that I needed to go about 150 degrees F higher than
the Bullseye recommended temperatures, and about four times as long on the ramp time of the initial cooling rate. This schedule generally
yields a well annealed blank.
Times are in minutes. Temperatures are in degrees F. AFAP = As Fast As Possible (I open the kiln lid to get the temperature down quickly so as not to linger in the devitrification zone). The above photo was taken during the rapid cool-down to the annealing temperature.
I use a home-made 8-segment ramp/soak controller to control my kilns for firing the telescope mirror blanks. Click the photo to see how I built the controller.
After the blank had finally cooled down to ambient, I took it out of the kiln, and removed the outer mold. Then it was time to start
cleaning the plaster out of the hexagons and partial hexagons. A very messy and delicate job. Fortunately the plaster is rendered quite soft
and powdery by the temperatures involved in casting glass. So it comes out fairly easily.
Here is the honeycomb back of the cleaned out mirror blank. The blank is a little over 12.5 inches in diameter and slightly irregular.
I need to turn it round and get the diameter down to size on my grinder. The blank is 1.5 inches thick. The pockets are 1 inch deep.
There is .5 inch of glass left on the front face for grinding a curve. It weighs only 10lbs, 9.5oz at this point. It will loose a
little more weight on the grinder. It's not perfect. There are a few tiny air bubbles that may be close enough to the front surface
to be encountered while grinding. Otherwise, I am very pleased with it.
Here is a photo of the finished 12.5 inch diameter honeycomb-back mirror blank after final edge trimming. It is side-by-side with a solid 12.5
inch blank of the same 1.5 inch thickness that I also cast in my kiln. The honeycomb blank weighs 10 lbs 3.6 oz. The solid blank weighs
17 lbs 10.4 oz. The honeycomb mirror is 42% lighter. I think I might be able to get close to a 50% reduction in weight with just some
slight tweaking of the design.
So, What's next? I plan on grinding and polishing this 12.5 inch mirror to see how well it takes a figure, and if the pattern of the back prints through. I am also planning on making larger diameter versions of this design. I will post updates in the future.
Here is the hardened log out of the mold. It is hexagonal in cross-section, and about 10 inches long. It turned out very nice. There are
a few more small air bubbles than I am used to with my hex-nut method, but no real problems. The plaster did not stick to the mold thanks to
the coating of vegetable oil. The wooden hexagonal base is visible to the right and below the log. I should be able to re-use this mold over,
and over again. I used a rasp to neaten up the log and remove the flashing.
I was pleasantly surprised to discover that the saw cuts through the log like a hot knife through butter. The cutting went very
quick and easy, and produced perfect hexagons.
In the end, I got 8 perfect hexagons out of the log. The 9th one, cut from the top of the log, was sunken in the center from shrinkage.
I discarded it. This was much quicker, easier and much less messy than making the hexagons the old way. This new technique will come in
handy in the future as I scale up my honeycomb-back design, since I probably won't be able to easily find hex nuts large enough to do it
the old way.
Here is a photo of the back of the above new mirror blank. I really took a different path on this blank. The mold was CNC milled instead of pieced together as in my previous hex
blanks. This allowed me to create a very complex design incorporating bearing points and sockets that would have been difficult to piece together. I know because I have
tried for the last couple of years to assemble such a mold by casting individual pieces. I finally gave up and decided the whole thing needed to be machined in one piece.
I also departed with the past by not trying to cast the blank as a single monolithic piece. I cast the honeycomb back separately, and then fused a sheet of glass on top to
be the surface to be ground and polished. This totally eliminates the problem of grinding into air bubbles that has plagued my previous cellular castings. This will probably
be the way I make cellular blanks in the future.