Making a glass optical dome for all sky camera

[Stefan Buda]

Members of the ASV's instrument making group have started building a batch of all sky cameras, myself among them, and I thought I challenge myself at making a glass dome for mine, rather than use a commercial plastic one.
Just recently I had built a small electric kiln so I started experimenting with glass slumping. After a number of trials I came up with a two stage procedure to get a roughly hemispherical glass blank that could be ground and polished into a good optical dome.
At the first stage I used a terracotta dish to slump a 99mm diameter glass disc to a concave shape.
At the second stage I slumped the preformed glass onto a 63mm stainless steed dome.
The main problem is that, while the inside is quite spherical, the edge of the glass has thickened to almost 5mm, and the center is 2.7mm thick, down from the original 3mm.
I will try to grind the outside spherical and achieve a wall thickness of at least 2mm.

[Leo.G]

This is an interesting project. How thick was the sheet of glass you started with?
When you perfect the process I'll have to send you the size of my clock face, a hand made timber clock from Italy in the shape of a ships steering wheel I purchased new in 1981 but when my son was 2 he slammed a door and the clock fell off the wall and broke the domed glass. I've never been able to get a domed glass replacement for it.
Though if I get a kiln (or make a gas kiln) I may have a go at it myself, nothing to lose but some cheap glass.

[leon]

Wow, Stefan that is an interesting project indeed, I hope you get the desired result you are looking for.

Leon

[AstroViking]

I've been reading Stefan's projects since I joined here and I'm thinking that there's nothing he can't do....

V.

[Stefan Buda]

Thanks for the comments. It's nice to know that there is interest in my little projects.

The rough grinding is now completed, down to #240 grit.
Lateral concentricity is better than 20 microns and wall thickness is about 2.35mm.
All done with hand tools.
If I make another one, I will invest some time in making fixtures so that I can machine away the extra glass.

[Rod]

That looks great Stefan. How long did it take working by hand? Did you use a tile tool?

Rod.

[Stefan Buda]

I must have spent something like 6 or 7 hours on it, over several days.
You can see the 3 principal tools on the attached picture, but most of the glass removal was done with a coarse grinding wheel and a big diamond file.

[Stefan Buda]

I finished the fine grinding and slumped a couple more discs for making the polishing laps.
I will try to polish the inside surface first because I think it is more likely that I break it while working the inside surface.

[Peter Ward]

Very impressive! I can also see why one would do this: optical glass domes are expensive.

I recall being in an optical surplus shop in Pasadena (think JPL's headquarters) in the pre-digital camera epoc, (that shop has long since closed) and pondered some optical domes...some were 6" in diameter and the asking price was about $US 20 depending on the size.

I though..cool..but what would I use them for?

If only..... :doh:

[Stefan Buda]

Thanks Peter!
I find it very strange that it is impossible to find any prices online, apart from the ones dedicated for underwater photography. And yet there is no shortage of manufacturers but they don't reveal their prices.

Anyway, the polishing is progressing better than expected.

[Stefan Buda]

Yesterday as I was about to finish the polishing of the inside surface, disaster struck! I must've got a bit overconfident and at one point I did not engage the driving pin properly. As a result, when I switched on the machine, the contraption instantly went into self destruct mode.

So, back to square one, it seems, but in fact not, because I now have most of the tooling and fixtures needed. Also I will start with a small batch so that I end up with at least two finished ones - one for a replacement spare if I ever need one.

[Leo.G]

That's the worst thing with glass Stefan.
At least you have everything set up as needed to get a faster start on the new units and the experience.
What was you using to polish the glass?
Was it some sort of sphere with diamond tiles or something?


Back 40 or so years ago I worked in glazing for a not extended period, the boss expecting me to carry big sheets of glass up an 8 metre ladder in rain and strong winds "shove it moron". There were no OH&S regulations back then.
I'd often find myself doing a pencilled edge on large sheets, hold it under the arm and run it evenly up and down the shaped grinding tool (large machine) and whatever you do don't twist the glass. The strangest bit of the entire process was using a cork wheel and belt to polish the ground edges to the nicest sheen. At least I've known how to cut glass when I've needed to.

[Stefan Buda]

Leo,
The polisher was plain synthetic pitch with cerium oxide - nothing unusual.
The difficult bit is the extreme curvature which would require a special purpose polishing machine. I had to make a curved driving pin and used pure spin polishing.
Anyway, I have an artisan glass cutter just around the corner and I got half a dozen new discs cut the other day.

[Leo.G]

I always keep cerium oxide here just in case., probably couldn't find it if I need it but I have it here.

[Stefan Buda]

I did a lot of glass bending over the last week and now I have 4 good blanks.
I've also done the rough grinding of the inside surfaces.
Next I need to make a fixture for diamond machining the outer surfaces.

[By.Jove]

Well, in case they all break... $US15... min order 1 off, apparently glass, not acrylic...

https://vyoptics.en.made-in-china.com/product/pwbGNqLHbThA/China-Custom-High-Precision-Underwater-Glass-Half-Hemisphere-Dome-Port-Lens-with-Groove-for-Camera.html

[Stefan Buda]

Thanks for the link. Interesting find but those domes are huge and it is interesting to see an optional US$258 quality inspection associated with them.

Some months ago I managed to find some suitable sized glass domes on one of the online markets but the price was over $400.

[Stefan Buda]

I made a holder for machining the external radius. Not having a suitable size fly cutter, I just used a hole saw to generate the required radius.
For the glass machining I happened to have just the right size diamond plated tool.
For attaching the domes to the holder I used a mixture of paraffin and bees wax.

[Leo.G]

Is that being held in a chuck on a milling machine or a lathe?
I avoid working with glass after experience working in glazing back in the early 80s.

[Leo.G]

I figured it out, it's one of those off axis rotational chucks used on milling machines. I've yet to purchase one. I'm yet to weld up a stand for a milling machine which came as an attachment to a lathe. I purchased a separate cross slide table and the milling machine still sits in it's box until I get a suitable stand welded up. I didn't want it bolted to the lathe when I read the notes stating it may either need packing to fit properly or, machining of the milling machine upright mounts. That would have been a big pain so I purchased a separate compound table to set it up as a stand alone machine.

[Stefan Buda]

It is an ordinary dividing head, tilted upwards. Very useful for generating spherical surfaces.

[Stefan Buda]

Mission accomplished!
I have two polished domes and two unbroken blanks left.

[Rainmaker]

Excellent work Stefan !:thumbsup:

[Leo.G]

They look amazing mate!

[Stefan Buda]

Thanks Matt and Leo!

Now I have to figure out how to mount them.
I'm thinking of gluing each into a 2mm deep groove using polyurethane or silicone rubber.

[bratislav]

No groove. Make a step where outside is flush with glass bottom, so any water runs away, not inside. And machine/print that area to slope down, to drain water away.

[Terry B]

Now you need to find away of stopping dew when using it.

[Stefan Buda]

The reason for wanting to use a groove is that I think this way I can avoid turning the gluing job into a mess. If I put just the right amount of glue into the groove, spread evenly around, and then put the dome on, hopefully the glue will just fill the space between the glass and the metal plate without oozing out. I also want to drop three short pieces of wire into the glue, at 120 degree locations, prior to inserting the glass, to act as raisers.


I'm hoping that the RPi will generate enough heat to keep dew away.
That seems to be the case with the ASV prototype, but that one is in a plastic housing with a plastic dome and my version will have aluminium housing with a glass dome. I may have to add insulation to the housing.

[Leo.G]

The aluminium housing will draw (and hold) the cold won't it?
I guess if the Raspberry Pi emits enough warmth it will counteract the cold.
There's always a Peltier cooler used on the reverse side but it would likely steam the glass up. I think last time I played with one I may have burnt my finger on the hot side.


Still, the domes look amazing and I can only imagine how much work went into making them.

[Stefan Buda]

Leo,
The ASV prototype ASC is reporting a Pi CPU temperature of 38 degrees at the moment while the ambient temp is about 10 degrees. So, I think there is plenty of heat being generated inside the housing. It is just a matter of managing it so that the whole thing doesn't overheat on a hot summers day.

[Stefan Buda]

I mounted the domes using black silicone rubber.
The groove is only 1.5mm deep and I used 3 tiny u shaped spacers at the bottom of the groove, made from 0.4mm diameter nichrome wire.

[Leo.G]

They look amazing but seeing some of the other things you do I shouldn't be at all surprised.
Still, I worked in the industry (glazing for a short period of time which was still too long) and had to polish the pencilled edges on large sheets and the slightest twist and you have a large sheet of glass now in little shards on the floor and an angry boss. The diamond grinding wheel pencilled the edge, the cork belt/wheel polished it to a clear edge.


I even have a circle cutter here I've never bothered trying out because unlike the movies it's not as simple as scribe a single line then pull the centre patch out. I learnt how to do that and it involved a lot of scribe (cutter) lines and a lot of broken glass, including occasionally the one I was trying to cut the circle out of (mainly for exhaust fans in old people's homes (aged care)).


But the ability to cut glass has come in very handy over the years.

[Stefan Buda]

I can barely cut straight lines and won't even contemplate circles, so I'm lucky to have a shop nearby where they make all sorts of glass art objects.

[Leo.G]

I've in the past repaired a leadlight cabinet from the 1940s I used to keep my camera collection in. All freehand cutting of some quite complex shapes along a marker line traced on the glass to keep it as original as possible.


Since then my once young son (Turns 30 in 13 days, HOPEFULLY (severe depression)) put a toy truck into a panel and I simply covered the crack line with silastic so no sharp edge for him to cut himself on and while in storage till I get to redo the few cracks a cat knocked a suitcase over and made a huge mess of several panels.
One day, maybe.
I'm not lazy I just don't get to half the stuff I should get to with chronic migraines on a daily basis. No actual shed to work in doesn't help, not everything can be done in the yard and the house I rent is way too small to do it in here.

It's actually a valuable cabinet if I get to doing the work.