SatinSnow ground glass...
 

Anyone have experience using the SatinSnow GGs in non-static adapters? Not much came up in a search of this forum -- certainly nothing conclusive.

Large format photographers seem to really like them, and they're very reasonably priced for custom GGs.

I remember when everyone was on the GG cd's that the satin snow thing kinda fizzled because the Satin Snow people didn't have a way to make circular glass. I don't remember anyone trying it for a spinner or other moving adapter.

Your right... Satin just got back to me saying they can't do circles.

I haven't given up on them, though. I know someone who can cut glass into circles, so I may order a rectangular piece and do the rest here.

It's a significant risk in time, though, if it can't be cut.

Gah!

I'd imagine it can be cut, just doing it right the first time is a problem. I cut quite a few glass circles when I was working on wax GG adapter and I broke just as many as I didn't, but then again if you got someone who knows what they're doing you should be good to go.

I think I'm going to do just that... I emailed SatinSnow back, and they said there shouldn't be an issue cutting it after it's been finished. Thanks for the feedback.

No problem, let me know how it works out. I thought about trying it before, but like I said I felt bad enough with a 50% success rate when it was glass from the dollar store. Can't imagine doing it with stuff like that.

I definately know what you mean... but I think it's worth the risk. Actually, I decided not to go with the SatinSnow just yet (the idea of a 3 month wait was killing me -- I want my mini 35 NOW ;)). I actually ordered a 100mm square piece from Thor Labs yesterday, and it arrived today! (That was with the CHEAPEST shipping). It might be even more pricy than the SatinSnow, but it sure was fast.

The way I see it, the GG quality is as vital to the image as the lens and the CCDs, so I just have to go for it and take the risk with the good stuff. This whole thing is still winding up being many times cheaper than buying any pre-made mini35 out there.

It's the 1500 grit stuff, and just holding it up behind a 35mm lens, the diffusion characteristics already look better. Also, it doesn't have that greenish tint that lower quality glass has when viewed from the side.

The GGs from OptoSigma aren't big enough for a spinner, so it's either going to be the Thor Labs one or SatinSnow.

Justin,

You say you know someone that will now cut the GG into a circle. How exactly will this be done, and once done, what are your planned methods for spinning?

1500 grit does not diffuse completely.

Agreed. I had the same with the Optisigma 1500

Really? That's a bit troubling news... but surprising. I was examining it before and the outlines of things in the background couldn't be seen at all. It seemed to diffuse as well as the much courser grain GG I have in the adapter now... though I won't know for sure untill I test it.

Ben, were you using the Thor Labs GG? I've read contrasting opinions here about Thor vs OptoSigma, and while it doesn't seem to make sense, I DO believe the method of grinding makes a difference. Someone correct me if I'm wrong, but isn't that why aluminum oxide must be used instead of normal sandpaper? Just for the heck of it I tried sanding a garbage piece of GG with normal (not AO) 1500 grit sandpaper, and it actually seemed to polish the glass (it started to become transparent, and no longer diffused the light sufficiently). But the Thor Labs GG I have now doesn't do that at all, it seems to diffuse the light just as well as my other GG did (before I took sandpaper to it).

Hmm, this is disappointing though if 1500 is really too fine. Thor doesn't make an inermediate glass between 1500 and 600, and OptoSigma's are too small.

Wayne, this peson owns a glass shop, and I don't actually know how he cuts it. It comes out as a circle, so I'm happy...

The GG is attached to a 20v DC motor running at a maximum 18v (two 9-volts in series -- 1 battery actually might be enough power). I can't remember the torque constant, but it's powerful, quiet, and cheap. The GG on it now is just epoxied to an aluminum plate on the motor's spindle. This was a tempo thing so I could test the first GG and work out mirror alignment, intermediate lenses, etc. The new setup (which I'm working on, and maybe completing tomorrow) will be as follows: The motor shaft will be threaded, and an aluminum back plate screwed on. The motor shaft will extend out from the plate, and through a hole in the GG's center just big enough for the shaft, and into another (threaded) plate. But that's not it completely. The plates will not be tightly screwed together so that the GG is squeezed rigid. Between the GG and each plate go thin layers of foam. The Plates will only be screwed snugly, and the foam should allow the centrifugal force of the spinning GG to stabilize it completely. Optimally, the GG will sit in this mount perfectly evenly without having to be spinning, but if it's off even a few micrometers, than the foam should give the GG enough play to stabilize itself. ISSUES: If the GG is attached too loosely, the gyroscopic force of the plate will resist panning/tilting and precess, changing the flange focal distance. The GG must be attached tightly enough to prevent this, yet with enough "play" in the foam to allow self-stabilization.

I'll post closeups of all this once it's finished (hopefully some time this weekend).

Does that all sound reasonable? I was amazed how something that seems so simple (attaching a piece of glass to a motor) has turned into such a project. Cutting circular glass and drilling holes in glass were some of the pitfalls I intended to avoid with my original (oscillating) design... oh well.

Gah, threading the motor shaft didn't work... I wound up ruining a motor.

The glass isn't back from the cutter yet, but because I couldn't thread the shaft (may use a pre-threaded motor in the future), I struck a compromise in my design.

I'm going to glue the GG to the motor plate again (carefully), but instead of attaching it directly to the metal, I'm gluing it to a thin foam pad, and that to the metal. If there's any wobbel at all (it should be less than .5mm to start with), there should be just enough play that the GG will spin itself stable from centripetal force.

I was able to glue the old GG (which was in direct contact with the metal) well enough that there was no visible woble in the for-aft direction, and the laser test yielded just slightly less than perfect results. I'm hoping the foam will fix that all together.

On a side note, I pretty much know what the second version of this adapter will look like (which I will provide building instructions for)... and it should be very easy to build.

Except for air friction resistance and the professional look of the innards, there is probably no other reason for spinning a round groundglass in preference to a perfectly square groundglass other than difficulty in centering and balancing and perhaps a smaller enclosure the round groundglass enables for a usable image area.

For a flip adaptor using right-angle prisms the square spinning groundglass would not be usable in a design like mine where one prism overhangs the groundglass.

For wobble (runout), provided the groundglass itself is balanced, mounting the motor on a soft compliant mount and running the motor at about 3000 rpm which for the CD drive motors is co-incidentally about 3V should enable a thick groundglass to stabilise itself gyroscopically.

Compliant mounts will work best for an out-of balance state if they are positioned around the plane of the groundglass so the out-of-balance state does not sit outboard of the centers of force the compliant mounts deal with.

At those positions, widely spaced and around the outside of the groundglass, I expect you could get good results with soft foam doughnut shaped pads or even with an entire cylindrically shaped foam support.

Just a thought.