Coulter's Smithing Home.

phototube and mounting jig On recently acquiring a large collection of tubes of various kinds, it became possible to do this project, one I've wanted to do for awhile. This photon counter (or at least I'm told it is possible to count single photons with this tube) can be fitted to the telescope for astronomy purposes, or used with some scintillator material as a radiation detector. Key design issues here are that it has to be easy to mount to things in a light-tight way, and the electrical leakages have to be kept very low. The first was satisfied by the welded metal tubing shown, which just fits the standard 1.25" telescope accessories. The second was done as well as I can, by getting the various insulators very clean, and using ceramic sockets on the end. Most of the 8 pin tube plug pins will be grounded to keep the approximately -1000 volt supply current from leaking into the output pin. I will probably build a small transimpedance amplifier right on the connector socket external to this, so it can drive cables better. I used 470k ohm resistors for the divider chain, and may add bypass capacitors to it later as I've seen done in other photon counters. In other words, I need to make this so I can take it back apart too. I will probably use the time honored approach of pushing on the bottom cup and wrapping it all in black tape.

The tube pictured is an old RCA 931a I got "new" in the box, manufactured in April, 1945 according to same. Well, unless air has leaked into it, it should be fine, there's not much to go wrong with one of these.

Construction was simple, sort of. First tubing of the right sizes was obtained from McMaster-Carr. Then the larger piece that contains the tube was put into the mill and a slot cut in it where the light is to enter. The other piece was then notched with a bimetal hole saw. This part was WAY too exciting, and almost broke the drillpress. It's just not rigid enough to keep the teeth of the hole saw pulling themselves into the metal too fast, and won't go slow enough. So all this was moved to the milling machine at 60 rpm, and there it went fine. The pieces were clamped in a vise and wire welded.

Next, end caps were needed. I could have just cut some sheet metal and welded it on there, I suppose, but I wanted something a bit more elegant, and removable. So, a jig was made by using a 1-5/8" Forstner bit to drill about halfway through a piece of very hard oak from some 100 year old flooring. This became the die. For the punch I just used another piece of the 1-1/2" inch tubing with the edges rounded a bit. Plugging in a 2" copper flashing washer and pushing in the pipe with a press made the caps. The top cap was then soldered in place with the normal plumbing stuff, and all tested for light tightness.

For the base, I punched a hole in one of the copper caps to hold an octal tube base. This is held in with a snap ring, then J&B weld for light tightness. The piece of PVC pipe in the picture is an insulating spacer to keep the resistor chain from shorting to the pipe, and to keep the tube more or less in place. Not shown is a piece of cardboard I intend to add as a shim to make the tube a friction fit.

Of course, to be cool, anything telescope related must be black, so I painted it all with flat black spray paint. Unfortunately, the 1.25" tubing is that, exactly, and so are the eyepiece holes in the scope gear, so that paint and some metal had to go to make things fit.