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Anonymous
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I have a device in my shop that measures MTF. It consists of a photomultiplier tube mounted inside a rotating lucite drum. The drum has a diffraction grating. A microscope objective/eyepiece assembly focuses the image of the filiment of a bright projector bulb onto the diffraction grating. The lens or optical system to be tested is placed between the bright bulb and the microscope focuser so that the photomultiplier "sees" the filiment projected on the rotating diffraction grating. The results are monotered on an oscilliscope. There is a "desired" wave form. This device allows the user to compare one lens with another to determine which lens has better contrast.
It was originally used by Delco Optical to select the best aerial cameras from a given lot. For example, Delco would contract 100 lens assemblies and use this device to select the 10 best, and scrap the remaining 90.
I've used it a few times, but my bench isn't long enough since the lens has to be "seen" at prime focus (eg.: long focus refractor lens). Good for eyepieces though. I'm told by the maker that it can also test mirrors, but I haven't tried yet.
The down side is that 1400 VDC is fed into the photopultiplier...sort of dangerous in my book. I power it with a step up transformer borrowed from a high power laser. From time to time I see "lightening" striking from the wiring to the bench. I suppose I should use shielded wiring. Other than that, I wonder why Ben Franklin wasn't hit by lightening. I guess it's "inventor's luck."
Joe Donahue
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Anonymous
Unregistered
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How much amperage is pushing that 1400VDC? I've never worked with any DC that high, but I have a healthy respect for it.
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Anonymous
Unregistered
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About 1/2 amp.
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rboe
Numbfinger
Reged: 03/16/02
Posts: 39735
Loc: Phx, AZ
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That's enough to kill you on a bad day. But voltage is usually not that bad a thing, it's the amps that will get you. I remember from somewhere that an 1/8th amp is all you need if it hits your heart at the wrong time.
That said, it still won't feel good! Those good shockers you get while getting out of the car or shuffling across the carpet are in the 4000 volts and much higher range. But little amperage.
Dry air makes a good insulator, have you noticed if the humidy is up when you see the "leakage"? I recall looking under the hood on my first car, a 1967 Thunderbird. Regular light show was going on with all those bad sparkplug wires. Very cool to see!
So Joe, are you going to test some eye pieces for us?
--------------------
Ron
NS11GPS
Pronto
16" dob
15X70 Obies
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Anonymous
Unregistered
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Eyepieces? I'm shocked that you'd ask! (get it?)
Next time I have it up and running I'll test what ever you like.
What this thing is best at is comparing two or three identical eyepieces to see which one is best.
Before I'll use it again I have to set up some grounds.
Joe
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Anonymous
Unregistered
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I hope you replaced those plug wires with something a little more shielded! When I was building Buick 455 street engines that would have been a nightmare to see under the hood.
Yeah, with DC it doesn't take much amperage to do you in. I was curious. I worked with 400VDC with about an amp pushing it. Nasty stuff if you're not paying attention.
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