Wednesday, November 14, 2007

48mm Camera Filter Tap & Die

48mm Camera Filter Tap & Die

There is a very common camera filter thread that I believe is M48x0.75. I am having trouble finding a source for tap and die for cutting threads for this size. I've tried the obvious places, like McMaster-Carr and MSC Direct, and I've searched the web. Any suggestions of who might have such a tap and die set?

UPDATE: I received a number of good suggestions, and amazingly enough, it does not appear that anyone has such a tap available off the shelf at less than $150.

The reason that I need this ability is that my 5" refractor uses an astonishing piece of optics called an Aries Chromacor, which is made by one of those Ukrainian optical wizards who I gather used to work for the Soviet military--and after the collapse of the Soviet Union, put his expertise to work on peace dividend stuff.

In this case, the Aries Chromacor is a piece of rather specialized glass that does two things, simultaneously:

1. It corrects the chromatic aberration that is common to all but apochromatic refractors.

2. It also corrects for whatever spherical aberration your refractor has. If you have a 1/6th wave undercorrected achromatic refractor, you buy an Aries Chromacor that has a 1/6th wave overcorrection built in.

Anyway, it works very well--turning what would otherwise be a so-so cheap refractor into something that is perhaps 85-90% of an apochromat for about 30% of the price. A detailed review that I wrote several years ago is here.

As with all clever compromises, there are couple of irritations about the Chromacor. Because it goes inside the telescope tube, at the eyepiece focuser end, something has to hold it place. The Chromacor has a male, 48mm filter thread on it. Refractor diagonals (at least some refractor diagonals) have the end that goes into the focuser tube threaded to accept 48mm filters, so the Chromacor just screws onto the end.

This works fine, except that the diagonal adds enough length to the optical path that you can't attach a camera at prime focus--the focal point won't be at the focal plane of the camera. To take prime focus photographs requires removing the diagonal--and thus the Chromacor. What is needed to solve this problem is something that goes inside the focuser tube, and has 48mm filter threads on one end. That's why I was looking for a way to tap M48x0.75 threads.

Well, it turns out that others have had this problem, and one solution is this Televue 2" eyepiece barrel extension. I'm told that it has 48mm threads (probably at both ends) so that you can attach it to a 2" diameter eyepiece barrel, and lengthen the eyepiece. I think what I will do (once I have verified the thread details) is buy one (or perhaps two), and then machine a piece of aluminum that will slide into the focuser tube. On one end, it will be wide enough that it doesn't slide all the way into the tube. On the other end, it will be small enough that the threads on the barrel extension will slide in, and I can use some epoxy to hold everything together. Then I can screw the Chromacor into the other end of the barrel extension.

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