HI,
To
birddog and
blue wizard and anyone else who is interested.
This whole subject depends on the results you are after. But first, please read the old thread at the following link:
http://audiokarma.org/forums/showthread.php?t=87723
The subject was explored pretty thoroughly by me with many other chipping in with their own ideas.
In order to clear up some confusion I will restate the goal. It is to accurately determine stylus wear by observing the tiny facets that develop on a worn stylus. I'm not concerned with general stylus examination for dirt, debris and dust bunnies. A hand held magnifier will get that job done. But a real microscope is needed for wear determination. Real microscopes are the subject of this discussion.
I'm going to lay down some guidelines to get you thinking in the right direction:
1. Maximum Magnification: Is of prime importance. 100X can work but only barely. 200X is ideal. Anything less than 100X will not show the wear facets. They are tiny. 200X defines most of the other aspects of the scope design because that, my friends, is pretty high magnification under which everything else becomes critical.
2. Variable magnification: At 200X, placing the stylus tip in the field of view is very difficult. Thus, the Shure scope has a zoom eyepiece that zooms from 100X to 200X overall. Use 100X for tip placement and 200X for examination. Even at 100X, placement is not so easy. I would prefer 50X.
3. Micrometer stage adjustments: Rather than moving the cartridge or stylus around by hand trying to get it in the field of view, the stage x/y axis adjustments are used. They provide very fine control of placement. Doing it by hand is crude and frustrating even at 100X. At 200X it is nearly impossible. There is a reason why all but the least expensive microscopes have micrometer stages. I consider this to be an essential feature for any microscope beyond a toy.
4. Other Mechanical Features: Smooth controls are important and nice to work with. Focus and stage adjusments, in particular, must be smooth. Mechanical stability is important. Any decent microscope has adequate stability and weight.
5. Lighting: This is very important and is one of the unique features of the Shure scope. The tip must be lighted from above and to each side. Most microscopes illuminate from the bottom. The lighting angles must be right to allow the wear facets to reflect light into the scope objective lens. The facets act as tiny mirrors. Without the proper lighting, the facets become invisible.
6. Optics: Must be reasonable for sharp images but don't have to be great. Color fringing should be minimal. But most important is the choice of magnifications for the objective lens and the eyepiece. The Shure uses a 10X objective lens and a 10X to 20X zoom eyepiece. These were carefully chosen with the 10X objective being the most important factor. There are a number of combinations that could result in the overall magnification of 200X. The 10X objective gives the best working distance and the deepest depth of field of any common objective and, when used with a 20X eyepiece, gives 200X overall magnification. Unfortunately for someone trying to design a stylus microscope, like me, a ZOOM 20X eyepiece is impossible to find. I’m doing it a different, but inferior, way.
I define working distance as the distance from the plane of focus (the stylus tip) to the surface of the objective lens. Why is this important? Well, you need room to work. The stylus is being moved around quite a bit and you need adequate space to keep the stylus from accidentally contacting the lens and breaking off. I did this when I was playing around with a 40X objective (very small working distance) and a 10X eyepiece. Fortunately, I was using an old, worn out Shure stylus.
Depth of field is also important. The 10X objective gives adequate DOF to see the entire wear pattern. 20X is too shallow to work with. 40X is impossible.
All in all, I think Shure chose the best combination of lenses.
6. Anything hand held is useless. Just try to use a 100X (or even 20X) magnifier hand held and you will see what I mean. But they are good for dust bunnies.
Sparky
