Mitchell's Telescope Making Website

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Mirror Testing Equipment

The types of testers I have built are called Ronchi (pronounced ronki) and Foucault (pronounced fou-co) tester. These are the two most common testers. The Ronchi is a very simple and extremely easy test. I can use it all the way up to figuring and some during figuring. Then I mostly use the Foucault tester that’s a bit more advanced for taking precise measurements. For that I originally modified my Ronchi tester a little. Now I built an entirely new tester which works great!

The Testers:

The R1: I don't have any Pictures of the R1. But it was basically the black box in the picture below, with the LED shining out the front and the silver adjustment knob at the back for brightness. Then the grating was taped crudely onto the front. It was functional, but just barely. The box was in the way so you couldn't get your eye close enough to see the whole mirror at once and you had to strain your neck a lot to look.   

The R2: The R2 is a vast improvement over the R1. You can now see the whole mirror much better and you can get your eye right next to the grating where it needs to be. The adjustment box is screwed to a piece of 2X4 and I just extended the wires out to the LED's new position. It is held in place by half of a light switch face-plate. Taking images is much easier too because the camera doesn't have to be sideways. You don't have to strain your neck as much either. Now this tester has fine adjustments because it rides atop my Foucault tester stage. The Ronchi works very nicely with this setup, the Foucault tester is working pretty well, just a few bugs. The R2 is very easy to look through and use and easily puts the R1 to shame. And because of my latest modification to my Foucault tester I can use my small spotting scope for this too! 

A view from the backside of the Ronchi Tester where you look through^

The F1: My first Foucault Tester! It uses my old Ronchi Tester to hold the Knife Edge. Then it all rides atop a fine adjustable stage with dial indicator to take test readings. Like the R1, it has a few bugs to work out. I have now also added an old spotting scope to make images larger, so it is easier to take readings! I will mount it on a hinge so I can simply tip it over out of the way, use my eye if I wish then tip it right back up and get a much nicer, bigger image. It is a nice trade up from just the eye when your not ready to shell out a bunch of $$$ for a whole camera system. And I have the added benefit that I can still use both my eye and scope. With the scope I don't need my glasses, which is nice too. It's an old 6 X 20 finder. The only thing is images in this scope were a little dark and you lost too much contrast.

Here is a side view of my Foucault Tester. You can see the old Ronchi Tester riding on top. Just below the LED is the Knife Edge hidden by the spotting scope. All the way to the right is the dial indicator to take readings with and the two silver bolts with large washers are adjustment knobs. Looking underneath you can see it all rides on some cheap teflon chair pads atop a copper pipe. It is very simple and quick to change between Ronchi and Foucault Tests.

The F2: I finally got around to making a new tester! This one is a *VAST* improvement over all my previous testers. The base is stained and polyurethane along with the adjustment knobs for looks and personal pride. I shined the copper bar by spinning it against a brillow pad. I put the bolts in a drill chuck and spun them against fine sand paper to smooth them. I used the same dial indicator. I also used T-nuts to put threads in the wood instead of pounding in regular nuts. For the "V" channel to ride on the bar I used two pieces of 1" V channel side by side. They hold in place (2) 4"x1" Magic Sliders (these are the BEST), the same as the tilt adjust rides on. With how everything is set up nothing "torques" or lift up, etc. It just moves buttery smooth. Because of this, the tension system is just a 7" rubber band which works perfectly over the 1.75" of travel the tester can do even though the dial indicator stops at 1". I used the same black box as my old tester which allows the LED brightness to be adjusted if needed. The red button turns the LED on and off. I tried a new way of hold the KE and Ronchi screen by using electrical wall outlet plates. I also found a 2.5X, 17mm opera monocular made by Newkon which is nice as it is low power so you don't lose contrast but gives a little power boost and then I don't need my glasses while testing. I mounted it on a wooden block for the proper height. This tester is about 1/2 the size, and 1/3 the weight of my previous one. 

I have gotten to the point where I can make a tester that works beautifully mechanically!!! I just need to sort out one last little problem. The switching of tests, it is a bit awkward to pull the LED out and then switch plates. Plus I need to be sure they stand straight up. But these issues I'm sure I can come up with a creative solution to. Anyway, this tester is GREAT.

Here's the Tester from a back-side profile.

This view is from the front. You may notice it doesn't have the "second deck" in the previous picture. I hadn't yet done final securing down with screws and I took it off between pictures for some reason. I leave it on all the time because it helps keep things stable with its weight and give a little more height so my chin has more clearance from the dial indicator.

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