As explained previously, in order for your lens to work well it must be dewedged. This is the process of removing the "wedge" or difference of thickness around the edge of the glass surfaces. No matter how good your glass may have been when you started, by now it will generally have some wedge from hogging out. To test for the wedge you need a "wedge tester," an easily constructed test jig. The intent of this tester is to measure the relative thickness around the piece of glass. The wedge tester consists of a flat plate, which can be made of wood, on which you attach three balls at the apexes of an equilateral triangle. You will place your lenses on top of these balls, so make the triangle just large enough for the lens to sit on comfortably. The balls can be small (1/4") ball bearings, or better yet, hemispherical furniture glides made of nylon. If ball bearings are used, then it's wise to cover their tops with cellophane tape, otherwise as the lens slide over them it can get scratched. The flint is especially prone to this. This set of balls determines the plane from which you are going to measure so make sure that the lens doesn't hit the base of the tester!
Over one of the balls you will need to position a high quality dial indicator which reads to at least 0.0005"/0.013mm. And on either side of the same ball (not too close by!) you will need to afix two "stops" against which you will push the lens and contact its edge. These stops can be vertical posts of some kind or flat pieces of steel or aluminum but they must be firmly fixed down. These two pins set the glass so that it doesn't move in and out when you are measuring the glass and thus change the readings. A model for such a wedge tester can be found in "Sky and Telescope," (June, 1987), p. 665. Newport Glass Works also has a drawing of a tool for measuring wedge in it's article on lens grinding.
The balls and the stops act to locate the lens with respect to the dial indicator, keeping the latter at a constant distance from the lens's center (assuming the lens blanks are accurately circular as they should be, and will be if you order the glass from a reputable supplier or edge it carefully yourself).
The general procedure for evaluating wedge is to set the blank on the balls, push it up against the stops, set the tip of the indicator on the lens's top near its periphery, and then slowly turn the lens while watching the arrow on the indicator rise and fall.
A simple jig can consist of an 1/8" thick steel plate 12" by 12" on which you can draw the outline of your lens blank in permanent marker. Then using 5 minute epoxy, carefully epoxy the balls or glides onto the steel at a short distance INSIDE of the lens outline. On either side of one ball at about a 1" distance (depending upon the diameter of the lenses you are making- we're doing a 6" lens here), epoxy your stops in place just OUTSIDE the line. Let the epoxy cure an hour or more. Next cover the balls with cellophane tape to prevent scoring the glass and place your lens on top of them. If you can get a standard magnetic base, two clamps, and an extra post that will accommodate your indicator, then you'll be comfortably positioned to do the testing. Otherwise, you'll need to rig up a solid support of some kind for the indicator to keep it accurately positioned over the lens. Plumbing fittings could work here. The advantage of five minute epoxy is that it cures rapidly and can easily be removed from the steel later with a putty knife and a hammer. Pliers and or razor blades will take it off the balls. As to the actual dewedging, once you've figured out how much the indicator rises and falls during a rotation of the lens, you'll also need to mark the location of the thickest area on your lens. This is where you'll direct heavier grinding.
The wedge tolerance of our Fraunhofer is as following: the crown element should show no more than 0.001"/0.025mm total thickness variation by the end of fine grinding. The flint should show no more than 0.0005"/0.013mm. If you can achieve less wedge, then so much the better.
At the 40 micron stage of grinding, you should test your lenses and make sure that their wedge is now no greater than about 0.005"/0.125mm. If it is greater, then you may want to go back to #220 Carbo to remove some of it. By the end of 40 micron grinding you'll want no more than 0.002"/ 0.050mm of wedge. After 20 micron those numbers should be halved and after 9 micron you should reach the final wedge tolerance. Hand polishing should have no effect on the wedge and is useless at reducing it.
The procedure for dewedging by hand is discussed at Texerau, chapter 10-5 ("Rough Grinding, Fine Grinding and Smoothing"). The tool should be placed over the thickest part of the lens and extra pressure should be applied there to thin that side down. Make sure you have a good bevel on the glass! It doesn't matter which lens face you choose for dewedging, but at the end of every dewedging session, you should spend an additional 20-30 minutes doing normal stroking to return the lens face to a sphere. It can be a good idea to cover the lens surface with black marker, especially over the thinnest area and keep grinding until all of the marker disappears and the grind looks totally even. You may be surprised at how long it takes to even the surface out after a dewedging session. But you MUST persist in grinding it smooth and even again or you will not be able to polish out and the lens will be useless. Don't skimp on time if you want a fine optic! Also, while you are doing this, you don't lose the radius of curvature that you have developed or you will be doing all this for nothing.
After every dewedging and smoothing session, return the optic to the tester and revaluate the residual wedge. Over time you will notice that the high spot migrates around the optic. This is normal and no cause for worry unless the wedge doesn't become smaller! Then your technique is defective and you need to reevaluate what you're doing. But usually there is no problem and gradually the wedge goes away.
Proceed down through the rest of the grits now to 9 micron keeping the channels on your lap open, dewedging and getting a good uniform grind. Take your time and do lots of wets. This will save you anguish at a later stage.
One final word of caution: at the 9 micron stage the flint can scratch pretty easily. Treat it gently and don't let the lap ever stick to it. Most scratches come from surface adhesion in the fine grinding stage and rarely from grit contamination, unless you've been really careless. Grind cleanly, clean up well between grits, wash your hands, and scrub under your fingernails and scrub the grinding laps. If you pay attention and work slowly and conscientiously, you'll finish sooner and with less anguish. Haste and fine optics don't go together!
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