Starting the Grinding Process

Make sure that you know which glass is which! The supplier will probably mark the packaging, but if you lose track, remember that the flint will be noticeably heavier than the crown. Otherwise they should look identical. You might want to mark the sides of the disks with the glass types in permanent black marker or an abrasive wheel in a Dremel Tool (do this very lightly!) to avoid confusion. Along with the disks you'll receive documentation indicating the types and properties of these glasses. Keep the documentation. If you have raytracing software, it's easily possible to slightly adjust the radii which I specified earlier in order to accommodate the refractive indices of your actual disks. These will differ somewhat from the catalog values, if you know them. Chances are good that the differences will be trivial. Index variations of even 0.00100, though large, will not seriously degrade the performance, because achromats are so forgiving. Later on, I'll tell you how you can tweak the performance of your lens to improve it, if the indices or you fabrication have differed materially from the design given earlier.

If you've decided to keep the disks, as is probable, then the first thing to do is to carefully bevel the edges as you'd do to a mirror blank to prevent them from chipping. Use a fine carborundum stone, whetstone, or a diamond abrasive pad if you have one and water. Fine aluminum oxide sand paper on a flat backer may also be useful. BE CAREFUL! Crown and especially flint glass are much softer than Pyrex. The disks will be supplied with sharp, unbevelled edges and are easily chipped. It won't take much pressure to start the beveling operation. Be patient and work slowly. You don't need to beat on this glass anywhere near as hard as pyrex to get it to do things. And DON'T run very cold or hot water on the glass for ANY REASON. This is NOT a low expansion glass, like Pyrex, and drastic changes in temperature could crack it, especially the flint glass.

Now is the time to check the diameters carefully. A machinist's caliper is very useful here, though a wood caliper might work too. The disks need to have the same diameter to within about 0.003," otherwise there is a risk that you'll end up with visible lateral color in the star images as the one lens shifts with relation to the other lens. Lateral color, as opposed to longitudinal chromatic aberration, smears star images into short spectra side to side. The same error will arise if you do not keep within the wedge requirement. "Wedge," a construction error which I've mentioned before, is the unevenness of thickness around the periphery of a lens. If a lens measures, say, 1.000" thick on one side and 1.020" (or some such) on the opposite side (i.e. 180 degrees around its periphery), then the lens will act like a slight prism and smear stars into short spectra. The object of wedge-testing and de-wedging is to remove this difference of thickness, so that the lens is equally thick all the way around its periphery. The amount of wedge from the glassmaker is not really specified other than not being obnoxious, is not precisely controlled when the curves are placed on the glass. It may vary from 0 to several thousandths of an inch easily.

Another way to think about wedge error is that it causes one or more surfaces of your lens to be tilted with respect to the others. If, for example, your crown element has the wedge, then in effect the first surface of your crown is tilted with respect to all the others, since the flint sits squarely on the bottom of the lens cell and R2 of the crown sits squarely on the flint. If, on the other hand, the flint is wedged, then R4 still sits square, but R1, R2, and R3 lie tilted on it. Refractive surfaces that lie tilted with respect to one another act like prisms that are included in the shape of the lens.

In the same way, if your crown and flint disks come in different diameters, then one or the other of them will likely lie in the cell slightly askew with respect to the other, making the centers of the crown and the flint not always concentric with each other. The effect of this is to tilt one or more lens surfaces slightly, which will again cause lateral color. If you would rather not deal with the differing diameters by grinding one down, you will then need to make the lens cell so that both lenses have the same clearance, either by putting something around the smaller lens or by stepping the cell for the smaller glass. don't forget that the smaller lens needs to go into the cell first if you do this and the step can't touch the larger diameter glass! This may mean that the crown goes into the cell first if it is the smaller lens so take care.

So, as a result, you must make sure that your lenses are edged to within a few thousandths of an inch of one another in diameter. If you have ordered them to be edged this way, then you should have no problems. But if you are using undocumented glass, then you must find way to edge them. Local optical shops should be able to do this. If not, then I suggest you follow Texereau's hints on edging in chapter 2-8 ("Preparing the Mirror Disk"), chapter 8-6 ("Edging") and chapter 10-3 ("Cutting the Central Hole and Edging"). Improvise with whatever materials you can get hold of. No optical methods are sacred. Just keep clearly in mind what you're trying to achieve, and don't work in a dangerous fashion.

If you only have a wood caliper, then use it to figure out which disk is the larger. Next, set the caliper to measure this and transfer it to the smaller disk. If you can pass more than a single sheet of writing paper or a postage stamp between the caliper and the disk when the caliper is positioned across the disk's diameter, then the difference in size between the disks is probably too great. In this case Texereau's method in chapter 2-8 could be applied with the least bother to the larger disk. 20 micron grit should quickly bring it down to size. Aluminum flashing, sold in hardware stores, can be used in place of his zinc strip as the exact metal is really unimportant to the job - Texerau found that zinc coated steel readily at hand and thus he used it.

If you find it necessary to do more extensive edging, then it is best to edge both disks at the same time. To do this you must temporarily join the disks together carefully. One way to do that is to wax or pitch the disks together. Gently heat them for a prolonged period in an oven at the lowest heat possible (it is helpful to level the crown disk, if possible before starting). Once they are quite warm but not really hot (about 170F), then open the oven and place a large number of small chips of paraffin, or candle wax, or medium hard pitch on top of each disk and close the oven door. You'll want to have lined the bottom of the oven with heavy duty aluminum foil or an old tray to catch any drips so that they don't fall on the electric filament and make acrid smoke. It is also recommended that you handle the hot glass with oven mitts or your hand may pull enough heat from the glass to make it shatter from the thermal differences! You may also stupidly drop the glass because it is too hot to handle.

Once the wax or pitch begins to melt turn off the oven and carefully grasp the flint disk and invert it on the crown (you may/will need gloves for this). Placing a suitable weight on top of the flint can accelerate the process, but be careful, make sure any weight is also warmed at the same time. Now close the oven and keep an eye on the flint. You don't want it to slide off the crown, but to come down gradually on top of it. Blocks to hold it in place are always nice but remember to have them in the oven when you warm things up. The wax or pitch should spread out and make an even, thin layer. Excess will drip out onto your foil or tray. Keep the flint well centered on the crown and let them gradually cool off in the oven, you can use some pieces of tape to keep them properly located - just don't block off the ability of the wax or pitch to leak out. Heating and cooling the disks in this way doesn't pose much risk. Just be sure to let them heat slowly and don't cool them off suddenly.

Gradually, the mass will cool in the oven. When it has returned to room temperature, you should remove the glass and wash it in cool water. Take a sharp single-edged razor blade and cut or scrape off the excess wax or pitch from the outside surfaces. Try to keep the excess from going down your drain. Also check to see that the two pieces of glass are essentially concentric with each other. Gradually rewarm and move if necessary. Remember that the glass doesn't like to have rapid changes in temperature so be careful!

Now you're ready to do the edging. When you finish, you can reverse the process of bonding by heating the mass slowly in the oven until you can slide the flint off the crown. Remember to cool them slowly in the oven after you separate them.