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POLISHING


The polishing procedure is simple. The polishing strokes will be virtually identical to the fine grinding strokes but we shall no longer talk about "wets". When our work becomes dry we shall simply add a tiny bit of cerium oxide and a little warm water, quickly smeared over the mirror face, and continue to polish. Our new problem will be the care of the pitch lap. It must continually be pressed to our mirror curve.

It is preferred that the polishing begin immediately once the pitch lap is firm, and continue without interruption until the mirror is bright. Convenience usually dictates otherwise. If it becomes necessary to interrupt the polishing for a short period of an hour or two, the mirror may simply be left centered on the wet lap. We shall call this "wet pressing". If it is left this way for longer periods, particularly in warm, dry weather, the mirror may stick to the lap as it dries. For longer interruptions we shall "dry press" the lap. When we must leave the polishing for several hours or more we separate the mirror from the lap and dry the mirror face with a soft, clean cloth. Allow the pitch lap to dry by evaporation, and set the mirror on to press - being careful not to slide the mirror face across the dry lap. The cleaning cloth may be allowed to accumulate cerium oxide. It may be occasionally shaken, but need not be laundered. If the mirror should stick to the lap, run cool water through the grooves and beat the mirror and lap apart with a rubber mallet or your fist.

It is important that the pitch lap be kept pressed in this way by the mirror face until the polishing and figuring are finished and the mirror is ready to be aluminized. It is also important that no foreign matter or broken pitch be allowed to stick to the face of the lap. If the foreign matter or broken pitch stick, they may become embedded in the face of the lap and raise the level of the surrounding area. These raised areas will then polish too strongly and groove the mirror face. If the lap becomes dirty it may be scrubbed with a clean brush under running water.

The polishing may be expected to take ten or fifteen minutes per inch of aperture and entails considerably more work than grinding because the glass is now harder to push. Usually the harder it pushes the faster it cuts.

It pushes harder with greater pressure and with greater contact between the mirror and the lap. If it pushes too hard, you can control it by re-grooving the lap to decrease the size of the squares, or by adding too much polishing agent. The polishing agent that actually does the polishing is embedded in the pitch and is in contact with the mirror face. Excess polishing agent serves as a lubricant and decreases contact between the mirror and the lap. We shall use this fact later in "figuring" to control the area of contact between the mirror and the lap.

When the mirror has been "dry pressed" for a period and we are ready to resume work it is advisable to soak the pitch lap in warm water for five or ten minutes before we begin. We then put a tiny bit of cerium oxide on the mirror face, quickly smear the surface with warm water, and start to polish. At first we polish very gently with very short strokes and little pressure until the mirror face has had time to warm up. Then gradually we increase the length of our strokes and the pressure. When the polishing is underway the mirror face is heated by the polishing itself.

Note: Make sure that during polishing, the grooves in the mirror do not get squeezed shut. If this happens it will be necessary to re-groove the lap. Heat the lap very gradually, first in warm water, then in hotter and hotter water until the pitch is soft enough to be re-grooved with a wet stick. When our mirror curve has been polished until the haziness left by fine grinding has virtually disappeared, we are ready to "figure" the mirror (to correct the curve toward a parabola of revolution). The deviation of our finished curve from our desired paraboloid should be held within two millionths of an inch.

Essentially, the "figuring" process consists of two very different operations, "reading" the curve and re-shaping it. First, we must be able to "read" the curve on the mirror face in order to determine the nature and magnitude of its defects. Second, we must be able to alter our polishing strokes so that we selectively polish away those portions of the mirror face that are too high.

Fortunately, there are several excellent methods for "reading" the defects in our mirror curve down to one millionth of an inch. Unfortunately, most of the methods are a little complicated and require special equipment because the measurements are made at the center of curvature of the mirror face.

A spherical curve has a center of curvature but only an approximate focus (focal point). A paraboloid has a focus, but only in approximate center of curvature. Testing methods which "read" the curve at the center of curvature have a null test for a sphere but complications must be introduced for the determination of a paraboloid. Testing methods which "read" the curve at the focus have a null test for a paraboloid. We shall test the focus.

In this booklet we shall restrict our discussion to what we consider to be the simplest and most straightforward method of "reading" our curve. We shall "read" it at the eyepiece of the telescope where we have a null test for a paraboloid. We shall "read" it under use conditions and with the entire optical train in tow.

We must get a bright star in focus at the eyepiece and then throw the star out of focus by pushing the eyepiece first one way and then the other. Compare the distribution of light in these two out-of-focus discs. If the mirror curve is a paraboloid, the out-of-focus image of the star when the eyepiece is pushed in too far will be identical to the out-of-focus image of the star when the eyepiece is pulled out too far by the same amount. If the two out-of-focus discs are not identical, we shall know by the differences in the distribution of light in those two discs what are the defects in our curve.

Any portion of the mirror which bundles too much light in the out-of-focus disc beyond focus is too flat and needs to be dug.

This method of "reading" needs no mathematics, no complicated measurements, and no extra equipment.

READING THE MIRROR CURVE

The following are examples of possible (and common) problems and how to remedy them during the final stages of polishing (figuring).

PROBLEM: Middle of mirror is too deep.

RESULTS:Bright in the middle inside of focus and dark in the middle outside of focus.

REMEDY:Polish with short strokes center over center.

PROBLEM:Middle of mirror is too shallow.

RESULTS:Dark in the middle inside of focus and bright in the middle outside of focus.

REMEDY:Hang over the edge while polishing and take long strokes. For correcting high centers it is convenient to pour a smaller lap which may be used without endangering the edge of the mirror.

A bright ring at the edge of the disc beyond focus indicates what is called a "turned down edge". The mirror is too flat near the edge (not at the edge). This defect can sometimes be remedied by going around near the edge with the thumb as a pitch lap, running the fingers around the outside edge of the glass to guide the thumb's position over the high (flat) area.

© 1976 JOHN DOBSON

CAUTION!! SUNLIGHT REFLECTED OFF THE FACE OF A TELESCOPE MIRROR CAN CAUSE BLINDNESS OR START A FIRE. TELESCOPE MIRRORS WHICH HAVE BEEN FINE GROUND OR POLISHED SHOULD BE HANDLED INDOORS OR IN THE SHADE!