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Grinding Errors

Failure to curtail the stroke length or the lateral overhang during fine grinding usually shows up in the polishing stage as a central region which refuses to polish.

Putting too much pressure on the overhanging edge of the mirror usually shows up as a hazy ring perhaps an inch or so wide from the edge which refuses to polish.

Carelessness in separating the discs during fine grinding, or using cold water between them after the back side of the mirror blank has been warmed by the hands usually results in an edge area that refuses to polish.

If we are fortunate enough to have our polishing lap poured on some surface other than our own grinding tool, these defects may easily be remedied by further grinding without re-pouring the lap.

If we were thoughtful enough to grind two mirrors alternately on the same tool, we can have a sun telescope and a night telescope which will fit on the same rocker.

Testing the Focal Length

If, for some reason, the sun is not available for testing the crude focal length or our mirror curve we may test it in the dark with a narrow beam flashlight. But in using this method we must bear in mind that the distance between the mirror and the crude image of the flashlight formed by the wet mirror face increases as the flashlight is brought closer to the mirror. A meaningful measurement is obtained, however, when the image is at the same distance from the mirror face as is the flashlight itself, namely at the cruder center of curvature. In this case the distance from the flashlight (and from the image of the flashlight) to the mirror face will be approximately twice our crude focal length. A mirror that will form a crude image of the sun at five feet will form a crude image of a flashlight at ten feet if the flashlight itself is at ten feet.


If a finished mirror, lying face up, were to be bent by downward pressure on the northern and southern edges and outward pressure on the eastern and western edges then the curve along the east-west diameter would have a slightly shortly focal length than the curve along the north-south diameter. This sort of defect is called astigmatism.

If when we read the mirror curve at the evepiece, the out of focus discs are â??out-of-roundâ??1, we may have astigmatism. If the orientation of the out-of-roundness does not rotate with the eye or with the eyepiece or with the objective then the defect is most probably in the optical alignment or the diagonal. If it is in the alignment, fix it! If it rotates with the eyepiece its in the eyepiece. If it rotates with the objective, go back to 400 grit!

There are several possible causes for astigmatism in a mirror. The release of internal strain in the mirror blank during grinding and polishing may introduce astigmatism. Or it may be introduced by grinding or polishing the mirror face up on a very flat workbench when the back of the mirror is not flat. It may also be introduced by wrong turning habits. If instead of turning the mirror considerably in on direction one turns it for awhile in one direction and then for a while in the other direction, one may expect astigmatism.

If your turning habits are bad, mend them! If you bench is too flat, put thumbtacks in it under the mirror. If your glass is under undue strain it may be annealed or replaced. In any case, the loss cannot exceed one mirror blank and the time spent working on it - which should never have exceeded one hour per inch of aperture.


One should be careful not to break one's tool or turn it over near the end of fine grinding so that the edge of the flat bottom surface grinds off the edge of the mirror blank. We call the Number 2 Boo-Boo. It may require returning to rough grinding to repair it.

Number 1 Boo-Boo is dropping the mirror and breaking it. This boo-boo becomes worse the longer you delay. If the mirror is broken, make another! If the mirror is badly chipped but not broken, the shape of the curve may be altered sufficiently to return to 320 grit.

1 Look for "out-of-roundness" by examining the discs on close to focus on either side of focus: an "out-of-round" disc will usually appear as a horizontally oriented ellipse (rather than a circle) on one side of focus, and a vertically oriented ellipse (rather than a circle) on the other side of focus.