Below is a photograph of the two spherometers used in the construction of the Maksutov-Cassegrain (Scale is in inches). The large three footed spherometer on the right was used for the corrector lens, it is seated on an 8" diameter x 3/4" thick plate glass flat used for zeroing the readings. The small ring spherometer on the left uses a 0.001" reading dial gauge (the same gauge used in the wedge control device); it was used in making the secondary mirror. The three conical soft steel posts in the bottom left corner were used when measuring the convex side of the corrector lens in the rough grinding stages. The shorter posts for measuring the concave lens surface (obscured by the spherometer arms) are of similar construction. The spherometer arms were cut from a single piece of 3/8" thick aluminum. The carbide tip of the micrometer head was capped with a 45o tapered point.
When taking a reading with this spherometer the micrometer barrel was very gently turned with one hand, while one of the spherometer arms was lightly rocked between the fingers of the free hand. Using this technique it was possible to consistently judge when the micrometer tip made contact with the surface by finding the position where the spherometer just began to rotate on the micrometer tip. For this reason, when using a "micrometer style" spherometer, it is best that the spherometer be as light as possible (without introducing arm flexure). If a dial gauge is used, then a heavy base is probably preferable. The spherometer was zeroed on the flat before each measurement. Repeatability of the zero measurement at each reading was ±0.0001" (in fact, the reading never varied by more than 0.0001").
NOTE: This does not mean that the absolute value of the depth measurements were of 0.0001" accuracy, only that readings were repeatable to this precision. However I did make the following observation concerning the absolute accuracy of my depth of curve measurement. During the grinding, the depth of the center of the lens was calculated by finding the thickness of the lens somewhere near the edge and the measured radii of curvature of both sides of the lens. Once fine grinding was completed, the thickness of the lens measured in this way was found to be 0.749 ± 0.002". Later, when the central plug was removed, a direct micrometer reading of the thickness gave 0.749 ± 0.0001". This agreement strongly suggests that the absolute value of the sagitta measurements were in fact close to the repeatable reading precision of the micrometer (about ±0.0001").
For more useful information about spherometry, (i.e. how to correctly interpret the results) follow this link.
Return to Maksutov Page...