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SJAA Members Grind Mirrors at Houge Park

Ralph Seguin

SJAA Veep Bob Havner makes sure his blank has a good bevel.

Tom Whittemore demonstrates mirror blank handling for students in the SJAA ATM class.

ATM students practice adding a protective bevel to the edge of their mirror blank. Photos by Ralph Seguin.


Attendance and enthusiasm were fantastic for the first of the SJAA telescope and mirror making classes. Tom Whittemore did an excellent job of getting everybody going. He brought a number of 6", 8" and 10" mirror kits with him which quickly sold. These kits included the mirror blank (Pyrex glass), a tile coated tool (cement, or other material), and an ample supply of grit. Some people brought their own blanks or mirrors in process, including myself (I need to start polishing...).

Tom first talked of the importance of maintaining a bevel on the glass to avoid chips and damage to your blank. He then demonstrated good technique using a aluminum oxide grinding stone to put a nice 45 degree, 5mm wide bevel on the glass.

Next up was a brief discussion about how to "hog out" the center of the glass and cut down the edge of the grinding tool. He discussed sagitta, and radius of curvature, and talked of how to measure the sagitta by using coins (pennies). I have promised to bring a spherometer with a direct readout dial indicator to future classes to help in measuring sagitta.

A few ATM (Amateur Telescope Making) terms for everybody's reference:

Radius of Curvature is twice the focal length.

Sagitta is the "depth" of the concavity in the mirror.

Focal length is the distance from the mirror (or lens) to the focal point/plane.

Focal ratio (or f/number) is the focal length divided by the diameter of the objective. For example, a 6" mirror with a 48" focal length is said to be an "f/8" mirror.

The higher the f number, the "slower" a mirror is said to be (for photography only). A high f number implies a longer focal length. The lower the f number, the "faster" a mirror is (for photography only). Lower f numbers mean a smaller focal length, and thus a wider field of view, but at the expense of being harder to grind, polish and especially "figure."

Check the schedule on the first page for the next class. The more the merrier, so please come along, bring friends and bring glass to push!


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