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Vernier Setting Circles Good vernier setting circles are invaluable on an equarorial mount and even though I intend to fit digital setting circles in the future, I couldn't build this mount without this useful feature. I wanted the scale to be easily readable so I looked at various engineer's rules and scales for examples I liked. The good ones have different lengths of index marks so the eye can see the measurement without having to count too much. I decided to have different lengths for the 1 hour scale, the 1/2 hour scale or the 1/6 hour scales. (the DEC scale was layed out similarly.) |
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I decided a lathe would be the perfect basis for scribing the scales with the addition of a pair of index plates for the accurate positioning of the spindle. I'm lucky that the company I work for has two Strippit CNC Punch Presses. These machines can punch any pattern of holes at a rapid rate to a very high accuracy. It was a simple matter to write a program to punch a plate each for R.A. and DEC. |
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A simple jig was made to fit on the back end of the lathe spindle, incorporating a pin for accurate indexing and the scales were mounted in the 3 jaw chuck and a V tool mounted sideways in the 4 way toolpost allowed accurate scribing. |
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The designations were stamped usung a set of 3mm number punches. I made a jig to hold the punches square and in position and to support and allow rotation of the scale underneath the stamps. I wrote all the designations in felt tipped pen and tripple checked everything before the first stamp was struck. I spent a nice quiet afternoon in my shed tapping away with a few CDs my only distraction I then mounted each scale in the lathe and with a few strokes of a fine file I had removed the raised areas around each stamp. Finally, all designations were filled with black enamel and buffed off. |
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The RA scale measures to 10 minutes on the primary scale and the vernier will divide into 1 minute of RA. |
The DEC scale measures to +/- 1 degree on the primary scale and the vernier will divide into +/- 5 minutes of arc. |
| Last updated - 31th December 2002 | Return to Dave's HomePage | email Dave (d a v e g @ t p g . c o m . a u) |