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George,

Span symmetry (2 TAN) is performed by eye using the central reticle within the SLR view finder. Each data point image is a best fit selected from a clutch of images taken over a ten second period or so. The data is expected to follow the 2TAN function but a linear approximation is simpler and sufficient. There hasn't been an asymmetrical effect observed so far and both upper and lower limb observations are consistently useable for each image. Where inconsistencies occur are in the editing process. The editor can reduce these inconsistencies by always measuring the same way. To make the editing procedure practical while underway a cropping tool is used. The cropping tool displays a line that is placed tangent to the limb and horizon. The cropped image width is then the altitude in pixels. This method is easier on the eyes when bouncing about below decks. Your concerns for underway images are warranted for the 200mm lens which sometimes blurs. The 100mm works ok. The 50mm works no matter how bad the conditions. Most 50mm opportunities are taken at 1/4000 of a second which freezes hand and vessel motion completely. Metal sextant results are achievable as long as the F stop is kept at F22 and the focus at infinity. Film sensitivity can vary between ISO 100 and 800. Shutter speed should be as fast as possible. 1/4000s is recommended but 1/1000s will work. Don't think that camera CN can be done in automatic mode. Automatic mode constantly changes settings to achieve an ideal exposure. Camera CN falls outside of the idea exposure range requiring manual mode. Also recommended is black and white mode for better contrast of the image.

The next time I'm in a gale with a camera CN opportunity I'll snap off a few shots for you George. I don't do such things on purpose anymore. A sign of getting older.

Greg Rudzinski
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