Using a Plumb Line To Indicate The Lens Aperature Position

On the last set of local noon observations I had to estimate the position of the lens aperature after the fact. Today while setting things up I used a plumb line to help determine the aperature position (see image). The simplest arrangement I could come up with was placing a rubber band around the lens holder and attaching the plumb line with a bent paper clip. The plumb line had a slip knot to permit the adjustment of the string's length. One can tell if the paper clip is positioned in the middle of the aperature by sighting along it through the lens. One moves one's head sideways until the entire paper clip appears to be in the same direction and does the same moving forwards and backwards. If the paperclip is not centered in the aperature then one has to adjust its position on the rubber band to center it properly. The lens magnifies distance slightly so only small corrections are necessary. One should also sight along the rubber band on both sides of the lens holder to make sure that it is lined up with the string. The plumb line should swing freely and be slightly above the graph paper to reduce paralax. To stop the plumb line from swinging one can clamp it gently between two fingers to dampen out the oscillations and then release it by slowly moving one's fingers sideways and it will stay fixed at the position beneath the aperature. Sighting from the north and east allows one to determine the coordinates, (λ,μ), of the lens aperature position.


I did a set of solstice observations of the Sun's track about local noon today but it will take a little while to "reduce" the data. The image size used was 14 megapixels which resulted in the 1 inch grids being about twice their actual size in MSPaint. The finer lines on the grid turned out much clearer.