Saturday, May 25, 2013

Aiming for the Sun

When examining the early designs of "helioscopes", specifically those of Christoph Scheiner, one thing always seemed to be puzzling to me. What method did they use to align the telescope with the Sun?
These days, whenever a solar filter is employed, we tend to use the "least shadow method", that is the smallest shadow cast by the optical tube. If the shadow is fairly circular, you're close, and it then simply becomes a matter of minor adjustments. These early telescopes weren't so forgiving. For one, they had a very narrow field of view, especially the Galilean designs. You could employ the least shadow technique, but you now had the problem of sunlight hitting your screen. That makes the following image somewhat puzzling.

We know that many of these early designs, especially those of Scheiner, had a shield of sorts. In his first design, there was not just one shield but technically two; a smaller one towards the front, followed by the main shade. The main shade was approximately the same size as the "tabella", the rear of the instrument, where the "chartis" was positioned. Since this was all one instrument, all one had to do was align the shadow of the large shade onto the "tabella", and the instrument would be pretty close.

On my little experiment, I chose to try a second shade, towards the back of the instrument. Both shades are about 8" (200mm) square. 

By aligning the shadow of the forward shade onto the rear shade, we were able to aim the telescope.

Not enough contrast, but you get the idea.

In late designs, Scheiner appears to have dispensed with shades. In a more advanced version of his helioscope, the instrument is mounted on an early equatorial mount and aimed through a small hole in the ceiling through which the Sun is visible. This design is advanced yet somehow awkward.

In all the documentation, no mention is made of how the telescopes were aimed. This is purely speculative, but appears to work and truly makes using the "helioscope" a much simpler affair.
(EDIT - It appears as if the second design did have a finder of sorts. According to an article written by H.D. Curtis, "Popular Astronomy", Volume 20 (1912), "the little holes at ε and H served as a "finder."" This is a pretty simple system, using small holes, aligned up with the top of the assembly. Very clever, and definitely present. - RL)

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