Wednesday, August 2, 2017

Filtering and masking

A solar filter is required for scope aiming, focusing and DSLR orientation prior to the eclipse. This filter is only removed during totality or else the camera will be fried. It must be fast and easy to remove without applying much force to the setup. The perfect solution for this is the solar safety film from Baader Planetarium. This is high quality, cheap and easy to build into a filter cell of your own making. My daughter and I had a fun rainy afternoon doing this using just some carton, gaffer tape and a circular cutting tool.
Building our Solar filter using Baader film.
Another important tool is the focusing mask. The solar disc does not offer much focusing help - you basically are left with gauging the fuzziness of the sharp horns of the eclipsed Sun. By using a simple mask with two opposing holes at the edge of the main objective two distinct images of the Sun will result. Even when slightly out-of-focus these two images will not overlap. Only at precise focus do they merge into one image, see an example below using the moon. Such a mask is a great tool for achieving tack-sharp focus on eclipse day. The last focusing tweak should be done 10-15 minutes before totality, thereby minimizing the risk of focus drift.

Solar filter and focusing mask.

Focussing on the moon using the mask. When out of focus two distinct images
are formed. Only at perfect focus do the merge into one.

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