Overview and Background

I intend to create an image of polarisation in the solar corona during the upcoming total solar eclipse. This requires installation of a large polariser in front of the primary optics, which must be set up in a way that can polarisation can be selected during the imaging sequence at totality.

I designed this adapter to allow me to install a Lee 105mm linear polariser on my APM 115/805 telescope. The FP105 ring can be screwed directly onto the adapter which is then a press fit into the lens cell. The image shows the telescope with adapter (blue) then ring and polariser installed.

There are 4 pry slots at 90 degrees to one another and 4 index slots at 45, 135, 225 and 315 degrees. These allow the user to orient the polariser reasonably accurately.

Note that the installation of a polariser will increase the T-stop of a telescope by about 1.6 stops. Although I will operate my f/7 refractor telescope at f/5.2, the polariser will work as an aperture stop and also absorb light (roughly 1.6 stops.) The effective T-stop is then T/7.6. Exposure calculations must take this into account.

Lesson Plan and Activity

Using the device for corona polarimetry

I assume that the telescope is mounted on an equatorial platform which is tracking at the Solar rate.

Orient the polariser north-south then collect flat field images.
Ensure that the primary solar filter is closed.
Collect dark frame and bias frame images.
Place the camera under computer control (using something like Eclipse Maestro).
Point the telescope at Sol.
Collect the image sequence during the partial phase (from C1-C2).
After totality commences, open the primary solar filter.
Half way through totality. rotate the polarisation filter through 90 degrees.
Close the primary solar filter before the end of totality.
Continue with the partial phase images until the end of the eclipse. (C3-C4).
Collect a second set of flat field images corresponding to the second polarisation orientation.
Download and process the images as is standard for astronomical images. It may be useful to map the NS polarisation to (say) red, EW polarisation to (say) green and common to white.

Materials Needed

Mine was made from PLA - approx 26 grammes. The approximate size is 122mm (dia) x 12mm high.

Telescope adapter for Lee Filter Ring