Calibration Sequence

In order to perform the calibration it is necessary to mount the polarimetry optics over the AO and CT mirrors (near the primary focus). The mounting is very heavy, and sits on top of some very delicate (and expensive!) instrumentation, so please take great care while installing. The installed mounting can be seen in the figure below.

Figure 3: Polarimetry Calibration Optics

This calibration mounting consists of a red filter (1), a linear polarizer (LP) (2) and a removable retarder (3). During the calibration the LP is moved in steps of 45$^\circ$, starting at 0$^\circ$, and going to 315$^\circ$ (a total of 8 angles).

Figure 4: Figure 2: Describes the two ways of changing the angle of the LP

The angle can easily, and accurately, be changed by 90$^\circ$ (in one direction) by moving the lever from position 1 to 2 (see figure 4). Making steps of 45$^\circ$ requires you to rotate the angle scale (figure 3, right). Note that the angle you set is the angle at the white tape arrow (when the lever is at position 1). The most convenient stepping procedure needing the fewest movements of the optical components is 0$^\circ$, pos. 1-2 $\rightarrow$ 90$^\circ$, rotate angle scale at pos. 1 $\rightarrow$ 45$^\circ$, pos. 1-2 $\rightarrow$ 135$^\circ$, rotate ... $\rightarrow$ 180$^\circ$, 270$^\circ$, 225$^\circ$, 315$^\circ$.

For every LP angle step you should take images in 3 camera modes:

Polarimetry Sum

Repeats 5. For taking normal images, without the retarder.

Polarimetry Sum Calib

Repeats 1. For calibration images with the retarder, which steps through 365 degrees with a step size as defined in the POLAR_CFG field of the SOUP config file (see below).

Polarimetry Sum Dark

Repeats 5. For taking dark images, with the light blocked.

Frame numbers and step size should be set based on your calibration analysis software and sensitivity needs. The table below outlines three possibilities:

Mode	Frames	Step Size (deg)	Expected Time
Long	50	5	2 hours
Medium 1	25	5	1 hour
Medium 2	50	25	1 hour
Short	25	15	40 mins

The retarder steps are set in a config pointed to by the POLAR_CFG field of the SOUP config file. The default, polar.cfg, has 74 angles of 5$^\circ$ step size. If you are in a hurry, polar_coarse.cfg can be used which has 15$^\circ$ steps. You are now ready to run the calibration.

1. Stop both AO and CT systems.
2. Point the telescope to the center of the sun.
3. Carefully install the polarimetry optics mount.
4. Make sure all cameras are in focus mode.
5. Disconnect the wide band SOUP cameras from the connection set (uncheck connection set for each).
6. For the SOUP camera, disable SOUP but enable Polarimetry.
7. Now follow the checklist in appendix E (copies should be available in the control room).