The Rotatable Filter

There are two rotatable filter mountings which can be used to change the angle of incidence of a filter. They are typically used to scan through multiple angles. To use a rotatable filter:

1. Connect the rotating filter platform to a Newport control box.
2. Connect the Newport control, via serial cable, to the camera computer using port ttyS0 (i.e. the first serial port - other ttyIC ports do not allow hardware flow control). Note that there are two cables which are labeled for use with these filters. Do NOT use any other types of cable - the Newport uses different wiring.

The tilt of an interference filter with respect to the beam results in a shift of the transmission profile towards shorter wavelengths. Currently, a narrow (bandpass 1.1 Å) Ca II H filter is mounted in the rotatable filter holder. Tilting this filter under different angles provides the possibility to observe at various positions in the blue wing of the Ca II H line and thereby sample different heights in the solar atmosphere. The filter is specified as having center wavelength = 3968.8 Åat 0 degrees tilt angle, which is slightly redward of the line core (3968.5 å). The filter has to be tilted approximately 1.12 degrees in order to cover the line core.

To define your observing sequence you need to create a ROTF_CFG: line in the camera config file (see section 2.2). For example

ROTF_CFG:cfg/rotf.cfg

This file can be found on the control machines at ~obs/cameras/camctrl/cfg/rotf.cfg and looks like this:

---------------------------------------------------
#Configuration file for the rotating calcium filter
ROTF_TTY:/dev/ttyS0
ROTF_START:170.0
ROTF_STOP:190.0
ROTF_STEP:0.2

ROTF_SEQUENCE:179.9,181.2
#ROTF_SEQUENCE:179.9
---------------------------------------------------

This file has two parts, the ROTF_START:, ROTF_STOP: and ROTF_STEP: are calibration settings and ROTF_SEQUENCE is for running a sequence. In this case the calibration starts at a filter angle of 170$^\circ$, taking steps of 0.2$^\circ$ and stops at an angle of 190$^\circ$. The sequence that will be run are the two angles 179.9$^\circ$ and 181.2$^\circ$. To know at what angles (where in the calcium line) you like to run your sequence you must calibrate the filter (see section 7.2).

Please note that the rotf.cfg file is not used when running in focus mode.