ALFA Sky Angle Tracking

ALFA sky angle tracking

Rotating ALFA and taking data with ALFA is normally two mutually exclusive activities. This is not important for drift observations but poses a problem for any observing mode in which the telescope is moving while taking data, since the ALFA beams then will be rotating on the sky. Several observing modes have been dealing with that by rotating ALFA before taking the data and then do a new rotation before taking the next block of data. However, when tracking an object or when mapping a region by moving the telescope it is desirable to rotate ALFA and take data at the same time. This possibility has been implemented and on 16 April 2006, the first scientific data was taken using this new mode. The new mode is called 'sky angle tracking' and is available for all existing observing modes.

Observers who want to use sky angle tracking have to keep one important thing in mind: while tracking a source, the beams do NOT stay in a fixed position even when using sky angle tracking. The reason for this is geometry. When ALFA is projected onto the sky, the projection forms an ellipse. This ellipse changes its form depending on the azimuth and the result is that the six outer beams appear to trace out elliptical arcs on the sky. These arcs have a diameter of almost one arcminute. The following figure shows an example of how the beams move when tracking an object at Dec=+10 from rise to set with a sky angle of 19 degrees. The colored dots represents the center positions of the beams at 15 minute intervals.

This problem also affects mapping since the distance between the beams will vary during an observation. The next figure shows the distance between the beams in declination as a function of hour angle when the telescope is tracking a source with Dec=+10 from rise to set with a sky angle of 19 degrees. The colored dots represents the center positions of the beams at 15 minute intervals.

Equal beam spacing in declination

One way of dealing with the varying distances between the beams while mapping is to use 'Equal beam spacing in declination'. The following figure shows the distance between the beams in declination as a function of hour angle when the telescope is tracking a source with Dec=+10. The sky angle used in this figure is not constant but has been varied to give equal spacing between the beams at each moment. This special tracking mode has been implemented and is available for use.

The corresponding movements of the beams when using 'Equal beam spacing in declination' is shown here:

Align three beams at center declination

A third way of dealing with the varying beam positions is to keep three of the ALFA beams at the center declination. This can be useful for certain types of drifting observations. In this case, the sky angle is kept close to zero degrees but is allowed to vary by a few degrees to achieve the alignment goal. This mode was implemented in CIMA version 3.1! The plot showing the movement of the beams is here:

The track of the beams on the sky with the 'Align three beams at center declination' mode is shown here:

ALFA rotation control window

The sky angle tracking is controlled from the 'ALFA rotation control window' which You can access via the 'ALFA rotation' button in the 'Telescope pointing control menu'. The window currently looks like this:

Sky angle tracking can also be controlled inside a command file by using the commands: TRACKSKYANGLE and STOPSKYANGLE. An alternative way that is available starting with CIMA version 3.1 is to store an ALFA set-up in a configuration file (together with a signal path set-up) and then let the LOAD command apply this set-up.