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.
Follow fixed sky angle tracking
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.
You can return to the main CIMA page by clicking here.
This page is administered by Prakash Atreya
( patreya (a) naic . edu ) and was last updated on 4 December 2008.