Drift Chase Observations

Ultra Deep Survey (AUDS)

Implementation of Drift Chase Observations

Introduction

The basic strategy for our observations is "drift & chase". The idea is to point a a few minutes ahead of the target area, stop the tracking, let the sky drift by and then point again to the original position. The results will be a short drift scan as shown in Figure 1. For a detailed justification, see the proposal.

Figure 1

Orientation of ALFA

Different sampling strategies are conceivable to map a particular area on the sky using drift scans. To reach the best possible sensitivity, AUDS chose an orientation of ALFA so that three beams are aligned at exactly the same declination. Other drift scan projects might choose a constant offset relative to this basic configuration.

The foot print of the ALFA on the sky is elliptical. The major and minor axis of this ellipse are a_min=329 and a_maj=384 arcsecs, and its orientation corresponds to the parallactic angle of the target. This is illustrated in Figures 2. The red footprint is for a parallactic angle of 0, the green one for a parallactic angle of 45 degrees without any rotation of ALFA. The observer has no control of the orientation of this ellipse.

Figure 2

To align the three central beams along constant declination, ALFA has to be rotated. The physical orientation of alfa is recorded in column ALFA_ANG of the FITS file. Projected on the sky, the individual beams move along the elliptical footprint when ALFA is rotated. In Figure 2, the green beam positions are the positions for ALFA_ANG=0 at a parallactic angle of 45 degrees. When changing ALFA_ANG, the beams will move along the green line. The beams drawn as dashed lines show the desired positions of the central three beams. Also shown is the angle by which ALFA_ANG has to be rotated to achieve this positioning. This angle is close to the negative parallactic angle, but not identical.

It can easily be shown that the desired angle for ALFA_ANG is:

y= a_min*sin(parallactic_angle-poff) x= a_maj*sqrt(1- (y/a_min)**2) ALFA_ANG=-atan(ypar/xpar)+poff+aoff where aoff and poff are fixed offset angles. The difference between ALFA_ANG and the parallactic angle for offset angles set to zero is plotted in Figure 3.

Figure 3

Position of outer Beams

The above formula for the setting of ALFA_ANG positions the central three beams at the same declination. Because of the projection effects shown in Figure 2, this will produce sky coordinates for the outer four beams which slightly differ as a function of the parallactic angle. This is illustrated in Figure 4. The black lines show relative beam position on the sky for a parallactic angle of zero degrees. The red and the green lines show the same for a parallactic angle of -45, respectively 45 degrees.

Figure 4

CIMA implementation

The "Drift and Chase" observing mode is implemented in a CIMA script which is selected in the "Spectra Line Observing" menu as "Drift Chase Map". This brings up the widget shown in Figure 5. The parameters for the procedures are explained in table 1.

A new FITS file is started when the script is invoked. It then goes through loops of drift scans. Each loops consists of the following steps:

Go to target coordinates
start tracking target
take on/off cal observations
stop tracking, start drifting
take data for one scan
go to start

A new FITS file is started after each predefined number of scans.

The setting of the angle as described above is computed within the CIMA tcl code. The code for the drift & chase widget is in /home/aoui/develop/library/driftchase.tcl, the computation of the angles is done in procedure track_alfa_para1 called by driftchase, both located in file /home/aoui/develop/exec/exec.d/wappmap.tcl.

Figure 5: Drift & Chase CIMA widget

Parameter	Explanation	AUDS value
Seconds per loop	clock time in seconds for each drift scan	100
Loops	number of drift scans	30
ALFA Offset angle (deg)	ALFA orientation on the sky	-0.5
PARA Offset angle (deg)	offset for parallactic angle	5
Offset algorithm	If set to 1, the Offset angle will be computed relative to the sky coordinates, i.e. taking into account the geometric projection as discribed abvoe. If set to 0, the offset angle will be computed without projection correction, resulting in ALFA orientation relative to the sky coordinates which depend on parallactic angle.	1
Caltype	If this parameter is set to hcal or low lcal, high respectvely low cals are fired at the beginning of each scan. Set to nocal, no cals will be fired.	hcal
Cal Secs	time the cals are on.	1
Scans after firing Cal	number of scans after each cal ON/OFF pair	1
Scans per FITSFILE	number of scans before a new fitsfiles is started.	1

Acknowledgements

We thank Mikael Lerner, Avinash Deshpande, Phil Perillat, Erik Muller and Steve Torchinsky for suggstions, discussions and help with our observations.

This page created for the AUDS group by Wolfram Freudling, and maintained by Barbara Catinella & Emmanuel Momjian

Last modified: Wed Feb 9 15:06:03 AST 2005