Proposed Observations
However, the number of calibrators in declination zones below
-30° drops by a factor of 4 as shown in Figure 1 and table 1.
The probability of not finding a calibrator within 3° from the
target is only 6% in northern hemisphere, but increases to 25% at
the declination zone [-30°, -40°] and 58\% at declinations
below -40°.
Table 1. Probability to find a calibrator in the disk of the specified
radius at any given direction in various declination zones.
| Zone | 2° | 3° | 4° |
| [-20°, +90°] | 70.2% | 94.1% | 99.5% |
| [-30°, -20°] | 64.6% | 90.7% | 98.5% |
| [-40°, -30°] | 45.1% | 75.1% | 91.6% |
| [-52°, -40°] | 21.5% | 42.3% | 61.2% |
| [-90°, -52°] | 21.3% | 41.4% | 61.1% |
Figure 1. Calibrator source density per steradian as a function
in various declination zones.
There is a need to extend the pool of calibrators further to the southern
hemisphere:
- VLBI observations with the southern VLBI network LBA need calibrators
for phase-referencing observations of weak sources. One of the
important applications will be follow-up observations of
γ-ray sources that will be detected with the GLAST mission.
Many of them will be weak at radio frequencies and will require
a nearby phase calibrator for detection.
- ALMA, as well as the SKA precursors in Australia and South Africa,
needs 50--100 strong compact calibrators over the sky. These primary
calibrators will be used for determination of ALMA antenna positions.
These primary calibrators will be used for the determination of ALMA
antenna positions (and similar astrometric calibrations for SKA) to
the specified accuracy. the positional accuracy needed for the
calibrators is about 1-mas level.
- Since it is difficult to predict which sources observed at S/X
band will be sufficiently bright or compact 300 GHz, the frequency
that ALMA to be used for most accurate astrometric observations, or
at < 1.6 GHz for the SKA, the number of candidate primary
calibrators found at S/X band should be at least 200.
-
ALMA and SKA will need several thousands secondary calibrator
sources for the separation of a target to a secondary calibrator
to be less than 2—3°. The grid of secondary calibrator
will be also used for differential astrometry,and their positions
should be known with accuracy better of a few mas in order to
obtain accurate absolute positions and to remove contribution
of second order phase effects. Thus, the position of these
secondary calibrators must be accurately tied to the global
set of primary calibrators already cataloged.
Recognizing these challenges, a program for a search of calibrators with
the LBA in the declination zone [-90°, -50°] has been
submitted to the LBA, and the first experiment is scheduled at
2008.02.05 (Refert to more details
)