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For reference, we first remind the observing strategy advised in the HERA
manual. Two main points will help us explain the behavior on Polaris.
- The array is rotated by a given angle that ensures Nyquist sampling
and optimal sky coverage in a single observation (i.e., each portion of
the sky is observed only by one of the nine pixels during a scan). This
requires two contiguous subscans.
- However, the observing strategy usually set up at Pico Veleta samples
regularly the projected sky and not the sky itself as naive
users tries to obtained rectangular projected maps. Moreover the HERA
receiver array is a perfect square in focal plane and on the sky, but
not in the projected map!
Under observer-friendly conditions, namely relatively small maps
near the projection center and maps near the equator, these rules
deliver the expected result. However, if those conditions are not met
(large field of view and the source is located at high declination), the
sky and the projected map will both be incorrectly sampled!
Maps near Polaris (declination 87:42:04.6) where observed during the
project 219-07 (PI: P.Hily-Blant). The maps extend typically from (0,0) to
(-1500,2000) arcsec (projected offsets). We focus here on the scans 26 and
40 covering the projection center, and scans 98 and 115 observed at the
largest distance from the projection center.
Figure
shows the scans observed near the projection
center. They do not show any particular effect visible by eye. The array
receiver is still square (rotated by 9.6 degrees3.5) on the projected map, and the coverage of the 2 scans is as
expected. Figure
shows the scan coverages in spherical
coordinates (i.e., without projection). The spherical sky is correctly sampled.
Figure:
Scans 26 (top) and 40 (bottom) relative coordinates
(radio projection) of project 219-07 observing Polaris
(declination 87:42:04.6). The observing strategy is the usual one,
i.e. there were 2 OTF subscans (red: first subscan, black: second
subscan), with a 9.6 degrees derotator angle, and a slight shift
from one subscan to another to fill the gaps. These 2 scans cover
the (0,0) reference position.
|
Figure:
Same as Fig.
(scans near the projection
center) but showing absolute coordinates. In this case, the
uniform mapping of the projected map results in uniform mapping of
the sky.
|
On the other hand, the scans observed far away from the projection
center
are highly affected by the projection
distortions. The projected receiver array is not square anymore. It is
parallelogram-shaped in the projected map. As a consequence, the rows
scanned in declination are shifted. Figure
shows
that the vertical scanning is not correctly sampled in the projected
sky. This can be understood by looking at the scan in absolute
coordinates (Fig.
): The scan does not follow a
South-North direction suited for the 9.6 degrees derotator angle on
the sky. To first order, the angle should have been different (the
exact value is not computed here)3.6. On the other hand, the scan along the right
ascension is correctly sampled, but the start and end points of the
rows are actually shifted in projected coordinates compared to the
scan 26 near the projection center. This leads to different edge
effects in right ascension according to the declination of the source
center.
Figure:
Same as Fig.
but showing scans 98 (top)
and 115 (bottom) relative coordinates (radio
projection). The same observing strategy is used. These 2 scans
cover the (-1500,2000) offset position, far from the reference
position. Note parallelogram shape of the receiver array in the
projected map, and as a consequence the unexpected coverage of the
vertical scan.
|
Figure:
Same as Fig.
(scans far from the
projection center) but showing absolute coordinates. In this
case, the attempt of uniform mapping of the projected map results
in unexpected mapping of the sky. In particular, the 9.6 degrees
derotator angle on the sky is obviously not suited here.
|
Next: Guessing ON and OFF
Up: Offsets of multi-pixel receiver
Previous: Description of the radio
Contents
Gildas manager
2023-06-01