Synopsis
advise(takeadvice, amplitudeloss, fieldofview, pixels, cell, facets, phasecenter)
Description
Advise on recommended values of certain parameters. Return these
values and optionally use them in Qimager.
The calculations are performed as following:
- cell
- The maximum uv distance in wavelength is found and then half of the
inverse is taken as the maximum cellsize allowed.
- pixels
- The field of view is converted to a number of pixels
using the calculated cell size.
- facets
- The number of facets on an axis is calculated in two
different ways. The first method simply requires that the peeling of
facets away from the celestial sphere should not cause an amplitude
drop of more than the argument amplitudeloss. The positions may
be incorrect, but all the sources will be removed correctly. The
second method requires that the source positions be accurate to the
same fraction of the beam specified by amplitudeloss. The
second calculates the second moment in w and in uv distance and
chooses the number of facets correspondingly. The first method does
the same but after fitting a plane to the sampling:
w = au + bv.
For an approximately coplanar array, the positions may be wrong but
the removal of sidelobes will be accurate. The number of facets
returned is the second, usually smaller, number. The formula used
is:
Nfacets = Npixels
|
(1.24) |
where
is the cellsize in radians, and A is
the amplitude loss. This formula can be derived from (a) the peeling
of facets from the celestial sphere, and (b) a quadratic approximation
for the beam size both in the plane of the sky and along the w axis.