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imager.advise - Function

2.3.1 Advise (and optionally use) parameter values


Description

Advise on recommended values of certain parameters. Return these values and optionally use them in Imager.

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:
                  ∘ -----------
                     √Δθ-wxrms
N xfacets = Nxpixels   -8δA--------
                      uvxrms
(2.2)

where Δθ is the cellsize in radians, and δA 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.

Arguments





Outputs

pixels

Number of pixels on a side

allowed:

int

Default:

cell

Recommended maximum cellsize

allowed:

record

Default:

facets

Recommended number of facets on one axis

allowed:

int

Default:

phasecenter

Direction of phase center as a measure

allowed:

string

Default:

Inputs

takeadvice

Use the advised values?

allowed:

bool

Default:

true

amplitudeloss

Maximum fractional amplitude loss due to faceting

allowed:

double

Default:

0.05

fieldofview

Desired field of view

allowed:

any

Default:

variant 1.0deg

Returns
bool
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