imager.advise - Function

Advise (and optionally use) parameter values

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 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:

N_facets = N_pixels $\displaystyle \sqrt{{{\Delta \theta}\over{\sqrt{8 \delta A}}}{w_{rms}}\over{uv_{rms}}}$

(1.2)

where $\Delta$ $\theta$ is the cellsize in radians, and $\delta$ 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.

Arguments

`takeadvice`	`in`	Use the advised values?
		`Allowed:`	`Bool`
		`Default:`	`T`
`amplitudeloss`	`in`	Maximum fractional amplitude loss due to faceting
		`Allowed:`	`Float`
		`Default:`	`0.05`
`fieldofview`	`in`	Desired field of view
		`Allowed:`	`Quantity`
		`Default:`	`'1deg'`
`pixels`	`out`	Number of pixels on a side
		`Allowed:`	`Int`
`cell`	`out`	Recommended maximum cellsize
		`Allowed:`	`Quantity`
`facets`	`out`	Recommended number of facets on one axis
		`Allowed:`	`Integer`
`phasecenter`	`out`	Direction of phase center as a measure
		`Allowed:`	`MDirection`

Next: imager.approximatepsf - Function Up: imager - Tool Previous: imager - Constructor Contents Index

	`Package`	`synthesis`
	`Module`	`imager`
	`Tool`	`imager`