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Basic Information on imcfn
Purpose: Compute confusion noise for an interferometer image
Categories: image analysis
IMCFN computes confusion noise in an interferometer image
owing to the presence of unresolved background sources.
At each location (x0,y0) in the output image, the variance
of the confusion noise is given by the product of 2 integrals
var(x0,y0) = I1 * I2(x0,y0)
where I1 is the source integral defined by
I1 = | N(S) S**2 dS
and I2 is the beam integral defined by
I2(x0,y0) = || [SB(x-x0,y-y0) * PB(x,y)]**2 dx dy
Thus, at the desired output location (x0,y0), the synthesized
beam is shifted to the location (x0,y0), and then the product of
the shifted beam and the primary beam is formed, and then that
product squared is summed over the size of the synthesized
The user is told what the source integral value is. Thus,
you can in fact rescale the output image to other flux density
ranges just by knowing the square root of the flux integral.
The program can run without an input beam or output image
whereupon just the source integral is done. This can be useful
because the computation of the beam integral is very slow, as
for each output pixel, a sum over an image the size of the
synthesized beam must be done.
The user inputs the differential and normalized source count
function N(S)/N0(S) where S is the flux density. The units
of N(S) are counts/Jy/steradian. The user also specifies
N0, which is a cosmological normalization, usually a power
law of S. This function is input via a power law and/or
polynomial. A break flux density is given below which the
power law is used and above which the polynomial is used.
In the help file, examples are given for keyword values for
the 20cm number counts. These results come from Windhorst
et al 1993, ApJ, 405, 409
Synthesised dirty beam image (must be power of 2). No default.
Confusion noise (sigma) image (1/2 size of beam image) in Jy
3 numbers (Jy). flux(1) and flux(2) define the range of flux
densities for which the confusion noise is calculated.
flux(3) is the break point below which the power law is
used, and above which the polynomial is used. Set flux(3)
above flux(2) if you wish to use only the power law. Set
flux(3) below flux(1) if you wish to use only the polynomial.
For the 20cm counts, the break point is flux(3) = 1E-4 Jy
The polynomial coefficients are good from flux(3) to
flux(2) = 10 Jy. The power law is good from
flux(1) = 1E-5 Jy to flux(3) = 1E-4 Jy. Below 1E-5 Jy
the function is unknown, but must turn over and converge
so as not to distort the CMB spectrum.
N(S)/N0(S) specified as a polynomial of up to 5th order fit to
log10(N/N0 counts/Jy/sr) vs log10(S Jy) You input up to 6
polynomial coefficients (low to high order).
For the 20cm number counts, the polynomial coefficients are
2.519192, -0.139680, -0.302235, 0.067539, 0.046945, 0.005320
See the flux keyword for the acceptable range.
N(S)/N0(S) (counts/Jy/sr) specified as a power law
a * S**p where S is in Jy and you give a and p.
For the 20cm number counts, the power law is a = 195
and p = 0.45 See the flux keyword for the acceptable range.
The normalization factor N0 = f * S**b where S is in Jy
You give f and b.
For the 20cm number counts, f = 1.0 and b = -2.5
Plot device on which to plot the differential normalized
and un-normalized source count functions. You can specify
a plot device but not an output or beam image if you just
want to see the plots but don't want to spend ages computing
the beam integral.
Revision: 1.8, 2013/08/30 01:49:21 UTC
Generated by email@example.com on 21 Jun 2016