AIPS HELP file for ACTNOISE in 31DEC22
As of Tue Nov 28 22:46:02 2023
ACTNOISE: Verb to put estimate actual noise in image header
INNAME Image name (name)
INCLASS Image name (class)
INSEQ 0.0 9999.0 Image name (seq. #)
INDISK 0.0 9.0 Disk drive #
PIXSTD RMS about mean pixel value
PIXAVG Estimate of zero level in
Use: ACTNOISE is a verb to put an estimate of the actual rms of the
image into the image header. Gaussian fitting tasks (SAD, IMFIT,
JMFIT) will use any positive ACTNOISE found in the image header.
If there is none, then they use all image pixels to fit the width
of the peak in the histogram in order to estimate this noise.
Such automatic fits can have problems in the actual fitting and
will use all pixels even if that is inappropriate (e.g., images
blanked with zeros, images corrected for the primary beam, ...).
A less obvious consideration: the fit rms is for signal-free
regions, while the noise after Clean is larger on source than off.
You may use this verb to estimate the on-source uncertainty.
PIXSTD may be set with verbs IMSTAT and TVSTAT. Do not include
any real signal while doing this, since the real signal will raise
the rms , making it not represent the real noise. The task IMEAN
may be used to fit the histogram for an rms in a sub-image --- or
better still, it may be used to plot the histogram so that you may
make a proper estimate of the actual noise. IMEAN will use and
update (if DOCAT > 0) the ACTNOISE and ACTMEAN keywords.
Note that SAD, IMFIT, and JMFIT use ACTNOISE only in estimating
the uncertainties in the fit parameters. IMEAN will use ACTNOISE
to help it fit for a better estimate of the actual noise in the
sub-image it is considering.
INNAME......Image name(name). Standard defaults.
INCLASS.....Image name(class). Standard defaults.
INSEQ.......Image name(seq. #). 0 => highest.
INDISK......Disk drive # of image. 0 => any.
PIXSTD......Estimate of the actual noise in the image in the same
units as the image. If PIXSTD <= 0, then the Gaussian
fitting tasks will fit the histogram to determine the
PIXAVG......Estimate of the zero level about which the PIXSTD is
distributed in the same units as the image.