MMM
Name
MMM
Purpose
Estimate the sky background in a stellar contaminated field.
Explanation
MMM assumes that contaminated sky pixel values overwhelmingly display
POSITIVE departures from the true value. Adapted from DAOPHOT
routine of the same name.
Calling Sequence
MMM, sky, [ skymod, sigma, skew, HIGHBAD = , READNOISE=, /DEBUG,
NSKY=, /INTEGER,/SILENT]
Inputs
SKY - Array or Vector containing sky values. This version of
MMM does not require SKY to be sorted beforehand. SKY
is unaltered by this program.
Optional Outputs
skymod - Scalar giving estimated mode of the sky values
SIGMA - Scalar giving standard deviation of the peak in the sky
histogram. If for some reason it is impossible to derive
skymod, then SIGMA = -1.0
SKEW - Scalar giving skewness of the peak in the sky histogram
If no output variables are supplied or if /DEBUG is set
then the values of skymod, SIGMA and SKEW will be printed.
Optional Keyword Inputs
HIGHBAD - scalar value of the (lowest) "bad" pixel level (e.g. cosmic
rays or saturated pixels) If not supplied, then there is
assumed to be no high bad pixels.
MINSKY - Integer giving mininum number of sky values to be used. MMM
will return an error if fewer sky elements are supplied.
Default = 20.
MAXITER - integer giving maximum number of iterations allowed,default=50
READNOISE - Scalar giving the read noise (or minimum noise for any
pixel). Normally, MMM determines the (robust) median by
averaging the central 20% of the sky values. In some cases
where the noise is low, and pixel values are quantized a
larger fraction may be needed. By supplying the optional
read noise parameter, MMM is better able to adjust the
fraction of pixels used to determine the median.
/INTEGER - Set this keyword if the input SKY vector only contains
discrete integer values. This keyword is only needed if the
SKY vector is of type float or double precision, but contains
only discrete integer values. (Prior to July 2004, the
equivalent of /INTEGER was set for all data types)
/DEBUG - If this keyword is set and non-zero, then additional
information is displayed at the terminal.
/SILENT - If set, then error messages will be suppressed when MMM
cannot compute a background. Sigma will still be set to -1
Optional Output Keyword
NSKY - Integer scalar giving the number of pixels actually used for the
sky computation (after outliers have been removed).
Notes
(1) Program assumes that low "bad" pixels (e.g. bad CCD columns) have
already been deleted from the SKY vector.
(2) MMM was updated in June 2004 to better match more recent versions
of DAOPHOT.
(3) Does not work well in the limit of low Poisson integer counts
(4) MMM may fail for strongly skewed distributions.
Method
The algorithm used by MMM consists of roughly two parts:
(1) The average and sigma of the sky pixels is computed. These values
are used to eliminate outliers, i.e. values with a low probability
given a Gaussian with specified average and sigma. The average
and sigma are then recomputed and the process repeated up to 20
iterations:
(2) The amount of contamination by stars is estimated by comparing the
mean and median of the remaining sky pixels. If the mean is larger
than the median then the true sky value is estimated by
3*median - 2*mean
Revision History
Adapted to IDL from 1986 version of DAOPHOT in STSDAS,
W. Landsman, STX Feb 1987
Added HIGHBAD keyword, W. Landsman January, 1991
Fixed occasional problem with integer inputs W. Landsman Feb, 1994
Avoid possible 16 bit integer overflow W. Landsman November 2001
Added READNOISE, NSKY keywords, new median computation
W. Landsman June 2004
Added INTEGER keyword W. Landsman July 2004
Improve numerical precision W. Landsman October 2004
Fewer aborts on strange input sky histograms W. Landsman October 2005
Added /SILENT keyword November 2005
Fix too many /CON keywords to MESSAGE W.L. December 2005
Fix bug introduced June 2004 removing outliers when READNOISE not set
N. Cunningham/W. Landsman January 2006
Make sure that MESSAGE never aborts W. Landsman January 2008
Add mxiter keyword and change default to 50 W. Landsman August 2011
Added MINSKY keyword W.L. December 2011