kai.mtf.mmm

Functions

mmm(sky_vector[, highbad, readnoise, integer, debug, ...])

Estimate the sky background in a stellar contaminated field.

Module Contents

kai.mtf.mmm.mmm(sky_vector, highbad=None, readnoise=None, integer=False, debug=False, silent=False)

Estimate the sky background in a stellar contaminated field.

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.

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 from Python to IDL – J. R. Lu 2010-05-30