MAKESNR - Creates a signal-to-noise array from an NDF with defined variances

Description:

This application creates a new NDF  from an existing NDF by dividing the DATA  component of the input NDF by the square root of its VARIANCE  component. The DATA array in the output NDF thus measures the signal to noise ratio in the input NDF.

Anomalously small variance values in the input can cause very large spurious values in the output signal to noise array. To avoid this, pixels that have a variance value below a given threshold are set bad in the output NDF.

Usage:

makesnr in out minvar

Parameters:

IN = NDF (Read)

The input NDF. An error is reported if this NDF does not have a VARIANCE component.

MINVAR = _REAL (Read)

The minimum variance value to be used. Input pixels that have variance values smaller than this value will be set bad in the output. The suggested default is determined by first forming a histogram of the logarithm of the input variance values. The highest peak is then found in this histogram. The algorithm then moves down from this peak towards lower variance values until the histogram has dropped to a value equal to the square root of the peak value, or a significant minimum is encountered in the histogram. The corresponding variance value is used as the suggested default. []

OUT = NDF (Write)

The output signal to noise NDF. The VARIANCE component of this NDF will be filled with the value 1.0 (except that bad DATA values will also have bad VARIANCE values).

Examples:

makesnr m51 m51_snr

This example divides the DATA component of the NDF called m51, by the square root of its own VARIANCE component, rejecting pixels below the default MINVAR value, and writes the resulting signal-to-noise values to an NDF called m51_hires.

Implementation Status:

• This routine correctly processes the AXIS, DATA, QUALITY, LABEL, TITLE, HISTORY, WCS, and VARIANCE components of an NDF data structure and propagates all extensions.

• The DATA values in the output NDF represent dimensionless ratios, and therefore the UNITS  component is not propagated.