SQRTERR-Generates an error array as Error = Square Root of (Data/Const)

Description:

SQRTERR produces an error array, given a data array and a constant. Each element of the data array is calculated as the square root of the equivalent element of the data array, after division by a specified constant factor.

Parameters:

IMAGE

The name of the file containing the data for which an error array is to be caluclated. The data can have any dimensions. If there is already an error array, its contents are replaced.

FACTOR

Calculating the errors as just the square root of the data is OK only for certain types of data, such as raw data from photon counting detectors. For this type of data, FACTOR should be set to 1.0. Some other types of data, such as raw data from CCD detectors, can be handled if the data value is first divided by a constant factor - the data number per photon value (or ADU/photon value). Other types of data should not be treated using this routine, in general.

OUTPUT

The name of the resulting image. This can be the same as the input image, in which case the error array is added to the original image, or different, in which case a new file containing an error array is created.

Source comments:

 S Q R T E R R

 Sets an error array for which the value of each element is
 given by the square root of the corresponding data array
 element.  Optionally, the data array element value may be divided
 by a specified constant before the square root is taken.  The
 case where the constant is 1 is suitable for data such as
 photon-counting data is being used.  For CCDs and similar devices,
 the constant should be equal to the ADU/photon value.

 Command parameters -

 IMAGE  (Character) The name of the structure containing the image.

 FACTOR (Numeric) The value of the constant by which the data
        values are divided before the square root is taken.

 OUTPUT (Character) The name of the result of the operation.  This
        can be the same as for IMAGE.  If not, a new structure
        is created, with everything but the error array a direct
        copy of the input.
                                  KS / AAO 8th Jan 1987