FINDSP-Locate fibre spectra in an image

Description:

This routine locates spectra in a large fibre frame and produces a polynomial file. The polynomial file has a version 2 format. Version 1 format uses the coefficients of a Chebyshev series, while version 2 format uses ordinary polynomial coeffients.

The technique of this routine is to

1 Compress the data array, 2 Follow ridges from start positions by centroiding, 3 Fit a polynomial Y(X) to the centroids, 4 Write the polynomial coefficients to a text file.

The text file can be read by the applications OVERPF and POLEXT. Those applications will also be able to read text files in version 1 format.

Parameters:

IMAGE

The fibre frame - one with distorted fibre spectra equally spaced.

BLACK

The data value below which the image display is to have the background colour. The display is scaled linearly between the data values specified as BLACK and WHITE.

WHITE

The data value above which the image display is to have the foreground colour. The display is scaled linearly between the data values specified as BLACK and WHITE.

NUMFIB

The total number of fibres used in the observation, including any dud fibres.

NORDER

The order of the polynomial to be fitted along each spectrum. The default is 6 and the maximum order allowed is 10. An even order is suggested by the presence of 'barrel' distortion.

NPTS

The image is compressed in the X (wavelength) direction before the centroids are determined. This parameter fixes the number X-direction bins in the compressed frame. This parameter also by definition is the number of points along the spectrum to be used for fitting the polynomial. Choice of this parameter is a trade-off between having enough points along the spectra that the the curved spectra can be reliably followed and having enough S/N in the compressed image to determine a reliable centroid.

FWCENT

The 'full-width' of the centroiding range in the vertical direction.

CFW

To get the initial centre for the centroiding the program does a linear extrapolation from the last two centroids. The program searches out from the previously determined central centroids. In order to supress large fluctuations that sometimes occur it is necessary to have damping in the extrapolation. If fact this 'Centroid Weighting Function' parameter is the constant that the true gradient of the linear extrapolation is multiplied by to guess the next centroid. Hence a value of CFW less than 1 damps the extrapolation toward the horizontal.

YFIRST

The position of the centre of the first spectrum. This is expressed as the number of pixels up from the bottom of the image as viewed on the ARGS. Note that the default is only a guess from the size of the image.

YSEP

The average number of pixels separating each spectrum in the input image. Again the default value represents a guess.

PFILE

The file to which the results of the spectrum fitting performed by FINDSP is to be written. If no extension is specified, `.pol' is used.

ADJUST

Used to ask whether centroid start points need adjustment.

CHGPAR

Used to ask whether analysis to be repeated with changed parameters.

REJECT

The number of a fibre to be rejected.

CHGREJ

Used to ask whether the set of fibres to be rejected should be revised.

Authors:

jrl: John Lucey (AAO, Durham)

hme: Horst Meyerdierks (UoE, Starlink)