Finding Beams

After the number of beams to extract has been determined, it comes time to locate the beams on the detector. The spectral image first has any residual bias level removed by subtracting a multiply clipped mean, and it is then collapsed along the spectral axis to form a profile spectrum.

To find the beams, the profile spectrum is turned into a five-pixel wide image which is made up of the original profile flanked by symmetric half- and quarter-strength copies. This step is non-parametric, and can prefer faint blips over strong beams, although in practice the correct beam is found.
[_FIND_PEAKS_BY_MAKING_IMAGE]

If the number of beams found does not equal the number of beams calculated in the previous step (see Counting Beams ) then spectral extraction will not occur. If flux calibration is to be performed, then processing skips to division by standard (), if division by standard and flux calibration is necessary.

IRIS2differs in that the entire spectral image is not collapsed to form the profile. Collapsing the entire image risks producing spurious peaks due to noisy data near the edges of the array, so a profile is formed by collapsing a region 0.05 microns short and 0.15 microns long of the central wavelength.

After the beam locations have been determined they are filed with the calibration system to be used for faint sources, if necessary.

The beam detection step described here does not modify the Group file.
[_EXTRACT_FIND_ROWS_]