Catalogue Parameters in the SHS
Note that for the broad-band R magnitudes from the short-red (SR) exposures, the emulsion is Tech-Pan and not IIIaF. Hence although the broad-band filter (RG630) is the same as for R magnitudes in the SSS from IIIaF plates, the emulsion is different. The emulsion sensitivity curve is slightly different and the emuslion cut-off is slightly less extended in the red. Fortunately the differences are slight and good agreement between the R-magnitudes obtained from both Tech-Pan and IIIaF are obtained and the colour term very modest.
In the galactic plane, the vast majority of images are stellar with modest numbers of true galaxies being detectable. However, due to the crowded nature of the plane fields, many stellar images are blended with 2 or more components which often leads to the Image analyser determining galaxian image parameters for these blends (this problem is also present at higher galactic latitudes but at a much reduced frequency). In light of this a decision was taken to assume all detected images in the SHS are stars for the purposes of photometric determinations. This was assumed for both the H-alpha and matching SR scans. Hence the ocassional `true' galaxy'detected in the SHS IAM data will have the wrong magnitude. However it is assumed that anyone interested in finding galaxies in the Southern Galactic Plane will use DENIS or 2MASS detections.
H-alpha R-equivalent magnitudesIn the narrow-band H-alpha images (fwhm of inteference filter is only 70Angstroms) magnitude estimates are not strictly applicable. However, for point source continuum objects it is clear that the same images are visible on both the H-alpha and matching SR and that indeed the depths are very similar. Thus for convenience of comparison between the broad-band and narrow-band images an R-equivalent magnitude was adopted for all images detected in the H-alpha exposures with values determined in the same way as if they were R broad-band magnitudes. Hence a continuum source in the H-alpha exposure would have an R-equivalent magnitude very similar to that in the broad-band case. True H-alpha emitting point sources such as be stars and T-tauri stars would have brighter R-equivalent magnitudes.
Word of caution: Unfortunately the H-alpha R-equivalent magnitudes and their broad-band counterparts will not always be sufficient to identify true H-alpha emitters. This is because many late type stars have a prominent TiO band which falls in the H-alpha band-pass. The stronger TiO bands redward of this are outside the band-pass. This can lead to elevated R-equivalent magnitudes for such sources when compared to the standard R-magnitude for the same object. When the Galactic Plane I-band survey is incorporated it will be straightforward to identify and eliminate such contaminants.
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