The input catalog
|Fig.1 This figure shows how ConvPhot defines the overlap region between the detection and the measure image in case of different sizes and offsets, provided by the keywords CPHXOFF and CPHYOFF in the measure image. The objects whose segmentation is partly inside the overlap region are flagged bound.|
As mentioned above, ConvPhot relies on the output of the popular SExtractor code to detect and deblend the objects and to define the object position and main morphological parameters. In the following, we shall also use the SExtractor naming convention to specify several image characteristics.
The user must first obtain a reliable SExtractor catalog of the detection image prior to running ConvPhot. In practice, the required output from SExtractor consists of the SEGMENTATION image and of a few parameters: NUMBER, X_IMAGE, Y_IMAGE, XMIN_IMAGE, XMAX_IMAGE, YMIN_IMAGE, YMAX_IMAGE and BACKGROUND, LOCAL or GLOBAL. In addition to this, the user must provide the detection image and the measure with its MAP_RMS image. It is important to remind that the measure image may be shifted and/or have a different size with respect to the detection one: it is only required that the images are aligned (i.e. have been same pixel size and be registered). This may be useful when dealing with the follow-up of large HST mosaics. Fig.1 shows the case of a measure image having a different size and shifted respect to the detection one. In this case, the relative offset (in pixels) between the two images must be provided by filling two keywords (CPHXOFF and CPHYOFF) in the measure image. The measure image and its RMS must have the same size. As shown in Fig.1, ConvPhot works on objects whose segmentation lies in the overlap region between the detection and the measure image. In the detection image, objects which partly lie inside the overlap region are flagged as bound and will be handled keeping into account the fraction of total flux inside the overlap region.