Extraction Region

In older ETC’s, the group of pixels over which the signal to noise ratio was calculated was referred to as the “photometric aperture”. We found that the use of the word ‘aperture’ sometimes caused confused with instrument apertures, such as slits or the location on the detector, so in the APT ETC, we have adopted the term “extraction region”.

When using imaging with point sources, the ETC will calculate the fraction of the total flux that is contained within the user selected extraction region using tables that are functions of size and the effective wavelength. For extended sources, counts are calculated by synphot per square arc second and are then converted to per pixel and multiplied by the number of pixels. When performing spectroscopy, the fraction of counts at a particular wavelength is calculated for some rectangular area (usually 1 pixel wide) using tables using the actual wavelength and region size.

Because of the Lyot stop, the point-spread function is different when performing coronagraphic observations with the ACS so there are separate tables for coronagraphic imaging observations.

For point sources, the present ETC offers the possibility to select among a wide variety of extraction regions. The user should take into account that increasing the aperture size implies an increase in the detector and sky noise/background. In general, in a noise limited regime, the extraction region should be small. For bright object observations, for which poissonian noise from the source dominates, the apertures should be large to include most of the source’s flux.

The default aperture selected by the instrument team (see table 1) should be fine for most observations.

Table 1: Default Extraction Regions

The full set of extraction region tables are given in Appendix B.