Extraction Regions

In older ETCs, 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 confusion with instrument apertures, such as slits or the location on the detector, so in the 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 pysynphot 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 resolution element wide) using tables indexed by the actual wavelength and region size.

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 extraction region 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 extraction regions should be large to include most of the source’s flux.

The default extraction region selected by the instrument team (see tables 1 and 2) should be fine for most observations. See Spectroscopy: Regions for Rate and Counts and Spectroscopic Plots and Tables for information about whether the extraction width or the resolution element is used in producing tables and plots for the spectroscopic modes.

Note that using extraction regions smaller than 0.1 arcsec in radius will produce incorrect results, as the enclosed energy tables do not sample below this size.

Table 1: Default Extraction Regions for Stare Modes

Instrument Imaging: Stare Mode Spectroscopy: Stare Mode
Point Extended Point Extended
ACS/HRC 80% circle [2] 2 pixel by 2 pixel box 9 pixel x 2 pixel box 2 pixel x 2 pixel box
ACS/SBC 80% circle 2x2 box 15x2 box 2x2 box
ACS/WFC 80% circle 2x2 box 5x2 box 2x2 box
COS/NUV 0.4 arcsec circle [1] 0.4 arcsec circle 8x3 box 8x3 box
COS/FUV 0.4 arcsec circle 0.4 arcsec circle 35x6 box [3] 35x6 box [3]
STIS/CCD 80% circle 2x2 box 7x2 box 2x2 box
STIS/NUV 80% circle 2x2 box 7x2 box 2x2 box
STIS/FUV 80% circle 2x2 box 7x2 box 2x2 box
WFC3/IR 0.2 arcsec circle 5x5 box 3x1 box 3x1 box
WFC3/UVIS 0.2 arcsec circle 5x5 box 5x1 box 5x1 box

[2]A circle of sufficient size to enclose 80% of the light from the source.
[3](1, 2) For G130M/1055, G130M/1096, G130M/1222, and G140L the extraction regions for point and extended sources are different from 35x6. G130M/1055 has 65x8 and 63x8 boxes for segments A and B, respectively. G130M/1096 has 63x9 and 59x9 boxes for segments A and B, respectively. G130M/1222 has 47x6 and 45x6 for FUVA and FUVB respectively. G140L/1105 and G140L/1280 have 57x6 for both segments, and G140L/800 has 30x6.

Table 2: Default Extraction Regions for Scan Modes

Instrument Imaging: Scan Mode Spectroscopy: Scan Mode
Point Point
WFC3/IR scan length x 5 pixels scan length x 1 pixel
WFC3/UVIS scan length x 5 pixels  


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