Synphot Data User's Guide


2.4 Encircled energy (ACS, WFC3)

2.4.1 Background

In April 2003, a new capability was introduced in synphot for use with the Advanced Camera for Surveys (ACS) imaging modes. This functionality is new to synphot and is currently only available for ACS and WFC3. Users are now able to specify a circular aperture (radius in arcsec) to calculate the source counts within this aperture. If no aperture is defined (or if a 4.0 arcsec radius is specified), synphot calculates the total number of counts in an infinite aperture.

This makes synphot more flexible and accurate, particularly in cases such as red targets that are observed at long wavelengths. At wavelengths greater than 7500 A (HRC) and about 9000 A (WFC), ACS CCD observations are affected by a red halo due to light scattered off the CCD substrate. An increasing fraction of the light as a function of wavelength is scattered from the center of the PSF into the wings. This problem affects particularly the very broad z-band F850LP filter, for which the encircled energy depends on the underlying spectral energy distribution the most.

In the currently available APT ETC, the treatment of such an effect has been ameliorated but not solved. In fact, the encircled energy fraction is at the effective wavelength which takes into account the source spectral distribution. This fraction is then multiplied by the source counts. (The effective wavelength is the weighted average of the system throughput AND source flux distribution integrated over wavelength, that is,

where P is the (dimensionless) passband throughput, and f is the source flux distribution).

However, this does not account for the variation in enclosed energy with wavelength. As a consequence, to obtain correct estimated count rates for red targets, observers are advised to use the synphot package in IRAF/STSDAS for which this proper integration over wavelength has now been incorporated for encircled energy.

To quantify this new synphot capability, we compare APT ETC results with synphot for a set of different spectral energy distributions and the observation mode WFC,F850LP. In the following table, the spectral type is listed in the first column. The fraction of light with respect to the total integrated to infinity is listed in the other two columns, for the ETC and synphot calculations respectively. These values are derived for a 0.2 arcsec radius aperture for the ETC and synphot calculations.

Sp. Type

The ETC results are off by up to 3% for these star types. If this small effect is relevant to particular observations, then the synphot software package can be used.

2.4.2 Usage

The "aper" keyword allows the user to select an aperture (radius in arcsec) and indicate this value by typing "aper#value". When calling "aper#0" the user will obtain the number of counts in the brightest pixel, i.e. the peak counts of the source centered at that pixel.

A typical obsmode would now read:

From the cl command line such an obsmode should be entered within quotes, or synphot complains:

 cl> calcphot "acs,wfc1,aper#0.2,f850lp".

GOs can always "epar" the program they want to run, or use Pyraf instead of cl. Users can also provide their own input spectrum, which must be in the same format as the user-provided spectrum for the ETC. The file should be placed in the directory from which synphot is run. Please refer to the ETC help pages for more specific information about this format.

Supported apertures depend on each instrument, and are listed below.

Arbitrary aperture sizes are also permitted but are not recommended, because synphot provides only a linear interpolation between supported apertures, which is a poor approximation, especially at small apertures.

2.4.3 Supported ACS Aperture Sizes

Currently, the following apertures are supported: every tenth of arcsec between 0. and .6 arcsec, 0.8, 1., 1.5, 2. and 4. arcsec.

2.4.4 Supported WFC3 Aperture Sizes

Currently, the following apertures are supported: every 0.05 arcsec between 0.1 and 0.3 arcsec; every tenth of arcsec between 0.3 and 0.6; 0.8, 1.0, 1.5, and 2.0 arcsec.

Space Telescope Science Institute
Voice: (410) 338-1082