The Photometric Performance and Calibration of theHubble Space TelescopeAdvanced Camera for Surveys

Abstract

We present the photometric calibration of the Advanced Camera for Surveys (ACS). The ACS was installed in the Hubble Space Telescope (HST) in 2002 March. It comprises three cameras: the Wide Field Channel (WFC), optimized for deep near-IR survey imaging programs; the High Resolution Channel (HRC), a high-resolution imager that fully samples the HST point-spread function (PSF) in the visible; and the Solar Blind Channel (SBC), a far-UV imager. A significant amount of data has been collected to characterize the on-orbit performance of the three channels. We give here an overview of the performance and calibration of the two CCD cameras (WFC and HRC) and a description of the best techniques for reducing ACS CCD data. The overall performance is as expected from prelaunch testing of the camera. Surprises were a better-thanpredicted sensitivity in the visible and near-IR for both the WFC and HRC and an unpredicted dip in the HRC UV response at ∼3200 A ˚. On-orbit observations of spectrophotometric standard stars have been used to revise the prelaunch estimate of the instrument response curves to best match predicted and observed count rates. Synthetic photometry has been used to determine zero points for all filters in three magnitude systems and to derive interstellar extinction values for the ACS photometric systems. Due to the CCD internal scattering of longwavelength photons, the width of the PSF increases significantly in the near-IR, and the aperture correction for photometry with near-IR filters depends on the spectral energy distribution of the source. We provide a detailed recipe to correct for the latter effect. Transformations between the ACS photometric systems and the UBVRI and WFPC2 systems are presented. In general, two sets of transformations are available: one based on the observation of two star clusters; the other on synthetic photometry. We discuss the accuracy of these transformations and their sensitivity to details of the spectra being transformed. Initial signs of detector degradation due to the HST radiative environment are already visible. We discuss the impact on the data in terms of dark rate increase, charge transfer inefficiency, and "hot" pixel population.

New cataclysmic variables and other exotic binaries in the globular cluster 47 Tucanae*

“…We obtained A(U300)=0.26 ± 0.13 and A(B390)=0.18 ± 0.09. For the optical filters we used the conversions given in Sirianni et al (2005) which result in A(B435)=0.16 ± 0.08, A(Hα)=0.10 ± 0.05, and A(R625)=0.11 ± 0.05.…”

The progenitors of the Milky Way stellar halo: big bricks favoured over little bricks

“…For consistency, we convert all magnitudes into Sloan Digital Sky Survey (SDSS) bandpasses. JohnsonCousins magnitudes are converted to gri SDSS filters using the relations in Jordi, Grebel & Ammon (2006), and Hubble Space Telescope (HST)/Advanced Camera for Surveys (ACS) filters are converted into Johnson-Cousins bandpasses using the procedure outlined in Sirianni et al (2005). The magnitudes and colours we use have been corrected for extinction following the prescription of Schlegel, Finkbeiner & Davis (1998).…”

Irradiated, colour-temperature-corrected accretion discs in ultraluminous X-ray sources

“…When converting the WFPC2 magnitudes, we used values of zero-points and photflam from the WFPC2 data handbook, along with filter wavelengths and bandwidths from the WFPC2 filter archive. 9 For the ACS data, we used zero-points from Sirianni et al (2005), as used by Gladstone et al (2013), appropriate photflam and photplam values for the observation dates calculated with the HST ACS zero-point calculator, 10 and photbw from the HST ACS data analysis web page. 11 Having been converted into units of photons cm −2 s −1 , XSPEC readable spectra were created for each HST observation using the FTOOL FLX2XSP.…”