A Revised Characterization of the WFPC2 CTE Loss

📝 Abstract

Charge-transfer loss on the Wide Field Planetary Camera 2 (WFPC2) onboard the Hubble Space Telescope is a primary source of uncertainty in stellar photometry obtained with this camera. This effect, discovered shortly after the camera was installed, has grown over time and can dim stars by several tenths of a magnitude (or even more, in particularly bad cases). The impact of CTE loss on WFPC2 stellar photometry was characterized by several studies between 1998 and 2000, but has received diminished attention since ACS became HST’s primary imager. After the failure of ACS in January 2007, WFPC2 once again became the primary imaging instrument onboard HST, restoring the importance of ensuring accurate CTE corrections. This paper re-examines the CTE loss of WFPC2, with three significant changes over previous studies. First, the present study considers calibration data obtained through 2007, thus increasing the confidence in the reliability of the CTE corrections when applied to recent observations. Second, the change in CTE loss during readout is accounted for analytically. Finally, a reanalysis of the CTE dependencies on counts, background, and observation date was made. The resulting correction is significantly more accurate than that provided in the WFPC2 Instrument Handbook (Dolphin 2002 and updates through 2004), resulting in photometry that can be enhanced by over 5% in certain circumstances.

💡 Analysis

Charge-transfer loss on the Wide Field Planetary Camera 2 (WFPC2) onboard the Hubble Space Telescope is a primary source of uncertainty in stellar photometry obtained with this camera. This effect, discovered shortly after the camera was installed, has grown over time and can dim stars by several tenths of a magnitude (or even more, in particularly bad cases). The impact of CTE loss on WFPC2 stellar photometry was characterized by several studies between 1998 and 2000, but has received diminished attention since ACS became HST’s primary imager. After the failure of ACS in January 2007, WFPC2 once again became the primary imaging instrument onboard HST, restoring the importance of ensuring accurate CTE corrections. This paper re-examines the CTE loss of WFPC2, with three significant changes over previous studies. First, the present study considers calibration data obtained through 2007, thus increasing the confidence in the reliability of the CTE corrections when applied to recent observations. Second, the change in CTE loss during readout is accounted for analytically. Finally, a reanalysis of the CTE dependencies on counts, background, and observation date was made. The resulting correction is significantly more accurate than that provided in the WFPC2 Instrument Handbook (Dolphin 2002 and updates through 2004), resulting in photometry that can be enhanced by over 5% in certain circumstances.

📄 Content

arXiv:0906.3557v1 [astro-ph.IM] 19 Jun 2009 PASP In Press A Revised Characterization of the WFPC2 CTE Loss Andrew E. Dolphin Raytheon Company, Tucson, AZ 85756 adolphin@raytheon.com ABSTRACT Charge-transfer loss on the Wide Field Planetary Camera 2 (WFPC2) on- board the Hubble Space Telescope is a primary source of uncertainty in stellar photometry obtained with this camera. This effect, discovered shortly after the camera was installed, has grown over time and can dim stars by several tenths of a magnitude (or even more, in particularly bad cases). The impact of CTE loss on WFPC2 stellar photometry was characterized by several studies between 1998 and 2000, but has received diminished attention since ACS became HST’s primary imager. After the failure of ACS in January 2007, WFPC2 once again became the primary imaging instrument onboard HST, restoring the importance of ensuring accurate CTE corrections. This paper re-examines the CTE loss of WFPC2, with three significant changes over previous studies. First, the present study considers calibration data obtained through 2007, thus increasing the confidence in the reliability of the CTE corrections when applied to recent observations. Second, the change in CTE loss during readout is accounted for analytically. Finally, a reanalysis of the CTE dependencies on counts, background, and observation date was made. The resulting correction is significantly more accurate than that provided in the WFPC2 Instrument Handbook (Dolphin 2002 and updates through 2004), result- ing in photometry that can be enhanced by over 5% in certain circumstances. Subject headings: data analysis and techniques 1. Introduction Shortly after its installation onboard the Hubble Space Telescope in 1994, the Wide Field Planetary Camera 2 (WFPC2) was observed to suffer from charge-transfer loss, a phe- nomenon in which charge is removed from a star’s image during CCD readout (Holtzman et al. – 2 – 1995a). The effect of this inefficiency is to reduce the star’s apparent brightness, especially those with large Y (parallel-read direction) values on the CCDs (and, to a lesser extent, those with large X values). An initial solution to this was to cool the camera from −76◦C to −88◦C to reduce the magnitude of the effect, and to apply a correction that scaled from linearly from zero for stars with Y = 0 to a maximum of 0.04 magnitudes for stars with Y = 800. Holtzman et al. (1995a) also noted that the CTE loss appeared to be a function of background level, and recommended no CTE correction for images with backgrounds over ∼250 electrons. This dependency of CTE on background, as well as dependencies on star brightness and observation date, were quantified by Whitmore & Heyer (1997) and updated by Whitmore (1998), who found that the CTE loss in the Y direction could be corrected by an equation with the form CTScorr = CTS × [1 + f1(CTS) × f2(BG) × f3(DATE) × Y ], where CTS is the star’s brightness on the read-out image; BG is the background level; DATE is the date of observation; and f1, f2, and f3 are used to denote functions. (Whitmore et al. used power law functions for both f1 and f2, and a linear function for f3.) In addition to reports from the WFPC2 team, characterizations of CTE loss in the refereed literature were made by Stetson (1998), Whitmore, Heyer, & Casertano (1999), Saha, Labhardt, & Prosser (2000), and Dolphin (2000a, hereafter D00). Differences in the assumed functional form of the solution and in the photometry techniques resulted in some- what different correction prescriptions. However, D00 found reasonable agreement between the corrected magnitudes being produced by each of the four. Since that time, there have been several significant issues requiring continued examina- tion of the effects of CTE loss on WFPC2 stellar photometry. Most notable, given the time dependence of CTE loss, is the question of the applicability of published CTE corrections to data being currently obtained. Especially given the increased utilization of WFPC2 dur- ing cycles 15 and 16, reinvestigation of CTE issues can greatly increase confidence in the photometry being generated. Other concerns are related to the functional form of the CTE corrections themselves, due to limited ranges of star brightness and background level of the calibration observations. Whitmore & Heyer (2002) examined the validity of existing CTE correction equations on stars significantly fainter than had been previously examined. While it was encouraging that the D00 corrections were valid when applied to stars as faint as 100 electrons (thus, the corrections are accurate for the vast majority of data used by WFPC2 science programs), for yet fainter stars these corrections appeared to overestimate the CTE loss by ∼0.2 – 3 – magnitudes. Likewise, there is the open question of the applicability of existing CTE corrections to higher background levels (specifically, those typical of deep science exposures). Early work on WFPC2 CTE loss relied primarily

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