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Hubble Space Telescope
CTE Photometric Correction Formula

Results from Cycle 20 Observations and Updated Coefficients

Analysis of the Cycle 20 External CTE monitor program (CAL/ACS 13155, P.I. Chiaberge) indicates that the CTE trends are continuing according to the model presented in Chiaberge, M. ACS ISR 2012-05. We derive new coefficients and provide an updated correction to be used for aperture photometry of ACS/WFC drizzled images taken after Servicing Mission 4 (May 11 2009). The updated correction allows an average photometric accuracy of better than 3% for stellar sources located at any distance from the amplifiers. In order to achieve the highest accuracy of photometric correction, we encourage use of the CTE correction Web tool (with updated coefficients) available from the STScI ACS website.

Data

Observations were taken as part of calibration program CAL/ACS 13155 (PI: M. Chiaberge). The target of the observing program is a field 7’ West off the core of the globular cluster 47 Tucanae. Two different filters (F502N and F606W) and a range of exposure times (between 30s and 400s) enable sampling of CTE losses for at least five different background levels. For a more extensive description of both the characteristics of the datasets and the method for the data analysis we refer the user to Chiaberge M., ACS ISR 2012-05. In the following we summarize the main steps and the latest results.

For each epoch, filter and exposure time combination, stars are grouped in bins of stellar flux (in e-) measured in each “pair” of images (0,0 and 0,1). Each star is assigned to a particular flux bin based on the lowest flux measured in each “pair”. In most cases six bins are obtained. A non-weighted linear fit to the data is performed and the value of Δmag2000 (Δmag for 2000 pixel transfers) is derived from the slope of the linear regression.

These measurements are used to perform a global fit and derive the coefficients pi, qi, p’i, q’i, (i=1, 2) of the following model correction

Δmag (Y, t, SKY, FLUX) =                                                                                                                           1
[p1 Log(SKY) Log(FLUX) t + p2 Log(SKY) Log(FLUX) + p’1 Log(SKY) t + q1Log(Flux) t + p’2 Log(SKY) + q2Log(FLUX) + q’1 t + q’2] * Ytran / 2000,

where t is the time the observation was executed (in modified Julian days, e.g. August 24, 2012 at 0h 0m 0s = MJD 56163.5), FLUX is the flux (in e-) measured within a 3-pixel aperture radius, SKY is the background level (in e-/pixel) measured as close as possible to the star, and Ytran is the number of parallel transfers to the serial register.

The values of the coefficients derived using data from Cycle 17 through Cycle 20 are reported in the following table.

Coefficient Slope σ Intercept σ
(1) (2) (3) (4) (5)
p p1 = 3.090 x 10-5 8.276 x 10-6 p2 = -1.584 0.461
q q1 = -5.119 x 10-5 8.352 x 10-6 q2 = 2.614 0.465
p' p'1 = -1.275 x 10-4 2.704 x 10-5 p'2 = 6.533 1.505
q' q'1 = 2.330 x 10-4 2.629 x 10-5 q'2 = -11.92 1.463

Expected accuracy

The updated formula was tested for different levels of background and stellar flux. Stars in the field range from ~100 to ~100,000e- (measured within 3-pixel aperture radius), depending on the exposure time. The global accuracy is better than 3% for all background levels. Note that the global accuracy is measured averaging out all stars in the calibration field. In Fig. 1, we show the magnitude loss for 2000 parallel transfers (Y axis of the detector), for stars of different fluxes. The average sky level of 41 e- was obtained with an exposure time of 400s and the F606W filter. This corresponds to a typical background level for science images. The black points are the losses measured when no CTE correction is performed (from photometry performed on DRZ files). The red points are obtained by correcting the photometry of stars on the DRZ files with the photometric correction formula. The blue points are derived using photometry on DRC files, i.e. corrected using the pixel-based CTE correction currently available in the MAST pipeline. Note that at the time of this analysis, the pixel-based CTE algorithm slightly overcorrects for CTE losses.

Fig1. Magnitude loss for 2000 parallel transfers (Y axis of the detector), for stars of different fluxes. The black points are the losses measured when no CTE correction is performed (from photometry performed on DRZ files). The red points are obtained by correcting the photometry of stars on the DRZ files with the photometric correction formula. The blue points are derived using photometry on DRC files (i.e. corrected using the pixel-­based CTE correction currently available in the MAST pipeline.