STScI Logo

Hubble Space Telescope
CTE Photometric Correction Formula

Results from Cycle 21 and 22 and updated coefficients

Analysis of data from the Cycle 21 and 22 External CTE monitor program (CAL/ACS 13592 and 13955, P.I. Chiaberge) show 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. 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, we encourage users to utilize the CTE correction web-tool available in the ACS team website at STScI to correct photometry.

Data

The target of the calibration program is a field ~7’ West off the core of the globular cluster 47 Tucanae. Images are taken using two different filters (F502N and F606W) and with a range of exposure times (between 30s and 400s), in order to sample 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 measured as close as possible to the star, and Ytran is the number of transfers.

The CTE correction web-tool offers the option of using a 5-pixel aperture radius. Note that such a correction is still in beta version, since it is only based on 3 epochs.

The values of the coefficients (for 3 pixel aperture radius) derived using data from Cycle 17 through Cycle 22 are reported in the following Table.

Coefficient σ Coefficient σ
(1) (2) (3) (4)
p1 = 1.501 x 10-5 4.326 x 10-6 p2 = -0.703 0.243
q1 = -2.805 x 10-5 4.581 x 10-6 q2 = 1.327 0.257
p'1 = -5.893 x 10-5 1.393 x 10-5 p'2 = 2.732 0.783
q'1 = 1.465 x 10-4 1.419 x 10-5 q'2 = -7.108 0.796

We encourage users to utilize the web interface to correct their data.

For a CTE correction cookbook, please refer to Chiaberge, M. ACS/ISR 2012-05.

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 40 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.

The formal accuracy of the beta correction for photometry performed using a 5-pixel aperture radius is also better than 3%. However, since the parameters were derived using data from three cycles only, it should be used with some care.

Fig1. Magnitude loss for 2000 parallel transfers (Y axis of the detector), for stars of different fluxes. Photometry with 3 and 5-pixel radius is shown in the left and right panel, respectively. 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. Data are from Cycle 22 (observations performed in November 2014).