Mitigation of CTE Losses in ACS/WFC:
Optimal Background Parameters From
Simulated Images!
Roberto J. AVILA, Jay ANDERSON, and the ACS Team!
Space Telescope Science Institute, Baltimore MD!
Putting the electrons back where they belong!
ABSTRACT!
The Advanced Camera for Surveys (ACS) team has been exploring ways to further mitigate the effects of charge
transfer inefficiency (CTI) on the wide field channel (WFC), in particular at low background levels where losses are so
large that the current methods cannot recover the original signal. Using post flash increases the background levels and
mitigates losses, although this also increases the effective background noise. We simulated images to represent typical
astronomical scenes with various levels of post flash and explored the relationship between signal preserved and noise
added. From this we present guidelines for guest observers on how best to optimize signal to noise ratios in their
observations. !
INTRODUCTION!
!Over the last few years the HST/ACS Team has explored
ways to alleviate the problem of CTE losses that CCDs
experience in space due to harsh radiation conditions. In June
2012 the team released CALACS 2012.2, the new image
calibration pipeline that delivers products with CTE correction
(amongst other corrections) using the Anderson & Bedin
(2010) method. Work has also continued on improving the
CTE Photomotric Correction Formula (Chiaberge, 2012).!
! CTE losses are more severe when the background is
low (Anderson & Bedin, 2010; Chiaberge, 2012; Avila et al.,
2011) because signal packets have a higher chance of being
caught in charge traps and effectively vanish during readout.
This is detrimental to short exposures, UV images where the
background is low, and images taken with narrow band filters.
The white paper by MacKenty & Smith (2012) discusses
further methods that can be employed to alleviate CTE losses.
For ACS/WFC in particular they recommend increasing the
background. By increasing the background, the chances of a
signal packet getting caught in a charge trap is reduced
because the charges in front of it fill those traps. When
observing with ACS/WFC the background of an image can be
increased by taking a longer exposure, using a broader filter,
or by using the post flash capabilities of the instrument. The
post flash method uses an internal LED lamp which
illuminates the CCD and floods it with extra electrons. !
! This study is an extension of our study of simulated
images with low background levels (Avila et al, 2011). We
explore how photometry of point sources is affected by CTE at
high background levels. !
! In addition to alleviating the losses, the increased
background level helps the correction algorithm work better.
The green points in figure 1 show the photometry for stars
after CTE correction has been applied. At the highest
background level the ratio between the input and output flux
remains within 10% down to a few hundred counts. !
!Figure 2 shows the signal to noise ratio (left) and the
magnitude uncertainty (right) for CTE corrected and
uncorrected photometry. The left panels show that even
though there is an increase in noise due to the higher
background, at high backgrounds the preservation of the
signal increases quickly enough that the hit to signal to noise
is not too severe. !
CONCLUSION!
! The problem of CTE degradation in space bound
CCDs will never go away, but new methods are continually
being developed to help alleviate the problem. Now, in
addition to using a photometric correction formula, or an
image correction algorithm, we recommend increasing the
background in images by at least ~100 electrons in order to
reduce CTE losses of points sources. This, in combination
with the other established methods ensures that even faint
point sources retain enough signal for good scientific use.!
Figure 1 – Results from 0.1” aperture photometry. Left column shows results
in terms of instrumental magnitudes. Right column is in counts. Each row
corresponds to a different background level. Green points are for simulated
FLC images and red points for FLT images.
!The result of each simulation is three images: one that
simulates an observation where the CCD has 100% CTE,
one with the observed CTE losses (FLT images), and one
with CTE correction (FLC images). These are used to
measure how much CTE degrades the photometry, and how
well the correction algorithm recovers the true flux of the
stars.!
IMAGES!
RESULTS!
!We created images with 5 different background levels:
0, 25, 50, 100, and 200 electrons. The images mimic the
process of image acquisition, CTE loss, and CCD readout.
We ignore bias level subtraction and flat fielding since these
steps have no impact on the amount of CTE loss. We pay
close attention to the points in the process where noise and
CTE losses are introduced. !
!
•  Each image contains 4640 stars with brightness ranging
from -14 to -2 instrumental magnitudes. !
•  The desired background is added.!
•  Poisson noise is added.!
•  CCD readout is simulated with and without CTE losses.!
•  Read noise is added.!
!
!Figure 1 shows the result of the aperture photometry in
terms of both input instrumental magnitudes and fluxes. The
red points show the photometry for stars in the simulated FLT
images. We see that at background=0 even bright stars
(~1000 electrons) lose ~30% of the flux within the aperture.
The loss levels decrease with increasing background, and
they level off at a background of ~100 electrons. At that
background level, even stars with only a few hundred
electrons only lose ~20% of their flux. This result is close to
that found in Figure 6 of MacKenty & Smith (2012). The main
difference being that those tests use real images with more
noise than these “ideal” simulations. !
!
Figure 2 –The signal to noise (left) and magnitude uncertainties (right) from
the simulated photometry. Each row corresponds to a different background
level. Green points are for simulated FLC images and red points for FLT
images.
REFERENCES!
!   Anderson, J. & Bedin, L. R. 2010, PASP, 122, 1035–1064!
!   Avila, R. et al, AAS meeting 219, 241.07 !
!   Chiaberge, M., ACS ISR 12-05, STScI, Baltimore, MD !
!   MacKenty, J., Smith, L., White Paper, !
! http://www.stsci.edu/hst/acs/!
!