Mitigation of CTE Losses in ACS/WFC:!
Post-Flash Capabilities!
Sara Ogaz, Jay Anderson, Marco Chiaberge, Linda Smith, and the ACS Team!
Space Telescope Science Institute, Baltimore MD!
Putting the electrons back where they belong!
ABSTRACT!
The charge transfer efficiency (CTE) correction that is currently being applied in the Advanced Camera for Surveys Wide Field Channel (ACS/WFC) pipeline effectively corrects lost charge for most cases. However, when the
background level is low a large percent of the original signal is lost during read out and cannot be recovered by the CTE correction algorithm. To address these cases, the ACS team is investigating the post-flash capabilities of
ACS. Here we present results from our initial analysis of the repeatability, count rate, and gradient across the CCDs. !
Why You May Want To Use Post-Flash!
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Figure 1: Each postage stamp represents a sigma clipped mean of 100 stars in Omega Cen positioned furthest from
the readout. The vertical axis represents the expected flux measured in a long exposure. The horizontal axis
represents different values of post-flash plus sky . The yellow number in the bottom right corner of each postage
stamp represent the measured counts from a 3 by 3 pixel aperture. There is a dramatic increase in the measured flux
when the background increases from 8 to 25 e-.!
Figure 2: Example of a post-flash reference file in units of electrons per second. The lamp is centered between the
two chips and has a gradient in the y and x directions. There is a signal decrease of approximately 50% from the
brightest sections near the chip gap to the darkest sections in the outside corners. This image has not been flat
fielded, which is the main source of the small irregularities in the image.!
!
!   The post-flash file in Figure 2 is a median combined image of 12 dark frames, each with
Figure 1 shows the effect of increasing the background to mitigate CTE loss. The
measured flux in electrons (shown in yellow) approaches the expected flux (shown on the yaxis) as the background is increased.!
!   The positive effect of increasing the background is the reduced loss of flux due to CTE
an exposure time of 0.5 seconds, and a post-flash time of 72.0 seconds. The final image
was then normalized to electrons per second. A longer than average post-flash time was
used to obtain good signal to noise images.!
loss. As the background is increased using the post-flash there is a definite improvement in
the recovered flux. This continues until the 56e- sky bin for most flux bins. !
!
!   As the background is further increased the improvement in CTE loss levels off. At high
!   After examining multiple post-flashed images with the same settings, we have confirmed
levels of sky background the noise in the image is significantly increased. The faintest stars in
Figure 1 disappear into the noise. The optimum post-flash level appears to be between 25
and 40e-, when the decrease in CTE loss is significant compared with the increase in noise.!
that the post-flash illumination is stable over short time scales.!
The values in the image range from ~7 to ~15 electrons per second. The variation
between the brightest part of the post-flash and the darkest is approximately a factor of two.!
!   We have found that the illumination has decreased at a slow rate over the years.
!   The ACS Instrument team is developing several tools to assist users in choosing a correct
When
comparing current post-flashed images to those taken in 2006 (Cox, 2006) we see a 25%
percent decrease in illumination. This is due to radiation damage (Kniffin et. al, 2000).!
post-flash setting. The Astronomerʼs Proposal Tool (APT) has been updated to allow
observers to chose their level of post-flash.!
!   Due to the uneven illumination of the post-flash lamp, users will have to be aware of
REFERENCES!
!   Cox, C. 2006, ACS Post-Flash Measurements, ACS ISR 2006-07!
!   Kniffin, S. Reed, R. Kim, H. 2000, Proton Effects Test Results for the Optodiode OD800 Light Emitting
Diode, NASA/GSFC Report!
where their objects are placed on the CCDs to achieve their desired background.!