A NEW CTE PHOTOMETRIC CORRECTION FORMULA FOR ACS Marco Chiaberge
TIPS meeting 05/16/2012 Slide 2 CHARGE TRANFER EFFICIENCY (CTE) per
pixel Defined as CTE = 1 - Q/Q = 1 - CTI For an ideal CCD CTE = 1.0
For real CCDs CTE < 1 Manufacturing imperfections in the
crystalline lattice Radiation damage (increasing with time) CTE on
ACS was not 1.0 at lunch! The total CTE is CTE N significant effect
for large CCDs CTE depends on flux, sky level, # of transfers Slide
3 The effect of CTE on stellar photometry is to reduce the measured
flux A significant fraction (all?) of the lost flux goes into the
tail Slide 4 Timeline of ACS CTE corrections Time dependent formula
based on 3 epochs March 2003 Feb 2004 Riess & Mack ISR 2004-006
Revised formula Chiaberge et al. ISR 2009-01 New approach for data
analysis, 4 epochs Increased accuracy mag = 10 A SKY B FLUX C
Y/2000 (MJD-52333)/365 Linear in log mag v log flux and log sky!
Anderson & Bedin 2010 pixel-based CTE correction Included in
the new CALACS Slide 5 WHY DO WE STILL NEED A PHOTOMETRIC
CORRECTION FORMULA? We need to keep monitoring the time dependence
and make sure that photometry is correctly recovered Some users may
prefer to use a correction formula. The correction formula may be
more accurate for some regions of the parameter space What should
we use for extended sources? Slide 6 Photometric test WFC B A D C
Y1 Y2 Y Transfers= Y=Y1-Y2 For y=1024 Y=0 mag=0 Slide 7 Post-SM4
Observations Programs: CAL/ACS 11880, 12385, 12730 FILTERS: F606W,
F502N EXP TIMES: Between 30s and 400s 5 Background levels between
~0.1 and 40e - Low sky CR-REJECTION, no dithering 1 epoch/cycle
Target: 47 Tuc (7 off center) Cycle 19: + pointing 3 south of 47Tuc
center for the lowest sky level 9 external orbits Slide 8 47 Tuc 7
off center ~ 2000 stars 47 Tuc 3 off center ~ 7000 stars Slide 9
ANALYSIS PROCEDURE (semi-automatic, thanks to Pey Lian!) 1 Generate
clean, deep, drz image using all data 2 Identify saturated pixels
and mark them on the DQ extension of FLT files 3 Mask out area
around the saturated stars 4 Find stars on the deep mosaic, then
measure flux of all stars that are detected on (both of) the
single_sci files (aperture phot) 5 Fit mag vs # of transfers for
different bins of flux (rejecting outliers with iterative sigma
clipping) 6 Find the best fit model parameters to reproduce the
dependence of mag y=2000 on Sky and Flux levels Slide 10 A linear
fit is performed for each bin of flux (red lines) Rms errors on the
slope are estimated (yellow lines) At y = 2000 Mag loss of 0.56
0.07 mag F502N 30s November 2011 Slide 11 CTE Correction formula
assumed dependence on flux mag = 10 A SKY B FLUX C Y/2000
(MJD-52333)/365 Cycle 17 0.3e - 32e - Cycle 19 0.6 e - Y=2000 2
problems: CTE improves at low Signal levels??? Large deviations
from the assumed linear dependence Slide 12 Photometry with
different detection thresholds: 3 and 10 Slide 13 100e - 50e - Far
from amplifiersClose to amplifiers Detection threshold N Flux mag
Average flux I measure in that bin BUT THIS IS WHAT USERS NORMALLY
DO! Slide 14 CTE Correction formula assumed dependence on flux mag
= 10 A SKY B FLUX C Y/2000 (MJD-52333)/365 Cycle 17 0.3e - 32e -
Cycle 19 0.6 e - Slide 15 Cycle 19 0.6 e - mag = Log flux + sky),
sky) 14e - Much better represented by a linear relation in mag v
Log sky Slide 16 mag = [ Log(flux) + y/2000 p Log(sky) + q = p
Log(sky) + q mag = p Log(sky) Log(flux) + q Log(flux) + p Log(sky)
+ q p, q, p, q = p(t), q(t), p(t), q(t) Slide 17 mag = p Log(sky)
Log(flux) + q Log(flux) + p Log(sky) + q p, q, p, q = p(t), q(t),
p(t), q(t) Linear fit using R, for each epoch The new time
dependence does not assume CTE = 1 at T = T launch T T launch (d)
Slide 18 Comparison with pixel-based CTE correction CTE formula is
more accurate than the pix-CTE correction at the lowest background
levels Slide 19 FUTURE WORK (from TIPS 2009) New observations after
SM4 using CR-REJ and possibly dithering Procedures should be made
automatic (or semi-automatic) Formula for different aperture radii
Better data might lead to a better characterization Different form
of the formula? FUTURE WORK Better estimate of the errors on the
coefficients Formula for different aperture radii Extended sources?
Webpage for correction Thanks to Jay, Linda, Roberto and Pey Lian
RESULTS A new accurate CTE photometric correction formula is now
available The accuracy is comparable to (or better than!) the
pix-cte correction Does not depend on the assumption of CTE = 1 at
T = T launch Slide 20 Photometric test Allows to measure the total
flux lost and provides correction formulae for photometry. Stars
are positioned at different distance from the readout amplifier
thus changing the number of transfers and therefore the impact of
CTE. WFC D B C A Slide 21 47 Tuc F606W 40s exp Slide 22 Long vs
short: find stars in F606W 400s Measure the flux of all stars in
F502N 30s Slide 23 No correction Corrected i in F606W 400s Slide
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