We present the results and measurement of Charge Transfer Efficiency (CTE) of the WFC3 UVIS detector, based on data acquired during the monthly internal Extended Pixel Edge Response (EPER) observations over a 2 year period. We present an algorithm for CTE assessment and fit a power-law to CTE measures versus signal level. We find that at each signal level, CTE declines linearly over time and CTE losses are worst at the lowest signal levels. 1

B. Hilbert

L. Petro

Operated For Nasa Aura

S. Baggett

V. Kozhurina-platais

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SP ACE
TELESCOPE
SCIENCE
INSTITUTE
Operated for NASA by AURA
Instrument Science Report WFC3 2009-20
WFC3 SMOV Program 11798:
UVIS PSF Core Modulation
E. Sabbi
November 17, 2009
ABSTRACT
The image quality analysis carried out during the third thermal-vacuum campaign (TV3)
revealed that vibrations of the UVIS shutter cause changes in the width and in the central
pixel ﬂux of point sources in short exposures. During the Servicing Mission Observatory
Veriﬁcation (May-August 2009) we observed the ﬂux standard star GD153 with a series
of 0.5 sec, 1.0 sec, 3.0 sec, 10.0 sec and 20.0 sec exposures to investigate the impact of
the shutter vibration on data acquired in orbit. Our analysis shows that in the very short
exposures (<3.0-10.0 sec) the PSF is signiﬁcantly degraded, but that the ﬂux is preserved.
Introduction
An anomaly in the characteristics of the WFC3/UVIS point spread function (PSF) was
discovered during the ground tests in the TV3 campaign. In series of short exposures the
width of the PSF and the peak signal fraction for a point source were observed to modulate
for alternate exposures. This behavior was strongly dependent on exposure time, with the
greatest modulation occurring at 0.5 sec exposures (the shortest exposure time possible)
and was ascribed to vibration of optical train components induced by the UVIS shutter
mechanism (Hartig 2008).
Copyright c   2008 The Association of Universities for Research in Astronomy, Inc. All Rights Reserved.Accelerometer measurements showed that whenever the shutter servo electronics were
enabled, a signiﬁcant amount of vibration was induced in the optical bench (Rossetti et
al. 2008). These ground tests showed that, on the ground, the amplitude of the vibration
depended on the orientation of the instrument and led the engineering team to conclude that
on-orbit e ect could be as much as 3 times that observed from the ground (Hartig 2008).
Proposal CAL-11798 was designed to evaluate the impact the shutter vibration on the
stellar PSF in the on-orbit data. This proposal was executed during the 4th Servicing Mission
Observatory Veriﬁcation (SMOV4) that followed the Servicing Mission 4 (SM4) in May 2009
during which WFC3 was installed on HST. In this ISR we describe the exposures, the analysis
of the data and discuss the impact of the PSF modulation on science data.
Data
Five series of short exposures of the ﬂux standard star GD153 were acquired during
SMOV4 as part of Proposal CAL-11798, to measure the impact of the shutter vibration on
the quality of the photometry. Table 1 summarizes the observations: in total we acquired
49 0.5 sec and 51 1.0 sec exposures in the F606W ﬁlter, 50 3.0 sec exposures in the F505W
ﬁlter, 44 10.0 sec exposures in the F698M ﬁlter and 37 20 sec exposures in the F645N ﬁlter.
Images were acquired using the standard 512x512 pixel subarray in quadrant A to
minimize the impact of bu er dumping, with no dithering. Each exposure was bias and dark
subtracted and ﬂat ﬁelded using the standard calibration pipeline CALWF31 and, since
no calibration ﬁles from space were available, the most updated version of calibration ﬁles
aquired in TV3 were applied.
Analysis
For each exposure we measured several parameters using di erent pyraf routines: in
particular we measured the full width half maximum (FWHM) with imexam, the sharpness
with daoﬁnd, and the magnitude of the star within an aperture radius of 3, 5, 7, and 10
pixels using phot.
Figure 1 shows the variation of the stellar FWHM and sharpness in the 0.5 and 1.0 sec
1A complete description of CALWF3 is provided in the HST Data Handbook for WFC3 v1.0, Quijano et
al. 2009)
2Dataset Start Time End Time Exp. Time (s) Filter # Exp
IABV01011 2009-07-23 07:06:46 2009-07-23 08:10:12 0.5 F606W 24
IABV01021 2009-07-23 08:11:06 2009-07-23 08:33:07 0.5 F606W 25
IABV01031 2009-07-23 08:34:01 2009-07-23 08:56:02 1.0 F606W 25
IABV01041 2009-07-23 09:39:39 2009-07-23 10:02:35 1.0 F606W 26
IABV01051 2009-07-23 10:12:54 2009-07-23 11:22:33 3.0 F555W 25
IABV01061 2009-07-23 11:23:27 2009-07-23 11:46:18 3.0 F555W 25
IABV01071 2009-07-23 11:47:23 2009-07-23 13:01:58 10.0 F689M 22
IABV01081 2009-07-23 13:02:52 2009-07-23 13:25:26 10.0 F689M 22
IABV02011 2009-07-29 13:24:18 2009-07-29 15:04:45 20.0 F645N 37
Table 1: Logbook of the observations: the 1st column reports the name of the association
table, the 2nd and 3rd columns list when the observation started and ended, the used ﬁlter
is reported in column 4, and the number of observations acquired is listed in column 5.
long exposures. Results from the ground data are also shown for comparison. The same
odd/even modulation (Hartig 2008) observed in the ground data is present also in the data
acquired from the space. In the 0.5 sec exposures this modulation is   2 times higher than
in the ground data, in the 1.0 sec exposures the odd/even modulation is clearly visible, and
it is   14 times stronger than in the ground data acquired at the same exposure time.
Figure 2 shows the results for the 3.0 sec and 10 sec exposures from space. The odd/even
e ect is still present, but is strongly attenuated, especially in the 10.0 sec exposures. The
most striking feature in this plots is the jump occurring around the   20th exposure visible in
both series of observations. Because of coma, stars in the four corners of the UVIS detector
are elongated. During its orbit around the earth, HST is subject to temperature changes
that cause variations in its focus. This behavior is called telescope breathing. Because of
the breathing the PSF of each HST instrument changes within an orbit. The jump visible
around the   20th exposure is due to the telescope breathing and to the fact that the
observations for each time series were not acquired within the same orbit (images before the
jump were acquired at the end of one orbit, when the telescope is on focus, and images after
the jump were acquired at the beginning of the following orbit, when the e ect of breathing
is maximum). This e ect is illustrated in Figure 3 where two 0.5 sec images of GD153 in the
F606W ﬁlter are compared. At the beginning of the orbit (left panel) GD153 is elongated
along the top-left/bottom-right diagonal direction. At the end, on the contrary, the star
3Fig. 1.— Stellar FWHM and sharpness as function of exposure number for 0.5 sec (left
panels) and the 1.0 sec (right panels) exposures. Open circles and red continuous lines are
for the space data, x points and dotted black lines for the ground. Green points refers to
side B of the shutter, black points to side A.
looks more symmetrical (right panel). Both the images have been acquired with side B
conﬁguration.
Table 2 summarizes the impact of the shutter vibrations on the PSF FWHM. In partic-
ular it shows that in the 0.5 sec exposures from the space the FWHM is 15% broader on side
B with the respect to side A. In the 1.0 sec exposures the di erence in the FWHM is 7.3%,
and (once the impact of breathing is taken into account) 2.4% in the 3.0 sec exposures. At
4Fig. 2.— Stellar FWHM and sharpens as function of the exposure number for the 3.0 (left
panels) and the 10.0 sec (right panels) exposures. Green circles refers to side B of the shutter,
black to side A.
10.0 sec the di erence is less then 0.5%.
In order to evaluate the impact of the odd/even e ect on the photometry for each
data-set, we measured the magnitude of the star at 3, 5 7 and 10 pixel radii using aperture
photometry. Figure 4 shows the magnitude variations for the various exposure times. For
each set of exposures the impact of breathing on the quality of the photometry is clearly
evident in the set of analysis performed with the smallest aperture radius (3 pixels). These
5Fig. 3.— Magnitude of the GD153 star as derived from a 3 (black), 5 (red), 7 (green), and
10 (blue) pixel aperture photometry for the 0.5 sec (A panel), 1.0 sec (B panel), 3.0 sec (C
panel), 10.0 sec (D panel) and 20.0 exposures (E Panel). The dashed lines mark where an
orbit ends and a new one begins.
plots show that although the ﬂux from the star is preserved, but because of the broadening
of the PSF, there is an uncertainty of 2% in the magnitude of GD153 in the 0.5 sec exposures
when an aperture of 3 pixels is used. However at r=5 pixel the uncertainties is 0.8%, and in
the 1.0 sec and longer exposures the uncertainties in the r=3 pixel aperture photometry is
always less than 0.7%. Figure 3 also shows that for the shortest exposures, 0.5 sec, telescope
breathing and shutter vibrations have comparable impact on the quality of the photometry in
6Exp. Time. FWHM (side A) FWHM (side B)
0.5 2.04±0.08 2.40±0.08
0.5  1.98±0.03 2.15±0.05
1.0 2.05±0.08 2.21±0.07
1.0  1.85±0.02 1.86±0.02
3.0 2.01±0.17 2.07±0.14
3.0  2.08±0.03 2.13±0.03
10.0 1.78±0.05 1.79±0.04
10.0  1.80±0.01 1.81±0.01
20.0 1.83±0.04 1.85± 0.05
Table 2: Average FWHM measured in images acquired with di erent exposure time and
di erent side of the shutter. Values marked with an asterisk are for ground based data for
0.5 and 1.0 exposures. The   symbol indicates the values obtained before taking into account
the telescope breathing.
the 0.5 sec exposures, and that in relatively short exposures (e.g. 1.0 and 3.0 sec) breathing
is the dominant source of uncertainties. Figure 5 shows the variation in ellipticity of GD153
as function of time for di erent exposure times. The passage from one orbit to the next is
clear.
Conclusions
We measured the impact of shutter vibrations on the quality of photometry of a point
source using observations of the ﬂux standard star GD153. The star was observed in quadrant
A with a standard 512 512 pixels subarray. Our analysis shows that in very short exposures
(T=0.5 sec) the vibration of the shutter introduce an uncertainty of   2% in the magnitude
of the star when an aperture of radius smaller than 5 pixel is used and that the FWHM of
stars acquired with shutter on side B is systematically larger than that acquired on side A.
As a consequence stars will appear fainter in the corners of the detector if measured with a
small aperture radius.
We conﬁrm that shutter vibrations have less e ect for longer longer exposure times, and
that already for exposure times  1.0 sec the uncertainties induced by the odd/even e ect
are less then 1%. Our analysis shows also that telescope breathing increases the astigmatism
that a ects UVIS data in the corners, and that in short exposures acquired at the beginning
7Fig. 4.— Ratio between the minor and the major axis of GD153 as function of the observing
time. Values obtained for the 0.5 sec exposures are in black, for the 1.0 sec exposures in red,
green is for the 3.0 sec exposures and and blue for the 10.0 sec exposures.
of an orbit sources in the four corners of the detector will be fainter than in exposures
acquired with the same exposure time at the end of the orbit if using photometric aperture
with a radius smaller than 4 pixels.
8Acknowledgments
I thank M. Lallo and G. Hartig for their help and suggestions in investigating the impact
of breathing in the photometry.
References
Hartig, G.F. 2008, ”WFC UVIS Shutter Vibration-Induced Image Blur” STScI ISR WFC3-
2008-44 10, 100
Kim Quijano, J., et al. 2009, WFC3 Mini-Data Handbook, Version 1.0, (Baltimore: STScI)
10, 100
Rossetti, D. 2008, ”WFC3 UVIS Shutter Jitter Characterization”, WFC3 System Engineer-
ing Report WFC3-SER-MECH-030, 2008
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