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Astronomical Data Analysis Software and Systems IV
ASP Conference Series, Vol. 77, 1995
R. A. Shaw, H. E. Payne, and J. J. E. Hayes, eds.
CCDs at ESO: A Systematic Testing Program
T. M. C. Abbott
European Southern Observatory, Casilla 19001, Santiago 19, Chile
R. H. Warmels
European Southern Observatory, Karl­Schwarzschild­Straúe 2, D 85748,
Germany
Abstract. ESO currently offers a stable of 12 CCDs for use by visit­
ing astronomers. It is incumbent upon ESO to ensure that these devices
perform according to their advertised specifications (Abbott 1994). We
describe a systematic, regular testing program for CCDs which is now
being applied at La Silla. These tests are designed to expose failures
which may not have catastrophic effects but which may compromise ob­
servations. The results of these tests are stored in an archive, accessible
to visiting astronomers, and will be subject to trend analysis. The test
are integrated in the CCD reduction package of the Munich Image Data
Analysis System (ESO­MIDAS).
1. Introduction
At the time of writing we at ESO, La Silla offer 12 CCDs for use by visiting
astronomers. These CCDs range in quality from a venerable RCA with read
noise of 32 electrons per pixel to the most recent, a thinned Tektronix 2048 2
pixel device. Supporting all of these CCDs poses some unusual problems. ESO
serves a very broad community, and the astronomers who use our CCDs range
in ability from those who are quite new to the field to those with many years of
experience in the use of modern, state­of­the­art detectors. We must be aware at
all times of the current status of our CCDs so that even the most exacting visiting
astronomers can be satisfied that their data is of uniformly high quality and that
they are completely informed of any problems or limitations. Likewise, we must
work to protect the less experienced astronomers by convincing ourselves that
our CCDs are providing data of sufficient quality to ensure the success of a
broad spectrum of observing programs. It is, therefore, not sufficient that we
trust our CCDs to remain in the state determined when they are commissioned,
nor that we depend on visiting astronomers to identify problems as they arise.
Instead, we must make a concerted effort to regularly investigate the quality of
the data delivered, whether or not any problems are known. To that end, we
have instigated a systematic program of standard CCD tests at ESO, La Silla.
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2. The Test Data
We currently test one CCD each week, and thus each CCD is tested every 3
months. These tests are not intended to be as thorough as might be performed in
a specialized CCD lab; instead, they should expose as many problems as possible
with minimal technical intervention and under the simple setups available with
the CCD on the telescope. We can then investigate any problems that we identify
with more sophisticated methods, or, if we judge the CCD to be functioning
satisfactorily, the test results provide a baseline of its performance.
For each test, we collect the following data: (1) nine bias frames, (2) sixteen
pairs of flat fields (both of each pair have the same integration time) using a
stable light source and with exposure levels ranging from just above bias to
digital saturation, (3) nine low­count­level (of order a few hundred electrons per
pixel) flat­fields with stable light source, (4) one flat­field exposure obtained with
64 rapid shutter cycles, (5) three 30­minute dark images, and (6) the time taken
to read out and display an image. All images include bias overscan regions in
both dimensions, cover the entire light­sensitive, unbinned area of the CCD and
are collected under the same circumstances as normal observing.
The light source used to obtain the flat fields may be either an LED or
a beta light. Beta lights consist of a fluorescent screen stimulated by fi decay
from a small bulb of tritium. Since these present a possible radiation hazard and
are prone to variation with temperature (¸ \Gamma0:3% per ffi C (Florentin, private
communication)) we are in the process of replacing them with compact light
sources consisting of a battery­powered LED regulated by feedback from a photo­
diode. Like the beta lights, these are small enough to fit within a normal filter
wheel in most La Silla instruments and exhibit a flux/temperature dependence
of ¸ 0:2% per ffi C (we expect to improve on this in future versions).
3. The Results
The information we expect to obtain from each test data set is as follows:
1. A 16­point transfer curve (Janesick et al. 1987, Figure 1a) generated for
any window onto the images obtained.
2. Two 16­point linearity curves. We find that the linearity curves are most
useful when expressed as count rate versus true exposure time (Figure 1b).
We determine the mechanical shutter delay either by linear extrapolation
of the normal linearity curve (observed counts versus exposure time), thus
assuming the response of the CCD is linear, or by adjusting the exposure
times such that the count rate curve is closest to a straight line, thus al­
lowing for a first­order nonlinearity in the response of the CCD. We obtain
the 16 pairs of frames in two groups of eight---the first with increasing ex­
posure times and the second with decreasing exposure times, interleaved
with those of the first group. In this way, we can reject trends observed in
the CCD response that are probably caused by the effect of temperature
variations on the light source. The linearity curves may be generated for
any window onto the images obtained.

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Figure 1. a (left): Sample transfer curve (TK#36). The abscissa is
the mean counts per pixel in a 200 2 pixel region centered on the CCD.
The ordinate is the variance of the same region in the image that results
from the difference of two images of the same mean counts. b (right):
Sample linearity curves expressed as count rate versus mean counts
in an image using the same light source throughout (TK#36). The
straight lines are linear fit to the data. The exposure times have been
corrected for a shutter delay of 1.4 seconds.
3. A map of hot pixels in bias frames (obtained from a median stack of the
nine raw bias frames).
4. A map of traps and other defects (obtained from a median stack of the
nine low count level frames).
5. An estimate of bulk CTE in the horizontal and vertical directions (by the
EPER method (Janesick et al. 1987)).
6. The amplitudes and frequencies of interference signals (from a Fourier
analysis of raw bias frames).
Figure 2. a (left): Sample dark current map (TK#36). Contours
are labeled in electrons/pixel/hour. b (right): Sample shutter delay
map (TK#25). The contours are at 0.016 seconds and 0.024 seconds.
Note the hexagonal shape caused by the iris shutter.

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7. The current values of all bias and clock voltages (normally measured by
the CCD controller and recorded in image headers).
8. A map of dark current across the CCD (Figure 2a).
9. A map of the shutter pattern on the CCD (e.g., a star­shaped pattern in
the case of an iris shutter (Figure 2b), obtained by analysis of the image
made with 64 shutter cycles).
4. Implementation and Documentation
The test program has been integrated in the CCD reduction package of the
Munich Data Analysis System (ESO­MIDAS) (ESO 1993), and will be available
in the 94NOV release. Therefore, in addition to the already available pipe­line
and interactive processing tools for CCD direct imaging data, the CCD package
will also offer standard tools for testing the detector quality at ESO and at other
institutes.
We issue a full report on the condition of a CCD each time a new CCD test
data set is collected and reduced. We are in the process of developing an on­
line test data archive to store the raw and reduced test data (ESO 1994) and a
World Wide Web interface for browsing these data. These software systems are
accessible via the Internet at the ESO Home Page 1 . We use the data obtained for
normally functioning CCDs to define baselines for their performances. Trends in
these data expose possible slowly developing problems and thus allow realistic
preventive maintenance, reducing the probability of catastrophic failures. The
most recent test data set combined with the history of a CCD's behavior provides
the astronomer with an indication of the current performance and reliability of
the device.
Acknowledgments. S. Deiries of ESO, Garching designed and built the
stable LED light source. We are grateful to the ESO, La Silla Astronomy
Department and CCD group for their cooperation in collecting the test data
necessary for the success of this project.
References
Abbott, T. M. C. 1994, ESO CCD Catalogue
Janesick, J. R., Elliot, T, Collins, S., Blouke, M. M., & Freeman, J. 1987, Optical
Engineering, 26, 69
ESO 1993, Document MID­MAN­ESO­11000­0002/0003/0004,ESO­MIDAS User
Manual, Volumes A, B, and C (Garching, ESO)
ESO 1994, Document OSDH­SPEC­ESO­00000­0002/2.0, EMMI/SUSI Calibra­
tion Plan for an On­Line Calibration Database (Garching, ESO)
1 http://www.hq.eso.org/eso­homepage.html