The ESO CCD Testbench
Written by P.Amico, T Bohm,
Updated and adapted by C.Cavadore
INTRODUCTION
ESO's plan for optical detectors ref[1] are mainly concentrated on commissioning
CCDs systems for all the VLT optical instruments, with a total of about
40 scientific systems in the next 5 years. These systems will all be driven
by the new FIERA controller ([2], [3]). In addition, all new detectors
maintained by ESO in La Silla will also be "embedded" in FIERA systems.
While routine testing is usually done at the telescope site, a first
complete characterization of each device is carried out at ESO's headquarters
in Garching. For example, all the CCDs operating at the La Silla Observatory
(12 mounted at the telescope plus about the same amount available for backups)
have been tested with the existing ESO testing facility, equipped to operate
with VME and ACE ([4]) controllers.
The same procedure is foreseen for all the detectors needed by the
VLT instruments. The introduction of the new FIERA controller as well as
the growing size of CCDs, which cannot be handled by the "old" testing
facility, have made the construction of a new testbench essential for the
characterization of the future generations of scientific detectors at ESO.
The new testbench prototype for FIERA controlled CCDs, named the TAMeR
(Test And Measuring Rig) is at present being developed at ESO/Garching
and has been available since 1998. The new design provides the possibility
to test big detector sizes (8kx8k pixels and bigger) in a one-step process.
Fundamental CCD parameters are tested in fully automated modes, while specific
additional tests can be performed interactively. The testbench software
is integrated with the VLT Standard Central Control Software (CCS, [5])
and some of its modules, which will be also available at the telescope,
fully comply with VLT standards.
The project is carried out by the ESO Optical Detector Team.
2. TESTBENCH HARDWARE AND DESIGN
>The selection of the hardware components has been a lengthy but fruitful
process, whose starting point was not only the work done at ESO with the
actual testbench facility but also by other laboratories. A useful source
of information for first step design has been provided by Lesser and McCarthy’s
work for the Steward Observatory [6]. A subsequent step has been to create
a testbench simulator (by means of Microsoft Excel) to check the actual
output of the designed system. One of the questions we had to solve was,
for instance, whether the employment of a double monochromator, can provide
enough flux in the UV range at the detector's position. Although it provides
major improvement in signal/stray-light ratio with respect to a single
monochromator, this instrument introduces twice as many reflections, with
consequent loss of signal. The entire system's efficiency has its minimum
at the blue end of the spectral range of interest (320 – 1100 nm). Therefore,
the selection of many crucial hardware components (precisely the monochromator
and its parts), as well as the verification of small details, such as choice
of the lamp type or the selection of the coating for the integrating sphere,
have been taken into consideration and simulated.
A schematic of the final testbench design is given in Fig. 1.
Figure 1
Figure.1: Schematic of the testbench design The elements shown on the
table top (300 X 90 cm) are in scale (except for the dewar size). From
right to left: lamp and light intensity controller looped via the power
supply, the double monochromator system with two output ports. The secondary
port will input light into the optic fiber, used by PSF experiment; the
primary port feeds the integrating sphere. The flanges are screwed onto
the table top and the space in between light tightened. The CCD head is
mounted on a metallic plate fixed to the left flange and the dewar hangs
outside the box. All GPIB driven instruments are connected to the GPIB
controller, which in turn interfaces via SCSI to the FIERA
The system is mounted on a Newport 300 cm x 90 cm optical table top,
which provide static rigidity and flatness, in addition to the standard
configuration of sealed mounting holes. All hardware components are GPIB
driven and controlled through a GPIB controller, by National Instruments,
which holds up to 14 GPIB devices and is attached to a SPARC board with
embedded FIERA controller through SCSI connection. A PULPO monitoring unit
[7] for environmental variable control (temperature, humidity, etc.) will
also be part of the system.
The major system components are:
- Lamp housing and lamp: a QTH 100 W lamp with horizontally elongated filaments is hold in a standard convention cooled housing, equipped with a F/1 condenser, which produce a ~3 cm diameter collimated beam. This beam is then focused on the monochromator input slit by means of a secondary f/4 plano-convex lens, which matches the acceptance pyramid of the monochromator. The light system maximizes the total power into the monochromator and provides a smooth continuum within the desired wavelength range. The wattage has been chosen in order to have enough flux in the UV part of the spectrum. This condition is especially critical to achieve a good S/N for the absolute calibrated diode used for QE measurements.
- Power Supply and Light Intensity Controller: both produced by Oriel. The light intensity controller is directly connected to the lamp housing through a light sensing head, which monitors light variations, and interfaced to the power supply. It allows maintenance of constant light levels, for the duration of an exposure (exposure lengths vary from few seconds to about ten minutes) regardless of lamp aging, line voltage variations or changes in the ambient temperature.
- Monochromator: an Oriel Multispec 257 Double Monochromator in subtractive dispersion configuration. In the current setup, the output from the first unit is dispersed in the reverse direction by the second unit, thus homogenizing the light across the output slit. The net dispersion remains as that produced by the first monochromator, but the amount of stray light is greatly reduced, quoted by Oriel to be of the order of 10-7 of the unblocked signal. That is, almost three orders of magnitude smaller that the measured stray light for a single monochromator of the same kind. The two devices can be controlled via GPIB in either an independent way or together (using the first one as master). Both are equipped with microstepping motor driven slits and 600 l/mm ruled gratings, whose peak efficiency is at 400 nm. The usable wavelength region (that is, where the grating efficiency is more than 20%), goes from 250 to 1300nm (well beyond our requirements). With this configuration, a minimum bandpass of ~0.1 nm can be reached. Two motorized filter wheels, which hold up to five filters each, are attached at the input of the first monochromator. They control respectively the order sorting filters (2 filters, with cut-off wavelength respectively at 450 and 665 nm) and neutral density filters (4 filters with optical densities in the range 1-4). The first unit is also equipped with an integrated shutter, which can be controlled both via external TTL signals and through GPIB commands. The minimum exposure time setting is 20msec, the transition time ~2ms. Positioning the shutter before the light is inputted into the integrating sphere, instead that putting it at the exit port, has the advantage of eliminating the shutter pattern problem.
- Integrating Sphere: a 50 cm diameter "custom made" Labsphere. Its 8 inches exit port provides a uniform illumination, over an area bigger than the size of a 8k × 8k CCD or Mosaic (a typical 8kx 8k with 15 um pixel has a diagonal of about 17 cm; we will refer to this example throughout the paper). The primary output port is at 180 degrees with respect to the input port. A secondary output port (about 1.3cm), which hosts a photodiode, is drilled close to the primary output port. A baffle situated inside the sphere prevents that the output port “sees” directly the light source. The internal coating of the sphere is made in Spectraflect, a material that ensures a reflectance better than 98% in the range 400 – 1100 nm and better than 96% in the UV range. (320-400 nm). The best degree of uniformity across the illuminated field is achieved when mounting the CCD in close contact with the exit port. Otherwise, the degree of uniformity, defined as the ratio of the illuminance at the edge of the field to the illuminance on the axis through the center, is a function of the distance of the target form the source [8]. The second option has been chosen in order to have enough space between the sphere exit port and the detector to perform experiments (for instance, to put a lens and a target image to be projected onto the CCDs). The detector will be put at a distance of 50 to 75 cm from the sphere output port, so that, for a 8 inches light beam and a 8k X 8k 15 um pixel CCD, the degree of uniformity of illumination is always in the range 95% - 98%. A better than 1% uniformity is of course obtained for smaller detectors.
- Picommeters and diodes: the testbench will be equipped with two photodiodes, one permanently mounted at the secondary output port of the integrating sphere and the other, needed for absolute flux calibration of the system, put at the detector's position. A permanent solution, with the latter diode fixed as close as possible to the detector and sharing with it the same focal plane, is also planned for the future. Separate ammeters are attached to diodes through low-noise triax cables. A Keithley 486 is connected to the sphere's diode: a 5½-digit autoranging picoammeter designed for low current applications where fast-reading rates must be performed. The measurement range is between 2nA and 2mA, with a resolution of 10fA (@2nA range). The diode is a Hamamatsu 1cm2 Silicon Photodiode for precision photometry (NEP ~10-15) with good UV QE. The second diode is also a 1 cm2 silicon Hamamatsu photodiode, which has been absolute calibrated by reference to NPL (National Physical Laboratory, England) and to PTB (Physikalisch-Technische Bundesantalt) standards. At present is interfaced to a Keithley 619 Electrometer/Multimeter, with the same measuring range and resolution as the 486 model. Both ammeters are controlled via GPIB by means of the GPIB controller.
- Flanges system and light tight zone: the integrating sphere is attached to a flange, fixed onto the table top, through a flexible light shield, which allows a length span of ~25cm. A second flange, which will hold a custom made plate for each detector head (at least three different systems are foreseen for the VLT detectors systems), is positioned at a distance of 50 cm from the first flange. The dewar itself will be hanging from the outer wall of the flange. The space in between the two flanges will be closed by a wooden light-tight box, with lateral access door. The flanges, the box, and some other minor elements are being designed by ESO’s mechanical design office (see figure 1a).
Figure 1a, Overall view of the testbench hardware
Specifications for the hardware components provided by external manufacturers
are listed in the table below:
| Product | Manufacturer | Model |
| Optical table + Legs | Newport | M-RT-310-8(0.9x3.00)
NN4-28 or 23.5 |
| Integrating sphere | Labsphere | CSTM-US-200-SF |
| Light source System :
Lamp Housing + Power SourceS+1st Condenser Light Intensity Controller 2 nd Condenser |
Oriel | 6333+60043
60067
|
| Monochromator + accessories | Oriel | Multispec 257/77700 |
| Photodiodes | Hamamatsu | S2387-1010R
S1337-101 |
| Picoammeter | Keithley | 486 |
| GPIB SCSI board | National Instruments | GPIB-SPRC S 240 V |
3. TESTBENCH SOFTWARE
>The requirement to test new generation CCDs driven by the FIERA controller
has constrained the choice for the software development tools and for the
development platform. The latter must be a SPARC Board with the built-in
FIERA system under the Sun Solaris Operating System. The development language
is standard C to allow full compatibility with the FIERA and, more generally,
with the VLT software. LabWindows/CVI for Sun Solaris by National Instruments
is the software development environment of choice, since it offers a full
ANSI C and GNU C compatibility. Moreover, applications written under LabWindows/CVI
are independent from separate commercial run-time engines (which have to
be licensed). This requirement is essential because the data analysis modules
will be made available for testing at the telescopes.
In addition, LabWindows/CVI provides very attractive features for building
instrumentation software applications: run-time libraries, simplified GUI
programming, handling of GPIB instrument I/O, use of TCP and a set of instrument
drivers.
Fig.2 shows a schematic layout for the testbench software. It consists
essentially of a main interface window (GUI), which allows the selection
of the desired testing procedure, either a complete automated sequence
of all main tests or a single user-defined procedure. In all cases, scripts
provide interactions with three major and independent modules:
- 1. The hardware control software, which controls all the hardware peripherals driven via GPIB. It consists of graphical interfaces plus drivers to allow access and manipulation of all the instruments features. Among these, selection of the wavelength range, of the light intensity, of the bandpass through monochromators' control and collection of the data sent from the photodiodes to the ammeters. Flux readings from the ammeters (named control data on Fig. 2, because we plan also to use the readings from the sphere's photodiode to double-check exposure times), setup data from the monochromators and environmental data from PULPO are stored for later analysis.
- 2. The interface module to the FIERA system, which is in charge of sending to the controller instructions (exposure start, exposure time, I/O operation with data files, CCD initialization, voltages settings, etc.) for taking sets of exposures. Fits images are stored for later analysis.
- 3. The Data Analysis module, which takes the data stored by the other modules, process it and extract test results.
Figure 2
Figure 2: the testbench software flow. Different backgrounds distinguish separate software modules: left) hardware control, which interacts with all GPIB driven hardware; middle) FIERA control, which interact with FIERA software, i.e. with the CCD, and sets exposures sequences and times; right) data analysis, which collects data and process them into a final test report. Also shown is PULPO, used here for environmental control. All modules are controlled by a startup GUI, which allows test sequences, parameters definition and access to the calibration routines
The modules are independent in the sense they exchange data but in general
they can run in standalone mode. This requirement will allow the use of
the analysis software at the telescope site, thus providing a standard
tool for quick testing of the CCD performances when connected to the instrument.
The main interface accesses also a calibration routines module, used
periodically to check the system's hardware responses and perform wavelength/flux
calibrations.
figure 3, main testbench's software panels, left side to set and start test sequences, right to set up (manually) the CCD illumination.
4. TEST PROCEDURES
Once the data have been acquired, the FITS files are moved to a powerfull
PC (July-99) Dual PII 450MHZ processor, 512Mb RAM and processed by the
Prism software. This software
includes all the usual routines to get the CCD characterization parameters.
| Parameters | Measured relative Accuracy |
| Noise (according to the system's speed) | 0.4 % |
| Conversion factor | 1 - 2 % |
| Quantum efficiency | 3 % U,B 1% V,R,I bands |
| Photoresponse non uniformity | 0.4 % (photon noise limited) |
| Fringing | 0.5 % (photon noise limited) |
| Evaluation of cosmetics defects, using long dark exposure, flat field, bias images | Everything > 5 sigmas noise floor level |
| Linearity | less than 0.01 % |
| Dark current | 10% depends upon the number of exposures taken |
| Remanence | - |
| Amplifier glowing | Everything > 5 sigmas |
| Check for CCD contamination | 0.1 % |
| CTE using EPER method | 5% |
| Cosmic ray events | 5% |
| Crosstalk (multiple ports) | 2 ADUs / 65535 |
| Full well capacity | Depends upon the linearity level |
A standard report is made accordingly to the measurments, and the CCD
grade is rated according this measurmentrs for its future purpose..
This set up was also
used to generate this EEV 44 report.
5. CONCLUSIONS
Design and hardware implementation phases have been completed and the
testbench has been ran since August 1998. This set up has been used to test
all the CCD for the various instrument (FORS1/2,
UVES, WFI,...). All the parameters are measured with the accuracy
excepted and, this testbench is extremely useful to characterize CCDs,
monitor thier behavior according to time.
At that time, we are improving software tools to acquire the Data,
so as to end up with a homogenious software architecture. Also we plan
to make tests with differents voltages so as to get the best performance
possible, in an automated way.
Acknowledgments: all members of the Optical detector Team, both in Garching and Chile, have contributed to design and critique the development stages of the project. We thank M. Lesser for sharing with us his experience in building a test facility. We are also grateful to Fernando Pedichini, who greatly contributed to the project during his visits to ESO and who is still actively collaborating with us.
6. REFERENCES
[1] J.W Beletic, these Proceedings
[2] J.W. Beletic, R. Gerdes and R C. DuVarney, these Proceedings
[3] C. Cumani & R. Donaldson, these Proceedings
[4] Reiss R., "ACE, ESO's next generation of CCD Controllers for the VLT", in "Instrumentation in Astronomy VIII, 13-14 March 1994, Kona, Hawaii", SPIE Proceedings, vol. 2198
[5] Central Control Software (CCS0 User Manual, VLT-MAN-ESO-17210-0619
[6] M.P. Lesser and B. L. McCarthy, 1996, Proc. SPIE 2654B, “QE Characterization of Scientific CCDs”
[7] N. Haddad, P. Sinclaire, J. Anguita, A. R., these Proceedings
[8] R. Kingslake, Illumination in Optical Images, Applied Optics and Optical Engineering, Ed. R. Kingslake, Vol. II, Ch. 5, Academic Press, 1965.
See also :
ESO's new CCD testbench, p95, Astrophysic ans space Library, Optical Detector for astronomy, James Beletic, Paola Amico, KLUWER ACADEMIC PUBLISHERS