Testing and characterizing the CCDs of the OmegaCam wide-angle camera F. Christen2, C. Cavadore1, B. Gaillard, O. Iwert1, K. Kuijken2, D. Baade1, S. Darbon1 11 :: EuropeanEuropean SouthernSouthern ObseObservatoryrvatory 22 :: KapteynKapteyn InstituutInstituut (Groningen)(Groningen)
OmegaCAM will cover the 1deg x 1deg field of view of the ESO VLT Survey Telescope with thirty two 2k x 4k CCDs. Four additional chips of similar type will be used for the auto guiding and active optics control. Since the replacement of a unit detector or a re-arrangement of the mosaic after commissioning are not an option, every detector needs to be fully characterized in advance. These tests need to be carried out under realistic astronomical operating conditions.
Introduction Summary of the results obtained to date Linearity The test bench of ESO's Optical Detector Team ( ODT ) provides a platform well suited for The method consists of reading the CCD while it is illuminated with a characterizing CCDs. However, to make easier the test of more than 40 CCDs for Photon Response Non-Uniformity Quantum Efficiency light source (assumed to be constant on such short time scales), see OmegaCAM (including additional CCDs for other instruments), several enhancements were 14 Fig. 13. implemented in this set-up to increase its throughput. The last hardware improvements 100 optimized the turn-around time and the precision of individual CCD characterization. Two 90 12 Non Linearity 80 10 new detector heads accommodating two Marconi CCDs each have been assembled and 70 adapted. Scripts for a largely autonomous data acquisition and reduction have been written. 60 8 0.8 50 0.7
The implementation of these high-level procedures permits essentially an un-supervised (%) QE 6 0.6
40 (%) Rms 30 0.5 reduction of a complete data set. Finally, comprehensive test reports in HTML and PDF 4 20 0.4 format enable a convenient sharing and comparative evaluation of the results. (%) Rms 0.3 10 2 0.2 0 0 0.1 320 370 440 520 600 680 760 850 940 1040 320 420 520 620 720 820 920 1020 0
a s a r a s n s o x s s s s a o tli r m u o u d a u m u v Wavelength (nm) pu A u ajo le n r iu r pu pu a Wavelength (nm) A A r G g e u ael M cin o o L L P C Carin Dora F Hyd Mens nis Cir a Centaur Horol OmegaCAM Consortium Figure 4 : Quantum efficiency of 19 CCDs Figure 5 : Photon response non-uniformity of 19 CCDs C Chamae Netherlands : NOVA, Kapteyn Instituut Groningen, Leiden Quantum Efficiency is measured at an operating temperature of -120°C. These measurements are Figure 13: Illumination for Figure 14 : Peak-to-peak non- Figure 15 : Linearity for 19 CCDs, Germany : Universitäts-Sternwarte München, Göttingen. regularly checked by recalibrations. The repeatability is very good (Error : ± 0.2% max). The QE computing the non-linearity linearity= 1.5%, Rms= 0.4% Mean= 0.27% Rms= 0.15% Italy : Osservatorio Astronomico di Padova, Capodimonte. dispersion from device to device is increasing towards shorter wavelengths. The QE figures in the ESO : European Southern Observatory. range of 300- 400 nm are higher than the minimum specification. PRNU is increasing in the red due to After flatfielding ( the test is performed at 600nm where PRNU effects are the lowest), the fringing effects, and in the blue due to the backside laser annealing (diamond pattern, see Fig. 6) columns are averaged, applying a median filter. The residuals from a linear least-squares fit to the result represents the non-linearity (Fig. 14). Non-linearities are typically less than 0.3% OmegaCAM CCD Camera (Fig. 15). • 1 deg x 1 deg field of view Cosmetics of the CCD “Indus II” : • Sampling 0.2’’ / pixel The next images are three examples of 2kx4k full frame flat fields. The full test report includes 40 flat • 32 CCDs mosaic 16k x 16k (CCD EEV 44-82 1-A57) Electrical Parameters field images at different wavelengths, light levels and gains. • 40 Science grade CCDs ordered Conversion Factor (Gain) : derived from the measurement of the mean signal of flat field and • Contract with Marconi : CCDs to be tested by ESO its variance. • 4 auxiliary CCDs (two for guiding, two for image analysis Read Out Noise : spatial rms of the bias measured. for the active optic control for the VST) • Mosaic filling factor 93 % • Operating temperature -120°C Conversion Factor Read Out Noise Figure 1 : The OmegaCAM Camera 0.7 8 0.65 7 6 0.6 5 0.55 4 3 0.5 (e-) Rms
Marconi CCD44-82 1-A57 ADU) (e-/ CF 2 0.45 1 • Thinned back-side illuminated devices 2kx4k 15µm 0.4 0
r s s s pixels. lia r lia n o x m u u a o m jo s t jo us o us d u u s At Ara u u s A Ara rina r e n na gi Apus el rina leon um u s Apus el i r o Lep en Pav a Ma a aurus e in rado rnax i p u sa vo Ca c Grus Hydr Lup M C C a irc o Grus e up n Caelumis Ma Dora Fo rol • Single-layer Hf0 anti-reflective coating insures optimal nis nt D Fo L L Pa n ma Cir o 2 a am C Hydrus Me Centaua H C Ce h Ca sensitivity in the blue and near UV. C Horolog Ch Figure 6 : Flat field ( λ= 350nm, Figure 7 : Flat field ( λ= 600nm, Figure 8 : Flat field ( λ= 900nm, Left Port 50k Right Port 50k Left Port 225k Right Port 225K Left Port 50k Right Port 50k Left Port 225k Right Port 225K • Two serial read-out registers bandwidth= 5nm, 1500ADU) bandwidth= 5nm, 1500ADU) bandwidth= 5nm, 1500ADU) • On-chip Pt100 temperature sensor Figure 16 : Conversion factor for 19 CCDs, Figure 17 : Read-out noise for 19 CCDs, • Invar package providing high flatness level Between 420 and 870 nm the PRNU is photon noise limited. The acquisition of these images has Mean= 0.53 e-/ADU Rms= 0.03 e-/ADU Mean= 2.9 e- Rms= 0.4 e- at 50kpix/s - - • Required flatness : ± 10 mm three goals : general appearance of the images, identification of unexpected/expected defects (bright Mean= 4.4 e Rms= 0.7 e at 225kpix/s Figure 2 : Two Marconi 44-82 CCDs spot, patterns, 512x1K block stitching, scratches) and traps. Finally, all the defects are recorded for Conversion factor and read-out noise are measured for the two ports and at different speeds compliance with the contract (see Fig. 9). (50 and 225 kpixels/s). The test bench is actually not read-out noise optimized because the The Test Bench Cosmetic Defects : objective is to measure relative differences only. In 1996, Amico & Böhm[1] designed the new ESO testbench. After several improvements Six different kinds of cosmetic defects : (automatization, high-level scripts) this system is now optimized for mass testing. The Dark Current : A median-filtered stack of five 1-hour dark frames is used (temperature : main components are shown in the next picture (Fig. 3). • Hot pixels : provide a signal of > 60 e- /pixel -120°C, read out speed : 50kpix/s, high gain mode : 0.55e-/ADU) is computed. The horizontal The following measurements / hour. and vertical over scans pixel are considered to determine the dark current. It is mostly less - are routinely performed : • Dark pixels : have 50% or less than the than 1 e / pixel / hour (see Fig. 18). • Quantum efficiency (QE) average output for uniform illumination at a Charge Transfer Efficiency : Two methods have been used, one based on the extended 55 • Photon Response flat field level around 500 photo-electrons. pixel response through the image overscan area (EPER), the other used the standard Fe • Very bright pixels : provide a signal of > • Non Uniformity (PRNU) method. Cosmetic defects 200000 e-/pixel/hour. • Dark (short and long exposure) • Trap : captures more than 10 •Bias 100.0 electrons, measured with a flat field level Dark Current Charge Transfer Efficiency • Readout noise, conversion 100 100 100 34.6 around 500 photo-electrons. Pocket pumping 2.5 1.000000 factor acquisition technique is used to trace them. 0.999999 2 • Linearity 10.0 • Very large trap : captures more than 0.999998 0.999997 1.5 • Dark current 10 000 electrons, measured with a flat 0.999996 • Charge Transfer Efficiency 0.999995
field level around 90% of the full well CTE
Logarithmic scale (%) 1 0.999994 (CTE) 1.0 1.2 1.2 capability. Total non Total Traps 0.999993
DC (e-/pixel/hour) 0.5 • Cosmetic defects : (hot pixels, column defective • bad column : 10 or more contiguous hot or 0.999992 defective pixels 0.999991 pixels 0 very bright pixels, dark pixels, dark pixels in a single column or a very 0.999990 a m s s Sum of defects Contract specification li m o u s a s n s o x s s s a o t ia ra m jor u d m u u s v bright pixel or a very large trap. A Ara rus pu avo tl pu A a rin n ru iu r p n a traps, very large traps bad Apus leon giu A A a i G g d e e P rad G o P aelu tauru eleo c ora orna y L o Hydr Le Lupus C n a ir D F lo H M Caelu Mensa C e C ro ntaurus D Fornax anis M am o e mae Circinus orol C h H Figure 3 : The ESO CCD Testbench C C columns and coating blemishes) Figure 9 : Total balance of cosmetic defects C H Canis Major Cha Horizontal Transfert Vertical transfert With a mean per CCD of 66 hot and dark pixels (rms: 72) , 5 very bright pixels (rms: 4), 3 traps (rms: Capabilities : 3), 1 very large trap (rms: 2) and 2 bad columns (rms: 2), the overall cosmetic quality of the Figure 18 : Dark current for 17 CCDs at -120°C, Figure 19 : CTE for 17 CCDs at -120°C, • Long dark exposure time (several hours) Mean= 0.7 e-/pixel/h Rms= 0.6 e-/pixel/h Horizontal CTE : Mean= 0.999997 Rms= 0.000003 Omegacam device is very well within specifications. • Several modes and readout speeds driven by FIERA CCD controller Vertical CTE : Mean=0.9999995 Rms=0.0000002 • Uniform illumination up to a field of 20cm diameter provided by the integrating sphere Example of an unexpected defect : Parasitic light injected by on-chip ESD protection diodes. • Large wavelength coverage (300-1100nm) • Good spectral sampling (1nm) Conclusion • All devices can be controlled remotely by software • 88 CCDs ordered • Time to test 2 CCDs decreased from 2 40 science grade CCDs weeks to 3 days - 29 received, 29 characterized • Time to reduce and analyze data The procedure 16 engineering grade shortened from 1 week to 2 days - all CCDs delivered and tested • Ability to test 2 CCDs simultaneously After the delivery of a CCD, the data sheets and the CCD are checked, and the ESO 32 mechanical samples received • 3/4 of the OmegaCam CCDs have been database is updated. For the ease of reference each CCD is internally assigned a name • Streamlined CCD test procedure tested of a stellar constellation; they are also used below. A standard test cycle is executed and • Improved reliability • Results are generally very satisfactory some complementary measurements are carried out to check any particular parameters. • Accuracies have been assessed Comprehensive test reports are generated semi-automatically. The full report includes tables with the values measured, comparisons with the Marconi measurements and basic statistics. Compliance with the contract is checked for each parameter. [1] Amico, P., Böhn, T. (1997) : ESO’s New CCD testbench. In J. W. Beletic and P. Amico (eds) : Optical detectors for Figure 10 : Bias 50kpix/s, Figure 11 : Bias 225kpix/s, Figure 12 : Bias 625kpix/s, astronomy, Astrophysics and space science library, Vol. 228, Kluwer Academic Publishers, Dordrecht, Page 95-114 binning 15x15, color scale in binning 15 x 15, color scale in binning 15 x 15, color scale in ADU, gain= 0.55e-/ADU ADU, gain= 0.55e-/ADU ADU, gain= 0.55e-/ADU [2] : http://www.eso.org/~ccavador/testbench/Prism/CCDtest-US.html