Level A: Preliminary CTA System Performance Requirements

Transcription

1 Page : 1/25 Level A: Preliminary CTA System Author Laboratory Approved by Laboratory Konrad Bernlöhr (konrad.bernloehr@mpi hd.mpg.de) Oscar Blanch Bigas (blanch@ifae.es) Rodolfo Canestrari (rodolfo.canestrari@brera.inaf.it) Carlos Delgado (carlos.delgado@ciemat.es) Andreas Förster (andreas.foerster@mpi hd.mpg.de) German Hermann (german.hermann@mpi hd.mpg.de) Jim Hinton (jim.hinton@leicester.ac.uk) Bruno Khélifi (khelifi@llr.in2p3.fr) Giovanni Pareschi (giovanni.pareschi@brera.inaf.it) Richard White (richard.white@leicester.ac.uk) plus members of the CTA Design Study Work Packages German Hermann (Technical Coordinator), March 31 st, 2011 Project Committee, June 7, 2011 Applicable Documents No. Version Ref. Date Title TPC Reference Documents for Preliminary CTA Performance SPECS/110331c Requirements Level A and B Edition Date Observation New document Correction of few typos History Distribution CTA internal

2 Page : 2/25 Table of Contents 1. INTRODUCTION 3 2. SCIENTIFIC AND PERFORMANCE REQUIREMENTS OVERVIEW ENERGY RANGE FIELD OF VIEW FOR GAMMA RAYS ON THE SKY SYSTEM PERFORMANCE REQUIREMENTS Sensitivity Collection Area Angular Resolution Precision of source localization Energy Resolution 6 3. PHYSICAL REQUIREMENTS REQUIREMENT OF MORE THAN ONE TELESCOPE TYPE CAMERA FIELD OF VIEW PRELIMINARY DEFINITION OF TELESCOPES LST: large size telescope MST: mid size telescope SST: small size telescope, variant 1 (Davies Cotton optics) SST: small size telescope, variant 2 (secondary optics) LAYOUT ENERGY RANGES COVERED BY TELESCOPE TYPES ARRAY TRIGGER DATA ACQUISITION SYSTEM DEAD TIME DURING OBSERVATIONS DATA PRODUCTS ACCURACY OF THE ENERGY SCALE OPERATING CONDITIONS LIGHT LEVELS Background Light Level Signal Light Level DEFINITION OF OPERATING MODES Normal Operation Critical Situation Emergency Conditions Survival Conditions ENVIRONMENTAL CONDITIONS Altitude Operational Temperature Range Wind Speed and Power Spectral Density Seismic Activity Lightning Precipitation (rain) Snow and Ice Humidity LIFECYCLE, MAINTENANCE RELIABILITY, AVAILABILITY MAINTENANCE LIFE TIME SAFETY 24

3 Page : 3/25 1. Introduction This document collects fundamental performance requirements for a Cherenkov Telescope Array [RD1] (either Northern or Southern site) as a whole (level A). Subsequent documents (for now only to level B) in this collection will provide requirements and specifications at more detailed levels. Level B includes requirements at the sub system level (telescope types, array trigger ) which are independent of a specific implementation. A specific implementation would, for example be whether the photo sensors are photomultiplier tubes or semiconductor devices, whether signals are read out with analog ring buffers or with FADCs, whether a telescope trigger is based on pixel discriminators and subsequent sector wise multiplicity decision or it is based on FPGAs processing continuously digitized PMT pulses. Without reference to a specific implementation, some of the requirements and specifications both here at level A as well as in the companion level B document have to be rather general. Level C and D documents will eventually include specifications at an implementation specific level and in more detail. All of the documents in this series are only about the CTA instrument and will not attempt to deal with issues of site selection, with observatory operation etc. even though, for example, typical site parameters to be expected and consequences of changes of some site parameters on the instrument design may be addressed as well as data rates or other parameters relevant for observatory operation. In Section 2 (Scientific and ) we provide a first set of requirements at a global scale, addressing topics relevant to the astrophysicist who wants to make use of CTA. These include only basic parameters like the energy range to be covered, the field of view on the sky fully usable for science, as well as the sensitivity of a whole CTA instrument and a number of ingredients to achieving an excellent sensitivity. While this section is written without addressing specific sizes of telescope types or their layout, it should be emphasized that the resulting requirements are the result of detailed simulations and analysis codes, having tried a range of different layouts and settling to performance parameters we expect to be achievable with a class of CTA layouts providing a good compromise for a wide range of physics topics (see the Design Concepts document for a selection of physics topics envisaged as well a range of layout variants). Section 3 (Physical Requirements) introduces the types of telescopes which to our current understanding can be used to build a CTA instrument delivering or exceeding the requirements from section 2 within the anticipated cost frame. It also addresses other components needed for a CTA installation. Without the specific implementations at hand, these requirements again are rather general and may, for example, also be met by slightly smaller telescopes with higher quantum efficiency sensors than foreseen as a baseline, or by layouts with more medium size telescopes of wider spacing and fewer small telescopes etc. More detailed specifications will be the result of further optimizations of cost versus performance. Section 4 (Operating conditions) provides a collection of expected conditions which are generally not specifications for CTA itself but have to be kept in mind for the instrument design. Depending on the eventual site selection, some of these conditions will have to be detailed later, while other conditions may be relevant for issues of site selection as questions like maximum wind speeds, earthquake hazards, lightning or fires, ambient temperatures and pressure may lead to additional costs. Finally, Section 5 (Lifecycle and Maintenance) addresses issues relevant not just for building a CTA installation but also successfully operating and maintaining it over a period of many years.

4 Page : 4/25 2. Scientific and 2.1. Overview The Cherenkov Telescope Array should provide substantial improvements over current very high energy (VHE) γ ray instruments in a number of areas: Energy range covered. Sensitivity, in particular in the core energy range (about 100 GeV to 10 TeV), i.e. the energy range where the advantages of IACTs are largest in comparison to other detection techniques. Effective area. Gamma hadron discrimination. Angular resolution. Energy resolution. Field of view for gamma rays Full sky coverage, through sites on both hemispheres The wide energy range is required for distinguishing different physical processes that can be at action in a γ ray source. Over small energy ranges most processes result in power law spectra. Only observations over a wider energy ranges will show the characteristic deviations from power laws. Observations with a single instrument have a number of advantages for that purpose, in comparison to combining, for example, observations with Fermi and an IACT instrument: observations are strictly simultaneous and cross calibration errors are minimal. The sensitivity actually results from a number of factors, including effective area, gamma hadron discrimination and (at least for point like sources) angular resolution. The effective area determines the number of γ rays detected from a source of a given flux. The gamma hadron discrimination tells by how much the omnipresent background of cosmic ray protons and nuclei can be suppressed. In contrast to protons and nuclei, the additional electron background is difficult if not impossible to suppress. In either case the background suppression gets more difficult at lower energies because the numbers of interactions and particles involved are lower and therefore fluctuations in the shower development get more important. Remaining backgrounds will not only reduce the statistical significance of source detections but will also vary with off axis angle (w.r.t. to the instrument viewing direction), with zenith angle, with angle to the geomagnetic field, with atmospheric conditions etc. Systematic errors in the subtraction of such backgrounds are thus unavoidable and will limit the sensitivity when both, the numbers of detected γ rays and of remaining background events are high (or it would result in false detections). An improved angular resolution will both reduce the background on top of a point like source, and it will also allow to study the morphology of extended sources in more detail. Improved energy resolution can aid in the detection of possible line emission from dark matter annihilation or decay, in the accurate determination of changes in spectral index, cut offs etc Energy range The energy range to be covered with CTA is limited at the lowest energies by competition with space based instruments (e.g. Fermi) which achieve a much better background suppression by direct (non )observation of the incoming particles and/or the initial interaction products. The effective areas of space based instruments will be many order of magnitudes below that of CTA. The energy below which space based instruments can be more sensitive than CTA will differ between steady sources, where data of many years of Fermi observations can be added up, and variable sources, where only CTA will be able to detect a significant number of gamma rays in a short time. The sensitivity cross over is expected between 50 and 70 GeV for steady point like sources and 10 to 20 GeV for variable sources, depending on the time scale. At the highest energies, the flux of any source is so low that a huge area has to be covered with Cherenkov telescopes. Alternate detection techniques, either non imaging Cherenkov detectors or more traditional particle detector arrays, may ultimately become more cost effective despite their much inferior gamma hadron discrimination. A physically well motivated aim of detecting gamma rays of a few hundred TeV is well within reach of CTA. Such gamma rays

5 Page : 5/25 would result from interaction of PeV energy cosmic rays and would thus indicate the sources of such cosmic rays (often termed pevatrons). Specification: the accessible energy range of CTA should be 20 GeV to 300 TeV Implications on cost: Reduction of the lower limit would require larger telescopes and more of them, with more accurate control of background systematics (implying a larger camera field of view etc.). Extending the upper limit would require coverage of a substantially larger area with instruments sensitive to very bright showers Field of View for Gamma Rays on the Sky The field of view for gamma rays of a telescope system is the angle between the shower direction and the observation direction of the telescopes, at which detection efficiency for reconstructed gamma rays drops to 80 % of the on axis efficiency. In order to fully contain the shower images, the camera field of view needs to be larger than the field of view for gamma rays. A larger camera field of view also results in a larger field of view of a telescope system for gamma rays. In order to cover the full energy range, CTA is foreseen to be built of three types of telescopes of different sizes (see 3.1), large telescopes for the low energy section and medium size and small size telescopes for the mid and high energies, respectively. A uniform size of the gamma ray field of view (f.o.v.) across all energies would be desirable but is unachievable due to problems of mechanical stability resulting from high camera weights and extremely long focal lengths as needed for a good optical point spread function. With the low energy telescope being in competition to direct detection techniques and aiming mainly at transient and variable sources, looking necessarily point like, their requirements differ from those for the medium and high energy instrumentation. Low energy observations will mainly focus on point like sources, usually with a single (main) source as its target. In addition, small (few 0.1 ) sources and the small scale morphology of extended sources will be studied in the GeV energy region. Alternating off axis ( wobble mode ) observations of at angular resolution are required for optimal subtraction of cosmic ray and electron backgrounds. Specification: The gamma ray f.o.v. for the low energy regime should thus extend at least to 1.0 from the axis. Core energy regime and high energies: In addition to point like (often variable) sources, observations will include extended (steady) sources, in particular within a few degrees of the Galactic Plane. Multiple sources within the field of view are common and source free regions for proper background subtraction are rare unless the f.o.v. extends beyond the Galactic Plane. The number of sources which can be studied simultaneously should increase proportional to the f.o.v. solid angle (and thus the number of pixels in the camera) as long as the f.o.v. is still within the Galactic Plane, and then proportional to the f.o.v. diameter once extending beyond the Galactic Plane. Specification: For optimum overall performance the gamma ray f.o.v. radius for the core energy regime and high energies should extend to at least System The following system performance parameters quantify the performance requirements for CTA as next generation observatory. They are based on large scale simulations of many different possible implementations of CTA, analysis mainly by traditional methods (using Hillas parameters etc.) and selection of a near optimum configuration based on physics goals. It should be made clear that observations of different types of objects, with different spectra, may be better off with different configurations but the overall mix of observations of a wide range of object types is best served by a balanced configuration putting similar amounts of resources into the different CTA subset energy ranges (see figure 1).

6 Page : 6/ Sensitivity Description: The minimum flux of a point source that can be detected in given observation time. Specification: For 50 hours of observation time about 1 millicrab in integral sensitivity above a few hundred GeV (core energy range), about 10 millicrab above 10 TeV. For low energies above 50 GeV a sensitivity of about 10 millicrab should be achieved in only 5 hours of observation time. (see figure 2). The corresponding differential sensitivities are shown in figure Collection Area Description: The collection area, or effective area, is defined as the effective geometric area on the ground over which incident gamma ray showers can be detected and reconstructed. It can be calculated as the number of reconstructed showers in a given observation time divided by the incident flux and observation time. Specification: 0.05 km 2 at 50 GeV, 1 km 2 at 1 TeV, 5 km 2 at 100 TeV Angular Resolution Description: The ability of CTA to resolve an image spatially on the sky, expressed in degrees as the radius of a circle that contains 68% of the reconstructed image resulting from a pure point source. Specification: 68% containment radius of 0.15 o at 100 GeV, 0.05 o at 1 TeV, and 0.02 o at 100 TeV (see figure 4) Precision of Source Localization Description: The precision, at which the position of point sources and slightly extended sources can be determined. For sources that are detected with high count rate and good signal to noise ratio, the statistical error on the source location can be as low as a few arc seconds, or even below. The precision on the source location will then be limited by systematic errors of the instrument. Specification: The systematic error on the localization of point sources is required to be less than 5 arc seconds (rms) in space angle Energy Resolution Description: How well the energy can be reconstructed expressed as a percentage of the true energy. Specification: Better than 10% (rms) for energies above 1 TeV and better than 20% (rms) for energies between 100 GeV and 1 TeV (see figure 5).

7 Page : 7/25 Figure 1. Top: Illustration of a balanced configuration of CTA South. In this example with three large, 18 mid size, and 56 small telescopes (see 3.1). The telescope positions are in units of meters. The effective area of CTA may extend slightly beyond the area actually covered by telescopes (resulting in about 0.1 km 2 in the energy range only available to the large telescopes, close to than 1 km 2 in the energy range of mid size telescopes and about 5 km 2 in the energy range of the small telescopes, not counting for γ rays lost in the gamma hadron discrimination). Bottom: Illustration of a possible configuration for a Northern CTA array with emphasis on the low energy band, here consisting of 4 large and 17 mid size telescopes.

8 Page : 8/25 Figure 2 Integral sensitivity of the balanced CTA configuration (see figure 1 top) as a function of energy. For comparison the sensitivity curves of current generation of Imaging Cherenkov experiments are drawn, as well as of the Fermi LAT. Figure 3 Differential sensitivity of the same CTA configuration in 5 bins per decade of energy, for different observation times. The sensitivity is shown in 'Crab units', assuming an E 2.57 power law spectrum matching the Crab Nebula at TeV energies. The black lines are parameterizations of the best performance of any tested configuration. The balanced configuration is thus close to optimal at all energies and for all observation times, even though other configurations may be slightly superior at specific energies.

9 Page : 9/25 Figure 4: Angular resolution of the balanced CTA configuration as a function, showing both the 68% and 80% containment radius under different cut optimizations resulting in best sensitivity or in best angular resolution. Figure 5 Energy resolution (r.m.s.) of the balanced CTA configuration as a function of energy, again for different cut optimizations.

10 Page : 10/25 3. Physical Requirements 3.1. Requirement of more than one telescope type At the lowest energies large telescopes are required to focus as many of the few Cherenkov photons as possible into a camera. These are very expensive and their effective areas are limited by the characteristics of the Cherenkov emission in air. In addition, the detector dynamic range is limited to a few thousand photo electrons per pixel (PMT). Some pixels will thus saturate at high energies. Smaller telescopes will not be able to see the lowest energy showers but will be more cost effective for the detection of the higher energy showers. A single type of telescopes may reasonably cover an energy range of a factor of a few hundred between threshold and saturation. Coverage of the CTA energy range with only two types of telescopes would just barely be possible, leaving basically no overlap in the individual ranges and thus resulting in poor cross calibration. A two type configuration would also result in nonoptimal performance in the core energy range because the large telescopes would be in saturation and cover a too small effective area while the small telescopes would not yet detect the showers with good quality. The optimal design of CTA thus requires three types of telescope, with the middle size (MST: mid size telescope) matching the core energy range of the imaging atmospheric Cherenkov technique. Large and small telescopes (LST: large size telescope, SST: small size telescope) can then be built to have a comfortable overlap in individual energy ranges with the midsize telescopes (up to about a TeV for the large telescopes and starting at about a TeV for the small telescopes). For a site emphasizing observation of distant extragalactic objects (mainly AGN), the small telescope type would not be required since γ rays above the energy range of the mid size telescopes would be lost on their way due to interactions with the cosmic infra red background Camera Field of View Following the requirements on the field of view for gamma rays on the sky (section 2.3), the field of view required for the cameras of the different telescopes can be derived: LST: Images of gamma ray showers must be fully contained in the camera field of view as long as a telescope is in the light pool of the shower (image c.o.g. up to 0.9 offset plus ~0.5 length). The camera f.o.v. must thus be larger than the gamma ray f.o.v. Having more than the bare minimum f.o.v. will result in a more uniform response over the gamma ray f.o.v. and reduce any systematic errors in the background subtraction. A camera f.o.v. diameter of ~ 4.2 o 5.0 o is required. MST and SST: For MSTs, with inter telescope spacing not much exceeding the light pool radius, resulting camera f.o.v. diameters of 6 8 are optimal. For SSTs using large inter telescope spacings to extend the area coverage, the same gamma ray f.o.v. corresponds to optimal camera f.o.v. diameters of Preliminary Definition of Telescopes Experience with past Cherenkov telescope installations and detailed Monte Carlo simulations indicate that the desired performance for a full CTA array can achieved with a combination of three sizes of telescopes, as indicated below: LST: large size telescope Mirror collection area: ~400 m 2 Angular pixel pitch: ~ 0.1 o Camera FoV: ~ 4.2 o 5.0 o

11 Page : 11/ MST: mid size telescope Mirror collection area: ~100 m 2 Angular pixel pitch: ~ 0.18 o Camera FoV: ~ 7 o 8 o SST: small size telescope, variant 1 (Davies Cotton optics) Mirror collection area: ~10 40 m 2 Angular pixel pitch: ~ 0.25 o Camera FoV: ~ 7 o 10 o SST: small size telescope, variant 2 (secondary optics) Mirror collection area: ~10 m 2 Angular pixel pitch: ~ 0.2 o Camera FoV: ~ 7 o 10 o 3.4. Layout Due to the forward direction of Cherenkov light emission in air and the resulting Cherenkov light pool of fairly uniform illumination of about m diameter, inter telescope spacings of about 100 m are needed at the threshold, subject to optimization. Well above threshold, showers can be detected from outside the light pool, if the field of view is large enough. For an instrument consisting of several telescope type, the few (3 5) large telescopes (LSTs) will be placed in the center of the array. The more numerous (15 25) mid size telescopes (MSTs) will cover a larger area, again starting with inter telescope spacings of the order of 100 m in the inner region. These spacings may increase towards the edge of the MST part. The Northern CTA installation may consist of only the LST and MST parts since most sources unique to CTA North will be of extragalactic nature and no photons beyond tens of TeV may reach us. The Southern installation, with the inner Galaxy also in its view, will in addition also have many (25 100) small telescopes (SSTs) covering an even larger area, with inter telescope spacings from about 150 to 250 m. A few SSTs in the inner region covered by LSTs and MSTs can avoid gaps in area coverage at the highest energies where LST and MST cameras are in saturation and aid the sub system cross calibration. CTA South: 3 5 LSTs, MSTs, SSTs (see figure 1, top). CTA North: 3 5 LSTs, about 15 MSTs (see figure 1, bottom) Energy ranges covered by telescope types Description: Section 2.2 defines the required overall energy range of the CTA instrument, from about 20 GeV (for variable sources) or about 70 GeV (for steady sources) up to about 300 TeV. The lower limit, with overlap to space based instruments (Fermi) is covered by the LST. The upper limit of this range also defines the upper limit to the SST sub system. The overlap of the LST sub system with the MST sub system is required for at least two purposes: The LST telescopes need to be able to measure gamma ray showers up to at least 1 TeV for cross calibration with the MSTs. For independent operation of the LST sub array, showers should not saturate the cameras up to energies, where the sensitivity of the sub array is limited by signal statistics (at least 10 detected gamma rays). See reference [RD2]

12 Page : 12/25 In addition, overlap in energy ranges between MSTs and both LSTs as well as SSTs are required for reasons of crosscalibration. Specification: 20 GeV to 300 TeV, split up as TeV (LST), TeV (MST), <1 300 TeV (SST) Implications on cost: Reduction of the lower limit would require larger telescopes and more of them, with more accurate control of background systematics (implying a larger camera field of view etc.). Extending the upper limit would require coverage of a substantially larger area with instruments sensitive to very bright showers. A larger overlap between the different telescope types would require an increased dynamic range of the cameras Array Trigger The concept of CTA is based on the stereoscopic detection of air showers. Therefore it is advisable to combine the information of the different telescopes already online, and to acquire event data only for further processing, if certain, configurable inter telescope trigger conditions are fulfilled [RD3, RD4, RD5]. This helps to suppress background, to reduce system dead time and to increase the sensitivity of a telescope system. The CTA arrays therefore should be equipped with configurable array trigger systems Data Acquisition The bulk of data is generated by the cameras of the large size and medium size telescopes. The individual telescopes will generate pre array trigger about Byte of raw data per pixel and event, resulting in a raw data flow of the order of up to a GByte/sec per telescope. For a balanced Southern array (figure 1 top) the expected rate of detected air showers after array trigger will be of the order of khz, with an average telescope multiplicity of 5 6. For these events, the recorded waveforms of the pixels will be analysed and all calibration steps performed, resulting in pixel intensity values and possibly pulse arrival time values per pixel. In this processing step, the amount of data will be reduced by more than two orders of magnitude from about 50 GByte/sec to a few hundred MByte/sec of calibrated camera images, ready for stereoscopic event reconstruction. Detailed values derived from simulations can be found in the report of the WP MC trigger sub task group [RD6] The CTA data acquisition system has to provide the corresponding data transmission bandwidth and processing power, to allow for an online analysis including all pre processing and calibrations steps of the recorded events System Dead Time during Observations During normal data taking for observations, it is possible that telescopes can t process events, because either the camera readout or the data pipeline is busy with previous event(s). This generates dead time of the instrument, which leads to a reduced sensitivity of the system. Therefore, the dead time of individual telescopes during observations at normal operating conditions should be less than 5 %. In addition, the probability that at least one telescope is busy in an event should be less than 10 %. A second source of dead time during observation sequences can be delays in the response of the data acquisition and slow control software, e.g. between individual observation runs, preventing the telescopes from taking data and resulting in a loss of possible observation time. The intrinsic response time of these processes should be reduced to less than 10 sec.

13 Page : 13/ Data Products The telescope cameras should provide per pixel a measurement of the light intensity and possibly a measurement for the arrival time of the light pulses in the pixels. For the reconstruction of the events, additional calibration data will be required, like the optical efficiency of the telescopes, the light sensitivity of the individual pixels, the pointing of the telescopes in the sky. The corresponding calibration methods and devices have to be foreseen Accuracy of the Energy Scale The measurement of flux and energies of gamma ray is related to the amount of Cherenkov light emitted by the atmospheric showers. The camera instrumentation provides electric signals that are related to emitted photons by taking into account the electronics properties (e.g. amplification) and the efficiencies of all optical components (e.g. atmosphere, telescopes, camera). The relationship between the number of emitted photons and the measured signal is determined by the calibration process. It takes into account the atmospheric absorption (measured, e.g. using LIDARs), the global optical efficiency of the telescope (mirrors, structure and camera shadowing) and of the camera (entrance window, photo sensors) and its electronics properties (pedestal, amplification, linearity). Beyond the intrinsic limitation of the Atmospheric Cherenkov technique, the accuracy of calibration directly translates into the accuracy of the energy scale. Specification: <10% To achieve this accuracy, continuous atmospheric measurements are required on each observatory site. In addition each telescope should have a specific instrumentation to measure the telescope efficiency. In general, the largest inaccuracy is coming from the atmosphere properties.

14 Page : 14/25 4. Operating Conditions 4.1. Light Levels A telescope is exposed in a dark night to a) a persistent background light level from the night sky and b) short pulses of Cherenkov light Background Light Level Background light is due to star light, zodiacal light, air glow, scattered sun light, moon light and man made light pollution. In a dark site (little light pollution) at mountain level altitudes and under conditions of astronomical darkness, it is about corresponding to one magnitude 22 star per square arc second or some 200 magnitude 10 stars per square degree in the V (green) wavelength band, increasing substantially in energy flux from the UV to the green and red wavelength region [RD7]. Since Cherenkov light is most intense in the blue and UV region, photo sensors should provide as little sensitivity as possible in the green and, in particular, the red region. They should be completely insensitive to IR wavelengths. In a typical Cherenkov telescope of 100 m² mirror collection area and photon detectors as foreseen for an MST, this background light level will result in background photo electron rates in excess of 100 MHz per pixel, even for a very dark region of the sky. For a typical region in the Galactic plane, a level of twice that is common, with considerable spread. In observations while the moon is up, a level of five times that of dark sky is easily exceeded, depending on the phase and distance of the moon, the angle between telescope viewing direction and the moon etc. Observations under high light levels may increase the aging of the photo detectors. Even under dark conditions, the afterpulsing of photo multiplier tubes from background light may limit the energy threshold and low energy sensitivity of Cherenkov telescopes Signal Light Level The Cherenkov light of a 100 GeV gamma ray shower has a typical and fairly uniform density of some 10 photons/m² on ground (in the wavelength range 300 to 600 nm) within about 120 m from the shower core (see figure 6). This region around the shower core is referred to as the light pool. Beyond that the light density falls with radius R roughly like 1/R². The average central density depends also on the altitude of the site and the atmospheric profile, mainly due to different distance of the shower maximum to the site. At increasing energies E it rises for gamma rays a bit more rapidly than proportional to E, due to increasing shower penetration into the atmosphere. Proton showers are rather similar at TeV energies but are far less efficient than gamma rays (or electrons) at low energies. A typical telescope of 100 m² mirror collection will get some 1000 photons reflected to its camera from a 100 GeV shower and about 5 million photons from a 100 TeV shower. Depending on the photon detectors, this may result in close to 100 photo electrons at the 100 GeV detection level, of which about half may be detected in a handful of pixels, the remaining photo electrons being more spread out. For telescopes inside the light pool, most of the Cherenkov light will arrive within a few nanoseconds. Far outside of the light pool, arrival times may be spread out over tens of nanoseconds, due to geometrical reasons. In either case, the signal in each pixel can be rather short, with a typical r.m.s. width of 0.7 ns due to photon arrival times slightly narrower at the rim of the light pool but generally wider both inside and outside (see figure 7). Typical PMTs will add a similar spread due to their transit time spread, and the reflector, if optimized for point spread function rather than time spread, may add again a similar or even larger time spread. At the detection level, the pixels may thus see about 5 to 10 photo electrons, most of them within about 2 3 nanoseconds (FWHM). At energies of a thousand times higher than the detection energy threshold, the most intense pixels may have to deal with some pulses of photo electrons, a level easily exceeding the dynamic range of photon detectors and electronics.

15 Page : 15/25 Figure 6: Lateral distribution of Cherenkov light density (in the nm wavelength range) on ground for simulated gamma ray showers (left) and proton showers (right).

16 Page : 16/25 Figure 7: Distribution of photon arrival times on ground from simulated vertical gamma-ray showers in different ranges of core distance. In the absence of a time gradient along a shower image (around 120 m core distance), this is also the time distribution in a single pixel, for an ideal telescope and photo-detector. Time zero corresponds to when the primary gamma-ray would have arrived on ground, in case of no interactions Definition of Operating Modes Normal Operation The telescopes can be used for regular observations or be moved for maintenance. All deformations and the optical PSF have to be maintained within the limits for regular observations. All drive systems need to be operational at full speed and acceleration. The camera can be switched on and the lid opened (if in addition darkness criteria fulfilled) Critical Situation Observations / maintenance have to be stopped and the telescopes need to be moved to the safe position. No nonreversible deformations / damage may occur. All drive systems need to be operational at least at 70% of the full speed and of the full acceleration. The camera needs to be switched off and the lid closed Emergency Conditions In case the environmental conditions exceed the thresholds defining the emergency situation, the telescopes still need to be moveable to the safe position. No non reversible deformations / damage may occur. All drive systems need to be operational at least at 10% of the full speed and of the full acceleration. The camera needs to be switched off and the lid closed.

17 Page : 17/ Survival Conditions The telescopes need to be in the safe position. No non reversible deformations or damage may occur to the telescopes. The telescopes do not need to be movable. The camera needs to be switched off and the lid closed Environmental Conditions The environmental conditions described herein are not specifications for the CTA site, and are not intended as a complete description of the environment on site. They rather represent the current best knowledge of an expected site, and should therefore be treated as the site parameters relevant for the design of the telescopes Altitude Description: The operational altitude is the altitude at which the telescopes will be required to be operational. Specification: Operational altitude 1500 to 3800 metres above mean sea level. Justification: The geographical location of the CTA telescopes has not been identified yet, however site candidates have been identified at altitudes ranging from 1500 metres to 3800 metres above sea level. Qualification: The ambient air pressure and density is reduced as a function of site altitude. Items of electrical or mechanical equipment that are designed to operate at sea level (atmospheric pressure Pa) should be de rated for operation at the site altitude. Ultraviolet radiation also increases with altitude due to decreasing atmosphere thickness, ozone, cloud cover and aerosols. Impact on Performance: Cooling efficiency of items such as motors and fans will be impaired, hence these must be oversized for operation at altitude. As a general guideline ultraviolet solar radiation increases by approximately 10 % for every 1000 metres increase in altitude (estimated by the World Health Organisation). Materials such as thermoplastics and polymers are particularly susceptible to photodegradation due to ultraviolet exposure and increased UV levels can cause embrittlement and loss of extensibility, tensile strength and impact resistance. Impact on Cost: The requirement to de rate equipment for operation at elevated altitudes and specify ultraviolet resistant materials can substantially increase component and array capital cost. For example the use of ultraviolet resistant PTFE pneumatic airlines in lieu of nylon can increase costs by a factor of five Operational Temperature Range Description: The normal operational temperature range is defined as the bound formed by the upper and lower temperature limits for which the telescope will be required to conduct normal operations. The emergency operational temperature range is defined as the bound formed by the upper and lower temperature limits for which the telescope must be capable of being operated such that it may be either restored to its normal operating condition or be placed in a stowed position. The survival operational temperature range is defined as the bound formed by the upper and lower temperature limits which the telescope must be capable of withstanding without suffering permanent damage. Specification: Parameter Minimum Value Maximum Value Normal Operating Temperature 10 Celsius +30 Celsius Emergency Operating Temperature 15 Celsius +45 Celsius Survival Operating Temperature 25 Celsius +60 Celsius

18 Page : 18/25 Justification: The operating temperature ranges must incorporate both diurnal and annual temperature extrema for the candidate or actual site location accounting for both air temperature and gains from solar radiation. Qualification: Diurnal and annual air temperature extrema may be obtained from local or national meteorological data. Surface temperatures due to solar radiation may be estimated by calculation of the equilibrium temperature derived from incident solar radiation heat flux. Impact on Performance: Differential thermal expansion may cause structural damage, binding of moving parts, induce high strains in optical components, cause unstable operation or failure of electronic components due to high thermal gradients and degradation of polymers and composite materials resulting in a shortened operational lifetime. Impact on Cost: Athermalisation of the structure may require the use of materials having low coefficients of thermal expansion, e.g. austenitic stainless steels, invar alloys or composite materials. Use of these can significantly increase cost. Impact on Other Specifications: The structural properties of many commonly used engineering materials such as ferrous and non ferrous metals, polymers and composites vary widely with operating temperature. Variations of mechanical properties such as the Young s modulus, fracture toughness, fatigue resistance, stress strain behavior and thermal expansion coefficients must be considered Wind Speed and Power Spectral Density Description: The normal operational wind speed is defined as the wind speed at which normal telescope operation is possible. The critical operational wind speed is defined as the wind speed at which normal telescope operation shall cease and the telescope be moved at normal slew speed to its safe stow position. The emergency operational wind speed is defined as the wind speed at which the telescope must be immediately moved to its safe stow position. The maximum survival wind speed is defined as the maximum wind speed that the telescope will sustain without suffering permanent damage that would prevent immediate resumption of normal telescope operation. The wind power spectral density is defined as a function of mean wind velocity and frequency. Specification: Parameter Normal Operational Wind Speed Critical Operational Wind Speed Emergency Operational Wind Speed Maximum Survival Wind Speed Nomenclature: km/hr = Kilometres per hour Value Equal to or less than 50 km/hr Exceeding 50 km/hr but less than 65 km/hr Exceeding 65 km/hr but less than 100 km/hr Equal to 200 km/hr The wind power spectral density may be calculated either as a Von Karman Power Spectrum or by using an empirically based (Davenport) model. Justification: It is anticipated that static and dynamic wind loads will render normal telescope operation difficult at wind speeds exceeding 50 kilometres per hour. For wind speeds in excess of 50 kilometres per hour the telescope should be moved to the safe stow position. For the survival condition of 200 kilometres per hour the telescope should be locked in the stow position. Qualification: Wind speed magnitudes and power spectra are dependent upon environmental conditions at the telescope geographical location. Therefore site specific wind speed and power spectrum data must be obtained from local or national meteorological institutes.

19 Page : 19/25 Impact on Performance: The operational wind speed will directly impact upon the operational slew torque and the telescope pointing and tracking ability. The critical and emergency wind speeds will impact upon the slew torques required to drive the telescope to the safe stow position, the survival wind speed will dictate the magnitude of the static and dynamic wind loads that must be resisted by the telescope and the magnitude of the overturning moments that must be resisted by the foundation. Impact on Cost: The operational, critical and survival wind speeds will dictate the required drive torques and electrical power requirements and hence their capital and running costs. It is important to consider overall electrical power requirements for the array as the required infrastructure will incur a significant capital cost and operational power requirements will be a significant fraction of the annual running costs. The maximum survival wind speed will dictate the foundation and structure design and cost. Impact on Other Specifications: The maximum survival wind speed will directly impact upon the foundation specification, design and cost. Anchoring methods for the telescopes such as rock bolting or the use of a gravity base must be designed so as to withstand the survival wind speed Seismic Activity Description: Seismic activity is defined as the magnitudes of horizontal and vertical accelerations that the telescope must withstand during normal operation. The operational base earthquake is defined as the maximum earthquake level that the telescope must withstand without suffering damage that would prevent quickly resuming normal operation. The maximum likely earthquake is defined as the maximum earthquake level that the telescope will withstand without suffering such severe damage that it proves uneconomic to resume use of the telescope. Specification: The magnitudes of peak horizontal and vertical accelerations are site specific and must be obtained from local construction design codes or from national geophysical institutes. However as an example assuming South America as the telescope site the operational base earthquake horizontal acceleration magnitude will be specified to be 0.25 G and the maximum likely earthquake horizontal acceleration magnitude will be specified to be 0.34 G. The vertical acceleration in both cases will be assumed to be equal to 0.67 times the horizontal acceleration magnitude, with both the horizontal and vertical accelerations being applied simultaneously. Justification: Seismic maps indicating global peak ground acceleration magnitudes are available from GSHAP (Global Seismic Hazard Assessment Programme). The resistance of the telescope structure and foundation to seismic damage will typically be evaluated by specifying the magnitude of the operational base earthquake (OBE) and the maximum likely earthquake (MLE) in terms of acceleration amplitudes. These can vary considerably depending upon the site location. For example for South America the maximum likely earthquake (MLE) horizontal acceleration magnitude may be 0.34 G and the operational base earthquake (MBE) horizontal acceleration magnitude may be 0.25 G, whereas for West Africa these may be 0.08 G and 0.05 G respectively. Due to the potential for facility damage due to under specification or of incurring additional construction costs due to over specification it is extremely important that site specific acceleration magnitudes are obtained and adopted in the design. Qualification: General rules for the design of structures for seismic activity that may assist the designer when evaluating the resistance of the telescope structure to seismic activity will be given in local construction design codes. As an example within Europe the following design codes that may be relevant are Eurocode 8: Design of structures for earthquake resistance, Part 1: General rules seismic actions and rules for buildings, Part 5: Foundations, retaining structures and geotechnical aspects and Part 6: Towers, masts and chimneys. Impact on Cost: Foundation and structure cost may be considerably affected by the seismic acceleration magnitudes. Impact on Other Specifications: Acceleration of the telescope as a result of seismic activity will impose large overturning moments upon the foundation. Anchoring methods such as rock bolting or the use of a gravity base must be designed to withstand these.

20 Page : 20/ Lightning Description: A lightning protection system must be installed consisting of air termination points, down conductors, test points and earthing points. Specification: The lightning protection system must be designed in accordance with national standards, for example in the United Kingdom the relevant standard being BS (Protection against Lightning). Specification of the lightning protection system and modes of telescope operation during weather conditions likely to result in occurrences of lightning will be provided in the additional safety documentation for the telescope. Impact on Cost: The requirement to provide a lightning protection system for the telescopes within the array is anticipated to be a significant capital cost Precipitation (rain) Description: This is defined as the rainfall rate and velocity. Specification: For normal telescope operation the rainfall rate is specified to be zero. For the critical, emergency and survival conditions the rainfall rate and velocity must be derived from rainfall data obtained from local or national meteorological institutes. Justification: Rainfall rate and velocity is expected to be highly dependent upon local climatic conditions, hence local site specific rainfall data must be applied. Water tightness of enclosures containing critical electrical items must be specified by an ingress protection rating. Impact on Performance: Pooling or accumulation of rainwater upon or within the telescope may cause intermittent operation or failure of electrical items and increase the static loads imposed upon the telescope. Accumulation of rainwater within hollow members such as telescope truss tubes or mount will increase drive torques and power requirements and decrease slew rates. Accumulation of trapped or standing water will lead to corrosion and may also lead to structural damage as a result of ice formation at low temperatures. Impact on Cost: Specification of enclosures, cabling and electrical connectors that will be resistant to water ingress will incur additional cost Snow and Ice Description: Site specific snow and ice loads should be applied to the telescope structure in order to evaluate the resistance of the telescope structure to accumulated snow or freezing precipitation. Specification: Site specific snow loads and ice accretion data must be obtained from national or regional specific snow load maps. Justification: It is suggested that reference be made to national standards, e.g. ASCE 7 10 Snow Loads (United States), Eurocode 1 Actions on Structures Part 1 3 General Actions Snow Loads (Europe) or to the relevant national standards for the country concerned. Impact on Performance: Ice accretion or snow accumulation on the telescope will increase static loads upon the telescope due to self weight and increase static and dynamic loads due to wind resistance. These in turn will affect pointing and tracking ability and increase slew torques and drive power requirements and increase the total power requirements for the telescope array. Impact on Cost: Snow or ice accumulation will impose increased static and dynamic loads upon the telescope, the telescope structure must be designed to withstand these at a consequent increase in cost.

21 Page : 21/25 Impact on Other Specifications: Snow and ice loads will also increase static loads upon any telescope enclosure that may be specified Humidity Description: The normal operational humidity range is defined as the bound formed by the upper and lower humidity limits for which the telescope will be required to conduct normal operations. The dew point indicates the temperature at which water vapour will condense from the local air mass to form liquid water. The critical and emergency operational humidity ranges are defined as the bounds formed by the upper and lower humidity limits for which the telescope must be capable of being operated such that it may be either restored to its normal operating condition or be placed in a stowed position. The survival humidity range is defined as the bound formed by the upper and lower humidity limits which the telescope must be capable of withstanding without suffering permanent damage. Specification: Parameter Minimum Value Maximum Value Normal Operating Relative Humidity 5 Percent 95 Percent Critical Operating Relative Humidity 0 Percent 100 Percent Emergency Operating Relative Humidity 0 Percent 100 Percent Survival Relative Humidity 0 Percent 100 Percent Justification: The operating humidity ranges must incorporate both diurnal and annual humidity extrema for the candidate or actual site location. Local meteorological data must be consulted so as to assess the potential for dew forming conditions to occur that may prevent normal operation of the telescope due to degraded image quality. Impact on Performance: Low relative humidity can cause degradation of polymeric or rubber based compounds such as seals, couplings or drive belts and increase the possibility of electrostatic discharge, high relative humidity can cause condensation within enclosed spaces such as electrical enclosures. Should the dew point fall below the ambient temperature condensation may form on optical components and metallic surfaces and the optical image quality be adversely affected. Dewing of optical surfaces may be mitigated by use of anti dew heating elements embedded into the optics but these will considerably increase optical element complexity and electrical power requirements. Impact on Cost: Dew forming on optical surfaces may necessitate the inclusion of anti dew heaters into the optics, considerably increasing optic component costs and array operational costs.