The Milky Way viewed in VHE gamma-rays, in four bands of Galactic longitude [5]. 

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... first images of the Milky Way in VHE gamma-rays have been obtained in the last few years. These reveal a chain of gamma-ray emitters situated along the Galactic equator (see fig. 2), demonstrating that sources of high-energy radiation are ubiquitous in our Galaxy. Sources of this radiation include supernova shock waves, where presumably atomic nuclei are accelerated and generate the observed gamma-rays. Another important class of objects are "nebulae" surrounding pulsars, where giant rotating magnetic fields give ...

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... This light can be captured with optical elements and be used to image the shower, which vaguely resembles a shooting star. Reconstructing the shower axis in space and tracing it back onto the sky allows the celestial origin of the gamma-ray to be determined. Measuring many gamma-rays enables an image of the gamma-ray sky, such as that shown in fig. 2, to be created. Large optical reflectors with areas in the 100 m 2 range and beyond are required to collect enough light, and the instruments can only be operated in dark nights at clear sites. With Cherenkov telescopes, the effective area of the detector is about the size of the Cherenkov pool at ground. As this is a circle with 250 m ...

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... parts of the sky, maximise serendipitous detections, allow for optimal use of telescope time, and thereby ensure the legacy of the project for the future scientific community. Surveys of different extents and depths are among the scientific goals of all major facilities planned or in operation at all wavelengths. In view of both H.E.S.S. (see Fig. 2) and Fermi-LAT survey results, the usefulness of sur- veys is unquestioned, and many of the scientific cases discussed above can be encompassed within such an observational ...

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... telescopes of 7 m diameter with a 10 • field-of-view and 0.25 • pixels. The telescopes are distributed over ∼3 km 2 on the ground and the effective collection area of the array is considerably larger than this at energies beyond 10 TeV. The sensitivity of array E from detailed calculations and using standard data analysis techniques is shown in fig. 23. More sophisticated analyses result in sensitivities that are ∼20% better across the whole energy range. As fig. 23 shows, such an array performs an order of magnitude better than an instrument like H.E.S.S. over most of required energy range. Fig. 25 shows the angular resolution of this array, which approaches one arcminute at high ...

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... 2 on the ground and the effective collection area of the array is considerably larger than this at energies beyond 10 TeV. The sensitivity of array E from detailed calculations and using standard data analysis techniques is shown in fig. 23. More sophisticated analyses result in sensitivities that are ∼20% better across the whole energy range. As fig. 23 shows, such an array performs an order of magnitude better than an instrument like H.E.S.S. over most of required energy range. Fig. 25 shows the angular resolution of this array, which approaches one arcminute at high energies. The energy resolution of layout E is better than 10% above a few hundred ...

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... of array E from detailed calculations and using standard data analysis techniques is shown in fig. 23. More sophisticated analyses result in sensitivities that are ∼20% better across the whole energy range. As fig. 23 shows, such an array performs an order of magnitude better than an instrument like H.E.S.S. over most of required energy range. Fig. 25 shows the angular resolution of this array, which approaches one arcminute at high energies. The energy resolution of layout E is better than 10% above a few hundred ...

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... 0.1-1 TeV for the benchmark array. However, it is not clear that this result on relative performance at intermediate energies can be generalised to the much larger telescope array of smaller telescopes with which CTA plans to cover this energy range. Simulations of the 275-telescope array at 3700 m altitude are underway to address this question. Fig. 20 shows the impact of changing the (angular) pixel diameter (Θ p ) on the sensitivity of the benchmark array at 2000 m altitude. It can be seen that only modest improvements are possible with pixels below 0.1 • diameter. As the camera cost increases as 1/Θ 2 p , smaller pixels sizes are strongly disfavoured. The improvement of an- gular ...

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... described above is the focus of the current work within CTA and has been used to demonstrate the validity of the CTA concept. Fig. 21 shows some example events as seen in a candidate sub-configuration of this pro- duction array, demonstrating the high telescope multiplicity (and event quality) which is a key element of the CTA design. Fig. 22 shows how the angular resolution defined as the 68% containment radius, improves with the number of telescopes that record a shower image. With 4 images (as for instruments like H.E.S.S. or VERITAS) a resolution of about 0.1 • is reached, while with ≥ 12 images the resolution is ≤0.05 • . For the most energetic showers, resolutions of ...

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... efficiency curve of similar spectral shape (blue-sensitive) to that of current bi-alkali PMTs is assumed. This is a conservative assumption as ∼50% higher efficiency cathodes have recently been announced by several major manufacturers (albeit with larger after-pulsing rates, which may limit the advantage gained in terms of trigger threshold). Fig. 23 illustrates the integral flux sensitivity achieved with the three candidate CTA configurations shown above. The goal sensitivity curve for CTA is shown for comparison. It can be seen that these configurations (even with rather basic analysis methods) are close to achieving the goal performance in most energy ranges. At very high ...

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... spaced 12 m telescope array. This configuration provides superior hadron rejection and angular resolution (see later) but provides a more modest effective collection area at multi-TeV energies. Configuration E is a compromise array, which attempts to do well in all energy ranges using multiple telescope types and spacings. As can be seen from fig. 23, such an array comes closest to achieving the CTA performance ...

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... is important to study the potential sensitivity of CTA at much shorter observation times than the 50 hours used for reference. Fig. 24 shows how the sensitivity changes for 5 hour and 0.5 hour observations. The sensitivity scales linearly with time t in the regime limited by gamma-ray statistics and approximately with √ t in the background limited regime at lower energies. rays until 2007) would be possible in just over 30 minutes. Extreme AGN outbursts, which in the ...

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... by gamma-ray statistics and approximately with √ t in the background limited regime at lower energies. rays until 2007) would be possible in just over 30 minutes. Extreme AGN outbursts, which in the past have reached flux levels >10× the Crab flux, could be studied with a time resolution of seconds, under virtually background-free conditions. Fig. 24 also shows 50 hour sensitivity curves calculated using two independent analyses, illustrating a) that the conclusions on sensitivity presented here are robust and b) that the sensitivity can be improved using more advanced methods for background suppression over much of the CTA energy range. For the 50 hour curve two alternative ...

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... angular resolution for the CTA candidate systems is summarised in fig. 25. Res- olution at 1 TeV is in the 0.04-0.05 • range for configurations B and E, and somewhat worse for the larger area configuration C, illustrating the trade-off between collection area and precision at fixed cost. A simultaneous minimisation to find the best shower core and direction, using pixel timing information, provides a ...

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... worse for the larger area configuration C, illustrating the trade-off between collection area and precision at fixed cost. A simultaneous minimisation to find the best shower core and direction, using pixel timing information, provides a significant improvement over the tra- ditional intersection of image axes technique (see dashed line in fig. 25). The resolution approaches 1 arcminute at high energies. Fiducial cuts on core location and/or harder telescope multiplicity cuts improve this performance, at the expense of collection area. The energy resolution (for photon showers) as a function of energy is shown in fig. 26 for the candidate arrays B, C and E. The energy resolution ...

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... ditional intersection of image axes technique (see dashed line in fig. 25). The resolution approaches 1 arcminute at high energies. Fiducial cuts on core location and/or harder telescope multiplicity cuts improve this performance, at the expense of collection area. The energy resolution (for photon showers) as a function of energy is shown in fig. 26 for the candidate arrays B, C and E. The energy resolution is below 30% in almost the whole range of interest and ≤10% above about 1 ...

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... some of the very first Cherenkov telescopes were equipped with equatorial mounts, alt-azimuth mounts offer obvious advantages and have been adopted for all mod- ern instruments. Two main types of mounts are in use ( fig. 27): Circular rail system for azimuthal motion, supporting the dish between two elevation towers, as is used by H.E.S.S. and MAGIC. The elevation axis is positioned such that the dish is balanced and little or no counterweight is required. This support scheme will in general permit a large movement range in elevation, allowing the ...

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... unconventional mounting scheme is a lift-up mirror carried on a circular rail, which eliminates the elevation towers and, at least in some dish support schemes, allows the reduction of bending torques on the dish due to the camera support system. A conceptual design for such a scheme was worked out for H.E.S.S. (see fig. 28 left), but again did not offer cost advantages. With a different elevation mechanism, this support scheme has been considered for the medium-sized CTA telescope ( fig. 28 right). A drawback of such systems is that the centre of gravity moves as the telescope's elevation is changed, requiring significantly increased drive power compared to ...

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... support schemes, allows the reduction of bending torques on the dish due to the camera support system. A conceptual design for such a scheme was worked out for H.E.S.S. (see fig. 28 left), but again did not offer cost advantages. With a different elevation mechanism, this support scheme has been considered for the medium-sized CTA telescope ( fig. 28 right). A drawback of such systems is that the centre of gravity moves as the telescope's elevation is changed, requiring significantly increased drive power compared to balanced systems, where the drives only have to counteract friction, inertia, and certain wind loads. For the LST, only a rail design, as used by H.E.S.S. and MAGIC, appears ...

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... a stiff dish envelope, or via an extra elevation cradle as used in the H.E.S.S. II telescope. With a central positioner, the dish is supported from its centre, and loads at the periphery of the dish must be minimised. For the SST with its reduced weight and loads, it appears cost effective to use a central positioner type mount as illustrated in fig. 29 (left) or to support the telescope by elevation towers but replace the rail by a central azimuth bearing as is used in the HEGRA telescopes ( fig. 29, ...

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... centre, and loads at the periphery of the dish must be minimised. For the SST with its reduced weight and loads, it appears cost effective to use a central positioner type mount as illustrated in fig. 29 (left) or to support the telescope by elevation towers but replace the rail by a central azimuth bearing as is used in the HEGRA telescopes ( fig. 29, ...

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... a number of telescopes will be built. Simplicity, robustness, reliability and the ease to maintenance are therefore particularly important features. This led to the decision to build an early prototype. MC studies suggest that an f /d of around 1.4 and a FoV of about 8 • is required. Three groups within the consortium have developed their designs (Figs. 32, ...

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... main idea in the first design was to have the elevation axis close to ground level. This solution saves on the construction of elevation towers, but at the expense of a pit into which the lower half of the dish disappears when the telescope is parked with the camera at ground level ( fig. 32 left). The same team is working on a design that decouples the dish movement from the camera elevation. The second design was based on a light Figure 32: left: Putting the telescope into a pit reduces the height of the telescope. right: A CFRP dish on a steel mount. In both cases the dish is held at the edge and the azimuthal movement is ...

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... same team is working on a design that decouples the dish movement from the camera elevation. The second design was based on a light Figure 32: left: Putting the telescope into a pit reduces the height of the telescope. right: A CFRP dish on a steel mount. ...

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... was based on a light Figure 32: left: Putting the telescope into a pit reduces the height of the telescope. right: A CFRP dish on a steel mount. In both cases the dish is held at the edge and the azimuthal movement is realised by rails. and stiff dish, which consists solely of CFRP and is designed in a way that avoids CFRP joints to metal ( fig. 32 right). This design allows easy access to the camera and mirrors. For the elevation, two options were foreseen, a lift-up system and a more conventional swing-like ...

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... wireless communication using the Zigbee industry standard, with each actuator identified by a unique (48 bit) code. A broadcast mode is also available, which could be used to communicate the current elevation to all actuators allowing the controller to look up and apply the relevant individual correction values. A second solution can be seen in fig. 42. The upper part of the figure shows the motor, two actuators, and the micro-controller board of one mirror unit. This device uses servo motors with a Hall sensor attached to the motor axis which makes possible relative positioning of the actuator with high accuracy. The communication is based on CAN (Controller Area Network), a ...

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... requires the development of a single and reliable calibration device which can measure the flatfielding coefficients and the ratio between a single photoelectron and the number of digital counts recorded. This development will add to existing experience in building calibration devices, for example the H.E.S.S. II flatfielding system as shown in fig. 52. Overall, absolute calibration is achieved by reconstructing the rings generated by local muons. A special pre-scaled single-telescope trigger could be implemented to enhance the rate at which these are ...