By V. Vassiliev, S. Fegan

W A E $ 1 km2 array version:High Energy All Sky Transient Radiation ObservatoryHE-ASTRO By V. Vassiliev, S. Fegan Ground based g-ray Astronomy: Towards the Future. October 20-22, 2005 UCLA

Hardware implementation Approach Technologies Data Rates & Array Trigger

Input parameters (from simulations) • To provide “fly’s eye” operation mode (+- 45 deg sky coverage) FoV >=15 deg is required • To sustain CR trigger rate at below 100kHz and operate in ~30-50 GeV domain FoV <25 deg must be used • To trigger efficiently in this energy regime “effective” trigger pixel size must be in 0.1-0.2 deg range • To reconstruct events in “physics of the shower limited regime” image pixel size must be in the range 0.01-0.02 deg. • Optimal pixel size for triggering and imaging differ by a factor of ~10. (by 100 in units of number of “effective” pixels in the camera).

Cost Embarrassment • FoV =15 deg is ~180 deg2 • This is equivalent to ~104 “trigger pixels” or 106 “image pixels” • To have IACT telescope for <=$1M and FP instrument <= a few $100K the cost per trigger pixel must be in the $10-100 range, and the cost per image pixel must be in $0.1-$1.0 range • Current MAPMTs and MCP based MAPMTs are in the range $40-$60 per channel. Current PMTs are in the range above a few $100 depending on the size and some other factors. • Oups! To be suitable for imaging with required resolution in 10 years from now the MAPMTs must drop in price by a factor of 10 each year. This can only happen if market of these things will increase so much that every family on the planet will need one. This is not going to happen!

FP plate scale mismatch • Something very inexpensive with very high level of integration is required such as CCD or CMOS image sensors, or perhaps arrays of SiPMs if they are developed and made cheap. • Oups! This devices are very small and to be used as FP instrumentation they would require substantial optical processing of the image to change plate scale without further image distortion. This results in prohibitive light loss in many optical elements and to compensate this would require further increase of the telescope aperture thereby increasing plate scale mismatch problem. • Before optical image is conditioned to a small CMOS or CCD plate scale it must be amplified!

Array of MAPMTs Wide field of view optics; possibly of RC type Fine image resolution utilizing CMOS and CCD technology Moderate size primary (3-7m) Large aperture II (Electrostatic or MCP(?)) with extremely rapid image decay time Fast Gated Image Intensifier to reduce NSB 1963 Japan: Suga Italy: ? Russia: ?

The Story of Ground Based g-ray Astronomy (by Jelley & Porter)

Energy range > 1 TeV Readout Event Rate < 1kHz All sky covered with 80 mega pixels in the CMOS sensor arrays Optimized Baker-Nunn optical system with three corrector normal lenses made of acrylic resin and 1 m spherical reflector (spot size less than 1 arcmin, 0.016o, for parallel light rays incident at angles less than 25o). Focal sphere image intensifier, FIIT, of ~60 cm aperture

Focal Plane Instrumentation Fast random access CMOS sensor Image pixel size – 0.0146o Readout image –128 x 128 pixels Readout Image size – 1.875o x 1.875o Readout rate 30-40 kHz Optical or II-based delay Two-mirror modified RC optical system X-Y Optical Splitter Gate & Shutter OR X = ∑xi Y = ∑yi Amplified Image Gated Image Intensifier (MCP) (25-40 mm) Gate ~20ns Rep. rate ~40kHz P-43/P-24 , ~2msec Star tracker VETO Trigger Sensor ~8200 pixels with 0.146o Large Aperture Image Intensifier (Electrostatic or MCP) Photon detection efficiency ~30-50% Fast decay scintillator output screen ~25 ns Primary 3-7m Fov 15o e.g. Array of rate compensated Discriminators Slow Control

CMOS Image Sensor HE-ASTRO Image Sensor is not commercially available yet. However, industry is very close to meet specification. High-speed readout is achieved with pipeline and parallel technologies. Parallel processing macro-cell of 32x32 pixels (1024) can be readout with > 500 kHz, and 128 x 128 pixel image (16 macro-cells) with > 30 kHz Micron-MT9M413 1.3-Megapixel CMOS Active Pixel Digital Image Sensor Image pixel size – 0.0146o Readout image –128 x 128 pixels Readout Image size – 1.875o x 1.875o NSB per pixel – 0.032 (20 nsec gate) ADC – 8 bit (S/N improved, 10–>8) Pixel dimension 12mm x 12mm Sensor area – 12.3 mm x 12.3 mm Shutter exposure – a few msec

Ashra CMOS image sensor • Ashra collaboration had worked with FillFactory to develop Ashra Fine Sensor (2048x2048 pixels, 2D control of shutter/exposure and readout). • Prototyping from existing LUPA 4000M • Readout module is being developed by Toshiba • Ultimate goal 128x128 macrocells with 24x24 pixels • Status … unknown IBIS4 1.3 Megapixel Rolling Shutter Image Sensor (LUPA 4000M) Read out rate is probably a factor of a few x 10 lower than required

Ultrafast Imaging DRS technologies Inc. Variety of Ultrafast Cameras for Military applications CCD based 500fps to 100,000,000fps e.g. 350 KHz at 250 x 250 Pixel exposures from 5 nsec limited number of frames 120mm tank gun projectile Photron CMOS based high speed cameras Ultima APX-RS one of the fastest video cameras with 3,000 mega pixel frames per second (fps) or 250,000 fps at reduced resolution FASTCAM-X 1024 PCI is the first system to bring mega-pixel CMOS to your personal computer at usable speeds; capable of operating as fast as 1,000 fps at full 1,024 by 1,024 pixel resolution, or 109,500 fps through ‘windowing‘. Ultima APX-i2 uses a 25mm MCP Gen II image intensifier, directly bonded onto the APX's mega pixel sensor to provide unmatched image quality with 20ns gating. airgun pellet impacting a matchstick

Gated Image Intensifiers Left : C9016-2x Series & Controller Center : C9546 Series Right : C9547 Series Hamamatsu products Commercial products which almost satisfy requirements of resolution, repetition rate, and fast gating exist.

Trigger Sensor Hamamatsu H9500 Flat Panel 52mm square Bialkali Photocathode 16 x 16 Multianode 12 stage FoV: 15o Trigger pixel size: 0.146o Number of MAPMTs: 32 Effective Area Ratio : 89% Size: 312 mm

Trigger Sensor Hamamatsu H8500 Flat Panel 52mm square Bialkali Photocathode 8 x 8 Multianode 12 stage FoV: 15o Trigger pixel size: 0.205o Number of MAPMTs: 69 Effective Area Ratio : 89% Size: 468 mm

Trigger Sensor (alternatives) Courtesy of R. Mirzoyan BURLE85011-501 PLANACON™ 71 mm square ( 51.2 mm active) Bialkali Photocathode MCP-PMT8 x 8 pixels Nice single pe pulse FoV: 15o Trigger pixel size: >0.135o Number of MCP-PMTs: 91 Effective Area Ratio : ~52% Size: 710 mm Front illuminated SiPMs (Avalanche Geiger discharge) 3x3 mm2square 5625 pixels of 40µ x 40µ each FoV: 15o Trigger pixel size: 0.2o Number of SiPMs: 1 Effective Area Ratio : ~80% Size: 3 mm

II To trigger To single camera Increase of telescope aperture could be achieved by combining several midsize telescopes on the same mount and utilizing optical mixing (single camera) or digital mixing (multiple cameras – possibly more expensive). Various multiplexing options could be explored. Optical SystemRitchey-Chrétien configuration FP plate scale is matched with telescope aperture Field curvature coupled II For modified RC optical system the field curvature is convex toward the sky. Primary 3-4 m Fov 15o

Focal Plane Image Intensifier Electrostatic Image Intensifiers MCP Image Intensifiers Photek manufactures a range of 18, 25, 40, 75 and 150 mm active diameter image intensifiers (too expensive and too small) SIEMENS image intensifiers. Large aperture units (>40cm) are developed for X-ray imaging. Phosphor Scintillator P-47 - 80 ns <10%> decay time Lanthanum Bromide Scintillator, LaBr3 / LaCl3 - 25 ns <10%> decay time High QE photocathode in 200-400 nm, >25%, continues to be an issue

Telescope data pipeline ~80Mb/s TD & Veto Stars Trigger Sensor Disk Retrieve Image to disk L1 Position encoding X = ∑xi, Y = ∑yi Timing T Array trigger & L2 Broadcast 10 ms Memory Ring Buffer of Images Indexed by local trigger timestamp T II (int. ~20ns) OR L1i X,Y delay Timing Event Identification L2 Telescope Trigger ~40 kHz Gated Image Intensifier P-43 ~2msec Gate 20 ns ~2.3kb per image Shatter 2msec Zero suppression ~20kb per image 1024x1024 pixels Readout 128x128 Pixels 1.9o x 1.9o ADC 10 bits/pixel, 16384 pixels ~600 non-zero pixels (mostly NSB) Bitmask 20 bits ADC 10 bits ~1.0 Mpixel CMOS Image Sensor (1000-500 fps full frame) 15o x 15o SDSS 34 Mb/s LSST 10 Gb/s= 36 Tb/h Data rate = 80 Mb/s x 3600 s/h x 217 telescopes= ~62.5 Tb / array / hour

Array Trigger • Telescope Trigger decision (~30 kHz) • Local trigger → convert to GPS timestamp (good to 100ns) • Buffer timestamp locally • Broadcast “trigger packet” of timestamp and node identifier (~5 bytes) to all nearest and next-nearest neighbors (max. 2Mbps outflow rate) Distributed →Every node acts as its own array trigger • Local trigger together with any trigger of two telescopes from all neighbors and next nearest-neighbors is recognized as array trigger • Local processing at node • Receive trigger timestamps • Buffer trigger timestamps (10-20 μs) • Search for a coincidence (compensate for relative delays due to pointing) • Coincidence → retrieve pixel data, write to disk (~80 Mb/s) Data rate to center node ~24 Mbps @ 30 kHz

HE-ASTRO (specs) • Image pixel size – 0.0146o • Readout image – 128 x 128 pixels • Readout Image size – 1.875o x 1.875o • NSB per pixel – 0.032 (20 nsec gate) • ADC – 8 bit (S/N improved, 10– >8) • Pixel dimension 12mm x 12mm • Sensor area – 12.3 mm x 12.3 mm • Shutter exposure – a few msec • Image integration time - 20 ns • Optical system TBD • Array trigger protocol TBD • Data Rates ~80 Mb/secper node • Online data processing TBD • Array of 217 telescopes • Elevation 3.5km • Telescopes’ coupling distance 80m • Area ~1.0km2 (~1.6km2) • Single Telescope Field of View ~15o • FoV area ~177 deg2 • Reflector Diameter ~7m • Reflector Area ~40 m2 • QE 50% (200-400 nm) • Trigger sensor pixel size 0.146o • Trigger Sensor Size ~31.2cm • NSB rate per Trigger pixel ~3.2 pe per 20 ns • Single Telescope NSB Trigger Rate 1KHz • Energy Range 20–200 GeV • Differential Detection Rate Peak ~30 GeV • Single Telescope CR trigger rate ~30 kHz

Conclusions • Large array of moderate size telescopes may provide a viable cost effective solution to the problem of required large collecting area, large field of view, and low energy threshold at the same time, by combining new and reviving old ideas, e.g. using image intensifiers, but based on the contemporary technology. • Design of FP instrumentation requires innovative approach to resolution of a number of challenges. For example, cost vs # of pixels, large telescope aperture vs small imaging sensors (extremely light limited regime), large FoV vs high data rates. • Initial feasibility study of a possible implementation of FP instrument indicates that all critical to the project problems might be resolvable already in the next few years since the current state of technology is not too far from achieving required specifications. • Detailed feasibility study of proposed FP implementation concept as well as possible alternatives is required

By V. Vassiliev, S. Fegan