# Presentation: 928256

## Scouting High Altitudes with MACH

- Motivation.
- Technical Realization.
- Site, Expected Performance and Measurement Program.
- Conclusions.

- J. Buckley1, H. Krawczynski1, W.R. Binns1, R. Cowsik1, P. Dowkontt1, R. Stencel2, J. Bueler1, R. Chamberlain1, P. Crowley1, M. Franklin1, M. Israel1, D. Leopold1, P. McGuire1,I. Jung1
- 1 Washington University in St. Louis, 2 University of Colorado in Denver

## {M}obile {A}tmospheric {Ch}erenkov Telescope Experiment

- x 2

-  Small Stereoscopic System.
-  2.1 m Ø Mirror
-  8o FoV
- Mobile:
-  Complete Debugging at WU. STL.
-  Minimize work @4.3km

## {M}obile {A}tmospheric {Ch}erenkov Telescope Experiment

- Motivation:
- Stereoscopic observation of air showers at different sites, at different altitudes, and with different telescope-telescope distances (caveat: Ethr(MACH) > Ethr(STAR,Elf))
- >150 GeV Showerphysics.
- Testbed for new technologies:
- Detection of Air showers with Wide Field of View Optics,
- High QE detectors,
- High-performance low-cost readout including Giga-sample-per-second digitization, and multiplexing (FADCs, FPGAs, GB/sec fiber links),
- “Autopipe" compiler language for realtime processing of trigger, data compression, and analysis pipelines.
- Prototyping of components of various experiments:
- VLACT: large telescopes with large field of view;
- STAR: dense array of small telescopes;
- Elf: dense array of 10m telelscopes;
- Pevatron Observatory: sparse array of small telescopes.

## Optical Design

-  Ritchie-Chretien design:
-  isochronous and
-  large field of view.
-  Primary and secondary : cancel coma and spherical aberration.
-  Takes advantage of experience with designs of Antipodal Transient Observatory.
-  Design: analytical solution for conic constants, field curvature, and tolerances  Final optimization with ZEMAX.

- (J. Buckley)

**Notes:**
We derived a good compromise given the goals of minimizing the focal length, minimizing the secondary distance and minimizing the size (and obscuration) of the secondary while holding aberrations in check. The best configurations (and most practical for our mount design) tend to place the focal plane somewhat in front of the primary (negative back-focal distance). After considering a number of possible combinations of parameters, the best compromise was a the design with primary diameter D1 = 2.1 m, primary focal ratio F1 = 1.0, system focal ratio F = 1.2, back-focal ratio  = −0.25, secondary diameter ratio k = 0.34, primary radius of curvature R1 = 4.2 m, secondary radius of curvature R2 = 8.6 m, primary and secondary conic constants K1 = −1.72, K2 = −212, and distances to the secondary of t1 = 1.38 m, and from the secondary to the focal plane of t2 = 0.86 m. The field curvature was calculated to be m = 1.45m−1. To assess the sensitivity to misallignment of the secondary, the worst case aberrations over the full field were derived. We obtain coma of ATCDEC = −0.022 for a 1 cm decenter of the secondary, ATCTILT = 0.019 for a 1 deg tilt of the secondary, and a spherical aberration of ASADS = 0.034 for a 1 cm displacement of the secondary along the optic axis. We used these values in our mechanical design, and for generating budgetary quotes and as starting values to derive the optimum design using ZEMAX (ray-tracing software).

## Optical Design

- (J. Buckley)

## Optical Design

- (J. Buckley)

**Notes:**
Figure 2: Layout of optimized optical system (with representative rays) and resulting spot diagrams for 4 different field angles (1, 2, 3 and 4 off-axis). For comparison, a parabolic reflector with an f/#=1.2 gives an RMS radius of 0.18 at 4 offset, a factor of 2.5 worse than our design for the same f/#. This would essetially destroy the gamma-hadron separation and angular resolution at these field angles. Traditional Davies Cotton designs, on the other hand, result in unacceptable time dispersion that grows with mirror radius.

## Mounts and Mirrors

- Mirrors:
-  Slumped float-glass ground and polished to final figure.

- Mounts:
-  Mirrors and OSS: 700 lbs.
-  Adaptation of design of radio telescope (200 lbs dish).
-  Slew rate: 15o s-1,   acceleration: 5 deg s-2.
-  Mobility:   Construction at WU.ST.L. &  test at different sites.

**Notes:**
Working with DOTI, we are developing a means to check the tolerance on the local radius of
curvature and spotsize in multiple zones masked off by an annular aperture.

## Detectors and Camera

-  30x30 cm^2, ~7x7o2.
-  32 Burle Planacon multichannel plates-PMTs.
-  Each MCP-PMT:   64 Pixels, 0.15oØ per pixel.
-  2048 Pixel.
-  106 gain.

- Each camera:
-  31 MCP-PMT with Bialkali photocathodes.
-  1 MCP-PMT with InGaN photocathode with enhanced QE.

## Detectors and Camera

- QE:
-  40% @ 250nm
-  10% @ 400nm

## Electronic Readout

- Readout:
- Multiplexed FADCs (move zero-suppression upstream) with information which channel triggered.
- 16 MUX/FADC Boards per telescope with 128 channels each: use experience from VERITAS with low-power CMOS components.
- FPGA/PAROL boards which connect to fibers.
- FPGAs:
- programmable comparators (can fire for single PEs),
- nearest-neighbor pattern trigger,
- programmable-delay array trigger,
- FADC mode & STAR mode.
- Signal processing block for the FADC waveform optimal filter/deconvolution based on AutoPipe technology (Franklin & Buckley et al.).

**Notes:**
3ghz switch

## MCP/PMT

- After Preamp and Cables

- After Delay Line

- After RF Switch and Buffer Amp.

**Notes:**
Figure 5: Top: Photo of prototype electronics tested in our lab, showing the light-box containing the planacon, and prototype
boards for the front-end amplifier, receiving amplifier, RF switch, chip delay-line, and buffer amplifier. The output is connected
to one of the 10 input channels of one of the WU 500 MHz FADC boards. Bottom (left to right): single photoelectron signals
from: (a) the MCP/PMT output, (b) the output of the preamplifier and differential-pair cable, (c) the output of the delay line,
and (d) the output of the RF switch and buffer amplifier, as measured with a 1.5GHz 8Gsps scope.
I/O provided by parallel fiber-optic (or LVDS) links provided by 12-channel (GByte/ sec)

## Site

- Mt. Evans (Co):
-  The University of Denver (DU) Uses Site Since 1946.
-  2.3km – 4.3km with good infrastructure.
-   V band zenith within 0.5mag of natural background (21.6 mag/sq.arcsec; Stencel et al. 1994)
-  60+% photometric nights.

- 3.3 km

- 3.8 km

- 4.3 km

- 2.3 km

## Slide 13

## Expected Performance

- Performance:
-  After cuts: 86  hr-1, 		90 CR hr-1
-  Crab Sensitivity of ~6 hr−1.
-  Energy spectrum: 10 hrs.

- 4.3 km

- Rate before and after h-sep.

## Expected Performance

- Comparison 2.3km vs. 4.3km:
-  High altitude: factor 1.7 different peak energies:
- 800 GeV @ 2.3 km
- 470 GeV @ 4.3 km
-  Similar Crab Sensitivity at 2.3km and 4.3km.

- 4.3 km

- 2.3 km

## Observation Program

- 4.3 km + 2.3 km:
-  Elevation dependence of the energy dependent rate of detected -rays.
- 2.3 km:
-  Off-axis sensitivity.
-  Detection rate as function of telescope-telescope distance (30m, 80m, 300m) and FoV.
-  Synthesizing a larger telescope from the two small telescopes.
-  Cover part of camera with U, V, and B filters.
- In parallel: NSB intensity and spectrum (Cowsik et al.).

**Notes:**
 Measurments: June 2008-June 2009
The following measurements are planned:
Elevation dependence of the energy of detected -rays: At the highest altitude site, we will observe
the Crab Nebula in ”Standard Configuration” (80 m distance between telescopes) up to 30 hrs. This will be
the first high-altitude IACT data set ever taken. All previous measurements were taken at altitudes of 2.2
km and below. Weather permitting, after getting 30 hrs of good data, we will acquire an additional data set
with an increased distance between the two telescopes. The data will be used to derive high-quality energy
spectra that will be compared to equivalent energy spectra taken at the lower altitude sites.
Performance of the two-telescope system for off-axis sources: This and all measurements described
in the following will be taken at the lower-altitude sites where we have more Crab observation hours at our
disposal. We will observe the Crab Nebula at three different locations in the field of view. In addition to the
source close to the center, we will measure the source 2 and 3.5 off-center, proving that the optical design
achieves a high sensitivity over the entire field of view. Integrating over 5 hrs at 2 offset and 15 hrs at 3.5
offset, we will be able to measure the detection efficiency with an accuracy of 10%.
Measurements with different telescope-telescope distances: We will detect the Crab Nebula with
the two telescopes at different distances from each other. In addition to the standard separation of 80 m, we
will perform a measurement with the telescopes at 30 m and 300 m from each other. Compared to earlier
measurements of this kind [24], we will be able to actively vary the distance between telescopes from very
small to very large distances. Furthermore, our measurements will take advantage of the large field of view
of the telescopes which will make a decisive difference at energies above the peak energy. Using in software
only the information from the inner regions of the telescope cameras, smaller field of views can be mimicked,
and a detailed trade-off study for a sparse array of small telescopes can be carried through. We plan to
observe the Crab Nebula for 4 hrs at a 30 m distance and for 20 hrs at 300 m distance to achieve a sensitivity
estimate better than 20%.
Synthesizing a larger telescope from the two small telescopes: We will operate the two small
telescopes in “large aperture synthesis mode”. The two telescopes will be operated at a distance of 5 m from
each other, and the “STAR” trigger electronics will be used. The two 2.1 m diameter telescopes will thus
operate as a single 3.0 m telescope. We will merely take some cosmic ray air shower data and muon data in
this mode. Comparison of the expected and measured cosmic-ray detection rates will be used to prove that
the two telescopes can indeed functions as a single larger aperture detector.
Test of the high quantum efficiency photon detectors: Each telescope camera will consist of conventional
MCPs and of one high-quantum efficiency MCP. All the data acquired with the two telescopes
can be used to quantitatively test the high quantum efficiency MCP. For this purpose, we will carry out
single-photoelectron (single-PE) calibrations of all MCPs in the 30 min before the air shower measurements
begin. For each altitude, the net-gain in photoelectron yield can be determined by simple comparison of the
pulse height spectra found in different MCPs, when the pulse heights are measured in single PE units.
Ancillary measurements: At each of the three observation sites we will measure the intensity and spectrum
of the night sky background (NSB) with a photomultiplier set-up [25]. We will calibrate the quantum
efficiency of the PMT with an existing set-up at Washington University. At the site the calibrated PMT will
be operated in single photon counting mode and a filter wheel will be used to obtain information about the
NSB spectrum. We will measure the spectrum of the atmospheric Cherenkov light by covering 1/4 of a few
MCPs with calibrated U, V, and B filters [26]. Information about the spectrum of the Cherenkov spectrum
can be obtained by comparing the mean signal amplitude in the pixels before and after installing the filters
with each other. Taking several runs with filters covering different parts of the camera can be used to reduce
systematic uncertainties associated with drifts of the PMT gains.

## Summary - MACH: {M}obile {A}tmospheric {Ch}erenkov Telescope Experiment

- Mobile two-telescope system:
- Tests of several observations sites.
- Tests of observations at high altitudes.
- Testbed for new technologies:
- Wide field of view optics
- MCP-PMTs
- Multiplexed FADC
- FPGA comparators, trigger, delay lines
- Fiberlinks
- Parallel processing with Autopipe.