wide fov iac telescopes initial design considerations goal problem(s) designs: pros & cons where...
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Wide FoV IAC telescopes
Initial Design Considerations
Goal
Problem(s)
Designs: pros & cons
Where are we going?
Conclusions
Vladimir VassilievPierre-Francoys Brousseau
Stephen Fegan(UCLA)
"Ground-based Gamma-ray Astronomy: Towards the Future"
October 20-22, 2005, UCLA
km2 Telescope Target Parameters
Light collecting area: 40 m2 (QE=50%) – 100 m2 (QE=20%)
Effective Aperture: 7 m – 12 mField of View: 15 deg (0.26 rad)Viewing Solid Angle: 180 deg2
Image quality: 1’=0.017 deg (<2 deg) – 5’ (<7.5 deg) (?)Wavelength range: ~0.3 – ~0.6 micronFocal Plane Instrument:
Array of light sensors: ~1024x1024Pixel: 0.86’ per pixelPlate Scale: 0.5mm per arcmin (0.5 m diameter II) 3.8 mm per arcmin (3.3 m diameter mosaic
of MAPMTs [H9500] -> 1.6 m [H? 32x32])
If trend continues to 2020 and~ 1000 sources detected =>4 – 5 sources per field of view
"Ground-based Gamma-ray Astronomy: Towards the Future"
October 20-22, 2005, UCLA
Image quality problem (7.5 deg)
Whipple-like designs
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10
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
0
1
2
3
4
5
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
f=F/D - number
f=F/D - number
Spo
t S
ize
[arc
min
]F
P/D
rat
io [
1]
>5’
Would require f > 3 and focal plane size > D
Plate scale is mismatched
"Ground-based Gamma-ray Astronomy: Towards the Future"
October 20-22, 2005, UCLA
“Super-Etendue (throughput)” problem
D
[m]
f/ FoV [deg]
[deg2]
Etendue
[deg2 m2]
R [arcsec]
km2 10 0.25 15 1.8x10+2 1.0x10+4 >60
Ashra 1.8 0.22 50 2.0x10+3 5.0x10+3 60
LSST 8.4 1.25 (?) 3.6 10 270 0.5
SWIFT 8.4 1.5 (?) 1.5 1.8 100 0.25
UKST 1.8 1.68 5.4 22.9 60.2 4
Keck 10 2.5 0.02 3.1x10-4 2.5x10-2 0.25
HubbleACS/WFC
2.4 24 0.03 7.7x10-4 3.2x10-3 0.05
It is extremely difficult to maintain reasonable image quality and achieve high throughput factor by simultaneously having large aperture and large field of view. Traditional optical designs forbid this.
"Ground-based Gamma-ray Astronomy: Towards the Future"
October 20-22, 2005, UCLA
“Super-fast” problem
Duration of Cherenkov light flash is a few nanosec. Thus unlike optical telescopes the imaging cannot be improved through increased exposure. Because of this severely light limited imaging regime optical system of “1 km2 array” telescope must be composed from minimal number of optical elements.
Plate scale and FoV requirement are compatible with effective focal length 1.9 m (II, <) or 12.6 m (MAPMTs, <) suggesting f/0.19 – f/1.26
At present it seems thatf/0.19: VERY expensive telescope, REASONABLE cost cameraf/1.26: VERY expensive camera, REASONABLE cost telescope
"Ground-based Gamma-ray Astronomy: Towards the Future"
October 20-22, 2005, UCLA
Scalability
With the fixed FoV telescope design is scalable with the primary mirror diameter. However, this changes plate scale which may not be allowed due to limit on the number of optical elements in the system.
Replica of Newton's first 6 inch reflector
X
Telescope and Camera R&D are coupledTelescope prototyping is affected
"Ground-based Gamma-ray Astronomy: Towards the Future"
October 20-22, 2005, UCLA
Classical catadioptric wide FoV telescopes
Schmidt-Cassegrain Spherical primary mirror corrected by the Schmidt corrector plate, convex hyperbolic secondary mirror and a focal plane located behind the primary
Maksutov-Cassegraineither a spherical or parabolic primary mirror in conjunction with a meniscus-shaped corrector plate at the entrance pupil. The meniscus-shaped corrector plate allows for the use of an easily fabricated spherical secondary mirror rather than the hyperbolic mirror required for the Schmidt telescope.
3 optical elements design
Main disadvantage: does not scale up to large apertures (>2 m), since the corrector plate rapidly becomes prohibitively large, heavy, and expensive.
"Ground-based Gamma-ray Astronomy: Towards the Future"
October 20-22, 2005, UCLA
Primary aberrations / design requirements
Spherical ~1/f3
Coma (1st order) ~/f2
Astigmatism ~2/f1
Field curvature ~2/f1
Fast (small f-ratio) systems are severely affected by spherical aberrations and coma.
Design requirements:
Optical system consists of minimal number of optical surfacesSpherical and Coma aberrations freeTolerable Astigmatism and high order Coma
"Ground-based Gamma-ray Astronomy: Towards the Future"
October 20-22, 2005, UCLA
Single Mirror: Lessons
Parabolic mirror is free from spherical aberrations but suffers from Coma
Davies-Cotton design, a cleaver spherical aberrations free discontinuous mirror solution, reduces Coma but doesn’t meet large FoV specifications.
One mirror catadioptric design may be “aplanatic”, but it suffers from large Fresnel lens requirement
"Ground-based Gamma-ray Astronomy: Towards the Future"
October 20-22, 2005, UCLA
2-mirrors telescopes
Two mirror designs
Cassegrain
Gregorian
Dall-Kirkham
Ritchey-Chrétien
Spherical aberration and Coma free Ritchey-Chrétien telescope or RCT is a specialized Cassegrain telescope with a hyperbolic primary and secondary mirror.
Famous RCTsThe two 10m components of the Keck Observatory The four 8.2m components of the Very Large Telescope in Chile The 4m Mayall telescope at Kitt Peak National Observatory The 3.5m WIYN telescope at Kitt Peak National Observatory The 2.4m Hubble Space Telescope currently in orbit around the Earth
"Ground-based Gamma-ray Astronomy: Towards the Future"
October 20-22, 2005, UCLA
RCT & Schwarzschild theorem
Generalized Schwarzschild theorem:“For any geometry with reasonable separations between the optical elements, it is possible to correct n primary aberrations with n powered elements.” (1905)
Traditional for Cherenkov telescopesDavies-Cotton reflector compensates
spherical aberrations by discontinuous mirror. Discontinuous primary and
possibly secondary need to be explored for reduction of
aberrations in fast optical systems
sFs
Fp
convex
concave
F=Fp Fs / (Fs + s - Fp)
Traditional RCT design is inconsistent with small plate scale requirement
Discontinuous primary and continuous
secondary introduces comatic aberrations (!)
"Ground-based Gamma-ray Astronomy: Towards the Future"
October 20-22, 2005, UCLA
Non-traditional RCT & Abbe sine condition
sFs
Fp
concave
concave
F=Fp |Fs| / (|Fs| - s + Fp)
F/Dp > 1/2
Aplanatic
Highly aspherical non-conic mirror surfaces
Astigmatism and high order Coma can be
contained within specs for FoV ~15 deg.
Focal Plane Size, FPS, cannot be made arbitrary
small2222
90FPS4
Dπ
4
FoVπ
"Ground-based Gamma-ray Astronomy: Towards the Future"
October 20-22, 2005, UCLA
Ray Tracing: Design
Example of detailed ray tracingin modified RC design
Dp=10mDs=4.1mDf=1.6m
A(0)=0.81 x pi D2/4 A(7.5)=0.55 x pi D2/4
Spot size can be a few arcmin at the edge of the FoV
"Ground-based Gamma-ray Astronomy: Towards the Future"
October 20-22, 2005, UCLA
Ray Tracing
Simulations at 7.5 deg
Violation of Abbe sin condition in attempt to reduce plate scale rapidly deteriorates imaging quality (>=100’ at the edge of FoV).
"Ground-based Gamma-ray Astronomy: Towards the Future"
October 20-22, 2005, UCLA
Ray Tracing: Spot size
"Ground-based Gamma-ray Astronomy: Towards the Future"
October 20-22, 2005, UCLA
3 optical elements systems: RC-catadioptric
Needs detailed performance optimization
Plate scale can be further reduced
Fresnel lens aperture can be made acceptably small, however, preliminary analysis indicates strong accompanying vignetting
Not clear if Abbe sine condition can be satisfied and very fast systems can be made aplanatic
3 optical elements and Schwarzschild theorem insure high potential for aberration reduction. The prove is classical Schmidt-Cassegrain designs and its versions
Ligtht loss and cost increases
sFs
Fp
Fresnel lens
F/Dp < 1/2 (?)
"Ground-based Gamma-ray Astronomy: Towards the Future"
October 20-22, 2005, UCLA
Three-Mirror Telescope: Paul design
LSST: 8.4-meter primary mirror, 3.4-meter secondary mirror, 5.2-meter tertiary mirror. The light reflected by this tertiary mirror then passes through a 1.4-meter lens to the camera detector.
10 deg2 FoV, < 0.5’’ image quality
Needs detailed performance study for fast IACT applications
"Ground-based Gamma-ray Astronomy: Towards the Future"
October 20-22, 2005, UCLA
Emerging Options 2222
90FPS4
Dπ
4
FoVπ
Large ApertureD > 7m
PMT or MAPMT based camera
SiPMs Avalanche Geiger discharge
?
>1.5 m II ?
"Ground-based Gamma-ray Astronomy: Towards the Future"
October 20-22, 2005, UCLA
Emerging Options:
D < 5 m
Combine optical signals(MMT, Keck, SALT,…)“Ashra-like” approach
Combine electrical signals from all cameras operating
in single photon counting mode
“Star-like” approach
Telescopescould be deployed
individuallyor combined on a single
mount
To trigger
To single camera
II PMT
"Ground-based Gamma-ray Astronomy: Towards the Future"
October 20-22, 2005, UCLA
Ashra Optics
Modified Baker-Nunn optics
Primary Mirror: 1.8mFoV: 50 degResolution: 1 arcmin
Cost-performance balance
"Ground-based Gamma-ray Astronomy: Towards the Future"
October 20-22, 2005, UCLA
Cost Considerations
The largest challenge is to find cost-effective solution !
Large aperture large FoV Paul or RC-catadioptric designs requiring large focal plane plate scale are most likely prohibitively expensive (>>$1M per telescope) even if designed with moderate image quality of 1’.
Relatively small aperture (3-4 m diameter) modified wide FoV RC telescopes with small focal plane plate scale (<1m per 15 deg) allowing high pixel density focal plane instrumentation (MAPMTS, IIs) may provide basic integration element for construction telescopes with effective 8-13m aperture.
(D= 3m , A1=7 m2, A7=49 m2 (8 m), A19=133 m2 (13m))
"Ground-based Gamma-ray Astronomy: Towards the Future"
October 20-22, 2005, UCLA
3-4 m RC advantages
It appears to be consistent with virtually all proposed in this workshop telescope array concepts (1km2, STAR, small telescopes for high energy regime) and with operation in wide FoV sky survey mode
It appears to be compatible with potentially low cost high pixel density focal plane instruments based on MAPMT mosaics, IIs, and possibly SiPMs and APDs.
It might be utilized as a basic element for integrated moderate and large aperture telescopes for 1km2 array or (<10 GeV) large aperture telescope concepts via combining optical or electronic images
Utilizing innovative engineering designs have high potential for cost effective solution
"Ground-based Gamma-ray Astronomy: Towards the Future"
October 20-22, 2005, UCLA
Conclusions
Design of the wide FoV large aperture IACT optical system is driven by the high throughput, lowest light loss, small focal plane plate scale, low cost, and moderate image quality of ~1’.
Due to short effective focal length of optical system required to satisfy these factors design of the telescope is highly sensitive to spherical aberrations and Coma.
Aplanatic modified RC design with relatively small aperture may provide adequate solution as integration element
Optical group needs to be formed to further explore this primary option as well as Paul and RC-catadioptric design alternatives
"Ground-based Gamma-ray Astronomy: Towards the Future"
October 20-22, 2005, UCLA
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