Wide FoV IAC telescopes Initial Design Considerations

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Wide FoV IAC telescopes Initial Design
Considerations
Vladimir Vassiliev Pierre-Francoys
Brousseau Stephen Fegan (UCLA)

• Goal
• Problem(s)
• Designs pros cons
• Where are we going?
• Conclusions

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km2 Telescope Target Parameters

• Light collecting area 40 m2 (QE50) 100 m2
  (QE20)
• Effective Aperture 7 m 12 m
• Field of View 15 deg (0.26 rad)
• Viewing Solid Angle 180 deg2
• Image quality 10.017 deg (lt2 deg) 5 (lt7.5
  deg) (?)
• Wavelength range 0.3 0.6 micron
• Focal Plane Instrument
• Array of light sensors 1024x1024
• Pixel 0.86 per pixel
• Plate Scale 0.5mm per arcmin (0.5 m diameter II)
• 3.8 mm per arcmin (3.3 m
  diameter mosaic
  of MAPMTs H9500 -gt 1.6 m H?
  32x32)

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Image quality problem (7.5 deg)
gt5
Spot Size arcmin
fF/D - number

• Whipple-like designs

FP/D ratio 1
Would require f gt 3 and focal plane size gt
D Plate scale is mismatched
fF/D - number
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Super-Etendue (throughput) problem
D m f/ FoV deg W deg2 Etendue deg2 m2 R arcsec
km2 10 0.25 15 1.8x102 1.0x104 gt60
Ashra 1.8 0.22 50 2.0x103 5.0x103 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
Hubble ACS/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.
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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, lt) or 12.6
  m (MAPMTs, lt) suggesting f/0.19 f/1.26

At present it
seems that f/0.19 VERY expensive telescope,
REASONABLE cost camera f/1.26 VERY expensive
camera, REASONABLE cost telescope
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Scalability
X

• 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
Telescope and Camera RD are coupled Telescope
prototyping is affected
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Classical catadioptric wide FoV telescopes
3 optical elements design

• Schmidt-Cassegrain
• Spherical primary mirror corrected by the
  Schmidt corrector plate, convex hyperbolic
  secondary mirror and a focal plane located behind
  the primary
• Maksutov-Cassegrain
• either 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.

Main disadvantage does not scale up to large
apertures (gt2 m), since the corrector plate
rapidly becomes prohibitively large, heavy, and
expensive.
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Primary aberrations / design requirements
Fast (small f-ratio) systems are severely
affected by spherical aberrations and coma.

• Spherical 1/f3
• Coma (1st order) q/f2
• Astigmatism q2/f1
• Field curvature q2/f1

Design requirements
Optical system consists of minimal number of
optical surfaces Spherical and Coma aberrations
free Tolerable Astigmatism and high order Coma
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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 doesnt meet large FoV
  specifications.
• One mirror catadioptric design may be
  aplanatic, but it suffers from large Fresnel
  lens requirement

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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 RCTs The 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
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RCT Schwarzschild theorem
Traditional for Cherenkov telescopes Davies-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
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)
Fp
concave
Fs
s
convex
FFp Fs / (Fs s - Fp)
Traditional RCT design is inconsistent with small
plate scale requirement
Discontinuous primary and continuous secondary
introduces comatic aberrations (!)
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Non-traditional RCT Abbe sine condition
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 small
Fp
concave
Fs
s
concave
FFp Fs / (Fs - s Fp) F/Dp gt 1/2
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Ray Tracing Design
Example of detailed ray tracing in modified RC
design Dp10m Ds4.1m Df1.6m 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
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Ray Tracing
Simulations at 7.5 deg Violation of Abbe
sin condition in attempt to reduce plate scale
rapidly deteriorates imaging quality (gt100 at
the edge of FoV).
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Ray Tracing Spot size
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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
Fp
Fs
s
Fresnel lens
F/Dp lt 1/2 (?)
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Three-Mirror Telescope Paul design
Needs detailed performance study for fast IACT
applications
10 deg2 FoV, lt 0.5 image quality
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.
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Emerging Options
gt1.5 m II ?
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Emerging Options
D lt 5 m
PMT
Telescopes could be deployed individually or
combined on a single mount
Combine electrical signals from all cameras
operating in single photon counting
mode Star-like approach
Combine optical signals (MMT, Keck,
SALT,) Ashra-like approach
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Ashra Optics
Primary Mirror 1.8m FoV 50 deg Resolution 1
arcmin
Cost-performance balance
Modified Baker-Nunn optics
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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 (gtgt1M
  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 (lt1m 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 , A17 m2, A749 m2 (8 m), A19133 m2
  (13m))

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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 (lt10 GeV) large aperture
  telescope concepts via combining optical or
  electronic images
• Utilizing innovative engineering designs have
  high potential for cost effective solution

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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

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