wide fov iac telescopes initial design considerations goal problem(s) designs: pros & cons where...

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



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



“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.



“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



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



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.



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



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



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



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 (!)



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



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



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).



Ray Tracing: Spot size



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 (?)



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



Emerging Options 2222

90FPS4

Dπ

4

FoVπ

Large ApertureD > 7m

PMT or MAPMT based camera

SiPMs Avalanche Geiger discharge

?

>1.5 m II ?



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



Ashra Optics

Modified Baker-Nunn optics

Primary Mirror: 1.8mFoV: 50 degResolution: 1 arcmin

Cost-performance balance



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))



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



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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wide fov iac telescopes initial design considerations goal problem(s) designs: pros & cons where...

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