A Broad Band Imager for the European Solar Telescope Matteo Munari & Salvo Scuderi INAF –...

A Broad Band Imager for the European Solar Telescope

Matteo Munari & Salvo ScuderiINAF – Osservatorio Astrofisico di Catania

Massimo CecconiINAF – FGG Telescopio Nazionale Galileo

1st SPRING Workshop, Nov 26-28, 2103

Broad Band Imager

• The EST broad band imager (BBI) is one of the focal plane instruments foreseen for the EST Telescope.

• It is an imaging instrument whose function is to obtain diffraction limited images over the full field of view of EST at multiple wavelengths and high frame rate.

• Its scientific objective is the study of fundamental physical processes at their intrinsic scales in the Sun’s atmosphere.


Science CasesObserving Program Filters FOV Angular

Resolution SNR CadenceNumber of

simultaneous Channels

Chromospheric response to convective collapse (2.1.1)

CN band head, G Band, blue and red continua Ca II H core & wing, Ca II 845

22 (goal 33)Optimum 3030

0.1 100 5 s No

Internal structure of magnetic elements (2.1.2)



0.1 100 5 s ( 30 s) No

Emergence and evolution of fields in granular convection (2.1.3)



0.05 100 5 s No

Physics of flux cancellations in the solar atmosphere (2.1.4)

G Band, blue and red continua Ca II H core & wing, Ca II 845, H


0.1 100 10 s No

Physical properties and evolution of internetwork fields (2.1.5)



0.05 100 5 s No

Latitudinal distribution and structure of polar fields (2.1.6)

CN band head, G Band, blue continuum Ca II H core & wing, Ca II 845


0.05 100 10-30 s No

Network element dynamics (2.2.2) G Band 100100 TBD No

Highly variable phenomena in the chromosphere (2.3.2) Continua TBD 3030 0.2 30 s No

Sunspots (2.4.1) Continuum, G-band100100 Optimum 3030

0.05 0.05 s 2

Flares (2.6.1) Continua: 417 nm (Paschen), 840 nm (Brackett) 33 0.05 0.05 s 2


BBI CharacteristicsNumber of spectral channels 3 channels working simultaneouslyObservation modes - High Resolution

- to exploit the diffraction limited quality of the telescope + MCAO system (FoV: 1’x1’)

- Large Field of View - Seeing limited angular resolution to exploit the full

corrected telescope field of view (FoV: 2’x2’)Field of View 2’x2’Angular Resolution 0.04” @ 500 nmWavelength Coverage 390 nm – 900 nmWavelength Bandpass 0.05 to 0.5 nmWavelength Switching < 2 seconds Maximum bandpass shift 5x10-3nm (goal 3x10-3) @ 500nm, 30’’ from the field center

Transmission Total throughput > 30%


Operational Wavelenghts Filter Number Filter Name C [nm] FWHM [nm] Spectral Feature

BBI-WF2 F395W 395.37 0.5 Ca II H continuum

BBI-NF2 F397N 396.88 0.05 Ca II H core

BBI-WF1 F388W 388.30 0.5 CN band head

BBI-NF1 F396N 396.47 0.05 Ca II H wing

BBI-WF2 F395W 395.37 0.5 Ca II H continuum

BBI-WF3 F417W 417.00 0.5 Paschen continuum

BBI-WF4 F430W 430.50 0.5 G band

BBI-WF5 F436W 436.39 0.5 G band continuum

BBI-NF3 F656N 656.28 0.1 H

BBI-WF6 F668W 668.40 0.5 H continuum

BBI-WF7 F840W 840.00 0.5 Brackett continuum

BBI-NF4 F854N 854.20 0.05 Ca II IR


EST Light Distribution


Optical design: Constraints

Instrument Filters diameters < 10cm Detector format 4k x 4k Pixel size 10 microns Back Focal distance 500 mm

Telescope On axis Gregorian 4 meter entrance pupil diameter F/50, telecentric Focal plane of 2'x2' side Aberration free optical system


Optical design: Layout• Refractive Design

– After evaluation of all refractive and reflective/refractive designs an all refractive design with filters in telecentric configuration has been chosen on the basis of performances and simplicity

• Two arms– Two arms completely independent to optimize optical performances and throughput

through appropriate choices of optics, coatings and detector.• Blue Arm (for filters in the 380nm – 500nm range) two channels each divided in three sub–channels• Red Arm (for filters in the 600nm – 900nm range) one channel divided in three sub–channels

• 3 identical channels – Divided in 3 sub-channels.– The sub–channels share the same optics See the same aberrations .– Out of the three sub–channels of each channel, the first one hosts narrow band filters for

chromospheric observations, the second one hosts in focus wide band filters used as reference for Multi-Object Multi-Frame Blind Deconvolution reconstruction (MOMFBD) and photospheric observations and the third one hosts out of focus wide band filters for phase diversity reconstruction of photospheric observations.

– Filters located after the beam separation, that is, near the detector.


Optical design: Layout

• Each channel has two alternative observing modes – Realized with the use of pick-up relays– Large FOV mode direct – High resolution mode folded

• Flexibility Each mode available independently on the various channels


Optical Design: Characteristics

• Channel Dimensions: 4.5x1.0x0.5m (Biggest element first doublet ~210mm diameter)• Observing modes data

– High resolution: F/# =32; Scale=1.6’’/mm; Sampling=0.016’’/px; FoV size=64’’x64’’– Large FoV: F/# =17; Scale=3.0’’/mm; Sampling=0.030’’/px; FoV size=2’x2’– Both: Detector format: 4k x 4k; Pixel size=10 micron

• 2x2 lenses in each mode (one common doublet)• 2 glasses used in symmetric way (Corning B19-61 & Schott KZFSN5)


Optical Design: Generic Channel4.5 m

1.0 m

Common doublet

Telescope Focal PlaneHR doublet

Large FOV doublet

FPA

FPA

BS

Mirror


Performances: High Resolution Mode

2 px 4 px side

2px4px

2px4px

2px4px

1’x1’


Performances: Large FoV Mode

2 px 4 px side

2px4px

2px4px

2px4px

2’x2’


Performance: ThroughputLenses transmission (MgF2 coated)

wavelength (µm)


Mechanical Design Layout: Blue Arm

2400

mm1640 mm

1500 mm

1500 mm

4200 mm


Mechanical DesignThe mechanical components and assemblies included in each BBI channel are: optical bench, dust/light proof cover, entrance shutter, optical mountings, mechanisms, and detector cryostat.

UCSM1

M2

M3BS1

BS2

FPAFPA

FPA

Filter Holder

High Res Doublet

Large FoV Doublet

Linear Stage


Mechanisms• Filter Holders : Sizes and type depend on filters• Focusing

– Change of optical path: Filters & Telescope – Choice of most suitable optical element

• Mode switching : change between large FOV and high resolution modes

• Mechanism holding alignment target • Channel/Detector entrance shutter : detectors tests• Filters Temperature Control system • Polarizers : modulation of light intensity


Filters: Critical Issues Size

At least as big as the detector. Diameters of 6 cm quasi-standard request 8/9 cm still feasible but expensive in terms of money and time According to manufacturers bigger dimensions are unrealistic

F/# & spectral shape Large FoV F/18 & High Resolution F/32 Central bandwidth shift towards blue Broadening of the FWHM Careful Design

Thermal Control Temperature coefficients for interference filters 0.002–0 .02 nm/°C (Barr, Andover on line

docs) Spectral Band Shift Intensity fluctuations Temperature controlled within 1 degree or better depending on filter characteristics


F/# Central wavelength, FWHM (A)

Nominal 3964.70, 0.50

6562.80 ,1.00

8542.00, 0.50

12.5 3964.18, 1.16

6561.94, 2.02

8540.88, 2.30

25 3964.56, 0.59

6562.58, 1.14

8541.72, 0.78

35 3964.63, 0.53

6562.69, 1.06

8541.85, 0.60

Detectors

Format 4k x 4k 6k x 6k (FoV: 2’x2’)

Pixel Dimensions 10 – 20 micron

Full Well Depth > 60000 e–

Frame Rate 20 – 100 frame/s

Bit resolution 16

Read Out Noise < 10 e–

Dark Current Few e–/s

Quantum Efficiency > 60% @ < 400 nm> 80% @ > 400 nm


Conclusions

• Optical design -> Advanced• Mechanical design -> Preliminary• Filters -> Critical (contacts with manufacturers)• Detectors -> Critical• Cost Estimate -> 850 k€ (no detector)


A Broad Band Imager for the European Solar Telescope Matteo Munari & Salvo Scuderi INAF –...

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