An IFU for IFOSC on IUCAA 2m Telescope A. N. Ramaprakash Inter-University Centre for Astronomy &...

An IFU for IFOSC on IUCAA 2m Telescope

A. N. RamaprakashInter-University Centre for Astronomy & Astrophysics, Pune

Abhay, Atul, Hillol, Kalpesh, Mahesh, Moin, Mudit, Pravin, Ranjan, Shyam, Sujith, Swapnil, Vilas

4.12.2007 A. N. Ramaprakash, IUCAA 2

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Telescope

Design and Performance of the IFU

IFOSC performance with IFU / Summary

• Telescope and IFOSC parameters• Basic structure of the system• Constraints on IFU Design for IFOSC on IUCAA Telescope

• Basic Design• Selection criteria for parameters • Performance

Plan of the Talk


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http://www.cfai.dur.ac.uk/new/spectroscopy/spectroscopy.html

Techniques for 2-D Spectroscopy


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A glimpse of IGO

Built by Telescope Technologies Ltd., Liverpool, UK

Location Girawali - 80km NE from Pune, near Junnar Altitude - ~1000m above MSL

Telescope Specification - 2m, f/10, Ritchey Chrétien, 6’

radius field Wide Field Corrector to extend field to 21’

radius Mount – Alt-Az on hydrostatic bearings Focus – Cassegrain (1 direct & 4 side ports) Active pneumatic mirror support (20cm

thick) Integrated Autoguider


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Observatory Views


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IUCAA Girawali Observatory


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The 2m Telescope


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Site Vital Statistics

V-band median seeing in winter - 1.2" Extinction (mag./airmass)

B – 0.26 to 0.74 V – 0.14 to 0.38

Sky Brightness (mag/sq. arcsec) B – 21.8 V – 20.8 R – 19.3

Cloud cover over a period of six months (Nov. – April) 50% photometric 80% spectroscopic

(Das et. al, ’99,BASI 27,609)


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Instruments

IFOSC & Calibration Unit

PI CCD NIPI Next GenerationFive Year Plan


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IUCAA Faint Object Spectrograph Camera (IFOSC)

UBVRI filters 0.8”, 1”, 1.5”, 2” x 11’ slits 10 grisms (2 echelle & 2 cross

dispersers) R = 190 to R = 3700 = 0.35m to = 0.85m Polarimetry mode 2K x 2K EEV CCD with 13.5m2 pixels 40m/arcsec plate scale on CCD at

f/4.5 (demagnification of 2.2)


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Primary Mirror

Calibration Unit

IFOSC

CCD Detector

Lamps

IFOSC Object Plane/ Telescope Focal Plane

A & G Unit

Side Port

F/10 Beam

Direct Cassegrain Port


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Primary Mirror

Calibration Unit

IFOSC

CCD Detector

Lamps


A & G Unit

Side Port

Folding Mirror

Calibration Unit


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Primary Mirror

Calibration Unit

IFOSC

CCD Detector

Lamps


A & G Unit

Side PortInstrument

Folding Mirror

Cassegrain Side Port


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Dimension of the CCD + De-magnification of the IFOSC

Maximum Dimension of Projected Fibre Slit on the IFOSC Object Plane ~ 59 mm

Mechanical Constraints

Can’t put Fibre-slit directly at the

IFOSC object plane

IFU on IUCAA Telescope : Design Constraints


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Area–Solid Angle Product (AΩ) : Invariant

Optical System

A1

Ω1

A2

Ω2

A1 . Ω1 = A2 . Ω2

AΩ : Fixed by Telescope’s plate scale and F/number

For 1 arc-sec sampling

AΩ = 94.265 μm2 - steradians



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Teles-cope

Spectro-graphFore-Optics Lenslet + Fibre

UnitOutput Optics

IFU

• Necessary magnification

• Proper sampling.

• Telecentric Output

• To Sample the sky image

• Feed light into the fibre with proper f/#

• To couple fibre Slit to the Spectrog.

• Change the f/# of the beam

• Telecentric Output

Basic Design Configuration of the IFU


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Primary Mirror

Lamps


A & G Unit

Fore - OpticsLenslet

+Fibre Unit

Fibre Slit

Output Optics

Image of Fibre Core ~ Slit

Integral Field Unit(IFU)

This distance has To be > 80 mm

(Opto-Mechanical Design Constraints)

Mechanical Configuration of the IFU


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Focal Ratio Degradation (FRD) (F/#)out < (F/#)in

• Due to Micro-bending/ManufacturingDefects

• Statistical property

• Results in increase of (AΩ) product


FRD should be minimized


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Focal Ration Degradation

(Carrasco & Parry, 1994, MNRAS, 271,1-12)


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To minimize FRD : (F / #)in < 5 so that,

(F / #)out ~ (F / #)in

Further, invariance of AΩ

Lower the (F/#)fibre

Better Spectral Resolution

Smaller the Fiber Core Diameter

Smaller (F/#)fibre is desirable

Design Optimization of the IFU


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IFOSC accepts ‘F/10’ beam

F/# IFOSCF/10Output Optics

Fibre Core

Magnification = 10 / (F/#) = M

BUT

The projected slit width = M X Fibre Core Diameter

‘M’ affects the Spectral resolution



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The maximum length of the Fibre Slit = 59 mm / M

‘M’ restricts the total numbers of fibres

It is preferred to keep ‘M’ as low as possible.

(F/#)fibre should be made as large as possible

Recipe for Design Optimization

+Aberrations &Throughput



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The sky area would be sampled by 100 Lenslets + Fibres

Three spatial sampling scales 1.0" per fibre, 0.8" per fibre and 1.2" per fibre are being used.

The sky area sampled by IFU ~ 14 arc-sec X 7 arc-sec (For the case of 1 arc-sec per fibre)

Basic scientific constraints of the IFU


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f2 ~ 508mm, v = 10mm, Lenslet Φ= 2.1 mm

Fore-optics, Lenslets and Fibres

F

f1 f2

L1

L2

Telescope Focal

Plane

Lenslet + Fibre

Unitu

vd

For different sampling scales, MFore-optics=f2/f1 (17.2, 21.8, 25.75); L2 & v kept fixed; Only L1, u and f1 changed.

Singlet field flattener for telecentric output

Achromatic doublet


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Magnified Image by Fore-Optics

Dpupil

f

DL= 2.1 mm

/#

/#L

pupil samp

teles

FD D

F

F/number of the Lenslet = 4

F/number of the telescope = 10

Sampling Diameter at Telescope focal

plane

Lenslet and Fibre

Lenslets provide continuous sampling of sky; they feed light into the fibre with the appropriate f/#.


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Output Magnification ‘M’

F/4 IFOSCF/10Output Optics

Fibre Core

Magnification = 10 / 4 = 2.5

Projected slit width = 2.5 X 70μm (Fibre Core Dia.)

Maximum length of the Fibre Slit, = 59 / 2.5 ~ 23.6 mm

Output Optics

Telecentric output


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Fore-optics layout – 1" per fibre sampling


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Lenslet Optical Layout


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Fore-optics Spot Diagram (1" sampling)


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Output Optics Layout

This distance to be kept > 80 mm


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Output Optics and IFOSC


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Spot diagram at IFOSC object plane


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Spot diagram at IFOSC detector plane


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Fore-optics encircled energy diagram (1" sampling)


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Encircled energy at IFOSC detector plane


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Slit width - 220 μm:

Spectral Resolution

Grism Peak (Angstro

m)

Wavelength range

(Angstroms)

Spectral resolution

(Angstrom for 2.2 arc-sec slit)

IFORS 5 4000 3300 – 6300 20.02

IFOSC5 6000 5200 – 10300 20.24

IFORS1 4000 3300 – 5540 9.02

IFOSC7 5000 3800 – 6840 9.68

IFOSC8 6000 5800 – 8350 8.14

(Ranjan Gupta et al., 2002, BASI, 30, 785)


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Sensitivity

An IFU for IFOSC on IUCAA 2m Telescope A. N. Ramaprakash Inter-University Centre for Astronomy &...

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Transcript of An IFU for IFOSC on IUCAA 2m Telescope A. N. Ramaprakash Inter-University Centre for Astronomy &...