High Resolution Echelle Spectrograph for Chinese Weihai 1m Telescope. Leiwang, Yongtian Zhu, Zhongwen Hu Nanjing institute of Astronomical Optics Technology

High Resolution Echelle Spectrograph for Chinese Weihai 1m Telescope.

Leiwang, Yongtian Zhu, Zhongwen HuLeiwang, Yongtian Zhu, Zhongwen Hu

Nanjing institute of Astronomical Optics TechnologyNanjing institute of Astronomical Optics Technology

National Astronomical Observatories, CASNational Astronomical Observatories, CAS

OutlineOutline

•Background

•Spectrograph design

•Expected performance

•Status and planning

BackgroundBackground• WHRS, a high-resolution fiber-fed echelle spectrograpWHRS, a high-resolution fiber-fed echelle spectrograp

h is currently under development for Chinese Weihai h is currently under development for Chinese Weihai 1m telescope at Weihai observatory of Shandong univ1m telescope at Weihai observatory of Shandong university.ersity.

• To complement the current photometric work of the To complement the current photometric work of the Chinese Weihai 1m telescope, the Nanjing institute of Chinese Weihai 1m telescope, the Nanjing institute of astronomical optics technology and Weihai observatastronomical optics technology and Weihai observatory of Shandong university, started the WHRS project ory of Shandong university, started the WHRS project in the end of 2008.in the end of 2008.

• The design of WHRS was guided by the scientific need The design of WHRS was guided by the scientific need for a high-resolution spectrograph with a high optical for a high-resolution spectrograph with a high optical efficiency and very high wavelength stability. This instefficiency and very high wavelength stability. This instrument will significantly enlarge the scientific potentirument will significantly enlarge the scientific potential of Chinese Weihai 1m telescope.al of Chinese Weihai 1m telescope.

Spectrograph design-design goalSpectrograph design-design goal

• Wavelength coverageWavelength coverage：： 380nm-880nm380nm-880nm

• Resolving powerResolving power ：： 38000-5500038000-55000

• Image qualityImage quality : within 2x2 pixels (E80) : within 2x2 pixels (E80)

• Optical efficiency: >30%@500-600nmOptical efficiency: >30%@500-600nm

(without telescope and fiber)(without telescope and fiber)

• With high wavelength calibration accuracyWith high wavelength calibration accuracy

and stabilityand stability

Spectrograph design-optical Spectrograph design-optical designdesign

• Probably , white-pupil is one of the best Probably , white-pupil is one of the best solution for high resolution spectrograph.solution for high resolution spectrograph.

• Benefit: Benefit:

1) can be freed easily from scattered light 1) can be freed easily from scattered light produced at the echelle, avoids produced at the echelle, avoids vignetting vignetting

2) with smaller cross-disperser size2) with smaller cross-disperser size


The standard equation for a slit spectrograph’s The standard equation for a slit spectrograph’s resolving power is: resolving power is:

was first given by Binghamwas first given by Bingham If the collimated beam overfills the echelle, the equation becomes follIf the collimated beam overfills the echelle, the equation becomes foll

owing forms, which use the collimator focal length or collimated beaowing forms, which use the collimator focal length or collimated beam size:m size:

Or, Or,

sin cosB

F

LR

D

)tantan1)(tan1(

tan2

B

Bcoll

w

fR

)tantan1)(tan1(

tan2

BF

Bcoll

D

AR


• R2, R2.6,R4 Echelle Grating?R2, R2.6,R4 Echelle Grating?

• Higher blaze angles imply smaler collimator camHigher blaze angles imply smaler collimator camera focal lengthsera focal lengths

020000400006000080000

100000120000140000

50 100 150 200 250 300 350

BEAM SIZE(mm)

R

(arc

se

c) R2-ECHELLE

R2.6-ECHELLE

R4-ECHELLE


• Prism or grating cross-disperser ?Prism or grating cross-disperser ?• Double prism cross-disperser:Double prism cross-disperser: with high efficiency with high efficiency at all wavelengthat all wavelength• VPH grating cross-disperser:VPH grating cross-disperser: with higher peak efficiency but with higher peak efficiency but suffers from a limited wavelengtsuffers from a limited wavelengt

h range side of the blaze wavelength h range side of the blaze wavelength Two VPH gratings is Two VPH gratings is an alternative solution an alternative solution without appropriate prismwithout appropriate prism material. material.


Resolving power

38000 Without slit

55000 With slit

Wavelength range 380~880nm(89 orders)

Fiber entrance aperture 2.6arcsec

Spectrograph layout

White -pupil configuration

On 1.5× 1.6m vibration-proof optical bench

2 separated collimators

Collimated beam size

Focal length

Focal ratio

92.5mm in diameter

925mm in diameter

F/10

Echelle grating

Blaze angle

Spatial frequency

Active size

Newport spectra physics

R2.9( 71° )

31.6 g/mm

128× 254mm

Double cross disperser 2 LF5 prism, apex angle :41°

Dioptric camera aperture 116mm in diameter

Image quality (E80) <13.5um

Detector Pixel size

Active pixels

Cooling

13.5× 13.5um

2048× 2048

TE 5-stages peltier ,Cooled down to –100℃

Estimated efficiency Peak efficiency >30%

Table. Main parameters of the WHRS spectrograph


Figure. Optical layout and tracing of the WHRS spectrograph

Spectrograph design-Spectrograph design-telescope telescope interfaceinterface• The telescope interface links the spectrograph The telescope interface links the spectrograph

through optical fiber to the telescope. It is installed through optical fiber to the telescope. It is installed at the Carssegrain focus of the 1m Telescope. It at the Carssegrain focus of the 1m Telescope. It allows flexible combinations of WHRS with allows flexible combinations of WHRS with photometric CCD camera. optics and calibration photometric CCD camera. optics and calibration sources.sources.

• Calibration sources:Calibration sources:

--Tungsten lamp to provide a continuum spectrum --Tungsten lamp to provide a continuum spectrum for flat fielding for flat fielding

--Thorium-Argon lamp for wavelength calibration --Thorium-Argon lamp for wavelength calibration

--Iodine cell for wavelength standard--Iodine cell for wavelength standard

Figure. The side view of the telescope interfaceFigure. The side view of the telescope interface

Spectrograph design-Spectrograph design-telescope telescope interfaceinterface

Spectrograph design-mechanical Spectrograph design-mechanical baselinebaseline

• Highly stabilized environment is a key point for Highly stabilized environment is a key point for precision RV measurement. It include:precision RV measurement. It include:

--high degree of thermal stability--high degree of thermal stability

--high mechanical stability--high mechanical stability

----there are no moving parts on the bench apart of there are no moving parts on the bench apart of the CCD shutter.the CCD shutter.

-- The shutter will -- The shutter will operate as bi-stableoperate as bi-stable

((no power is need to remains in open or closed position)no power is need to remains in open or closed position)

Spectrograph design-mechanical Spectrograph design-mechanical overviewoverview The The

spectrograph will spectrograph will be mounted on a be mounted on a vibration vibration absorbing optical absorbing optical bench and bench and covered with a covered with a thermally isolated thermally isolated chamber. This chamber. This bench is placed in bench is placed in a air-condition a air-condition room to limit room to limit temperature and temperature and humidity humidity changes.changes.

Spectrograph design-thermal isolated Spectrograph design-thermal isolated chamberchamber

Temperature in the chamber

Temperature accuracy:

±0.05℃(PV) one light

±0.017℃(RMS) one light

Expected performance-resolving Expected performance-resolving powerpower

• With a beam size of 92.5 mm and a sky aperture With a beam size of 92.5 mm and a sky aperture of 2.6arcsec, the R2.9echelle grating delivers at of 2.6arcsec, the R2.9echelle grating delivers at the 1m Telescope a resolution of 38000the 1m Telescope a resolution of 38000

• By narrowing the adjustable slit at the fiber exit, By narrowing the adjustable slit at the fiber exit, resolution can be increased to 55000resolution can be increased to 55000

About 19% of the total flux is lost at the highest About 19% of the total flux is lost at the highest resolution (slit width ~70% of the fiber diameter). resolution (slit width ~70% of the fiber diameter).

Resolution as function of slit w idth

0

10000

20000

30000

40000

50000

60000

70000

80000

90000

100000

110000

120000

0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 2 2.1

Slit w idth (arcsec)

Res

olu

tio

n

Expected performance-optical Expected performance-optical efficiencyefficiency• Optical efficiency was a main issue through the design,Optical efficiency was a main issue through the design,

procurement and optical element manufacture. procurement and optical element manufacture. Spectrograph element Efficiency

Two collimators Mirror reflectivity 94.5%

Fold mirror Mirror reflectivity 98.6%

Prism Total prism transmission 88%

Total entrance /exit transmission 91.6%

Echelle Echelle grating overfilling 89%

Absolute efficiency 73%

Camera All lenses total transmission 81%

CCD Quantum efficiency 89%

Entrance/exit transmission 93%

Total peak efficiency: 31.9%Total peak efficiency: 31.9% @ 500~600nm @ 500~600nm

without telescope and fiber

Expected performance-image Expected performance-image quality quality

Spectrograph spot Spectrograph spot diagrams on CCD diagrams on CCD image. Wavelength image. Wavelength range: 375~880nm. range: 375~880nm.

From Up left to From Up left to bottom right is blue bottom right is blue to red. Each to red. Each 13.5um box 13.5um box corresponds with corresponds with 11××1CCD pixels. 1CCD pixels.

Expected performance-image Expected performance-image qualityquality

WHRS echellogram WHRS echellogram with 90orders simwith 90orders simulated on the 27.6ulated on the 27.6×27.6mm×27.6mm

Status and planning

• Now, the design of WHRS has been Now, the design of WHRS has been finished, the optical element are being finished, the optical element are being polished by Nanjing institute of polished by Nanjing institute of Astronomical Optics Technology.Astronomical Optics Technology.

• And plan to start integration at Weihai And plan to start integration at Weihai observatory of Shandong University in observatory of Shandong University in Dec. 2009.Dec. 2009.

Thanks

Documents

High Resolution Echelle Spectrograph for Chinese Weihai 1m Telescope. Leiwang, Yongtian Zhu, Zhongwen Hu Nanjing institute of Astronomical Optics Technology