Kai Zhang Yongtian Zhu Lei Wang Nanjing Institute of Astronomical Optics & Technology, National...

Kai Zhang

Yongtian Zhu

Lei Wang

Nanjing Institute of Astronomical Optics & Technology,

National Astronomical observatories, CAS

Feasibility Experiments and Prototype Design of a Multi-object Exoplanet Survey

System (MESS)

Content

Introduction of a Multi-object Exoplanet Survey System

– Principle of operation– Feasibility experiments

Prototype design– Optical and Mechanical design– Temperature Sensitivity– Anti-vibration & Optical processing problem

Principle of Operation

–Theoretical basis:

External Dispersed Interferometer (EDI)

——David Erskine et, al. in 1997 –System Composition:

Telescope(2~4m); Fixed Delay Michelson Interferometer; Medium Resolution Spectrometer (R5,000~20,000)

Advantage:

Interferometer fringes and spectrum yield Moiré effect to enhance the measuring precision of radial velocity shift.

Principle of OperationFigure of Principle:

With EDI, spectrum phase shift is measured

Without EDI, spectrum line shift is measured

Spectrometer

EDI

devices

Telescope

Phase shift of Moiré fringes is obviously more 5~6 times sensitive than spectrum line shift without EDI.

Principle of Operation

System:A Multi-object Exoplanet Survey System based on

LAMOST is being developed.– 4m LAMOST telescope and its fibers– Multi-object Fixed Delay Michelson Interferometer– LAMOST Low Resolution Spectrometer (LRS)

(works in Medium Resolution mode)Observatory objects:

– 60% dwarf, mainly sun-like type; 30% G, K type Giant and sub-Giant; 10% M type dwarf.

– 8~12V

Fig. Scheme of system based on LAMOST

Principle of Operation

Feasibility Experiment

In Oct. 2008~Feb. 2009, some feasibility experiments were processed in lab.

Result: – Basically prove the principle of EDI– Gain some data of interferometer spectrum– Aware of some problem about developing the devices

… …

Feasibility ExperimentFigures of feasibility experiment:

Fig. Experiment in Oct. 2008

Interferometer

Light Spectrometer

Fig. Experiment in Jan. 2009

Optical Fiber

Feasibility ExperimentFigures of feasibility experiment: (λ 0.51~0.55um)

Fig. Binary objects’ Interferometer spectrum of Mo, in Jan. 2009

Fig. Interferometer spectrum of Hg, in Oct. 2008

Fig. Interferometer spectrum of Mo, in Jan. 2009

Fig. Phase shift without any anti-vibration, in Jan. 2009

Prototype Design

According to the feasibility experiment and results, prototype design is given.

Design:

Multi-object Fixed Delay Michelson Interferometer (FDMI) is the key device of MESS.

– Optical and mechanical design• Modularization (3 units per a module)• Simplify and minimize the structure• High installation accuracy

– Temperature Sensitivity– Anti-vibration & Optical processing problem (Ongoing)

Optical and Mechanical Design

Figure of prototype design :

Fiber

Fiber

FiberSlit

Fig. optical design of a module

Optical and Mechanical DesignFigures of prototype design :

Fig. Mechanical design of a module

Optical and Mechanical Design

Some difficulties:– Multi-object (~40 object)– Limitation of slit (0.16~0.32mm; limited space) – Low optical efficiency (fiber coupling; transmission;

vignette effect)– Performance index (fixed delay 1~2mm; 4~6 fringes)– High installation and stability accuracy

… …

Temperature Sensitivity

Analysis:

Analysis of temperature sensitivity has been done, especially about the fixed delay.

Delay of interferometer changes with wavelength, temperature and incident angle. So the analysis was done in the following way:

– Plane effect (influencing factors)– Glass cube (Data and evaluation)– Defocusing effect (Optimization)– Radial velocity shift with temperature

Plane EffectThree influencing factors:The delay of Michelson interferometer, also called OPD, is

required to be a fixed one, like a plane. But temperature and incident angle, which are respectively along two different orientations, like two forces to twist the plane.

Third force, wavelength, also contributes the change of delay, like an elevator to lift the plane up or down.

For compensating these forces, the interferometer design borrows ideal from WAMI*, and set up seven conditions.

… …*WAMI, firstly introduced by R. L. Hilliard and G. G. Shepherd, in 1965

)(2 2211 lnlno 02

2

1

1 n

l

n

lw 0

o0

w

0T

w0

To 0

2

To

Glass CubeTwo compensation processes:A process of choosing different length and materials of

arms to give the required delay is known as field compensation since it allows a much larger acceptance angle for the interferometer.

Another process of choosing the better glass pair from all of samples, is suggested as evaluation function to degrade its temperature and wavelength sensitivity.

So, three coefficients of every glass type is given. The gain coefficient of refractive index with wavelength

Thermal line expansion coefficient

The gain coefficient of refractive index with temperature

n

n

1

T

l

l

1

T

n

n

1

Glass Cube

Glass cube:

Actually, seven above conditions can’t be perfectly and stimulatingly satisfied by real glass types. So a kind of glass data, we call ‘glass cube’, is given to evaluate every sample of glass pair.

Every glass cube is an array data of four dimensions that respectively represents delay, wavelength, temperature and incident angle. And their data are gain from some function of three above coefficients.

… …

21 w )()( 2211 Tw

)()( 222211

21 nnT

Defocusing effect

Optimization:After two above processes, the number of samples

decrease from 2883 to 40.For further widening angle at a wide waveband, a

defocusing effect is suggested as optimization method. The result is shown that makes acceptance angle larger by 10~20%.

Finally, a manual selection is done for choosing the best one of all the above samples.

In the following figures, the results of defocusing effect are respectively shown.

——Ref. SCHOTT Glass Data, 2009.

Defocusing effectAnalog figures:(N-LAK34 / BAF10 )

Fig. without defocusing effect Fig. with defocusing effectIf the max required delay error is 0.05 waves @527.4nm (blue dot line) With defocusing effect, the acceptance angle widens by ~13%.

Defocusing effect

Analog figures:(N-LASF31 / N-LASF45)

Fig. without defocusing effect Fig. with defocusing effectIf the max required delay error is 0.05 waves @527.4nm (blue dot line) With defocusing effect, the acceptance angle widens by ~19%.

Radial velocity shift with temperature:

with analysis of delay, radial velocity shift with temperature is given by the Doppler function.

If keeping the shift under 1m/s, the temperature would be stable in a small range. So a smaller radial velocity shift with temperature required becomes an important condition for selecting the suitable sample.

Radial Velocity with Temperature

TT

ccv

oo

Radial Velocity with Temperature

Figures of delay shift with temperature:

Fig. delay shift with temperature of Fig. delay shift with temperature of

N-LAK34 / BAF10 N-LASF31 / N-LASF45

Figures of Radial velocity shift per degree:

Fig. Radial velocity shift of Fig. Radial velocity shift of

N-LAK34 / BAF10 N-LASF31 / N-LASF45 According to two above figures, the environment’s temperature respectively

needs to keeping in a range of 2mK for radial velocity shift of 1m/s.

Radial Velocity with Temperature

Ongoing

Other problems:– Anti-vibration

Phase stability is required to 1/8 waves per 30 min. – Optical processing

Internal reflection mirror with optical wedge.– Optical contact

Epoxies used to join up the components might exhibit some level of creep as the epoxy shrinks.

– Beam splitter coatingA particular non-polarizing coating is designed to yield a

close 50-50 split in a large range of incident angle.

Ongoing

Figures of beam splitter:Optical path through a beam splitter is a dual path. So

the ideal transmission is 25% per split ray.

Fig. beam splitter with designed incident angle of 45°and field of view of 20°

Fig. beam splitter with designed incident angle of 30°and field of view of 20°

Analog figures of beam splitter coating:

Fig. Beam splitter coating 30° Fig. Beam splitter coating 45°

Two above figures are respectively two kinds of dual transmission of non-polarizing 50-50 coating with incident angle of 30°and 45°.

Ongoing

Analog figures of beam splitter coating:

Fig. Beam splitter coating 30°(-10°) Fig. Beam splitter coating 30°(+10°) Two above figures are respectively the dual transmission of non-polarizing 50-50 coating with boundary angle of 20°and 40°.

Ongoing

Analog figures of beam splitter coating:

Fig. Beam splitter coating 45°(-10°) Fig. Beam splitter coating 45°(+10°) Two above figures are respectively the dual transmission of non-polarizing 50-50 coating with boundary angle of 35°and 55°.

Ongoing

Thanks for your attention

Japan. Oct. 2009

张 凯