India to build world's largest solar telescope

Proposed National Large Solar Telescope

Jagdev Singh

Indian Institute of Astrophysics, Bangalore 560 034, India.

e-mail: jsingh@iiap.res.in

Abstract.

Sun's atmosphere is an ideal place to study and test many

magnetohydrodynamic (MHD) processes controlling turbulent plasma. We

wish to resolve some of the finest solar features (which remain unresolved

presently) and study their dynamics. Indian Institute of Astrophysics has

proposed to design, fabricate and install a 2-meter class solar telescope

at a suitable site in India to resolve features on the Sun of the size of

about 0.1 arcsec. The focal plane instruments will include a high resolu-

tion polarimeteric package to measure polarization with an accuracy of

0.01 per cent; a high spectral resolution spectrograph to obtain spectra in 5

widely separated absorption lines simultaneously and high spatial resolu-

tion narrow band imagers in various lines. The Himalayan region appears

to be a good choice keeping in view the prevailing dry and clear weather

conditions. We have started detailed analysis of the weather conditions in

the area and at some other locations in India. The site characterization will

be done using the Sun-photometer, S-DIMM and SHABAR techniques to

determine the seeing conditions.

Key words.

Solar telescope—adaptive optics—spectropolarimetry—

location.

1. Introduction

The Sun offers an excellent laboratory to address the key processes that take place in

highly magnetized plasma that govern the many astrophysical phenomena and thus

provides a stepping stone to infer physical parameters in other more complex stel-

lar systems and of the universe at large. All the observatories in India (Kodaikanal,

Nainital and Udaipur) have made significant contribution to solar physics. Kodaikanal

observatory has obtained photoheliograms and Ca-K and H-

a

spectroheliograms of

the Sun over a period of about 100 years and formed unique datasets to study the long

period variations in various physical phenomena on the Sun because of uniformity in

the data (Singh & Bappu 1981; Singh & Prabhu 1985; Sivaraman

et al.

1999; Makarov

et al.

2001). Apart from the study of long period variations, one needs to investigate

very small scale structures such as  ux tubes that play an important role in all the activ-

ity and physical processes on the Sun. One meter Swedish vacuum solar telescope at

La Palma (a beautiful sight with very stable and transparent sky conditions), Canary

Islands which is being operated for the last 5 years by the Royal Academy of Sciences,

345

 

346

Jagdev Singh

Sweden has yielded great details about the solar surface features (Scharmer

et al.

2002)

of the order of 0.3 arcsec. The 65-cm telescope at Big Bear Solar Observatory has been

used to take images and make polarization measurements in 1565 nm line recently

(Cao

et al.

2006a, 2006b). They could achieve a spatial resolution of 0.3 arcsec

using adaptive optics. Kiepenheuer Institute of Solar Physics, Germany is planning

Gregorian, on-axis, Alt-Azimuth, 1.5 m open telescope to be installed at Izana. Big

Bear Solar Observatory is planning to fabricate and install 1.6 m off-axis in place

of 65 cm existing telescope. The upgraded facility will be used for the existing pro-

grams and magnetic field measurements with better spatial resolution and photometric

accuracy.

A large solar telescope known as Advanced Technology Solar Telescope (ATST) of

4-meter size, capable of taking diffraction limited images of the Sun (spatial resolution

of 0.03 arcsec) and make observations in the infrared wavelength region, has been

proposed by the National Solar Observatory in collaboration with many organizations

(ATST report 2000). The proposed telescope will be capable of observing the solar

corona also. It is likely to come up in the near future. There are many scientific

objectives of the telescope:

• How are the highly intermittent magnetic fields observed at the solar surface

generated by dynamo processes and how are they dissipated?

• What magnetic configurations and evolutionary paths lead to  ares and coronal

mass ejections?

• What mechanisms are responsible for variations in the spectral and total irradi-

ance of the Sun and solar-type stars?

• Progress in answering these critical questions requires a study of the interaction

of the magnetic field and convection with a resolution sufficient to observe scales

fundamental to these processes. Recently a 50-cm telescope known asHinode has

been launched in space by a Japanese group in collaboration with USA and UK

scientists and has obtained diffraction limited images with a spatial resolution

of 0.2 arcsec in G-band, Ca-H line and have made magnetic field measurements

(Hinode 2006). The images indicate that  ux tubes still need to be resolved by

making observations with higher spatial resolution. ATST is planning to achieve

this and has many other objectives that require a large size aperture of about

4 m. We, in India plan to have limited scientific objectives such as to resolve

the  ux tubes, make magnetic field measurements with required accuracy and

some other objectives listed in Table 2 for which a 2 m class telescope seems to

be sufficient.

It is well known that existing observational facilities for solar research in India, the

latest being Solar Tower Telescope installed in the year of 1960, are grossly inadequate

for high resolution observations of the Sun. Absence of a large and versatile telescope,

which can facilitate simultaneous measurements of the solar atmospheric parameters

and of the vector magnetic fieldswith high accuracy, has been a serious handicap for the

solar astronomers. Efforts have been made for the past 20 yearsor so to have a good and

large solar telescope, equipped with state-of-the-art focal plane instruments using the

present day technology but without success. Recently Udaipur Solar Observatory has

succeeded in designing a moderate size telescope and placed an order with a company

(Venkatakrishnan 2006). There is therefore, an imperative need for a state-of-the-art

solar observational facility, comparable to the best in other countries. A 2-meter class

 

National Large Solar Telescope

347

large solar telescope is hence being proposed as a national facility for implementation

during the 11th five year plan.

2. Scientific objectives

The proposed telescope will address the fundamental questions about the nature of

solar magnetism, will aim to resolve  ux tubes and measure their strength; address

the development of magnetic fields on the Sun which are responsible for almost all

the observable phenomena on the Sun such as solar dynamo, solar cycle and solar

variability that determine and control the space weather. The other scientific objec-

tives are: (i) MHD waves by resolving small structures and determining periods of

oscillations which may be responsible to transport the energy to the upper atmosphere

of the Sun; (ii) Dynamic evolution of small scale structures by making high cadence

observations; (iii) Evolution of active regions and their role in triggering solar  are,

prominences, filament eruptions, CMEs, etc.; (iv) Thermodynamics of the chromo-

sphere by making the observations in the infrared wavelengths; (v) Weak and turbu-

lent magnetic field measurements using Hanle effect which are as important as the

strong magnetic fields and have now become possible to be measured because of the

development in technology. All these objectives will be achieved by making obser-

vations with high spatial resolution using adaptive optics, high spectral resolution,

high temporal resolutions, multi-wave length capability of imaging and spectroscopy

focal plane instruments, high photon  ux and sensitivity of the detectors and using the

infrared part of the spectrum for observations. Table 1 gives a summary of the scientific

objectives.

3. Telescope specifications

Keeping in view the scientific objectives and technical requirements of observations,

it has been proposed to have a 2 m class telescope of Gregorian format. Direct light

from the Sun falls on the primary mirror M1 of aperture 2 m and of a focal ratio of

f/2.0 which forms a solar image of about 36 mm size at the prime focus F1. In the

process of forming the solar image, the primary mirror concentrates nearly 3 kW of

heat at the focus. A heat trap (not shown in Fig. 1) with an efficient circulating coolant

placed close to F1 takes away more than 98% of the heat by directing most of the solar

radiations out of the telescope to the atmosphere. A field stop with a hole of about

6 mm diameter allows only a small portion of the solar image (about 300 arcsec) to

pass through further to the secondary mirror M2. This helps to reduce the heat fur-

ther within the telescope. The secondary mirror M2 magnifies the 300 arcsec of the

Sun's image and brings it to the secondary focus F2. The beam is allowed to pro-

ceed further to the tertiary mirror M3 of the Gregorian system which magnifies the

image at the secondary focus and brings the 300 arcsec image of the Sun to the final

focus point F3 via a chain of  at mirrors M4, M5, M6, M7, and M8 into the labo-

ratory at the lower level as shown in Fig. 1. M6, M7, and M8 form a de-rotator that

compensates for the rotation of the field of view. Mirror M9, serves as the tip tilt

mirror (active optics mirror) and M10 is the deformable mirror DM of the adaptive

optics. The mirror M11 feeds light to the chosen focal plane instruments for various

observational programs through set of beam splitters and mirrors. The primary image

formed by the mirror M1 will have aberrations (coma and astigmatism). These are

 

348

Jagdev Singh

Table 1.

Summary of science objectives and matching instrumentation.

Summary of science objectives

Physical pro-

Physical quantity to

Telescope/instrument

Instruments

cess/ region to be

be measured

requirement

proposed

observed

MHD waves and

Intensity variation of

High photon  ux need-

Narrow band

oscillations

1% or less

ing aperture of 2 meter

filters

and above

Velocities of

Spectral resolution ofa

High resolution

~ 200 m/sec or less

spectrograph

few mÅ

Magnetic field mea-

Polarization accuracy:

Spectropolarimeter

surements

10

or better

with spatial

-4

information

Properties of oscilla-

High time cadence High speed

tions

cameras

Different heights

Diagnostic parame-

Both visible and IR

Ca II K, H

a

,G

in solar atmo-

ters covering many

capabilities

band, He I1083 nm,

sphere

spectral lines

Fe I 1.56 microns

filters

Active region evo-

Velocity and mag-

High time cadence

Spectropolarimeter

lution

netic fields

large field of view of

with spatial

300 arcsec

information

Hanle effect Magnetic field

Polarization accuracy

Spectropolarimeter

~5–10 G

10

-4

properly corrected by mirrors M2 and M3 to produce the final aberration-free image

of the 300 arcsec of the Sun at the final focus. In addition, these two mirrors will be

mounted on hexapods to enable fine tuning of the alignment of the telescope sys-

tem. The f-ratio of the telescope system will be designed to be f/40 so that the final

magnified image of the Sun will have a scale of ~ 2

.

5 arcsec/mm at the final focal

plane.

One of the major focal plane instruments will be a spectropolarimeter package to

measure the magnetic fields with a high degree of accuracy and high spatial resolution.

The spectropolarimeter will use the new technology of Dense Wave Division Multi-

plexing (DWDM) filters and Fiber bundles to convert two dimensional solar images

into slit images to perform spectroscopy. This will avoid scanning of the image to

measure the magnetic fields and thus permit study of the temporal variations in the

magnetic fields before, during and after the solar  ares. This will help us to under-

stand the processes involved in the triggering of  ares. A high spectral resolution

spectrograph capable of taking spectra in 5 widely separated absorption lines simulta-

neously and a facility to take high spatial resolution narrow band images of the Sun in

many photospheric and chromospheric lines to resolve structures of about 50 km size.

The optical layout of the telescope is shown in Fig. 1 and the specifications are indi-

cated in Table 2. The narrow band filters will include Ca-K, H-

a

, CN-band, G-band

and UBF with 0.02 nm pass band. The telescope will employ active and adaptive

 

Figure 1.

Optical layout of the proposed 2 m solar telescope.

optics to stabilize the image and reduce the effect of variations in the atmospheric

conditions.

4. Location and site survey

The location of the telescope needs to provide a large number of clear hours for making

observations with very good seeing and transparency. To make observations in the

infrared wavelengths for high accuracy of magnetic field and velocity measurements,

water vapours in the air need to be very less. Himalayan regions appear to provide

such atmospheric conditions. A 2 m telescope has already been established at Hanle to

perform astronomical observations during night-time. The site survey done at various

 

Table 2.

Summary of telescope features.

Aperture (primary mirror M1) 2 meters

Focal length 4 meters

Optical configuration 3 mirror, Gregorian on - axis

Field of view 300 arcsec

Final focal ratio of the system f/40

Image scale 2

.

58 arcsec mm

-1

Optical quality

<

0

.

1 arcsec over the field of view

Wavelength of operation 380 nm to 1000 nm (upto 2500)

Polarization accuracy 1 part in 10,000

Active and adaptive optics To realize near diffraction limited performance

Spatial resolution

<

0

.

1 arcsec

Figure 2.

Part of the Pangong lake in the Himalayan region.

places has indicated that the lake sites provide better seeing conditions such as that at

Big Bear Solar Observatory. We, therefore, have planned to determine the atmospheric

conditions during daytime at Hanle (existing observatory), Pangong in the Himalayan

region (a big lake site, Fig. 2) and Devasthal near Nainital Observatory). We have

already collected the weather data at Hanle and at Devasthal. We have started to

determine the weather conditions at Pangong as shown in Fig. 3. We plan to measure

the seeing conditions at these places using S-DIMM and SHABAR techniques. We

have already procured one such instrument from National Solar Observatory, USA

which we expect to install at Hanle during the period of June 2007. We plan to fabricate

such systems to measure the seeing conditions at other places mentioned above and at

some other places chosen later.

 

Figure 3.

Weather station at Pangong to measure solar radiations, temperature, wind speed, etc.

Acknowledgements

This is based on the project report prepared by the members of solar group at Indian

Institute of Astrophysics and many others. The weather data at Hanle was collected

by the members of IIA and at Devasthal by members of ARIES.

Thanks

Jagdev Singh
Indian Institute of Astrophysics, Bangalore 560 034, India.
e-mail: jsingh@iiap.res.in

http://prints.iiap.res.in/bitstream/2248/1758/3/Proposed%20National%20Large%20Solar%20Telescope