Issue |
Astron. Astrophys. Suppl. Ser.
Volume 133, Number 2, December I 1998
|
|
---|---|---|
Page(s) | 285 - 292 | |
DOI | https://doi.org/10.1051/aas:1998312 | |
Published online | 15 December 1998 |
Fabry-Perot filter based solar video magnetograph
Udaipur Solar Observatory, Physical Research Laboratory, Udaipur - 313001, India e-mail: shibu@uso.ernet.in, arvind@uso.ernet.in, debi@uso.ernet.in, ashok@uso.ernet.in
Send offprint request to: S.K. Mathew
Received:
20
April
1998
Accepted:
11
June
1998
A tunable Lithium Niobate (LiNbO3) Fabry-Perot filter (FP) (passband
165 mÅat 6122 Å) based video
magnetograph has been designed and fabricated. This instrument is capable
of providing near simultaneous observations of photospheric longitudinal
magnetic field, chromospheric Hα , and photospheric CaI pictures
using the same telescope and back-end set-up. The magnetic field
measurements are made by using the polarization properties of the Zeeman
components of the photospheric CaI line at 6122 Å(Landé g
factor of 1.75). The CaI line has been chosen due to its low temperature
sensitivity and no blend with other solar or atmospheric lines.
A variable electro-optic quarter wave retarder, KD*P
(Potassium di-Deuterium Phosphate) along with a linear polarizer is used
for analyzing the circular polarization of the Zeeman components.
The filter tuned at 140 mÅaway from the line center in the blue
wing is found to give the best linear response for the field strength
up to 1500 Gauss. A field of view (FOV) of arcmin on the
solar disk is imaged using a
pixel Cohu CCD camera in
synchronous with the KD*P modulation. The
modulation is
achieved by applying ± 2100 volts to the KD*P to obtain alternate
frames of oppositely circular polarized images. These images are stored
in separate frame buffers of an image acquisition system. To achieve high
signal to noise ratio, a large number of images (maximum 256) are added in
the respective frame buffers and then the difference between the left and
the right circularly polarized images is obtained. This difference is
related to the magnetic field strength. On comparing the video magnetograms
(VMG) obtained at Udaipur Solar Observatory (USO) on 09 April 1997 at 09:32
UT with those taken by SOHO/MDI at 09:41 UT, it was found that all the
magnetic features matched very well in both the magnetograms.
In this paper we present the details of the instrument and examples of
observations.
Key words: instruments: polarimeter / Sun: activity; flares; magnetic fields
© European Southern Observatory (ESO), 1998