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CALTECH ASTROPHYSICS PREPRINT
THE CALIBRATION OF DOPPLERGRAMS

AND MAGNETOGRAMS AT BBSO

Zhongxian Shi*, Jingxiu Wang* and Alan Patterson

BBSO #0257

Big Bear Solar Observatory

California Institute of Technology

* Visiting Assocaite from Beijing Astronomical Observatory

Academia Sinica, Beijing, China

-1-
ABSTRACT

The calibration procedure for the BBSO videomagnetograph in which the radial velocity of the sidereal rotation of the Sun is used as a calibrator is described in this preliminary

report. One of the key points of the procedure is to eliminate the effects of the Earth's motion relative to the Sun and the temperature instability of the birefringent filter by tuning

the bandpass of the birefringent filter. The other is to make the light level of the direct

image of the videomagnetograph the same both in Doppler and in Zeeman modes in order

to reduce the errors introduced by imperfect linearity of the transfer curve of the camera

tube. Some practical problems of calibration are discussed for further improvement.

-2-
INTRODUCTION

The videomagnetograph at BBSO was built by Smithson and Leighton (Smithson and

Leighton, 1971; Smithson, 1972). In 1979 it was rebuilt with a digital image processor,

and in 1981 the capability was improved by installing RCA cameras with Newvicon tubes,

replacing the KDP crystal, and introducing computer programs for accumulation of an

almost unlimited number of frames. The calibration procedure using solar rotation was

begun by Patterson (Patterson, 1983). The principle of this method is well known, however,

several practical problems were not resolved completely, and quantitative work could not

be done at that time.

Practical calibration is a difficult problem for many reasons. The profile of the spectral

line used by the videomagnetograph is modified by the presence of the magnetic field and

by temperature changes. Neither the profile of the bandpass of the birefringent filter

nor the spectral line are perfectly symmetrical. Also the transfer curve of the camera

tube is not perfectly linear. Moreover, the spectral line is shifted by both resolved and

unresolved Doppler effects and the central wavelength of the filter bandpass is also shifted

by temperature instability within the birefringent filter. However, the videomagnetograph

has the advantage of high temporal and spatial resolution. This final point convinced

us to resolve the difficulties and attempt quantitative work. In the following, a practical

calibration procedure is described in detail, and some remaining problems are discussed.

THE VIDEOMAGNETOGRAPH

The BBSO videomagnetograph has an optical arrangement that is shown in Figure 1.

The elements comprise a prefllter and trim filters, a circular polarizer, a KDP crystal, a

quarter wave plate, a Zeiss birefringent filter and a TV camera. The TV signal is fed to a

Quantex digital frame store and processor, which is controlled by a PDP 11/44 computer.

The videomagnetograph system can be operated in both Doppler mode and Zeeman mode.

When the KDP is preceded by a circular polarizer the videomagnetograph is converted

into a dopplergraph. In the Doppler mode images obtained in the red and blue wings of a

single spectral line are differenced to produce a Doppler image. The KDP crystal, acting as

an electronically switched quarter wave plate, changes the bandpass of the birefringent filter

by tuning the narrowest bandpass element. The other elements are not tuned. Therefore

the spectral profile of the filter is different when tuned to the two wings. But the two

bandpasses are mirror images of each other so each line wing is treated equally.

For the dopplergraph the best spectral line to use is one that is not sensitive to

magnetic fields, i.e. one that has a Lande factor of zero. However, apart from the few

Universal models, birefringent filters have a limited tuning range. The VMG at BBSO

makes use of a Zeiss filter tuned for the Ca I line at 6102.727 A, a magnetoactive line with

a lande g factor of 2.0 . To generate magnetograms the "Doppler mode" circular polarizer

is removed.

-3-
at known positions in the east and west hemispheres and calculate the average signal in

the central part of each frame. This gives a calibration by which we relate our digital

signals to the solar rotation velocity at these points, calculated according to the standard

differential rotation equation.

In practice, considering that the band passes in the red and blue wings of the line may

not be perfectly symmetrical and that the Dopplergrams are not uniform, we often took

three Dopplergrams, one each in the east, west and central parts of the Sun. Each frame

was divided into square areas of approximately 100 arcseconds on a side and the data

within each square was averaged. The averages from each of the squares within one frame

were averaged, giving more weight to the squares in the central part of the frame. The least

squares method was used to get the calibration value. In all cases we avoided calibrating

on active regions so that the calibrations were not distorted by the active region velocity

fields. Usually the region to be observed is not at the disc center, so its average Doppler

signal is not zero. After the calibration and at the start of the observations on the region

we retune the filter to adjust the average Doppler signal to the half scale value. In this way

the velocity signal caused by solar rotation is eliminated. However, as the Doppler signals

from an active region are typically complex we do not retune the filter while observing

an active region at latitude B and longitude L, but we retune the filter at a quiet region

whose rotation velocity is the same as the interested active region. We can simply use the

region of latitude -B and longitude L if this region is quiet.

3. Magnetogram calibration

The magnetogram calibration is done by way of the Doppler calibration. For Doppler

observations there is a circular polarizer ahead of the KDP crystal. This selects one linear

polarization of light and converts it into purely circularly polarized light. For magnetic

field observations the circularly polarized light is detected directly by the equipment. The

magnetograph system behaves like two Doppler measuring devices in parallel, one sensing

right circularly polarized light and the other left circularly polarized light. The calibration

value of the Dopplergram can almost immediately be translated to a magnetic calibration

value.

The circular polarizer has a transmission of 30% so the intensity of light at the TV

camera in magnetic mode is three times the intensity found in the Doppler setup. Because

the transfer curve of the TV camera is not exactly linear and because of variations in the

signal due to the internal gain and automatic black level circuitry of the TV camera, the

intensity correction is much more complicated. The best way to correct for these differences

is by taking magnetograms with a 30% transmission neutral filter (NG-4 1mm) in the light

beam. The magnetograms obtained in this way can be simply calibrated by the previously

described Doppler calibration procedure. For a velocity of 1 km/s the Doppler shift is

0.02034 A at 6102.727 A. For Zeeman splitting the wavelength shift of each component

from the central wavelength is

-5-
DISCUSSION

This calibration method has been used since July of 1983. For quiet Sun magnetic

field observations it has proved very successful. For long integrations (requiring 512 pairs

of Zeeman frames and 68 seconds) in very good seeing conditions, the noise level is less

than 5 gauss. When the integration time is increased further the noise is reduced by

a factor of square root of the ratio of frame numbers which is used to create the final

magnetograms. One example of calibrated magnetograms has been presented by Wang

J. et al. (1985). Their Fig. Ib shows a calibrated magnetogram for 4 September 1983

taken at Big Bear Solar Observatory and a Kitt Peak magnetogram taken earlier that day.

The two magnetograms match each other quite well. Although we have made progress in

defining a calibration system some problems still remain. We would like to mention these

problems to end this report.

1. The signals generated by the videomagnetograph are not only affected by the

Doppler shift that results from the Earth's motion and other factors, but are also sensitive

to the shape of the spectral line. Our calibration is based on the average profile of quiet

regions which is certainly very different from the line profile in places of strong magnetic

field such as active regions and sunspots. The temperature enhancement in an active region

also affects the line shape so there is some error in applying the described procedure to

active region observations.

2. The overall accuracy of the Doppler calibration is limited by local motion and

non-uniformity in the videomagnetograph filter. Such non-uniformities could be reduced

by using a telecentric optical arrangement, but space requirements prevented this. The

magnetograms, however, are much less sensitve to the non-uniformities.

3. There is still no good method of reconstructing the magnetograms of sunspots by

evaluating the effect of stray light and nonlinearity of the transfer curve of TV camera

at low signal levels. When the magnetic field strength is greater than several hundred

gauss the measured field strength is sensitive to the intensity correction adopted. We have

measured the transfer curves at several light levels. Further work needs to be done to

see how much the magnetic signal is affected by stray light, and how it is affected by

the flattening of the transfer curve at low light level. As we mentioned in section 2, the

simultaneous measurement of Zeeman splitting and magnetograms in regions near the disc

center of the Sun provides an empirical way of estimating the intensity correction.

ACKNOWLEDGMENTS

Z. Shi and J. Wang thank Dr. Zirin for accepting them as visiting associates at

Caltech and Big Bear Solar Observatory. This work is supported by NSF under grant

ATM-8211002 and NASA under grant NGL 05-002-034.

-7-
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SOLAR ASTRONOMY 264-33

CALTECH

PASADENA, CA 91125

NASA NGL 05-002-034