The calibration procedure for the Big Bear Solar Observatory (BBSO) videomagnetograph in which the radial velocity of the sidereal rotation of the Sun is used as a calibrator is described. 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

Patterson, A.

Shi, Z.

Wang, J.

English

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CALTECH ASTROPHYSICS PREPRINT
https://ntrs.nasa.gov/search.jsp?R=19860011287 2020-03-20T15:17:02+00:00Z
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 veloc-
ity 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 mo-
tion 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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The calibratrion of dopplergrams and magnetograms at BBSO

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