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Astronomical Data Analysis Software and Systems VII
ASP Conference Series, Vol. 145, 1998
R. Albrecht, R. N. Hook and H. A. Bushouse, e
Ö Copyright 1998 Astronomical Society of the Pacific. All rights reserved.
ds.
Image Processing of Digitized Spectral Data
A. V. Boulatov and L. K. Kashapova
Institute of Solar­Terrestrial Physics, Siberian Division of Russian
Academy of Sciences, P.O.Box 4026, Irkutsk, 664033, RUSSIA
Abstract.
Every observatory has a library of old pictures taken by photographic
cameras. These measurements are not as such high quality as those from
modern CCD systems, but could contain important data about rare phe­
nomena or source data for long­period investigations (e.g., concerning the
solar cycle).
This paper presents methods for converting photographic images,
collected at the Large Solar Vacuum Telescope (Baikal Astrophysical Ob­
servatory) to digital images, similar to CCD pictures. Special algorithms
were used for this task and have been implemented in the IDL environ­
ment. The results of processing real spectral data and a comparison of
processed photographic images and images taken with a TEK CCD are
presented.
1. Introduction
The Large Solar Vacuum Telescope (LSVT) is one of the exceptional astro­
nomical instruments belonging to the Institute of Solar­Terrestrial Physics, the
Siberian Division of the Russian Academy of Sciences (Skomorovsky & Firstova
1996). Because of the excellent spatial (0.3 ## ) and spectral (0.2--0.5 š A/mm) reso­
lution of the instrument and also local seeing conditions (the telescope is situated
on the shore of Lake Baikal) we have the ability to investigate fine structure of
the Sun. These observations are used to investigate short­time phenomena, e.g.,
solar flares, and also for the study of long­period features, e.g., the solar cycle.
For this work series of observations are usually taken.
During the long period of solar maximum we used a photographic camera
with 35­mm film for imaging with our telescope. A considerable amount of
valuable data (photographic images on films and plates) was taken and then
collected for future processing and analysis. During recent years, instead of
the photographic camera we have begun to make observations with modern
devices such as CCD systems. These have many advantages in comparison with
photographic methods: linearity, higher sensitivity, and so on.
To analyze long­period solar features we have to use both old and new
observations, i.e., photographic images and frames from the CCD detector. Both
photography and CCD techniques have their own characteristics which influence
the image. In order to perform reliable analysis we need to convert the di#erent
detector data to a homogeneous type.
63

64 Boulatov and Kashapova
We have created a set of programs for such corrections based on the IDL
language. One of the research fields of our group is the mechanism of energy
transport and energy release in solar flares. It is studied using polarization
observations. We present the work of these IDL programs using the example of
our spectral polarization observations and data analysis.
2. Observations and Preliminary Processing
All observations were obtained at the LSVT as described above. On the one hand
the exposure time is determined by the instability of the earth's atmosphere and
on the other it is determined by detector sensitivity. So exposure times for the
photographic observations were 0.45 sec and the CCD detector allows exposures
of about 0.1 sec. We used a Wollaston prism and two #/2­plates to separate the
ordinary and extraordinary rays.
2.1. CCD Systems
The first CCD system, which we mounted inside the telescope spectrograph, was
a CCD system from a St. Petersburg firm with exchangeable CCD heads. There
were heads with 800x400 and 370x290 pixels arrays. Using this system we can
take frames with very short exposures even with a dispersion of 0.0051 š A/pixel.
This CCD system is controlled by microprogramming code and external Pascal
programs.
In addition to that system we also use one amateur class CCD system (SBIG
ST­6 with the supplied control program) at our telescope and, during the last
few months, also another professional CCD system (Spectroscopic Instruments
TEK CCD 512x512 with ST­130 controller and WinView). The basic di#erences
between these systems are their array sizes and the rate of obtaining sequential
frames (the minimum interval between successive frames). The main calibration
procedures (removing the dark current, accounting for the flatfield frames and
so on) are similar for all CCD systems (McLean 1989).
2.2. Photographic Methods
As mentioned above, photographic images were taken on the 35­mm film. In
Figure 1 a typical view of our polarization data is presented. The two spectral
bands correspond to orthogonally polarized spectral bands. You can see the
so­called ``moustaches'' (Ellerman bombs).
Digitizing of the photographic images was performed in 3 alternative ways:
. point scanning with a standard micro­photometer
. scanning by the CCD line on a micro­photometer coordinate device, sup­
plying precise micro­movements
. digitizing with the help of an optical bench and using the CCD array for
registration.
After such procedures we can write a digitized frame of the picture taken
from the film. To obtain intensity values from the photographic image we have

Image Processing of Digitised Spectral Data 65
Figure 1. The image of the moustaches in the H # line. This frame
was acquired using the photographic camera with 35­mm film.
to use the characteristic curve for the film and at the same time we should
take account of the transfer function of the CCD (used as the digitizer) and the
uniformity of light in the microphotometer. This is very important for future
accurate analysis of real solar data. Also at this stage we carry out procedures
which are specific to every frame, removing errors (e.g., dust or cuts on film),
trends and disturbances from the photographic image. After that we can use
the digitized image from photographic plate or film as input for astrophysical
investigation.
3. Analysis of Digitized Photographs and CCD images
After the calibration the real processing begins, in our example, with mapping
of the H # contour and obtaining the spectral profile. We take sections across
or along the dispersion direction over the region being investigated to make
e#ective use of computer memory. We also normally load a colour look­up­table
to make the processing of the selected profiles more convenient and clear.
Using spectral atlas data we then define some basic lines close to H # and,
with the help of the computer mouse, mark the position of these lines on the
frame and obtain the calculated position of the H # line centre on the frame and
the average dispersion for a set of frames. Once this is done we can operate
using wavelength instead of pixels on the frame. For the following polarization
calculation two spectral bands are shifted to each other in order to combine
the H # centre from di#erent bands. Results of such processing, showing the
corrected spectral profiles of the moustache, are shown in Figure 2. The thin line
corresponds to the ordinary ray and the thick line relates to the extraordinary
one. The next step is to obtain polarization vectors and additional checks for
artificial influences on the spectral image (e.g., high­order trends, instrumental
e#ects). Then we prepare the best visualization of the results.
Recently we have continued these observations with the help of new equip­
ment. In Figure 3 the image frame with the moustache acquired using a CCD ­
detector is presented. We can use the programs created for processing digitized
photographic images in order to process these data as well.

66 Boulatov and Kashapova
Figure 2. H # line profiles in the moustaches. 1-- the ordinary ray, 2--
the extraordinary ray.
Figure 3. The image of the moustaches, H # line. The frame was
acquired with a CCD detector.
4. Conclusions
The result of this work is a developed set of programs for spectral data pro­
cessing. We can say with certainly, that on the base of our set of programs,
processed photographic images become comparable to real CCD images. After
this rather di#cult processing is complete we can use the two sorts of data to­
gether as a common series as is required for the investigation of long period solar
phenomena (in particular) and many other problems of solar physics.
Acknowledgments. We are grateful to Dr. N.M.Firstova, as main ob­
server at LSVT, for her invaluable co­operation during observations.
References
Skomorovsky, V. I., & Firstova, N. M. 1996, Sol.Phys., 163, 209
McLean, I. S. 1989, Electronic and computer­aided astronomy: from eyes to
electronic sensors, (Ellis Horwood Limited), 193