Solar corona on July 11, 1991
After 8 years of developing of mathematical and software tools for total solar eclipse image
processing I was sure that nothing is able to surprise me too much. Nevertheless, the processing of
the image presented on this page broke down the classical paradigm of total solar eclipse photography
completely. The paradigm is that the sequence of images with different exposure times is needed in
order to obtain usable images of different parts of the solar corona and composing of these images is
the only simple way how to make a good image of a large part of the solar corona (I do not
discuss tools like radial filters, which have their specific problems). It is highly surprising for
me that it is not true.
It is obvious that the optimum sequence of eclipse
images would contain more long exposure images than short exposure ones. The reason is very low
contrast in the outer corona and therefore more images are needed to obtain acceptable image / noise
ratio. I have never made an attempt to find the optimum exposure sequence by means of mathematical
optimization methods and I estimated, for eclipses I observed, the sequence intuitively. The reason
for that was that I was skeptic about the trustworthiness of such optimization because of many
uncertain parameters influencing the contrast of corona images like instantaneous stage of the solar
corona, atmospheric conditions, optics etc. After processing the 1991 set of eclipse images taken by
Ronald Royer in Mexico, I surprisingly came to the conclusion that for modern color negative films
the sequence consisting of identical exposures is very near to optimum. These exposures must be so
long that the outermost part of the corona recorded in the image must be correctly exposed. Modern
negative films have multiple sensitive layers and contain DIR components, which makes the reaching of
saturation point by overexposure nearly impossible. The extremely overexposed part of the image is of
very low, but nonzero, contrast. This low contrast is actually an advantage because the innermost
part of the corona and the chromosphere contains extremely high contrast features like prominences.
Composing of several such images enables to obtain very good contrast throughout the corona and
nearly even spatial distribution of noise.
The necessary presumption for
achieving satisfying results is a good film scanner which is able to use the full advantage of the
negative film extreme dynamic range. Our new Epson Perfection V750 Pro scanner fulfills this presumption but
you need the following heavily gained know-how. The scanner is unusable with the original software,
Silverfast software must be used. The negative film must be scanned in slide mode. 48 bit TIFF HDR
format must be used for images saving - using of 48 bit TIFF causes artifacts in resulting image
which are of the same or higher contrast than corona features. Multisample scanning is unusable
because it makes motion-blurred images and so the same image must be scanned several times and these
scans must be aligned in some other software and than averaged in one image. Therefore acquisition of
a acceptable scan of one single frame takes more than one hour. Finally the scans are of excellent
quality. Epson Perfection V750 film scanner is a typical nowadays high-end photographic equipment
i.e. excellent hardware with horrible software (even the most expensive digital SLR cameras are of
the same type).
Ronald Royer used a piece of black cardboard hand-operated in
front of the aperture for exposing the images in order to prevent vibrations caused by the camera
shutter. It made short exposures impossible and so he made only long exposures of the outer corona.
Even though he varied the exposure times in some range, the Schwarzschield effect (reciprocity failure) of
the film caused that the images are nearly identically exposed. In 1991, from that time point of view, this
strategy was far from being perfect. For me it is amazing that after 16 years this strategy turned out to be
the optimal one. The
resulting image is one of the best from classical film era in the MMV project archive. It shows that for
total solar eclipse photography the classical negative film is still nowadays worth considerating
especially if we take into account the enormous progress in film quality in the period of mentioned
16 years. (See also this page)
The bright star clearly visible on the right is δ Geminorum of 3.5 visual
magnitude.
The corona during the total solar eclipse on July 11, 1991 was of the structure typical for the
solar cycle maximum.
Click on the image or on the following reference to display the
higher resolution image version (933 KB, PNG format).
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| Image | Tse1991rdd_c1.png |
| Date | 11. 07. 1991 |
| Time | 2nd contact 18:48:23 UT, 3rd contact 18:54:52 UT
Total eclipse duration: 6 min 29 sec |
| Place | Todos Santos, Baja California, Mexico |
| Coordinate | W 110° 17', N 23° 29' |
| Conditions | Clear sky, altitude of the Sun above the horizon: 82° |
| Optics | 10 inch F6 Newton reflector |
| Camera | Hasselblad |
| Film | Fujicolor 100 (6 × 6 cm) |
| Processing | Composition of 9 images. Images were aligned by means of phase correlation technique and then processed using Corona 4.0 software. Final processing was done by means of ACC 6.0 image analyzer.
Image processing by Hana Druckmüllerová and Miloslav Druckmüller |
| Scanner | Epson Perfection V750 Pro (6400 dpi resolution, 4× scanned each image) |
| Software | PhaseCorr 4.0, FilmCMP 2.0, Corona 4.0, Sofo ACC 6.0 |
| Note | The graph above the table shows the average values of the relative number of sunspots in which the red arrow indicates the time of 1991 eclipse. Graph was created according to data from Sunspot Index Data Center, Brussels. |
| Orientation | The image must be rotated 11.8° clockwise to achieve the standard orientation i.e. North top, East left |
| Copyright | ©1991 Ronald E. Royer, ©2007 Hana Druckmüllerová, Miloslav Druckmüller |
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Miloslav Druckmüller
Institute of Mathematics, Faculty of Mechanical Engineering
Brno University of Technology, Czech Republic
druckmuller@fme.vutbr.cz
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Page last update: 27.11.2019
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