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).

Date11. 07. 1991
Time2nd contact 18:48:23 UT, 3rd contact 18:54:52 UT
Total eclipse duration: 6 min 29 sec
PlaceTodos Santos, Baja California, Mexico
CoordinateW 110° 17', N 23° 29'
ConditionsClear sky, altitude of the Sun above the horizon: 82°
Optics10 inch F6 Newton reflector
FilmFujicolor 100 (6 × 6 cm)
ProcessingComposition 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
ScannerEpson Perfection V750 Pro (6400 dpi resolution, 4× scanned each image)
SoftwarePhaseCorr 4.0, FilmCMP 2.0, Corona 4.0, Sofo ACC 6.0
NoteThe 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.
OrientationThe 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


Miloslav Druckmüller
Institute of Mathematics, Faculty of Mechanical Engineering
Brno University of Technology, Czech Republic
Page last update: 18.8.2007