... the project

This project arose from a discussion with Arthur von Känel. He showed me his photographs and explained me what his problem was. The images of the stars were not pin points they became trails. These trails were clearly longer than what we could expect from the quality of the lenses, the mounting or the seeing. He gave me the initial hint that the trails came into being because of the zenith distance dependent variation of the refraction. Soon after that I recognized that the orientation of the hour axis contributes substantially to the length of the star trails. So we first took care of the alignment of the hour axis in summer 2001.

The investigation on the Internet and the contemporary literature did not bring anything else to light than the well known method of J. Scheiner better known as the star drift alignment method. With this method we could align the mentioned telescope within about four arc minutes. This is, as we learned later, really insufficient.

In the various books about celestial photography there was from time to time a hint about a certain person named E.S. King and his photographical method. Often there was a small paragraph with a schematic drawing how the star trails look like on a photograph of the polar region and an explanation how the hour axis has to be moved depending on the direction of these trails.

I found a first description of this method of E.S. King in an article of Luc Dettwiller et. al dated back in 1992. With this explanation we could do the data reduction of our first photograph of the polar region and then we moved the hour axis accordingly. At the beginning of October 2001 we set up the hour axis on the apparent (refracted) pole with a precision of about 10 arc seconds. First celestial photographs showed unambiguously that the stars were point symmetric.

At first I simply wanted to write a detailed manual about the procedure we applied. In summer 2001 I planed to take a one year sabbatical and therefore I had time to dive deeper. In fall 2001 I found E.S. King's original article Forms of images in stellar photography published in the year 1902 and after I studied it it became rather clear that there is more to do than just setting up the hour axis. E.S King's mentioned that if the hour axis points to the apparent pole the star trails should become shorter.

I found a further hint for this assumption, that the effects of the atmosphere can be minimized in the introduction of an article by A.V. Filippenko with the title The importance of atmospheric differential refraction in spectrophotometry. He wrote:

"The purpose of this paper is to remind spectroscopists of the large effects of atmospheric dispersion and to present convenient tables and graphs (...) of optimal slit orientation as a function of the object position in the sky".

At that time it was rather clear that the refraction and dispersion of the atmosphere are visible on a photograph, despite that the (scientific) consequences are not that dramatic as in the case of the spectroscopy.

I went on with the investigation of the available literature and I realized that the topic is voluminous and that it was hardly described in the books about celestial photography in the past years. I read many articles in the archives of the different news groups and mailing lists and I found that there was only little knowledge about the effects and most often the problems could only be treated qualitatively.

At that time the idea was born to write a manuscript about this topic. I finished the writing in September 2003. Because the effects are difficult to explain with words I programmed a simulation which produces photographs as a diffraction limited telescope would deliver in the focal plane. With this program I could study the effects extensively. For this manuscript I produced figures with a total of about 1200 hours pure observing time. These are about 150 nights. Under the real Middle European sky that is a task that needs 3 to 4 years to accomplish. On the computer this time shrunk to about 30 hours raw computing time.

...my person

I studied physics, mathematics and astronomy at the University Basel and got my PhD in the year 1993 in the area of the experimental particle physics and my topic was: Investigations about the π+ absorption in flight on 3He with the "Large Acceptance Detector System". The experiment was called LADS and was carried out at the Paul Scherrer Institute (PSI).

At the beginning of the seventies I became interested in astronomy an bought in 1974 an orange Celestron 8 which replaced a azimuthal mounted newton mirror with a diameter of 15 centimeter. This telescope, it is still my working horse, was a real challenge. After the first visual observations I wanted to take photographs what was obviously doomed to fail because this telescope was never really designed for that purpose. For that reason my telescope disappeared from the scene with the exception of a few moments. During the nineties it found a "permanent" place on the balcony of our apartment. In the mean time my knowledge about mathematics matured a little bit and I looked for method to align the hour axis without a direct sight to the polar region. Ingredients of this method are a map with a scale of 1:25000 and spherical geometry. I aligned the hour axis with a precision of about 0.1^o in both directions.

Illustration 1: Mars in the infrared light (wave length interval about 700 - 850 nm). Single picture taken with a Celestron 8, a ToUCam 740k and a IR-filter at 3. 11. 2003 18:31. The diameter of Mars is about 14 arc seconds, North is up and East to the left. Left the raw picture, in the middle with dispersion correction and at right unsharp masked.

Illustration 2: Title page of the Œuvre catalog.

Beside my "astronomical" and my professional activities I published together with Diana Fahrner the Œuvre catalog Kurt Fahrner, das gesamte Werk. We produced this Œuvre catalog in the years between 1993 and 1998.

Comments, questions, corrections: markus.wildi@one-arcsec.org

To the top