Comet 21P/Giacobini-Zinner

This week's photo is a good example of how not to photograph a comet. However, being a holiday weekend and considering the fact that I will not solve the problem anytime soon, I am posting it anyway. On a positive note, it demonstrates a few facts of cosmic life that may not be obvious to the casual observer.

Comet Giacobini-Zinner

This photo makes the comet look more like a flying stick than what we think of as a comet. However, you will note that the stars are points of light which indicates that the telescope was doing what it is normally expected to do, viz., tracking the stars with great precision. However, being much closer than the stars, comets impart their own motion relative to the background stars which can not be compensated for simply by moving the telescope as the world turns.

The result is that the comet nucleus is a streak whose length represents the distance it traveled during the 20 minutes the film was being exposed. This is most noticeable when comets near the sun as they move much faster than when far away. The proper way to take this photo would have been to have the telescope track the comet instead of a star. This would result in the stars being streaks but the object was to photograph the comet and this would have looked normal.

The reason I had to take it this way was because it was one of the compromises I had to make when switching from the 10" to the 16" telescope. The larger scope flexes too much with changing position to allow the use of a piggyback guide scope which could track the comet independantly of the stars.

For examples of "normal" comets:

Comet Hyakataki

Comet Hale-Bopp

As a point of interest, Comet Giacobini-Zinner was the first comet ever visited by a spacecraft. According to Sky and Telescope: "On september 11, 1985, the International Cometary Explorer (ICE) flew through its gas tail at a speed of 21 kps some 7800 km downstream from the nucleus. The nucleus was estimated to be 2.5 km across at its widest diameter. Just three days later, observers around the world watched G-Z passing within 2 degs of the still-faint Comet Halley, which was then on its way toward its much-anticipated 1986 apparition."

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