Captured Sept 15th, 2012_The Milky Way_Numbers

About two hundred years before the birth of Christ, Eratosthenes, the curator of the Great Library of Alexandria, decided he was going to measure the circumference of the Earth. He knew that at noon on a given day, in Syene, a city to the south, a stick planted perpendicular to the ground cast no shadow. So he did the same thing in Alexandria and on the given day, at noon, dutifully measured the angle of the shadow. He divided that angle into 360 and multiplied the result by the distance between the two cities. He then announced to the world the circumference of the Earth. Seems so simple, but he was right. He measured it to within two percent of the the number we have today. With two sticks. While he was at it, he correctly measured the distance to the Sun and Moon. But the powers that ruled the world at that time decided that the Alexandrians were just too smart for their own good. The library and almost all the knowledge contained within it were destroyed. Fifteen hundred years later, Columbus had a hard time convincing a crew to sail with him. They all thought they were going to fall off the edge of a flat world. 

Now for the numbers i want to talk about. If the earth didn't rotate at all, it would be the end of the world as we know it. But the astrophotography opportunities would be incredible! Not having to worry about rotation would allow me to collect HOURS of data without a hitch. At the Equator, the surface of the earth spins around the axis at just over 1000 miles per hour. Since where i live is so much closer to the axis, the speed is more like 700 miles per hour. Eight inches from the North or South Pole, the Earth is spinning at a robust 2.1 inches per hour. And that is about the speed my telescope mount rotates in the opposite direction. If i can get the axis of the scope to line up exactly parallel to the axis of the Earth, i can offset the rotation of the Earth and shoot pictures as if the Earth was not rotating at all. Not an easy task, and i spend an incredible amount of time on acquiring that alignment. But there are other ways....

The magic number is 600. That is the number i commit to memory. I don't even remember where i got it. I would assume i made some sort of google search on how to avoid star trails in photographs. In the next few lines i'm going to explain how to use it. 

The camera i use is a canon t2i. i bought it in a package deal containing two lenses. One lens is a 18mm to 55mm zoom, and the other is a 55mm to 250mm zoom. Nothing special, but not a bad package. The trick to photographing the stars off a fixed tripod is to limit each exposure to a time period that does not show discernible star movement. You can take as many photos as you like, they just have to be very short exposures. How short? Thats where the number 600 comes into play. If i wanted the greatest magnification possible from the lenses listed, i would take the larger lens and set the zoom at 250mm. If i divide 250 into 600 the result is 2.4. That is the maximum exposure i can use at 250mm. I can take as many pictures as i like to gather more light, but each shot can be no longer that 2.4 seconds. There will be no star trails. Just nice crisp stars. Several galaxies and nebula can be captured at these settings. If i were to set the zoom at 100mm, i could acquire shots at 6 seconds each. Once i have all the photos i want, i dump them into the freely available DeepSkyStacker. It does all the work of combining all the light data and aligning the stars in all the shots into one nice photo. 

The photo at the top of this post was taken at the Black Forest Star Party. I was using just the camera on a tripod. the lens was set to 18mm to allow 30 second exposures. (actually,  600/18=33.333 but 30 seconds is the highest autoexposure setting on my camera) The ISO would have been set at 1600 and the fstop as open as i could get it. I don't remember how many photos i collected, probably more than 20. It's a view looking toward the center of our home galaxy, the Milky Way. I used nothing more than the camera, tripod and a wired remote button.

(i know my daughter is going to read halfway thru this post and start mumbling "blah, blah, blah, tech, blah, tech, blah...". But that's ok, i'll explain it to my granddaughter in person.)

clear skies...