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Why a GEM for astrophotography?

*The GEM*

During a past on air hangout I spoke a bit about photographing Deep Space Objects and the equipment that I use to take those photos.  One of the things that was mentioned is that I use a GEM (German Equatorial Mount) on a very heavy tripod.  The GEM or Equatorial Mount is a really fancy way of saying it’s a tripod head that aligns the camera lens (either regular lens or telescope) with the earth’s axis.  I didn’t really explain in great detail why this is a must when you are trying to do very long exposures or “Stack” several shorter exposures.
Below is a photograph I took from the back deck of my house.  I had my camera mounted on a “Regular” camera tripod.  The region of the night sky may be recognizable by many people, it is the Orion Constellation area.  This photograph was achieved by taking several 13 second exposures (@24mm) and “Stacking” them using a free program called Deep Sky Stacker.  The total exposure on the image is just under 6 mins.
Since the earth moves, I had to move the camera to keep the Orion Constellation in the center of the viewfinder.  You will notice in the photograph that the in center of the photograph the stars are nice and round, as you move out from the center the stars start to take on an “Egg” shape, and at the very edges of the photograph, you can actually see what appears to be “Star Trails” and the image also gets a fuzzy look to it.  The reason for this is quite simply stated because a regular tripod mount does not rotate on the same axis as earth.  Each time I took another photo the Earth had moved and since my tripod head doesn’t move relative to the Earth’s axis the stars moved in the frame.
If you don’t mind this look, you can actually take shots with the equipment you probably already own.  If you’re looking to have an image like this where ALL the stars are nice and round you will need to purchase a GEM or something like Astrotrac, both of which move your camera with the axis of the Earth.


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