A mount needs to do more than track the motion of the stars across the sky to be suitable for piggyback work. It needs to be aligned with the Earth's axis of rotation. A computer-driven altazimuth mount can follow one object throughout the night. However, since it is not aligned with the Earth's axis, the stars around the object will appear to rotate. This is called field rotation. It doesn't make a difference to the visual observer. However, it does to the astrophotographer. Field rotation will create star trails on the film. This defeats the purpose of piggyback imaging, to record the stars and objects without trailing.
|German Equatorial Mount (GEM)|
Many Newtonians and refractors are used on German equatorial mounts (GEMs). As with all equatorial mounts, the right ascension axis needs to be aligned with Earth's polar axis to eliminate field rotation. To learn more about a GEM and its alignment, go to my polar alignment section. There are two prime locations where the camera can be fixed to a GEM. One is atop the telescope. The other location is on the declination axis. My telescope is a 10-inch Meade Starfinder equatorial and I mount the camera to the declination axis to do piggyback photography.
|Equatorial Fork Mount (EFM)|
The equatorial fork mount (EFM) is the other common equatorial mount. Many commercial SCTs are packaged with EFMs. My polar alignment section includes a detailed discussion of the EFM and how to polar align this mount. Similar to the GEM, there are two locations where the camera can be positioned. The prime location is atop the telescope. An alternate location is to mount the camera to one of the arms of the fork.
Now, let's look at some examples of piggyback astrophotography.
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Revised: February 11, 2002 [WDF]