Auburn Astronomical Society Astrophotography Page
Astrophotography is a combination of art and science. Unlike terrestrial
photography, astrophotography, even with today's fast films, requires exposures
of tens of minutes up to over an hour. And, because the Earth is rotating
during the exposure, the telescope must be slewing at the same rate, but
in the opposite direction to keep the object being photographed in the
center of the frame. This is done by a drive motor attached to the telescope's
mount which must be exactly parrallel with the Earth's axis; a technique
known as Polar
Alignment. Still, fine adjustments must be made constantly by the astrophotographer
to compensate for mechanical errors in the drive, changes in frequency
of the line voltage to the drive motor, and image shifting caused by the
shimmering effects of the atmosphere called Seeing.
This technique is called Guiding
and requires that the astrophotographer keep constant vigil at the guiding
eyepiece of the telescope for the duration of the exposure.
The technical hurdles of precise polar alignment, critical focusing,
and guiding
are among the most demanding of amateur astronomy's endevours. Try keeping
this in mind as you enjoy the astrophotography on these pages.
This
is a simulation of what it's like to guide during an astrophoto exposure.
Using an illuminated reticle eyepiece, one must precisely center the guide
star, while correcting (with a joystick or button controls)
the speed of the right ascension drive motor or correcting the declination
to correct for mechanical errors in the drive or atmospheric conditions.
As the guide star appears to drift away from the center of the reticle
box, quick reflexes are needed to return the star back to the center.