City of the stars
Photographing the stars and other galactic objects as static objects
is more complex because here, the more magnification you use, the
more the apparent motion caused by the Earth’s rotation is magnified
too. If you use a wide angle lens on a fixed tripod mounted DSLR camera
for example, a short exposure of less than 30 seconds will typically
show little in the way of star trailing. If the power is increased by using
a telephoto lens or even coupling the camera to a telescope, the same
length of exposure taken from a fixed mount will definitely show trailing.
Unfortunately, many deep sky objects require long exposures in order
to capture them well, and to do this it’s necessary to allow for the rotation
of the Earth, and compensate for it. The most common way to do this is
1211to use a camera and/or telescope mounted on a driven equatorial mount.
This is a mount with one axis set parallel to the rotational axis of the Earth
so that the motion of a telescope fixed to the mount follows the natural
motion of the stars and planets across the sky. If the mount is driven at
the same rate, but in the opposite direction to the rotation of the Earth,
the object that’s being looked at through the telescope no longer appears
to move. If the mount is set up accurately, it’s possible to take extended
exposures of the stars using a camera attached to the telescope.
However, even here there are limits and exposures greater than a couple
of minutes may still start to show small trailing effects. These occur due
to small errors in the mount's pointing accuracy and minor variations in
the telescope mount's drive rate.
For longer exposures, the use of an autoguider is recommended. This
is a more sophisticated setup that uses a second telescope physically
attached to the first with a camera dedicated to monitoring where
the second telescope is pointing. This autoguiding camera is, in turn,
controlled by autoguiding software. The basic idea here is that you take
an image through the autoguiding telescope, pick a bright star from the
image and then instruct the autoguiding software to keep that star in the
same position in the image field. In order for this to work, the computer
running the autoguiding software needs to be physically connected to the
mount. Many mid- to high-end mounts currently offer this facility.
Autoguiding is a lot easier than it used to be but it can still require quite a
bit of setting up. The connections necessary to make it work, including the
various power cables needed, can turn the business end of a telescope into a
veritable bird’s nest of wires if you are not careful. It’s also very important that
the scope and its various attachments are properly balanced on the mount.
A properly set-up autoguiding system will allow you to take exposures
lasting theoretically for as long as you like as long as the guide star is
above the horizon and visible. However, practicalities do apply here too,
it’s unusual for exposures to go beyond a maximum of 30 minutes.