PIGGYBACK ASTROPHOTOGRAHY - by Chris Picking

Extract from the Astrophotography Camp (Taupo, NZ - May 2005)

Piggyback Astrophotography is an excellent way to start imaging the night sky.
The equipment required is modest and relatively straight forward to master.

Mounts
In order to take images longer than a few seconds you will need a way of tracking the
the movement of the sky. To this you will need a suitable telescope mount. The three
most popular types of telescope mounts are Equatorial, Alt Azimuth, and Dobsonian. Of
these the Equatorial & Alt Azimuth are best suited for astrophotgraphy. It is also
essential that the mount is sturdy and has motors to track the sky.

Polar Alignment
To track the movement of the sky accurately the telescope mount will need to be polar
aligned. This is achieved by aligning the axis with the celestial pole. Once this is done
it's only necessary to make small adjustments in tracking to account for inaccuracies
in the motor driving the mount.

Guiding
Monitoring the tracking can be done using an illuminated reticule and telescope which is
fitted along with the camera on the mount. Some mounts are also capable of auto guiding which
can alleviate the need to constantly check the tracking.

Cameras
Cameras for astrophotography need to be capable of taking exposures lasting several minutes.
In the past 35mm SLR cameras have been favoured as they have a wide range of interchangeable
lenses and the camera shutter can be triggered manually using a cable release. Lack of suitable
films and the advent of low cost digital SLR cameras has lead to widespread use of digital
cameras such as the Canon 10D and 300D and Nikon D70.

Taking your picture
Once you have your setup your mount and are polar aligned you will need to adjust the camera
to frame the area of the sky or object you wish to image. It's a good idea to check star charts
or other images to see where to best place the camera frame. The length of the exposure you make
will depend on the type of object you are imaging. For example using a digital camera set to ISO 800
with a 50mm lens an exposure of 4 minutes will capture a great amount of detail of the milky way.
With digital images it's also possible to combine or stack many images together to enhance the details
and reduce background noise.