Is exactly what it sounds like: the camera rides "piggyback" on top of the telescope, making use of the scope drive to track the stars but shooting though its lens or small refractor instead of through the main telescope (Plate 76). Piggyback photography is often recommended for beginners since it eliminates—or at least reduces—the need for accurate guiding. Inaccurate polar alignment is also much less noticeable when doing piggyback photography.
The items needed to allow DSLR users to get started in piggyback photography are few. The main one is a bracketto enable the camera to be mounted on the scope. These are available from astronomy accessory dealers, are inexpensive in their most
Plate 76. (Piggyback)
A piggyback mount and a ball-and-socket tripod head allow a beginning imager to take attractive wide-field shots easily. Credit: Author.
basic forms, and mount easily in the "accessory holes" on the rear cells of CATs. One additional item, a "ball type" camera tripod head like that seen in Plate 76, is also desirable. If the camera is mounted directly to the piggyback bracket, it will be limited in the directions it can be aimed independently of the main scope. Finally, DSLR users will need a remote release cable (or a connection to a computer running a program such as Nebulosity) that allows the camera to take long exposures.
CCD cameras can also be used for piggybacking. Almost all the CCDs produced lately either include an integral tripod mounting socket or can be equipped with one. Naturally, "real" CCD cameras will need some kind of adapter to allow camera lenses to be mounted to them for wide field work (a very effective combination). The good news is these "T-to-camera lens" spacer-adapters are common and inexpensive.
For initial piggyback experiments, begin with a "normal" lens on the camera, a lens with a focal length of around 18 to 50mm. This will not only give a wide field of view, it will completely eliminate the need to guide the scope if polar alignment is even fair. With the scope set up and tracking, mount the camera on the piggyback bracket/ball head and point it in the general direction of a photogenic part of the sky (the southern Milky Way is especially good). Focus the camera as accurately as possible using either its viewfinder or, better, the big-screen display of a laptop program (don't depend on the infinity mark on a lens), and open the shutter with the remote release or PC program. Most SLR lenses deliver maximum sharpness if they are closed down an f-stop or two, so it's wise to try a couple of stopped-down shots and compare the results with "wide open" frames. A normal lens often has a speed of f/2 or faster, so don't expose for too long. Sky fog will set in quickly unless the skies are dark. Two or three minutes is a good start. Don't stop with one image, either. Take multiple images, as "stacking" them with image processing software will keep noise down.
The biggest challenge, it will rapidly become apparent, is not exposing images, it's processing them. Be forewarned, the pictures that initially come out of the camera will look awful. One of the most amazing aspects of the electronic imaging game is what a few simple processing steps will do to turn frighteningly ugly piggyback shots into detailed and beautiful celestial images. Where can a new astrophotographer learn this art? The books listed in Appendix 1 take novices through the process of "developing" digital images in step-by-step fashion.
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