Unless all observing will be done from home, batteries will be needed to power a current-hungry go-to scope. Even if all viewing is from the backyard, it may still be more convenient to power the CAT with a 12-volt DC source than to worry about extension cords and AC outlets. What is needed in a scope battery is current capacity. Batteries are rated for their capacity in "amp hours." If, for example, a manufacturer says a battery has a capacity of 12 amp hours, it will potentially deliver 1 amp of current for 12 hours. It could deliver less current—say 500 milliamps—for a longer time. The recommended lowest current capacity for the average go-to telescope is 17 amp hours. Not only are modern telescopes power hungry when slewing at high speed—often drawing more than 1 amp—the amp hour rating of a battery is only a ballpark estimate. If there is a frequent 1-amp current draw, a battery will likely lose current well before 17 hours elapse. And, 17 amp hours is a very commonly available capacity for batteries.

What is the best type of scope battery? A jump starter is—portable sealed lead-acid batteries are designed for jump starting cars with dead batteries. One feature common to all these units is 12-volt DC cigarette lighter receptacles, which makes them perfect for use at the scope since most scope DC power cords have cigarette lighter-style plugs. Jump starters often have other frills: built-in chargers, built-in lights, sometimes even built-in radios. Jump starter-style battery packs are available from scope retailers, but the best bet is an automotive discounter.

If you are like your old Uncle Rod, though, you are powering more than just a scope. There is the dew heating system and the laptop. Don't forget the CCD camera and the DVD recorder. For high-current situations, forget jump start packs and go with what we down here call a "trolling motor battery" (deep-cycle marine battery). Deep-cycle marine batteries with current capacities in the 75-amp hour range cost about what a 17-amp hour jump starter does. Deep cycle means that the battery can be completely discharged without harm, something that may come in handy for the CAT user on an observing field far from AC outlets for charging. As always, there are a few penalties for more-better-gooder. Marine batteries are heavy, and a good charger will also have to be purchased to go along with one. If "plenty of power" is the requirement, though, they cannot be beat. For more information on battery buying and care, see the CAT hacking chapter, Chapter 12.

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