## Electricity for telescopes

Most if not all computerized telescopes run on 12 volts DC. Several will also run on lower voltages. The Meade LX200 nominally requires 18 volts, but in fact my 8-inch (20-cm) runs quite contentedly on 12 volts; all that is lost is some slewing speed.

In the field, rechargeable lead-acid or NiMH batteries are convenient power sources. The capacity of a battery is specified in ampere-hours:

Battery life (hours) = Capacity<*mpere-hours)

A typical telescope draws about 0.5 ampere, or a bit more, and will therefore run all night from a 7-ampere-hour battery, but CCD cameras, dew removers, and other accessories add to the load. I use a 17-AH battery pack designed for jump-starting automobiles; I've removed its huge cables and added some small, convenient sockets. Car batteries are not designed for deep discharge and are not suitable.

The voltage of a battery is nearly constant until the battery is almost exhausted. For example, a 12-volt lead-acid battery may charge up to 13.1 volts, then settle at 12.3 to 12.6 V during most of the discharge cycle. When it's nearly

BATTERY +

Figure 3.8. Voltage can be measured across the battery at any time, but current (amperage) can only be measured by interrupting the circuit and inserting the meter into it.

empty, it will drop below 12 V just before giving up the ghost. A digital voltmeter is useful for monitoring these small changes in voltage.

Figure 3.8 shows how basic electrical measurements are made. Voltage is the pressure that drives a current through a resistance. Thus, to measure current (amperage) you must break the circuit and insert the meter into it, but voltage is measured across the battery.

There are only three ways to change a voltage: throw away some of the energy with a resistance (as is done inside the LX200 hand box, which is why it gets warm); use a transformer; or use a switching regulator. Transformers trade volts for amperes, but they work only with alternating current. Switching regulators, such as Meade's 12-to-18-volt DC converter, do much the same thing. They chop the incoming DC into high-frequency AC; run it through a transformer or an inductor that provides transformer-like action; and then rectify it back to DC, meanwhile regulating the voltage.

Current is measured in amperes (amps, A) or milliamperes (mA), where 1000 mA = 1 A. The current rating of a power supply is the maximum it can deliver, while the voltage rating is the actual value; thus you can connect a 12-volt, 2-ampere power supply to a 12-volt, 0.5-ampere telescope and only 0.5 ampere will flow.

Wattage (power) is a measure of the rate at which energy is being expended. It is the product of voltage and current:

Watts = Volts x Amperes Thus a 12-volt, 0.5-ampere dew heater consumes 6 watts.

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