Above: UPS made by APC, this model has a 1500 VAC rating and features a Serial Port for communicating to other devices (119,640 bytes).

Company Seven is involved not only in providing astronomical and other sensitive gear but we also build astronomical observatories, data centers and control rooms. A vital component of systems that are critical is the Uninterruptible Power Supply (UPS). Company Seven recommends the UPS even for the personal astronomical observatory where power may be required for critical shutdown operations; this can include closing the shutter of a dome and allowing time for an orderly shutdown and 'parking' of a telescope mount. Here we provide a brief overview of the UPS and how to select the capacity needed for your application.

The UPS: The Uninterruptible Power Supply is a device that provides there will be no lapse of continuous AC power to the connected device(s) if the primary AC wall current (mains) is interrupted. The UPS consists of a chassis housing electronic circuits with rechargeable batteries (usually sealed lead acid). The UPS plugs into an AC outlet, and provides AC outlets for electrical components. Incoming AC power is converted into DC current to charge the batteries. With loss of mains power the attached accessories are automatically switched over to run off the rechargeable batteries through a power supply that provides AC power out. Some UPS incorporate surge protection and/or line filtering circuitry to protect the attached device from current fluctuations or nearby lightning strike and/or to insure a highly perfected current, free of noise or voltage fluctuations, is delivered to the connected appliance.

Some UPS also incorporate surge protection and/or line filtering circuitry to protect the attached device from current fluctuations, line noise, or effects of a nearby lightning strike. These insure a highly perfected current is delivered to the connected appliance.

Telescopes of good quality can operate over a wide range of environmental, a span of temperatures for example greater than that of most other electronic appliances personal computers and UPS's included. The UPS recommended storage temperature range typically spans from -15 to 45° C (5 to 113° F), while ratings for nominal operation typically span from 0 to 40° C (32 to 104° F); so in some regions it may be best to keep the UPS and any associated personal computers in a climate controlled room or compartment.

UPS Capacity Ratings: the UPS set is typically rated by Volt Amps (VA), this is the maximum number of Volts * Amps it can deliver. Note VA rating is not the same as the current draw of the equipment, that is usually expressed in Watts.

Some but not all UPS's can continue to deliver power to the attached device(s) even if the VA rating of the UPS is exceeded, but a UPS can not provide power above their VA rating when the primary AC power fails. The circuitry of most UPS systems are designed to prevent exceeding rated output ratings, they may also protect the battery from being discharged too low. However, if the set lacks such features and you do miscalculate the requirements then to drain a UPS to beyond its capacity risks shortening the service life of the UPS's batteries and worse. In fact if some types of battery are drained fully then they may not recharge to their prior full capacity or may not recharge at all.

A UPS system cannot operate at 100 percent efficiency, some of the battery standby current is consumed by the device circuitry. So for purposes of calculating capacity we assign a factor in the calculation, typically 90 percent, termed "inverter efficiency". Also note the age of a battery can reduce the capacity rating.

Calculating UPS Capacity: to determine the suitable UPS capacity we calculate:

• battery power of the UPS (ie battery voltage) = inverter efficiency * battery voltage * battery amp-hour
• load = total current, in watts, demanded by the equipment that is necessary to perform an orderly shut down
• factor desired time to permit an orderly shut down; for example 20 minutes may be suitable.

For example if the UPS is a 1500 VA rated unit then this likely incorporates a 12 volt 12.5 amp hour battery. If the telescope mount's power supply draws 200 watts at 120v (1.7 Amp), and the observatory dome shutter control motor draws 373 watts at 120 volt (3.1 Amp), and we require 20 minutes of backup operating time (.33 hour), then:

Power Factor (PF) is another term you will run into that regards current draw, this is calculated by Watts / VA. A typical PF for workstations may be as low as 0.6, which means that if you record a drain of 100 Watts, you will need a UPS with a VA rating of 167. Some literature suggests that 0.7 may be a good conversion factor, but this will depend heavily on the machine.

Some general assumptions for calculations and considerations:

• a minimum inverter efficiency of 90%. In routine operation the baseline load should not exceed 50% of the UPS's rated capacity. So if you have a total 650 VA load from the telescope mount, shutter/roof motor, controlling computer, look for a UPS rated for at least 1300 VA.

• a tower style personal computer workstation may draw 500 watts or more when operating at full activity. An associated mid sized display panel can draw 70 watts.

• a regulated Power Supply that provides 12 volts DC at 5 amps may draw 200 Watts. For example of consumption, consider a hand-held drill may incorporate a ½ horsepower motor that may draw 373 watts.

Power Factor (PF) is another term you will run into that regards current draw, this is calculated by Watts / VA. A typical PF for workstations may be as low as 0.6, which means that if you record a drain of 100 Watts, you will need a UPS with a VA rating of 167. Some literature suggests that 0.7 may be a good conversion factor, but this will depend heavily on the machine.