There are many ways to distribute power to an astrophotography rig. At home and in the field, most of us try to arrange power so that the entire rig can be plugged into one large power bank or a single extension cord. When permanently set up at the observatory where power is readily available, some put every piece of gear on its own "wall wart". Somewhere in between lies a practical answer to the question, "How many power supplies?".
The important things to consider are whether each power supply (or the power supply) can provide sufficient current, whether the wiring can deliver that current without excessive voltage drop, and whether separating equipment provides a useful operational advantage.
Start With Enough Power
Before deciding how many power supplies to use, determine the maximum current your equipment could require at the same time. Include the mount while slewing, camera cooler at maximum power, dew heaters, computer, USB devices, focuser, filter wheel, flat panel, and anything else powered by the system. It's simplest to measure this in amps. (The other way to do it is watts, which is volts (12) times amps. Just doing it in amps is easier.)
Once you know how much current (amps) you need to provide, allow for plenty of reserve capacity. A simple rule of thumb is to choose a power supply rated for approximately twice your calculated maximum load. If your equipment could consume 8A under worst-case conditions, for example, a good-quality 15–20A supply provides comfortable headroom. The equipment will not consume 20A simply because the supply can provide it. Each device draws only the current it needs. But if the power supply is able to deliver 20A and you only ask it for 8A, you'll have fewer problems with heat and voltage drop.
Use good-quality regulated power supplies from reputable manufacturers. An inexpensive no-name supply advertised as “12V 20A” isn’t necessarily equivalent to an industrial-quality 20A supply. Regulation, ripple, transient response, protection circuitry, and the ability to continuously deliver the advertised current all matter. Mean Well, TDK-Lambda, XP Power, and TRACO Power are examples of established power-supply manufacturers. Power supplies supplied by reputable astronomy manufacturers (Pegasus Astro or Apertura, for example) are also reasonable choices.
Configuration 1: One Supply for the Entire Telescope
One of the simplest configurations is a single, generously sized 12V supply feeding everything on the telescope. The supply might feed an ASIAIR or a power-distribution box, which in turn powers the mount, main camera, guide camera, focuser, filter wheel, flat panel, dew heaters, and other accessories.
This has several advantages. There are fewer power supplies, fewer AC outlets, less wiring, and fewer potential failure points. With a properly sized supply and wiring, there is generally no electrical reason that a mount, camera, and other equipment can’t share the same supply.
The primary disadvantage is that the power supply becomes a single point of failure. A failure takes down the entire telescope. If you're using more than one supply (as described in one of the configurations below), you might be able to "borrow" power from another source until you can ship a replacement to the observatory. Furthermore, restarting one piece of gear by cycling the power requires taking down the entire pier.
For many installations, however, this is the simplest and most practical configuration.
Configuration 2: A Separate Supply for the Mount
Some astrophotographers routinely power the mount from its own supply. The argument for doing so is that motors create changing loads, particularly during slews. A separate supply prevents those changes from affecting cameras, computers, and other electronics. Conversely, a problem elsewhere in the system can’t cause the mount voltage to drop.
There is nothing wrong with this approach, but it usually isn’t necessary. If a shared power supply has ample capacity and the wiring is sufficiently heavy, the additional current required by a slewing mount shouldn’t cause a significant voltage change. Modern harmonic-drive mounts in particular consume relatively little power.
Separating the mount makes sense when there is a demonstrated power problem or when operational independence is desirable. It shouldn’t be necessary simply because a mount is a mount.
Configuration 3: Separate Power for Dew Heaters
There is a stronger argument for putting dew heaters on a separate supply. Dew heaters can represent one of the largest loads on a telescope, and their current consumption can vary considerably as a dew controller switches them on and off. They are also somewhat independent of the imaging equipment. There may be circumstances in which you want the dew heaters operating while cameras, mounts, or other equipment are shut down. A separate 5A or 10A supply for dew-control equipment can therefore provide useful isolation and independent control.
The disadvantage is simply the additional power supply and wiring. If the primary supply has adequate capacity and independent dew operation isn’t useful, there is no requirement that dew heaters be powered separately.
One consideration at AstroPeak is that dew is relatively rare. It may not be necessary to leave dew heater controllers active 24/7 in order to have dew protection when not imaging. If dew heaters/controllers turn on and off with the rest of the gear, that might be acceptable.
Configuration 4: Separate “Clean” and “Dirty” Loads
Another approach separates motors and heaters from sensitive electronics. Mount motors and PWM-controlled dew heaters are sometimes characterized as “dirty” loads because they can potentially introduce electrical noise onto the DC supply. Cameras and computers can then be powered from another supposedly “clean” supply.
There is some electrical basis for this approach, but it generally shouldn’t be necessary with good-quality equipment, properly designed power supplies, and good wiring. If electrical interference is actually observed, separating the offending equipment can be a useful troubleshooting step. Designing every system this way in anticipation of a problem that may never occur adds unnecessary complexity.
Configuration 5: Separate Telescope and Computer Infrastructure
At a remote observatory, there is a much stronger argument for separating telescope equipment from computer and network infrastructure. A mini PC may require 19V rather than 12V anyway, making a separate supply the natural solution.
Independent power provides valuable recovery options. You may want to power-cycle the mount, cameras, ASIAIR, or other telescope equipment without rebooting the computer. Conversely, you may need to restart the computer without disturbing the telescope.
Network equipment deserves similar consideration. If possible, avoid arranging things so that power-cycling a malfunctioning telescope also shuts down the Ethernet switch, router, VPN device, remote power controller, or other equipment you need to regain control of the system.
For remote installations, separating equipment according to what needs to be independently restarted is often more useful than separating it according to its electrical function.
Configuration 6: A Separate Supply for Everything
At the opposite extreme, every significant device can have its own power supply. This provides excellent isolation. A camera problem can’t overload the mount supply, and troubleshooting individual devices can be straightforward.
But it also produces a collection of wall adapters, occupies numerous AC outlets, increases cable clutter, and introduces many additional connectors and power supplies that can themselves fail.
Unless individual devices actually require different voltages or there is a specific reason for isolation, this is generally unnecessary.
Recommended: A Practical Hybrid Approach
For many systems, a good solution is a hybrid. Use one generously sized 12V supply for the telescope and imaging equipment, with a suitable power-distribution system near the telescope. Dew-control equipment can either share that supply or have a separate supply when independent operation is useful.
Computers and critical network or remote-control infrastructure can be powered separately so that telescope equipment can be power-cycled without losing remote access.
The goal isn’t to minimize the number of power supplies at all costs. It’s to use separate supplies where separation accomplishes something useful.
Wire Size Matters
Having a 20A power supply doesn’t help if the power has to reach the telescope through inadequate wiring. With 12V equipment, voltage drop is usually a more important consideration than the maximum safe current capacity of the wire. A wire might safely carry a particular current without overheating while still losing enough voltage to cause cameras, mounts, or computers to malfunction.
Wire resistance increases with distance. Resistance causes the voltage to decrease before it reaches the equipment at the other end. Heavier wire (that is, wire with a lower AWG value) offers less resistance and is recommended for longer runs.
The following table provides conservative guidelines for copper wire in typical 12V astronomy applications. Distances are the one-way distance between the power source and load.
Maximum Load |
Up to 5 ft Stays above or below the mount |
5–10 ft May cross the mount |
10 or more ft Crosses the mount |
|---|---|---|---|
1–3A |
18 AWG |
16 AWG |
14-12 AWG |
3–5A |
16 AWG |
14 AWG |
12-10 AWG |
5–10A |
14 AWG |
12 AWG |
10 AWG |
10–15A |
12 AWG |
10 AWG |
8 AWG |
These recommendations are intentionally conservative. They are intended to minimize voltage drop rather than merely prevent the wire from overheating. For short individual device connections carrying only a few amps, 18 AWG is generally adequate. For equipment that may consume 3–5A, 16 AWG is preferable, moving to 14 AWG as distances increase.
A main power feed carrying the combined load of an entire telescope deserves considerably heavier wire. A 10–15A trunk feeding a distribution box might reasonably use 10 or 12 AWG, depending on its length. Once the power reaches the distribution point, individual branches can use smaller wire appropriate to their individual loads.
There is very little electrical penalty for using wire that is larger than necessary. The disadvantages are primarily additional cost, weight, and stiffness. When choosing between two reasonable wire sizes, choosing the heavier wire is generally the safer choice.
Don’t Forget the Connectors
Wire gauge isn’t the entire circuit. Every connector introduces some resistance. Poor-quality barrel connectors, undersized adapters, cigarette-lighter plugs, inexpensive extension cables, and poorly made connections can produce significant voltage drops.
Use good-quality connectors appropriate for the expected current. For higher-current trunk connections, connectors such as XT30 and XT60 are useful because they provide low-resistance connections and are designed for substantially greater currents than typical astronomy barrel connectors.
The Simple Rules
Power systems don’t need to be complicated. Calculate the maximum current your equipment could consume simultaneously and choose a good-quality regulated supply with substantial reserve capacity. I like to calculate what I need, then double it. Use heavier wire as current and distance increase, and distribute power near the telescope rather than carrying large currents through long, thin cables.
Separate equipment onto different supplies when doing so provides a specific advantage, such as independent dew control, different voltage requirements, fault isolation, or the ability to power-cycle telescope equipment without losing remote access.