Running a Smart Telescope in the Cold

The best deep-sky nights of the year are cold, and cold is where smart telescopes are least comfortable. None of the problems are serious, but all of them will end a session early if you have not planned for them, and they tend to arrive in the same order every time.

Batteries: the problem that arrives first

Lithium-ion cells lose usable capacity as temperature falls. The chemistry slows, internal resistance rises, and the battery reports itself as flat while still holding charge it cannot deliver quickly enough.

The practical effect is that a telescope rated for six hours in mild conditions may manage two at −5°C, and the drop is not linear: most of it happens below about 5°C.

Three things help, in order of effectiveness:

• External power. A USB power bank kept warm in a coat pocket, connected by a cable, sidesteps the problem entirely. This is the single most effective cold-weather measure and it costs very little.
• Start warm. Bring the instrument out at room temperature rather than storing it in a cold car or shed. A battery that starts warm stays warmer for longer.
• Insulate. A neoprene wrap around the body holds several degrees for a couple of hours.

What does not help is charging in the cold, which some batteries refuse to do at all below freezing and which damages cells that permit it.^[1]

Dew: the problem that ends the session

Dew forms when the front optic falls below the dew point of the surrounding air. Because the optic radiates heat to a cold clear sky, it can drop several degrees below ambient air temperature, which is why it fogs while everything else is still dry.

This is also why the clearest nights dew the fastest: cloud cover reflects heat back and slows the process.

The fix is a dew heater strap, a low-power resistive band that wraps the front of the tube and holds the optic a couple of degrees above ambient. It runs from the same power bank you brought for the battery problem. This is the accessory that most extends a winter session and it is usually the cheapest one you will buy.

What to avoid: wiping the optic. Coated surfaces scratch, and you will be doing it in the dark with cold hands. It buys ten minutes and risks permanent damage.

Focus: the problem you will not notice

Optical components contract as they cool, shifting the focal point. The instrument cannot detect this, so it will keep collecting soft frames indefinitely.

Refocus once the instrument has reached ambient temperature, roughly twenty to thirty minutes after you set up, and again if the temperature drops sharply during the night. Two minutes of attention protects the entire session.

Condensation when you pack up

The failure that happens after the session. Bringing a cold instrument straight into a warm house causes moisture to condense on every surface including internal ones, and sealing it in a case traps that moisture against the optics.

Leave the instrument out of its case in the coolest room available and let it come up to temperature slowly. Put it away in the morning.

The short version

• External power bank, kept warm, connected from the start.
• Dew strap on before dew appears, not after.
• Refocus at thirty minutes.
• Do not charge below freezing.
• Do not case a cold instrument in a warm room.

That list turns winter from the season that defeats your equipment into the season that gives you the longest nights and the best targets, which is what it should be.

Notes & sources

1. Lithium-ion capacity loss at low temperature and manufacturer guidance against charging below freezing. ZWO Seestar FAQ, operating conditions; DWARFLAB Help Center ↩