When the telescope will not behave

Most of what goes wrong is not the telescope. Of the five below, three are not about the instrument at all — warm air in the tube, dew, and too much magnification; a fourth is where the mount is aimed. Only one is the instrument itself, and that one takes five minutes.

What you see, and what it is

Every star is a small comet with a tail, and all the tails point the same way.
It is the same at the edge of the view as in the middle, and focusing does not touch it.

That is collimation. The mirrors have drifted out of line — a Newtonian does it just by being carried about. There are three screws at the bottom and it is a five-minute job once you have done it twice.

The drive is running, but whatever you are looking at creeps out of the view.
It creeps the same way every time, slowly, over a few minutes.

A driven mount turns about one axis, and that axis has to point where the sky turns about — near Polaris in the northern hemisphere, near the faint Sigma Octantis in the southern. Aim it better and the drift goes.

It was sharp at nine o’clock and by eleven everything is dim and soft.
The sky is clear and the stars are still there to the naked eye.

Dew. Glass pointed at the open sky loses heat to it and falls below the air’s temperature, and the air’s moisture settles on it. A long shield in front, or a gentle heater, keeps it off.

The image boils and shivers, as if you were looking over a hot road.
You carried the telescope out of a warm house twenty minutes ago.

Warm air inside the tube rises through the light path and stirs it. Nothing is wrong with the telescope: put it outside and leave it half an hour before you expect much of it.

A stronger eyepiece makes the planet bigger and mushier at the same time.
There is a point past which nothing you swap in gets any sharper.

Every telescope has a limit — roughly twice its aperture in millimetres — and the air usually stops you before the glass does. Less power and a steadier picture shows more, which is the opposite of what the box promised.

Then try it as a game

🌙 Plan an evening outsideEvery object, one by one