What happens when you book a telescope

Booking a remote telescope feels like ordering something online, but what happens in between is more interesting than a delivery van.

1. You choose a target

A target is simply the thing you want to look at: a galaxy, a nebula, a star cluster, the Moon. Every object in the sky has a fixed address made of two numbers, in the same way a place on Earth has a latitude and a longitude.

The sky's version of longitude is called right ascension, measured in hours rather than degrees, because the sky turns once a day. The sky's version of latitude is called declination, measured in degrees north or south of the celestial equator. Together they point at exactly one spot.

2. We work out when it is actually visible

An object is only worth photographing when it is high enough above the horizon, and when the sky is dark. Near the horizon you are looking through much more air, which blurs everything. That is why every observation has a minimum altitude.

Objects rise and set on a schedule that shifts through the year, so a target that is perfectly placed in January may be behind the Sun in July. This is why we cannot promise every object tonight.

3. The telescope points itself

At the scheduled time the telescope slews to roughly the right place, then takes a picture and compares the pattern of stars in it against a star catalogue. That comparison is called plate solving, and it tells the telescope exactly where it is really pointing, so it can correct itself. It is far more accurate than the mount's own sense of direction.

4. It takes many short pictures, not one long one

A single long exposure would be ruined by the slightest tracking error, and bright parts of the image would burn out. Instead the telescope takes many short frames, called sub-exposures, and those get added together afterwards.

Adding frames together does something useful: the real signal adds up steadily while the random noise partly cancels out. Sixteen frames stacked look roughly twice as clean as one.

5. The files come back to you

When observing finishes, the files transfer to your account. That transfer does not use up telescope time — the telescope has already moved on to the next person by then, and your download can carry on in its own time.