Plan an Observation | The Schools' Observatory

Planning an observation for a telescope can feel tricky. You might know you want to observe a galaxy — but which one? And what does the telescope need to do to get the image or data you want?

The Go Observing pages guide you through the choices. This page explains the key ideas behind those choices, so you can understand what is happening and why it matters.

Coordinates: Where is it?

As the Earth spins each day and orbits around the Sun each year, the part of the sky we can see from a telescope changes.

We use coordinates to describe where an object is in the sky. The system can use these to work out whether an object is visible at a telescope at a particular time.

3D depiction of the celestial sphere with the sky "wrapped" around Earth at the center. The full equatorial coordinate system is shown, along with locations of the Sun in the sky at equinoxes and solstices. — Equitorial Coordinate System

Every star and galaxy has its own coordinates. These tell a telescope where to point to observe it.

Coordinates help with three useful questions:

North/South: Some objects are only visible from the northern or southern hemisphere. Coordinates help choose a site where the object rises high enough above the horizon.
Time of year: As Earth orbits the Sun, different parts of the sky are visible at night. Coordinates help work out which months an object can be observed after dark.
Time of night: As Earth spins, objects rise and set. Coordinates help find the best time to observe an object each night, usually when it is highest in the sky.

This is more complicated for the Moon, asteroids, and planets, because their positions change. However, we can calculate their coordinates for a particular time, so we can still plan observations.

Coordinates are also used to help make the coloured bars you might see while you Go Observing.

Instrument: What do you want to find out?

Professional telescopes usually have several different instruments. The best instrument depends on what you want to learn.

If you want a clear image, a camera is a good choice. You can also use a camera to measure the brightness of stars and galaxies.

If you want to find out what something is made of, you may need a spectrograph or spectroscope instead.

Photograph of a scientific instrument setup featuring optical components and prisms, with light being dispersed into a spectrum, used for astronomical observations or experiments. — The inside of the FRODOSpec spectrograph on the LT

Different instruments are designed for different kinds of science. For example:

Optical Cameras: These work like a very advanced phone camera, but are much larger and far more sensitive (meaning they can detect very faint objects).
Infra-red Cameras: Some objects emit light we cannot see. Infrared is part of the electromagnetic (EM) spectrum and can help us study cooler objects and see through dust clouds.
Spectrographs: These split light into colours like a rainbow. They can help measure composition, temperature, and the redshift of distant galaxies.
Fast-readout cameras: Some objects change quickly (for example, activity near a black hole, or a rapidly spinning neutron star). Fast cameras can take many images in a short time.
Sky Cameras: These look at a large part of the sky. They can help monitor the weather or search for rare events.

Field-of-View: How big is it?

Stars look like dots, but objects like galaxies and nebulae can look much bigger. How big an object appears depends on how big it is and how far away it is.

2 images of galaxies side by side, each against a black background with white dots of stars randomly scattered around in various sizes. On the left, the galaxy is small and sits centre-left, a white glowing sphere in the middle and the rest of it appearing as a fuzzy oval. On the right, the galaxy is centre and much larger, taking up most of the image. It has a glowing, white middle and fuzzy grey arms that spiral around it. — Comparing a small galaxy (NGC 23, left) and a large one (Messier 74, right)

Every instrument has a Field-of-view (FoV). This is how much of the sky it can see at once.

If an object is larger than the FoV, only part of it will appear in the image. In that case you may need to choose a different object, change instrument, or take multiple images to stitch together.

Astronomers often describe how big something looks using an angular size (measured as an angle). You can compare an object’s angular size to the camera’s FoV to see if it will fit.

For example, the FoV of the main camera on the Liverpool Telescope (IO:O) is 10 arcminutes (about 1/6th of a degree). Andromeda is about 6 degrees across, so it is far too big for IO:O. But the Fireworks Galaxy is just over 9 arcminutes across, so it fits well.

In Go Observing, we have selected objects that work well with the available fields of view.

Exposure Time: How long to look?

Telescopes have large mirrors and lenses so they can collect lots of light. This helps us observe very faint objects.

The longer a telescope looks at an object, the more light it collects. This is called the exposure time.

In theory you could expose for hours. In practice, telescope time is shared, so exposure time needs to be long enough to answer your question — but not longer than necessary.

Choosing an exposure time depends on several things:

Bright objects usually need shorter exposures than faint objects.
The instrument’s sensitivity matters (how much of the incoming light it can detect).
The quality of data you need matters. A “just visible” detection can use a shorter exposure than a precise measurement of a small brightness change.

Because this is complicated, Go Observing usually selects suitable exposure times automatically. Some advanced options may allow you to experiment with different exposure times.

Filters: What colour?

Most telescope cameras measure brightness. They do not measure colour in a single image.

Colour can tell you a lot about the Universe. For example, hot, blue stars are often very different from cooler, red stars.

Astronomers use coloured glass filters to let through only certain wavelengths. These filters select a range of colours, such as red or blue.

By comparing images taken with different filters, you can learn about an object’s colour and behaviour. For example, a blue star will usually look brighter through a blue filter than through a red one.

Filters are carefully made so astronomers can compare results from different telescopes.

For most observations in Go Observing, we choose filters that work well. Some advanced options allow you to experiment with different filters.

Seeing: How blurry can your image be?

"Seeing" is what astronomers call the blurring of images by the Earth’s atmosphere. Seeing changes over time, so the sharpest images are only possible at certain times.

Observatories are built in locations with good seeing, but it still varies. When planning an observation, it helps to decide how sharp the image needs to be for what you want to measure.

If you need the best conditions, the telescope can wait for excellent seeing. This can mean waiting longer, and it may need to fit around other observations that also need rare, high-quality seeing.

If you allow a wider range of seeing, your observation may be completed sooner — but the image may be more blurred.

In Go Observing, unless you are asked to choose seeing conditions, the system aims for a sensible balance between speed and image sharpness.

Sky Brightness: How faint can you go?

Many objects we want to observe are faint. Even with a big mirror and a long exposure, faint objects can be difficult if the sky is not very dark.

Professional telescopes are usually far from cities, so light pollution is less of a problem. However, twilight and moonlight can still brighten the sky and reduce what you can see.

Twilight: After sunset and before sunrise, the sky gets brighter. Even if your eyes cannot tell, the special cameras on telescopes can. For faint targets, we avoid “astronomical twilight”.

Moon: The Moon can brighten the whole sky. This depends on the Moon’s phase and how close it appears to your target in the sky. This is especially important for faint objects like galaxies and nebulae.

Two circular images side by side showing a partial view of a telescope, though most of the image shows the night sky. On the left, there are small white dots of stars and the dusty band of the Milky Way stretches in a slightly curved vertical line from top to bottom. On the right, there is a large white sphere to the right of the centre, and the sky is just dark. — SkyCamA images above Liverpool Telescope on different nights. On the left, the Milky Way is visible. But on the right, the moon is too bright for it to be seen.

Sky brightness can be predicted, so it is used to help make the coloured bars you might see while you Go Observing.

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