Plan an Observation

Planning an observation for a telescope is quite a tricky thing. You may know that you want to observe a galaxy, but which of the many millions we know of? And exactly what do you need the telescope to do to get the image or data that you want? The Go Observing pages will help you through all the decisions you need to make, but here you can learn a bit more about all the details.


Coordinates: Where is it?

Part of the sky with coodinate grid marked
Part of the night sky with a coordinate system marked.
Image created using Stellarium.

As the Earth spins around each day, and orbits around the Sun each year, the part of the sky that can been seen from any place on Earth changes. To help work out if something is visible at a telescope at any particular time, we use coordinates. Every star and galaxy has its own coordinate which tells us where it is in the whole sky around the Earth. At any time we can use that coordinate to work out where a telescope like the Liverpool Telescope (LT) would need to point to observe it.

The coordinates of objects are used to help to make the coloured bars that you might see as you Go Observing.


Instrument: What do you want to find out?

The inside of the FRODOSpec spectrograph
The inside of the FRODOSpec spectrograph on the LT
Image: LT/The Schools' Observatory

Professional telescopes usually have several different instruments. Which instrument you use will depend on what you want to learn from your observation. If you want to measure the size of a galaxy, or the brightness of a star you would use a camera to get a nice image, but if you want to find out what something is made of, you would use a spectrograph or spectroscope.


Field-of-view: How big is it?

Comparing NGC23 (small) and M74 (large) galaxies
Comparing a small galaxy (NGC23, left) and a large one (Messier 74, right)
Images taken by the LT for The Schools' Observatory

Although stars appear as dots in the sky, other kinds of object like galaxies or nebulae can look quite big. How big an object looks depends on both how big it actually is, but also how far away it is. The further away, the smaller it will look. Every instrument will have a Field-of-view or FoV, which is how much of the sky it can see. If you want to make sure you get the right measurements, you must check that the object you are observing fits into the FoV of your chosen instrument. If it doesn't, you will either have to choose a different object, change instrument, or take several observations to stick together.

There are many galaxies, nebulae, and so on in the universe. It can be quite difficult to find out or measure their size. In Go Observing, we have selected objects that fit into the FoV well.  

Exposure time: How long to look?

Short and long exposure images of the same nebula
Observations of a nebula with two different exposure times .
On the left an exposure time of 30 seconds, on the right 120 seconds.
Images taken for The Schools' Observatory by the LT

One of main reasons why telescopes have such big mirrors and lenses is to gather as much light as possible. This means that very faint things can be observed. Observations can be made even better by looking for a very long time. The longer you point a telescope at something, the more light you will collect. This is called the exposure time.

Because calculating exposure times is quite complicated, Go Observing will do the work for you. However, for some of the more advanced observations you may get to choose different exposure times to see how that changes your images.


Filters: What colour?

Filters for the Liverpool Telescope
Some filters from the LT

Most cameras on professional telescopes only measure the brightness of things. They are not able to measure colour at the same time. However, colour can tell you a lot about the universe. For example hot, blue stars are usually bigger and brighter than small red ones.

To find out about these colours, astronomers use special coloured glass filters. These only let through light of a particular range of colours, for example red. By looking at the differences between images taken using filters, you can find out a lot about the colours of the objects. For example, a blue star will be brighter in an image taken through a blue filter than in one taken through a red filter.

For most observations in Go Observing we have chosen the best filters. However there are some advanced options where you can experiment with different filters.


Seeing: How blurry can your image be?

Seeing example
Same nebula, different seeing
Images: The Schools' Observatory/LT

"Seeing" is what astronomers call the "blurring" of astronomical pictures by the atmosphere. The amount of blurring changes, and observations that need the sharpest possible images can only be done at certain times. Although the seeing above the LT on La Palma is usually much better than most other places in the world, it can vary. When planning your observations, you need to think about how blurry it can be before it stops you learning what you want to find out.

If you need the very best conditions, then the telescope can wait until the seeing is at its best. That will of course mean you might have to wait a long time. You may also have to work around other observations that need the same, rare high-quality seeing. On the other hand, if you let the telescope observe at any time, your observation might be done more quickly, but it could be very "blurry".

When you choose an observation in Go Observing, unless it asks you to choose the seeing conditions you want, it will choose a good balance between getting the observations done as quickly as possible and not being too blurred.

Students can learn more about the effects of astronomical seeing using our Seeing Workshop


Sky Brightness: How faint can you go?

The sky over the telescope with and without the Moon
All-sky camera images above the 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.
Images from LT SkyCamA

While some things in the night sky are quite bright like the Moon or some stars, many of the things we want to observe are quite faint. Even with a big mirror and a long exposure time, some faint objects are still difficult to observe, especially if the sky is not really dark. Fortunately on La Palma we do not need to worry about light pollution, but some other things, like twilight or the Moon, can make the sky too bright to observe the faintest objects.

The effect of sky brightness can be predicted, so it is used to help to make the coloured bars that you might see as you Go Observing.