Filters

Filters on the Liverpool Telescope
Credit: LT

Most astronomical instruments available today only measure the brightness or intensity of things - they are not able to measure colour at the same time.

However, colour can tell astronomers 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 taking 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.

Filters on professional telescopes are very carefully made to let through an exact range of colours. This means that filters on different telescopes are the same and so astronomers can easily compare images from different telescopes.

Examples of some filters:

  • R: A Red filter, often used to look at cold stars.
  • V: The "Visual" filter. This lets through most of the different colours of light that our eyes can see.
  • B: A Blue filter, good for finding very hot stars.
  • Ha or H alpha: This special filter only lets through a particular shade of red. Light of this colour is often made by the element Hydrogen when it gets hot. As hydrogen is the most common element in the universe, this filter is a good one to use when you are hunting for hot gas.
  • ND: Neutral Density filters are not coloured but grey. They do not select a particular colour or range of colours but cut out a certain fraction of all colours of light. This makes it possible to take observations of very bright things like the Moon or planets.

Sometimes the best filter can be different at different times, depending on the conditions at the time of the observation. For the NSO Go Observing system special computer algorithms are used to make the decision of the correct filter at the right time, so you may sometimes see an observation with the filter ALG (or sometimes ALG1, ALG2, ALG3 etc). You will be able to find out what filter was actually used after the observation is taken.