Filters

Filters on the Liverpool Telescope
Credit: LT

Most cameras on telescopes today only measure the brightness of things. They are not able to measure colour at the same time.

Colour is useful for astronomy. For example is can tell us how hot an object like a star is. It can also tell us if things are moving towards us or away from us. Often the colour of something will also tell us roughly how old it is.

To find out about these colours we use special coloured glass filters in front of the camera. These only let through light of a particular range of colours, like red, green or blue.

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 in a red one.
Filters on professional telescopes are carefully made to let through an exact range of wavelengths. The filters on different telescopes are the same and so that we can compare images easily.

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 yellow and green light that our eyes can see.
  • B: A Blue filter, good for finding hot stars.
  • Ha or H alpha: This is a special filter which only lets through a particular shade of red. This is made when hydrogen gets hot. Hydrogen makes up most of the Universe we can see. This filter is a good one to use when you are looking for hot gas. We get hot gas where stars are being born.
  • ND:  Neutral Density filters are not coloured but grey. These filters actually block out a fraction of all colours of light. This means you can take images of very bright things like the Moon or planets

Sometimes the best filter can change depending on the conditions at the time of the observation. For the NSO Go Observing system a special computer programme is used to decide on the best filter. You can find out what filter was used after the observation is taken by looking at the telescope information.