Resources: Worksheet, VAO Quick Observing Guide

Terminology: Apparent Magnitude, Local Sidereal Time, Hour Angle, Meridian

Tutorials: Observing Guide

Demonstrations: Sidereal time, EM on Earth, Filters

CCDs and Optics

Astronomers rarely look through the eyepiece of a telescope anymore when observing the sky for research. Research telescopes use electronic sensors called CCDs (charged coupled devices) to record the image coming in through a telescope's optics. They also use precise motors to point the telescope and to track objects in the sky as the Earth rotates. Often, the telescope will be entirely controlled by computer, so that an astronomer can schedule an entire night's worth of interesting observations without needing to manually control the telescope the entire night.

When using a simple optical telescope to look at the night sky you could just point it at random objects to see what you can see, but you would probably see a lot more interesting things if you planned ahead. You would look in an atlas or do other research to find out what interesting objects would be in the sky that night. You would do some checking to make sure that the objects will be visible with your particular telescope. When using a more advanced telescope with electronic sensors and computer-controlled pointing to take astronomical images for research, planning ahead and knowing what your instrument is capable of are also very important.

The CCD in a telescope camera works a lot like photographic film - a shutter in the camera opens to expose the surface of the chip to light from space. The longer the chip is exposed, the more light builds up. Very dim objects require long exposure times to see. If too long an exposure is used on a very bright object, the chip will be overexposed and the resulting image will not be usable.

Filters

Most astronomical cameras also have colored filters to block out certain kinds of light. These filters can allow one to see how bright objects are in different colors, or to look at the specific spectral line of a common element such as hydrogen. Look at the example image below. Hubble Space Telescope took this image of a galaxy using different filters. In the lower left, you can see the image taken in ultraviolet, then the next five filters were taken in the optical, or visual, regime of the electromagnetic spectrum, and finally the image on the lower right is an infrared image. While each image shows different properties and structure of the galaxy, it is best to combine all the images to produce a complete image, which is the center image of the galaxy.

While having a color image is helpful to understand properties of the object and they are beautiful to look at, we call these images false-color images. The colors don’t represent the true appearance of the object, instead we have assigned color filters to help bring out features. In many cases, like the red Hydrogren Alpha filter, it is tracing an atomic transition, not a real color that you would see with your eyes. Similarly, the green filter traces O III (oxygen 3), so when you look at a false color image and see green, you know it is showing you an ionized state of oxygen. It is convenient to use colors to map out these features, but the galaxy isn’t actually green.

Here is a nice summary of different properties of astronomical images, though it may be specific for Hubble Space Telescope at times.