Imaging astronomical objects using specialised CCD (charge coupled device) cameras has transformed our view of the universe at visual and near infrared wavelengths. Mass production of CCD chips used for commercial digital cameras has meant that even amateur astronomers can now purchase affordable very sensitive low noise cameras for astronomical imaging. The sensitivity of these cameras coupled with the low noise electronics has lead to quite remarkable images being captured even through small telescopes and in heavily light polluted skies! The sensitivity of CCD detectors is far greater than the human eye meaning they will capture faint detail and subtle differences in the brightness of an object whether it be a planet, nebula or galaxy in a way impossible with the human eye alone. This is because CCD's are linear devices with a large dynamic range. The human eye whilst also having a broad dynamic range is logarithmic in its sensitivity to light, meaning it is more difficult for us to distinguish between small differences in the brightness of a given object.
The main imaging camera we use in the observatory is a 4 Mega pixel cooled camera (model SXV-H16) made by the U.K. company Starlight Xpress. This is a monochrome camera rather than colour. Although colour cameras are available they have less resolution compared to a monochrome camera with the same number of pixels because of the bayer matrix that is placed over the CCD sensor to generate the colour image. Monochrome cameras are the type used in professional observatories and space missions. To produce a colour image using these cameras requires taking separate exposures through Red, Green and Blue filters and combining them in software to a colour image. Usually a fourth filter is used (which transmits most of the visual wavelengths) to produce a 'lightness image' that helps produce correct colour balance in the final image. This technique is called RGBL composite imaging.
An example of such an image, obtained using the C14 telescope in our observatory, is shown below. This image illustrates another feature namely the technique of combining many relatively short sub-exposures of an object into a single longer one. In the image of M42 below, 10 separate sub-exposures of 2min each were combined together to give a single integrated exposures of 20min. This was repeated through R,G,B and L filters and then finally all are combined into a colour image. This technique of 'stacking' exposures increases the signal to noise ratio of raw images. The raw images can then be processed using very powerful image enhancement software to bring out faint details in an object.
Finally it is worth stressing that the image of M42 below (indeed all of the deep sky images on these pages) were taken from our observatory, located as it is in one of the most light-polluted site in the U.K.! This exemplifies the power of CCD cameras coupled with image processing software to 'pull' detailed images of an object out of a heavily light polluted sky. Of course, dark-sky sites are preferable and would always give a higher signal/noise for any given exposure compared to a light polluted site. In the latter case we have to work harder and acquire longer integration times to achieve a given signal/noise ratio. Also the use of very narrow band filters can be a great help as they block much of the background 'sky glow' yet readily transmit light at wavelengths associated with emission nebulae such as hydrogen-alpha, hydrogen-beta, oxygen III and sulphur II lines.
A collection of images we have taken during the past year can be found by navigating the left hand menus. We will update these regularly as we acquire more images.
Here are some links to excellent interactive tutorials from the Faulkes Telescope website on CCD cameras and the technique of RGB imaging.