Using Colour Filters

Many Astrophotographers use Charge Coupled Device (CCD) Cameras which are designed specifically for Astrophotography. These cameras use thermoelectric coolers to keep the imaging chip at a low temperature to minimise noise and pixel bleeding which is essential for long exposure astrophotography. Many of the higher quality CCD cameras are monochrome (black and white). Adding colour decreases the sensitivity of the camera (unless a DSLR camera is being used) and also affects the signal to noise ratio, so using black and white cameras are preferable to colour in the demanding field of Astrophotography.

There are ways to turn black and white images into full colour by using specialised Red, Green & Blue (RGB) Filters. There is a lot more effort in taking colour images using filters as for every filter used you have to re-image the object. It is also advised to take a Luminous shot (L) as this will add to the depth of brightness and contrast in the final image. Taking all this in consideration the celestial object has to be photographed four times (LRGB), changing filters and refocusing in between images. This also means four times the image processing, but the end result is worth it.

The RGB filters mainly used are Wratten Red #25A, Wratten Green #58 and Wratten Blue #38A (this is the set I use). Becoming more popular are the CMY filters Cyan, Magenta & Yellow which transmit a larger portion of the visible spectrum than the RGB filters.

As a general rule, I take the same exposure times for the RGB images and a longer exposure for the Luminous Image usually around 1.5X longer. These images are then aligned, stacked and combined using dedicated stacking software (eg Astrostack, Registax, ImagesPlus). The stacked image can then be imported into image manipulation software such as Adobe Photoshop and the brightness, contrast and RGB intensity can be adjusted.

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                               L=Luminous                                       R=Red                                           G=Green                                           B=Blue

=    Final LRGB Image

                                                                                   Messier 33 - Triangulum Galaxy