Combination of 9, 3 minute exposures using an H-alpha filter.
SBIG ST-8XE CCD. 35mm f/2.8 Nikkon lens.
The use of an H-alpha filter when the CCD was attached to a wide-angle camera lens can improve the image quality considerably by reducing the size of stars and by reducing the sky background. The contribution of the stars can be regulated by adjusting the f-ratio of the camera lens:- the shorter the f-ratio, the more starlight gets through. Typically I find that an f-ratio of about 4 gives a nice compramise by allowing enough non-H-alpha light through to show stars, while keeping the star size small and allowing for longer exposures. The above image was shot at f/2.8 and so the effects of the fast focal ratio can be seen in some of the brighter stars.