Many of the most spectacular amateur ccd
images are tri-colour images of deep sky objects. One-shot colour
ccd's are increasingly available but these inevitably sacrifice some
sensitivity for the convenience of using internal filters on the
surface of the chip. In order to do tri-colour imaging with a
standard ccd camera a way of introducing filters in to the light
cone is needed. Filter wheels are popular with sct users whose
telescope produce relatively narrow cones. A f3.8 Newtonian poses
somewhat more of a challenge. The system I have chosen is that of a
filter slide to introduce filters between secondary and focuser. A
well described example is
here. Some
of the parts come from a dot matrix printer. A stepper motor driving
a threaded rod makes filter selection. The motor itself is
controlled by the same pc that looks after the dome and focuser.
