Starlight Xpress SXVR-H694C - Manual (Page 12)

Handbook for the SXVR-H694C Issue 1 March 2012
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It is fairly easy to find the correct focus setting for the camera when using a standard
SLR lens, but quite a different matter when the SXVR-H694C is attached to a
telescope! The problem is that most telescopes have a large range of focus adjustment
and the CCD needs to be quite close to the correct position before you can discern
details well enough to optimise the focus setting. An additional complication is the
need to add various accessories between the camera and telescope in order that the
image scale is suitable for the subject being imaged and (sometimes) to include a ‘flip
mirror’ finder unit for visual object location.
A simple, but invaluable device, is the ‘par-focal eyepiece’. This is an eyepiece in
which the field stop is located at the same distance from the barrel end, as the CCD is
from the camera barrel end.
When the par-focal eyepiece is fitted into the telescope drawtube, you can adjust the
focus until the view is sharply defined and the object of interest is close to the field
centre. On removing the eyepiece and fitting the CCD camera, the CCD will be very
close to the focal plane of the telescope and should record the stars etc. well enough
for the focus to be trimmed to its optimum setting
Several astronomical stores sell adjustable par-focal eyepieces, but you can also make
your own with a minimum of materials and an unwanted Kellner or Plossl ocular.
Just measure a distance of 22mm from the field stop of the eyepiece (equivalent to the
CCD to adaptor flange distance of the camera) and make an extension tube to set the
field stop at this distance from the drawtube end. Cut-down 35mm film cassette
containers are a convenient diameter for making the spacer tube and may be split to
adjust their diameter to fit the drawtube.
It is necessary to set up a good optical match between your camera and the telescope.
Most SCTs have a focal ratio of around F10, which is too high for most deep sky
objects and too low for the planets! This problem is quite easy to overcome if you
have access to a focal reducer (for deep sky) and a Barlow lens for planetary work.
The Meade F6.3 focal reducer is very useful for CCD imaging and I can recommend
it from personal experience. It does not require a yellow filter for aberration
correction, unlike some other designs, so it can also be used for tri-colour imaging. If
you use a focal reducer, using it at maximum reduction may cause the relatively large
chip of the SXVR-H694C to suffer from considerable ‘vignetting’ (dimming towards
the corners) and this will be difficult to remove from your images. Experiment with