Handbook for the TRIUS PRO-694C Issue 1 September 2020
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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 TRIUS PRO-694C to suffer from considerable ‘vignetting’
(dimming towards the corners) and this will be difficult to remove from your images.
Experiment with the distance between the reducer and the camera to optimise the
results. The longer the extension tube used, the greater the focal reduction will be.
As a guide, most CCD astronomers try to maintain an image scale of about 2 arc
seconds per pixel for deep sky images. This matches the telescope resolution to the
CCD resolution and avoids ‘undersampling’ the image, which can result in square
stars and other unwanted effects. To calculate the focal length required for this
condition to exist, you can use the following simple equation:
F = Pixel size * 205920 / Resolution (in arc seconds)
In the case of the TRIUS PRO-694C and a 2 arc seconds per pixel resolution, we get
F = 0.00454 * 205920 / 2
= 467mm
For a 200mm SCT, this is an F ratio of 467 / 200 = F2.34, which is much less than can
be achieved with the Meade converter and appropriate extension tube. However,
moderate deviations from this focal length will not have a drastic effect and so any F
ratio from about F4.5 to F6.3 will give good results. It is clear from this result that the
‘Starizona Hyperstar’ adaptor is very well suited to use with the SX-694C, as it
operates at around F1.95, so you might be interested in getting one of these.
