Starlight Xpress SXVR-H694 - Manual (Page 9)

Handbook for the SXVR-H694 Issue 1 March 2012
9
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-H694 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 SXVR-H694 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 H694, as it operates
at around F1.95, so you might be interested in getting one of these.
The same equation can be used to calculate the amplification required for good
planetary images. However, in this case, the shorter exposures allow us to assume a
much better telescope resolution and 0.25 arc seconds per pixel is a good value to use.
The calculation now gives the following result: