Starlight Xpress SXVR-M25C - Manual (Page 12)

Handbook for the SXVR-M25C Issue 1 June 2009
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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 for small
CCDs, if you have access to a focal reducer (for deep sky) and a Barlow lens for
planetary work. However, the CCD in the M25C is too large to be fully illuminated
by most standard SCT scopes, and you may not be able to take satisfactory images
with such an instrument. One very successful option that may be open to you is to fit a
‘Hyperstar’ lens (supplied by ‘Starizona’) in place of the secondary mirror. The
hyperstar turns your SCT into an F2 wide field astrograph and the latest versions (type
III at the time of writing) will fully illuminate the chip of the SXVR-M25C. The
hyperstar optics make a very fast, wide-field system which is ideal for large nebulae
etc. Another very good option is to use a well-corrected refractor, such as an FSQ106.
These superb instruments give pin-point stars over a very wide field and are an ideal
match to the M25C.
As a general 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)