Handbook for the TRIUS PRO-825C Issue 1 September 2020
15
Astronomical Imaging with the TRIUS PRO-825C
1) Getting the image onto the CCD:
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 TRIUS PRO-825C 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
