Handbook for the TRIUS SX-825 Issue 1 January 2015
9
pointers are modified to fit the new brightness distribution. Try experimenting with the pointer
positions until the image has a pleasing brightness and ‘crispness’.
At this point, you will have a working knowledge of how to take and process an TRIUS SX-825 image.
It is time to move on to astronomical imaging, which has its own, unique, set of problems!
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Astronomical Imaging with the TRIUS SX-825
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 SX-825 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,
