Starlight Xpress SXV-H9C - Manual (Page 12)

Handbook for the SXV-H9 Issue 1 June 2002
12
that any pixels that are to the left of the ‘black’ pointer are set to black and any pixels
to the right of the ‘white’ pointer are set to white. The pixels with values between the
pointers are modified to fit the new brightness distribution. Try experimenting with
the pointer positions until the image has a pleasing brightness and ‘crispness’. Most
daylight pictures suffer from having too much ‘Gamma’ (a way of referring to the
contrast response curve) and look unnaturally contrasty. Applying a ‘Power law’
stretch of about 0.5 power will often make them look better.
At this point, you will have a working knowledge of how to take and process an SXV-
H9C 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 SXV-H9C
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 SXV-H9C 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