Starlight Xpress SXVR-M25C - Manual (Page 11)

Handbook for the SXVR-M25C Issue 1 June 2009
11
Image enhancements:
Your first image may be satisfactory, but it is unlikely to be as clear and sharp as it
could be. Improved focusing and exposure selection may correct these shortcomings,
and you may like to try them before applying any image enhancement with the
software. However, there will come a point when you say, “That’s the best that I can
get” and you will want to experiment with the effects of image processing. In the case
of daylight images, the processing options are many, but there are few that will
improve the picture in a useful way. The most useful of these are the ‘Normal
Contrast Stretch’ and the ‘High Pass Low Power’ filter. The high pass filter gives a
moderate improvement in the image sharpness, and this can be very effective on
daylight images.
Too much high pass filtering results in dark borders around well-defined features and
will increase the noise in an image to unacceptable levels, but the Low Power filter is
close to optimum and gives a nicely sharpened picture.
The ‘Contrast’ routines are used to brighten (or dull) the image highlights and
shadows. A ‘Normal’ stretch is a simple linear operation, where two pointers (the
‘black’ and ‘white’ limits) can be set at either side of the image histogram and used to
define new start and end points. The image data is then mathematically modified so
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’.
At this point, you will have a working knowledge of how to take and process an
SXVR-M25C 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 SXVR-M25C
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 SXVR-M25C 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.