Starlight Xpress SXVR-H674C - Manual (Page 12)

Handbook for the SXVR-H674C Issue 1 March 2012
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colours until they are equally distant from the beginning of the colour data, but quite
close to it. This will both correct the colour balance (apart from any fine tweaks) and
increase the colour saturation.
In some images, the colour data may be balanced differently in the highlights and
shadows. For example, the dark areas may be quite neutral in colour, but the
highlights may be bluish in tint. This indicates a ‘slope error’ in one or more colours
and, in this example it is caused by the blue data increasing too quickly when
compared with the red and green data. To correct for this, the other histogram sliders
may be used as slope adjusters. In the case of the excess blue, try increasing the top
end (Max) value to greater than 255 (300 max is allowed). This will reduce the blue
slope and result in a more yellowish rendition. Alternatively, you can increase both
the red and green slopes by setting the upper red and green sliders below 255.
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-H674C image. It is time to move on to astronomical imaging, which has its
own, unique, set of problems!