Starlight Xpress SXV-H9C - Manual (Page 11)

Handbook for the SXV-H9 Issue 1 June 2002
11
The colour balance controls seem complex, but are really quite easy to use. The most
useful controls are the ‘Start’ and ‘Saturation Factor’ settings. Saturation factor will
simply vary the colour intensity, without any alteration of the colour balance, but the
‘Start’ settings for each colour will alter the colour rendering of the dimmer parts of
the image. As most astronomical images are badly affected by light pollution, which
affects the dimmer background colours in particular, the start point settings are
particularly helpful for correcting this. If you move the start point of the Red
histogram a few points to the right and click on ‘Apply’, the new image will have
LESS red in the background and will appear more Cyan. Similarly, if you move the
Green start point to the right, the image will appear more Magenta and moving the
blue start point will turn it more Yellow. Do not move the start points beyond the start
of the main burst of histogram data, or you will introduce colour errors into low
saturation parts of the image. Just move the sliders by small amounts in the clear area
below the main peak, until the background is nicely balanced.
In some cases, the histograms may all start a long way above zero (usually in astro.
images with a lot of light pollution). In this case, slide the start point settings for all
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.
Other 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, as above.
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