Starlight Xpress SXV-M7C - Manual (Page 9)

Handbook for SXV-M7C Issue 1 June 2004
9
Other options in the ‘Colour’ menu are ‘Apply Anti-Alias’ and ‘Adjust
Chrominance’. We have already used the Anti-Alias filter as an option during colour
synthesis, but it will be helpful for you to know its purpose. Basically, the AA filter is
designed to remove colour errors at sharp transitions of brightness. Because of the
spatial characteristics of the grid of filters on the CCD, a sharp edge in the image will
overlay certain filters, but miss adjacent ones. When the colour synthesiser equations
process the data from this region, there will be an incorrect result when these pixels
are processed and a line of abnormal colour will follow the transition edge. This is
well seen in images of stars, which often take on a strong colour bias, quite different
from the expected star colour. The AA filter scans the colour image data and searches
for fine details with rapidly changing colours. Wherever these errors occur, the filter
substitutes the equivalent and equal values of red, green and blue, thus changing the
deviant colour into neutral grey. The result is a much smoother colour image, with
clean transitions around stars and other fine details. The AA filter may be used as
many times as is desired, but its additional effects will be very slight after about 3
passes of the image.
The ‘Adjust Chrominance’ option is not needed for most images, but can be useful for
removing atmospheric dispersion from images of the planets. ‘Adjust Chrominance’
permits you to move the red and blue frames with respect to the green frame and so
compensate for the displacement of the colours caused by the atmosphere. To use the
AC function, shift the red and blue images until the planetary disc has an evenly
coloured periphery and any red and blue fringes disappear.
Other image enhancements:
Your first colour image may now be approximately the correct colour, 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
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 the effects of image processing. 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
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’.