SXVR-M26C handbook.pdf - Starlight Xpress

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Handbook for the SXVR-M26C Issue 1 March 2010 Relatively little nebulosity is visible at this time, but it will be revealed later. If you zoom in on the raw image, you will see that it has the regular grid of brighter and darker pixels, caused by the Bayer matrix filter grid on the CCD. Now apply the colour synthesis routine, using the same offsets that gave good results with the daylight image tests. 16
Handbook for the SXVR-M26C Issue 1 March 2010 The image should now look quite impressive and I hope that you are pleased with your first efforts! In many cases, a ‘Normal’ contrast stretch will give a good result, but may ‘burn out’ the bright regions and leave the faint parts of the image rather lacking in brightness. To combat this, many imagers will use a combination of ‘Normal’ and ‘Non-linear’ contrast stretches. The best settings are different for different objects, but performing a non-linear or power law stretch, followed by normalising the background to black with a normal stretch, is the usual procedure. Further small refinements are usually possible and you will become expert at judging the best way to achieve these as your experience increases. As a rough guide, the ‘Filters’ menu can be used to sharpen, soften or noise reduce the image. Strong ‘High Pass’ filters are usually not a good idea with deep sky images, as the noise will be strongly increased and dark rings will appear around the stars, but a ‘Median’ filter can remove odd speckles and a mild ‘Unsharp Mask’ (Radius 3, Power 1) will sharpen without too much increase in noise. Other things to try include summing several images for a better signal to noise ratio. Summing can be done in the ‘Merge’ menu and involves loading the first processed image, selecting a reference point (a star) then loading the second image and finding the same star with the mouse. Once the reference is selected, you can either add directly, or average the images together. Averaging is generally better, as you are less likely to saturate the highlights of the picture. The signal-to-noise ratio will improve at a rate proportional to the square root of the number of summations (summing 4 images will double the signal-to-noise), but different exposures must be used. Summing an image with itself will not improve the S/N ratio! Although I have concentrated on the use of a telescope for deep-sky imaging, do not forget that you have the option of using an ordinary camera lens for impressive wide- 17
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SXVR-M26C handbook.pdf - Starlight Xpress

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