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Nebulosity User’s Manual
prominent. Quite often, optimal results are obtained by using this tool multiple times. Each
time, make only moderate adjustments to the image and don’t worry about getting your
background very dark the first time or so through. Gradually hone in on your desired image.
(Don’t worry about the fact that you’re doing this multiple times and that it might cause
problems with the image range or values. Remember, Nebulosity does everything in 32-bit
floating point numbers internally.)
5.8.2 Digital Development (DDP)
Digital Development Processing (DDP) is a technique developed by K. Okano to make CCD
images look more like film images. Like the Power Stretch tool, it helps bring out faint detail in
the image and helps suppress skyglow. Okano’s technique passes a hyperbolic function to the
data to make the linear CCD response much more like the S-shaped “gamma” response of film.
When you select Digital Development, four sliders are presented. Default parameters for each
are pre-selected.
The first slider, labeled Bkg sets a level for the background in the output image (Okano’s b
parameter). The second slider, labeled Xover sets the cross-over point (where the transformation
shifts from a linear to a curved one – Okano’s a parameter). The third slider, labeled B-power
provides a method for darkening the background during the DDP process (not in Okano’s
description but may be set to 0 for a pure DDP processing). Finally, the fourth slider, labeled
Edge Enhancement controls the amount of sharpening done during the DDP process (part of
Okano’s description).
Unprocessed
DDP, No edge enhancement
DDP with Edge enhancement
5.8.3 Tighten Star Edges
Often, lack of precise focus or seeing will end up making your images slightly blurry. There are
numerous techniques astro-imagers have used to fix this. Standard “sharpen” or “unsharp mask”
techniques can be useful, but can be quite prone to over-sharpening the image, highlighting
noise, and leaving black halos around stars. Nebulosity takes a different approach to the
problem. The Tighten Star Edges tool first examines your image and performs an edge detection
Version 1.1.3 (1/31/2006)
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