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A note about saving 32-bit data from DeepSkyStacker

Let me start off by saying that I love DeepSkyStacker (here referred to affectionately as “DSS”).  I think Luc Coiffier is a hero to the astroimaging community for creating it and making it available for free.  DSS does 95% of everything you could ever want to do when calibrating, aligning, and stacking images.  The Drizzle algorithm is terrific. It has the most straight-forward, easy to use interfaces around.  Lately, I’ve been playing with PixInsight’s stacking tools, but I keep coming back to DSS for its simplicity and its image quality scoring, which makes determining which images to exclude from a stack so much less tedious.

I have been having an issue with it recently that I’d like to highlight to other users, though.  This could be user-error on my part, and I’d like to hear what I’m doing wrong if that’s the case.  Googling for similar issues, I think Jerry Lodriguss posted a similar issue to the DSS Yahoo board last February.

The issue I’m seeing is this:  if you save images (“Save picture to file”) with “Apply adjustments to the saved image” setting selected, the data are limited to a 16-bit resolution, even if the file is saved at 32-bits.  There is no problem if you use the autosave.tif file or save the image with “Do not apply adjustments to the saved image” setting selected.  I didn’t notice this issue until I started focusing more on narrowband imaging, where even 20-minute exposures are exceedingly dim.  I don’t usually use DSS for stretching my images, but at some point I experimented with it, and I left the “Apply adjustments…” box checked.  Even though I haven’t used DSS’s histogram tool in months, the setting remained in the “Save picture to file” dialog box, and I was getting a lot of posterization in my final images.

Below is a screenshot from PixInsight of a DSS-stacked image of IC 2177 (12 10-minute exposures taken through an H-alpha filter), saved with the “Do not apply adjustments to the saved image” setting selected as a 32-bit integer FITS. Note the smooth histogram.

IC2177 Do Not Apply

And here is a screenshot from PixInsight of the exact same image saved in DSS with the “Apply adjustments to the saved image” setting selected, also as a 32-bit integer FITS. Nothing was actually done with DSS’s histogram tool (“Apply” was never clicked, though it appears to apply the default stretch without clicking it).  Note the posterization and combed histogram.

IC2177 Apply

Once I get a free weekend, I need to go back through some of my recent images that show significant posterization and re-process them.

Again, DSS is amazing software that deserves to be on every imager’s computer, but this post is just a word of caution about what settings you use when you save your files.

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IC 2177, The Seagull Nebula

April 16, 2012 Leave a comment

This is a narrowband image of the nebula IC 2177, known as the Seagull Nebula.  Frankly, I think it looks a lot more like a parrot, even a phoenix (which would be a more dramatic name), but it’s hard to deny that it looks like a bird of some kind. It’s just a terrifically photogenic deep-sky object.

Also going for it is the fact that it was discovered by an amateur, Isaac Roberts, who published what Wikipedia calls ” the first popular account of celestial photography of the deep sky” in 1893.  I was going to complain that it took me four nights to collect the data for this image, but I’m sure Mr. Roberts had it a lot tougher than me back in the day.

Click to enlarge

Image data:

Exposures:  28 x 600s Ha , 23 x 900s O-III, 17 x 900s S-II (14h 40m total), all binned 2×2

Software:  guiding by PHD, stacking in DeepSkyStacker, processing in Photoshop CS3

Telescope:  Borg 77EDII 330mm f/4.3

Camera:  SBIG ST-8300M with Baader standard narrowband filters

Mount:  CGEM

Taken March 17, 18, 19, and 22, 2012 from Whitehouse Station, NJ.

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