Sky and Telescope - July 2017 - 70

Astrophotography

NO DARK DONUTS A "local deringing support image" (LDSI) at
bottom left is created using the StarMask tool, and prevents dark
circles from appearing around the brighter stars - a common
processing artifact that often results from deconvolution.

Next, you need to generate a point spread function (PSF)
image. This ﬁle contains the information needed to deconvolve your image properly. The PSF estimates how each point
source in the image has been spread out due to blurring. This
image is prepared by selecting PROCESS > Image > DynamicPSF and then manually clicking about 20 or more stars in
your target image. Try to choose fainter stars in your linear
image; the best choices will show a value of between 0.25
and 0.9 in column A. Once you've clicked on enough stars,
select them all in DynamicPSF with Control+A and click the
camera icon to generate the PSF image.
The last support ﬁle needed to perform deconvolution is a
local deringing support image (LDSI). The brightest stars in
the target image need extra protection to avoid introducing
artifacts that appear as dark rings in the deconvolved result.
The LDSI is a grayscale image that is completely black except
for the brightest stars, and can be made by applying StarMask
(PROCESS > MaskGeneration > StarMask) to the target image
using its default settings. The Deconvolution tool uses this
ﬁle to provide special deringing protection for the brightest
stars. To include more stars in the LDSI, reduce the value in
the Midtones slider.

Adjusting the Settings

APPLYING DECONVOLUTION Compared to the "before" image
of the Bubble Nebula, NGC 7835, at top left, the "after" image to
its right shows increased detail, and faint stars are made more
apparent. Getting this result required applying the three support
images at right with PixInsight's Deconvolution tool.

J U L Y 2 0 1 7 * SK Y & TELESCOPE

Now that you have created the support ﬁles, apply the
deconvolution mask image to the target image by selecting
MASK > Select Mask and choosing your deconvolution mask
image. Next, open the Deconvolution tool from the pulldown
menu (PROCESS > Deconvolution > Deconvolution), select
the External PSF tab in the deconvolution tool window, and
choose the PSF image in View Identiﬁer.
Check the box next to Deringing, as well as the Local
Deringing option, and choose your LDSI image in the Local
Support section.
For most deep-sky images, the Richardson-Lucy deconvolution algorithm works best. The Van Cittert algorithm sometimes works well on lunar and solar images, as we'll see shortly.
For both these algorithms, PixInsight includes a "regularized" version that suppresses noise during deconvolution. I usually tweak the settings on a small portion of the image made
using the Preview menu option (Alt-N). The Preview should
contain both high and low signal areas. This speeds up the
task of optimizing settings for detail restoration, background
protection and suppression of ringing artifacts around stars.
I focus on ﬁnding the best settings for Global Dark Deringing and the number of iterations. For Global Dark Deringing,
ﬁnd the lowest value that prevents ringing artifacts around the
stars. You can use a small amount of Global Bright Deringing
to suppress bright artifacts, but a gentle touch is recommended
since deconvolution is extremely sensitive to tiny changes.
Once you're satisﬁed with the Preview result, apply the
tool to the image by clicking the Apply square at the bottom
left corner of the tool window. At this early stage in the workﬂow, the improvements produced by deconvolution might
appear subtle. But make no mistake: They will deﬁnitely

Sky and Telescope - July 2017

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