Sky and Telescope - July 2017 - 71
LUNAR RESULTS For bright solar system targets such as lunar
close-ups, the Regularized Van Cittert algorithm produces signiﬁcant improvement. A range mask (made using the RangeSelection
tool) was used as the LDSI to prevent artifacts from appearing on
crater rims and other bright features.
u TRAILING FIX Using the Motion Blur PSF mode in the Deconvolution
tool can correct slightly elongated stars.
improve the amount of detail in the ﬁnal result. You can also
bring to bear sharpening tools later in the processing workﬂow to further enhance details recovered with deconvolution.
Images of solar system objects, including the Sun, Moon, and
planets, can greatly beneﬁt from deconvolution. For these
bright targets, the Regularized Van Cittert algorithm works
well in Parametric PSF mode, but it takes some experimentation to dial in the best settings. As with deep-sky imagery,
make a deconvolution mask of your target image as described
above. If you're processing a lunar picture, you can generate
the LDSI ﬁle from the deconvolution mask using the RangeSelection tool to select just the brightest features.
Once again, create a Preview on which to experiment and
try to ﬁnd the best setting for the StdDev slider ﬁrst, ignoring artifacts that might occur. Next, adjust the Global dark
and Global light sliders in the Deringing section to taste. As
for deep-sky objects, choose the lowest settings that achieve
the desired result. If the outcome still looks a little too sharp
after Van Cittert deconvolution, applying a slight blur can
give the image a more natural appearance. The Convolution
tool works well for this purpose.
PixInsight's Deconvolution tool can also ﬁ x slightly elongated stars, as long as the direction and amount of elongation
(eccentricity) of all stars are similar; it won't ﬁ x ﬁeld rotation
issues. Of course, it's much better to correct the root causes
of the eccentricity - usually an issue related to the optics,
tracking, or polar alignment. With realistic expectations in
hand, make a star mask from a stretched copy of the image
and apply it to protect nonstellar features and background.
Open the Deconvolution tool and select the Motion Blur
PSF tab. Here is where you'll adjust the length and angle to
match the amount of elongation in your target image. One or
two iterations of deconvolution is usually sufﬁcient, though
you'll need to experiment with the Global dark slider in the
Deringing section to home in on the best setting that prevents dark artifacts appearing around the stars.
Although the examples in this article demonstrate deconvolution using PixInsight, you'll ﬁnd many tutorials online on
how to perform deconvolution with other image-processing
software. Deconvolution has the ability to reveal the hidden
small-scale detail in deep-sky and solar system images, and
it can even help ﬁ x out-of-round stars. Don't be afraid to
experiment - the improved clarity and detail in your photos
will be well worth the effort.
¢ RON BRECHER hunts faint deep-sky targets from his backyard observatory in Guelph, Ontario.
s k y a n d t e l e s c o p e .c o m
* J U LY 2 017