66 ASTRONOMY • JUNE 2015Faint to medium-bright stars
in an image can be a distrac-
tion, making it difficult to
appreciate a nebula or galaxy.
Using a minimum or erosion
filter is a wonderful way to
de-emphasize star-crowded
fields and thereby dramatically
enhance your image.
The ideas of distraction and
optical confusion play a part in
our daily lives. Signage for
roads and the design of stairs
strive to minimize optical con-
fusion for public safety.
Thankfully, it is unlikely any-
one will injure themselves
while admiring your space art;
but using a minimum filter can
make it easier to see large
extended structures seemingly
hidden by the stars.
The standard method in
Photoshop is to use “Color
Range” to select only the bright
stars in an image (see Image
#1). Then follow these steps:- Choose “Highlights” as
the selection method, and
adjust the “Fuzziness” and
“Range” sliders until only the
stars appear in the preview
window with little nebulosity. - After pressing “OK” and
creating the selection, remove
any unwanted parts with the
“Lasso” tool in the “Subtract”
selection mode. - Next, “Expand” the selec-
tion by 3–6 pixels, and then
“Feather” by half the value you
chose. (Feathering by half the
value of a selection’s expansion
is an excellent rule of thumb.) - At this point, you can
apply the “Minimum” filter
using a fractional pixel value if
you desire, although 1 pixel is a
good choice for oversampled
images. Choose the “Preserve
Roundness” option, which is a
most appreciated kernel filter
that mitigates telltale artifacts
that used to plague this tool.
COSMICIMAGING
BY ADAM BLOCKMaximize a
minimum filter
FROM OUR INBOX
Alien visits
To dismiss and equate all UFO sightings (“Let’s cut the UFO
crap,” March issue, p. 9) to an “incredible naiveté” in the belief
of alien visits to Earth is a pretentious statement by Editor David
J. Eicher. After all, cosmological acceptance of inf lation and that
all matter evolved from a point source are just as incredible. It is
a wonder out there, and there remains much to be understood.
It’s too bad not to have more open-mindedness from the editor of
this magazine. — Barry Pfannebecker, South Deerfield, MassachusettsCorrections
On p. 42 in the March issue, #177 should have read “Wilhelm
Röntgen.” — Astronomy EditorsIn our March issue (p. 56), #252 should have read, “On August
28, 2003, Mars came closer to Earth than it had in 60,000 years.”
The next time Mars will be closer will occur August 28, 2287.
— Astronomy Editors- This one is a secret of mine.
Apply a very small “Unsharp
Mask” of 2 or 3 pixels with a
value of 20 to 30 percent. This
will keep the stars from wilting
by adding back a little brightness
and a small degree of edge
enhancement.
I also would like to offer a
method I developed that I have
not seen published elsewhere. It
modifies things after step 3
above and may be a powerful
tool in many situations. Make a
copy of your layer, and after
doing steps 1 through 3 above,
try the following: - Select “Create New Layer
Mask” by pressing the small icon
at the bottom of the “Layers”
palette. This will convert your
selection to a slightly bloated star
mask like Image #2. Remember
you can always convert a selec-
tion to a mask and vice versa. - On the upper layer, once
again choose “Color Range”
(“Highlights”) to select the stars
that have the same settings as
those you chose the first time,
but this time “Contract” the
selection by 1 pixel. - Now enter the layer mask
by pressing the ALT button and
clicking on it. The selection will
still be visible. - Finally, right click on the
selection, and choose “Fill.” Fill
the selection with black.
Your mask will look like
Image #3. This is something like
a contour or halo star mask.
The difference between the
expansion and contraction of
the selection determines how
large these rings are around
stars. The benefit is that any
modification you make to stars
using this type of mask will
affect most strongly the edges
or halos of stars. In our par-
ticular case, we employ the
minimum filter as described
above and now have much
greater control of the erosion
process — all without clobber-
ing the centers of stars!
In the next column, I will
investigate the stories that
images tell and how that
informs the choices we make
when processing our data.BROWSE THE “COSMIC IMAGING” ARCHIVE AND FIND VIDEO TUTORIALS AT http://www.Astronomy.com/Block.Image #1. Follow the author’s method
in Photoshop by first selecting the
“Highlights” choice from the pull-down
menu in “Color Range.” Then adjust the
sliders so that only stars are shown in
the preview window. ALL IMAGES: ADAM BLOCK Image #2. After duplicating your image and creating the expanded selection,
press the “Create New Layer Mask” but-
ton in the “Layers” palette to transform
your selection into a mask. You’ll find a
high-resolution version of NGC 2359 at
http://skycenter.arizona.edu/gallery/
nebulae/ngc2359.Image #3. Re-select the stars, and
“Contract” the selection by 1 pixel. Then
enter the star mask and “Fill” with black
in the selection. This will blacken the
centers of stars and leave the rings
shown here. This contour star mask is a
powerful tool in many applications.