70 September 2014 sky & telescopeImage Processingshouldn’t have a lumpy texture with bright areas scattered
around the image like the background model example
seen above. DBE probably chose a sample point on a
galaxy or bright star. As a result, the model has removed
desired signal, preserved unwanted signal (gradients), or
a bit of both. To get the DBE tool to function best, it takes
a little fi nessing of a few settings.
After opening the DBE tool, click within your target
image. This will place symmetry guide lines across the
picture and link the image to anything you do with the
DBE tool. Start by defi ning a small number of samples
per row in the Samples Generation menu. Although
the default of 10 sample points is appropriate, I prefer
a smaller number. Gradients usually present smooth
transitions that are well defi ned with only a few samples,
though severe cases may require more. For an image
that’s roughly 4,000-by-3,000 pixels and that displays a
smooth gradient when the STF AutoStretch is applied, six
samples per row seems a good number to start with. Once
you’ve made this change, click the Generate button.
Often the DBE tool may refuse to place samples in
certain areas of an image because they’re too bright or too
dark. The goal is to have the image’s entire surface well
sampled. One way to make sure DBE places samples in
brighter and darker areas is by increasing the Tolerance
value to 2 and decreasing the value of Minimum Sample
Weight to 0. This tells DBE to accept almost every sample,
no matter how bright or dark it is.
Click the Generate button again, to see where DBE
places the samples after the changes.With new additional sample points generated, make
sure they’re sampling areas free of stars or other subjects
you want to keep in the image, such as faint nebulosity.
Take a look at the point sample in the DBE window at the
top of the page. Although the star is rejected (the area in
black), the sample is poor: many pixels in the sample are
rejected, and the pixel edges around the black area that are
usually a bit brighter than the real background (darker in
the inverse sample image shown by DBE) may aff ect the
reading. It’s best to move or remove this sample point alto-
gether by clicking the red X at the top right of the window.
When you’re satisfi ed with the sample placement,
make sure that the Target Image Correction is set to Sub-
traction, and click the execute icon (green check mark).
After a few seconds, two new images are created. One is
the corrected result, the other a background model.
Again, examine the background model, this time side-
by-side with the original image and the location of the
samples. I often adjust the STF shadow/midtone/high-
light sliders on the background model image to increase
the contrast of the model to better display diff erences in
illumination (PROCESS > IntensityTransformations >
ScreenTransferFunction).
To illustrate what is going on in my background
model, I’ll place the samples from my corrected image
over the background model image. This is accomplished
by dragging DBE’s New Instance icon (the blue triangle
at bottom left) to the program workspace, canceling the
DBE process (red X icon at bottom left), clicking on the
background model image, and then double-clicking on
the DBE process I dragged onto the workspace.
See that sample over the brighter area in the bottom-
right in the image above? That’s the sample responsible
for that bright lump. This tells me that I should remove
that sample. The best practice is to remove samples from
bright areas that break a smooth transition, and to add
samples in dark areas. Besides adding and removing
samples, you can also move them around to more ideal
locations to produce the best model of the background sky.
Once you’re satisfi ed with all the sample points, apply
the changes to the original image. Make sure to transferFar left: The DynamicBackground-
Extraction tool (DBE) allows you to
carefully select and evaluate each
background sample to ensure the best
results. In this example, a bright star
was chosen as a background sample,
which produced the bright “lump” in the
resulting background map to its right.
Near left: Moving our DBE sample
points from the raw image to the fi rst
background map shows that the bright
“lump” at the bottom right corresponds
to a poorly chosen sample near a group
of galaxies.After exam-
ining and
removing
questionable
sample points,
the DBE tool
should produce
a smooth and
even gradient
of the target
image, like this.PixInsight.indd 70 6/23/14 12:17 PM