Sky & Telescope - July 2024 - 64

Imaging Tips
detail than true deconvolution and wavelet filters.
Most astronomical image-processing software includes
some form of deconvolution. These processes use small, iterative
steps to restore detail by setting the point spread function
(PSF), an estimate of the blur radius, and limiting the
number of iterations it performs. In PixInsight, I usually use
the Regularized Richardson-Lucy deconvolution (found in the
pulldown menu Process > Deconvolution) and set the StdDev
to around 1 when processing full-disk shots, and to 2 or more
for close-ups, with iterations set to anywhere from 10 to 200.
Experiment to find what works best.
Wavelet algorithms work in a very different way from
deconvolution. Wavelets increase what is called the high
frequency " bias " - how visible the fine details are by superimposing
these details directly onto the original image. I
prefer the wavelet function in PixInsight over those available
in MaxIm DL and RegiStax. It's found through the dropdown
menu Process > Wavelets > MultiscaleLinearTransform. If
you're working on a full-disk image, click on Layer 1 in the
tool window and then adjust the Detail Layer 1/4 Bias slider up
slightly, say, to 0.4. Now drag the caret in the bottom left of
the window onto your image to activate the process. If you're
working on a high-magnification close-up, click on Layer 2
and raise its parameter slightly higher to about 2, and then
apply this to your image. I rarely activate both layers on the
same close-up field.
If you don't have PixInsight, the wavelets in RegiStax are
still quite serviceable. Move the top two sliders to the right
to enhance the small-scale details in your photo. It's like
magic watching how much hidden detail these tools can
reveal! However, there can be too much of a good thing. Stop
increasing the levels of the wavelet sliders when either the
noise becomes objectionable or the details start to have a
bright outline. When you're satisfied with the result, be sure
to save your changes.
Deconvolution often raises the black level of an image, so
reopen the picture in Photoshop and use the levels tool (Image
> Adjustments > Levels) to readjust black level if needed (the
left end of the histogram). While you have the tool open, you
can adjust the overall brightness by moving the right slider
to the left. Be careful not to clip the brightest areas - stop a
little before the left end of the histogram. At this point, the
monochrome version of the image is complete.
If you're assembling a high-magnification mosaic from a
set of processed panels, Photoshop can automate the process.
Select File > Automate > Photomerge. In the command
window, click the Browse button, select the panels you're
stitching, then choose Reposition from the options in the left
of the window. In a few moments, you'll have your assembled
mosaic ready.
Introducing Color
You now have an excellent black-and-white image of the Sun.
Endowing your solar portrait with fiery reds, oranges, or electric
blues and violets will garner more attention.
An easy way to colorize your pictures in Photoshop is to
COLORING WITH CURVES
The Sun is essentially colorless,
so imagers typically photograph
it with monochrome cameras
and add color to the final result.
One easy method is to open
the Curves tool (right) in Adobe
Photoshop and adjust the red,
green, and blue channels separately
to achieve the desired result,
like the result seen above.
SELECTING HUE
Another easy technique
for adding color to your
monochrome images is to
use Photoshop's Hue/Saturation
tool. Simply move
the slider until it's positioned
below the color you
want your image to appear
and increase the saturation
slider as desired.
64 JULY 2024 * SKY & TELESCOPE
Sky & Telescope - July 2024

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