Sky & Telescope - July 2024 - 62

Imaging Tips
So how many frames should you stack? This has always
been a hot topic of discussion. A common rule of thumb used
to be to stack the best 25% to 50% of frames to make the raw
image for later sharpening. This methodology came about
from the early days of using inexpensive webcams. These
cameras had fairly noisy CCD sensors that required stacking
many frames in order to achieve a satisfactory signal-tonoise
ratio. Today, state-of-the-art CMOS video cameras used
for planetary imaging have excellent sensitivity and noise
characteristics. The old rules simply do not apply anymore. In
many cases, the more frames you stack, the blurrier the image
becomes because you're simply adding bad frames. Personally,
I stack no more than 50 of the typically 2,400 frames per
video I record.
Before processing your picture, it's important to consider
the final target audience. The steps you take and the final
appearance of the resulting images may be quite different
depending on the requirements of the intended recipient. For
example, if your primary goal is to submit images to organizations
such as the Association of Lunar and Planetary Observers
(ALPO), a crisp black-and-white image is mandatory.
However, most of us prefer to share our pictures online with
friends and family or present them on popular websites like
Astronomy Picture of the Day (apod.nasa.gov/apod/astropix.
html) that tend to favor colorful imagery. Some magazines
also generally prefer color, but it's advisable to keep the
saturation low in order to stay within the more limited
color gamut of the high-volume
printer's inks.
Beginning the Workflow
The same general processing
techniques are the same no
matter the type of solar filter
used. Once you've determined
your audience and created raw
images ready for enhancement,
there are several programs you
can employ to get the most out
of your data. I use Adobe Photoshop
(adobe.com), PixInsight
(pixinsight.com), Images Plus
(mlunsold.com), and Maxim DL
(diffractionlimited.com). Most of
these programs have a lot of overlap
and can perform the same
RAW CONTROLS Adobe Camera
Raw contains many tools to improve
your solar images. The Exposure slider
increases the brightness in your image
much like increasing the gain or ISO
in your camera. The Dehaze slider is
designed to reduce haze in landscape
photos but works well to increase the
contrast in the mid-range values.
62 JULY 2024 * SKY & TELESCOPE
processes, though some do certain tasks better than others.
I often bounce among Photoshop, PixInsight, and MaximDL,
and then return to Photoshop for the final tweaks before
declaring an image to be complete.
An important first step if you use a solar telescope with a
built-in diagonal mirror (found in many Coronado and Lunt
scopes) is to correct your image so that it's not mirror-reversed.
This is important because you'll want your images to be
directly comparable to others, particularly if you're concentrating
on a small region of the solar disk. In Photoshop, the steps
are: Image > Image Rotation > Flip Canvas Horizontal.
Depending on the camera settings you used, you may
need to set the black point and gamma, or midtone. Each is
adjustable with the Curve tool located within the Camera
Raw Filter in Photoshop (Filter > Camera Raw Filter). Nearly
all the fine details on the Sun are in the midtones, so pull the
middle of the curve downward to boost midtone contrast or
pull upward in order to reduce image contrast. Typically, you
want to slightly boost the contrast at this stage, so decreasing
midtones a bit will greatly benefit the next steps of processing.
Before hitting the OK button, set the black point in
the image by moving the lower-left end of the curve slightly
toward the right.
There are several excellent enhancement tools found
within the Camera Raw Filter. Among my favorites is
Dehaze, located in the Effects section seen along the right of
the filter window. This is an almost magic adjustment that
Before
After
http://apod.nasa.gov/apod/astropix.html http://apod.nasa.gov/apod/astropix.html http://www.adobe.com http://www.pixinsight.com http://www.mlunsold.com http://www.diffractionlimited.com
Sky & Telescope - July 2024

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