Sky & Telescope - February 2024 - 56

FEBRUARY 2024 OBSERVING
First Exposure
Start with the Exposure slider to
ARTIFICIAL INTELLIGENCE AT WORK In 2023, Adobe added an AI-powered feature called
Denoise to Photoshop and Lightroom. The software uses machine learning to differentiate
between fine details (including stars) and unwanted noise in high-ISO images. This before-andafter
comparison of a cropped-in photo of the Andromeda Galaxy shows its benefits. The original
30-second exposure (left) was shot at ISO 16,000 and displays a large amount of digital noise,
which appears as confetti-like colored speckles. A similar, standalone product called Topaz DeNoise
AI is also available from Topaz Labs.
Lightroom to +100, which makes any
color issues more obvious. Next, as you
adjust the Temp and Tint sliders, watch
the Histogram display and simply line
up the colored humps to maximize the
grey area in the middle of the graph.
I find this to be an extremely sensitive
way to color-balance my wide-field
astro images.
Once you've eliminated the color
cast, you can return the Saturation slider
to a more reasonable value. The final
setting is very much one of personal
taste, but I usually leave it around +15
or so. The Vibrance slider offers a more
subtle form of adjustment that increases
the intensity of the less-saturated colors
while leaving already-saturated colors
alone. Again, adjust to personal taste.
At this point, you'll have a neutral
white balance. This is fine, but you may
want to circle back to make your photo
a little warmer (more yellow) or cooler
(bluer) for purely aesthetic reasons. A
winter image may benefit from a cooler
tone, while a desert landscape may work
better with a warmer setting. Try some
minor white balance tweaks and see
what resonates with you.
Tone, Texture, Clarity, and
Noise
Astro images are odd ducks that often
have a whole bunch of information
packed into a fairly narrow luminance
range. You can use the sliders in Lightroom's
Edit panel to enhance the overall
brightness and contrast in your image.
This is where shooting in RAW format
pays off, because you can make more
aggressive adjustments than possible
with a JPEG image.
adjust the overall brightness of the
image. If you've followed my recommendation
to expose the original image
to protect the highlights, then you may
need to brighten the image a bit. If the
stars start to blow out, you can recover
some detail in them by reducing the
Highlights slider. If your image lacks
contrast, you can darken the sky background
by reducing the Blacks slider
level. (If you're old enough to remember
graphic equalizers on component
stereo systems, this set of sliders is the
visual equivalent.)
Use the Texture slider to increase the
visibility of mid-level brightness features
in your photo, without affecting
the fine details. You may need to zoom
in to 100% magnification to properly
judge its effects.
The Clarity slider is one of my favorite
tools for enhancing wide-field astro
images. It adds midtone contrast, which
helps bring out details in nebulae and
galaxies without affecting the highlights
or shadows in the photo. It's easy
to overdo it, though, so use it sparingly
- a little goes a long way.
If you're imaging from an urban
area, your images will suffer from light
pollution, which makes any atmospheric
haze look worse. Dragging
the Dehaze slider to the right can cut
through the fog and help reveal more
detail and color. It also helps add some
punch to a moonlit nightscape.
Noise Reduction and Lens
Correction
Quite often when imaging faint astronomical
objects you'll have to resort to
GRAPHIC EXAMPLE The screen at left shows the appearance of the Histogram in Lightroom when the image was initially imported. Note how the
red, green, and blue humps in the display aren't lined up. A simple trick to neutralize the color cast of a wide-field image is to line up the three humps
by adjusting the Temp and Tint sliders. And as the illustration on the facing page demonstrates, you can also use these sliders for creative purposes.
56 FEBRUARY 2024 * SKY & TELESCOPE

Sky & Telescope - February 2024

Table of Contents for the Digital Edition of Sky & Telescope - February 2024