Sky & Telescope - October 2024 - 56

OCTOBER 2024 OBSERVING
First Exposure
That processing flexibility is largely
POLLY VIEW The unprocessed RAW image (left) is pretty lackluster but full of potential. The
right-hand image is the JPEG version in which the camera optimizes the photo internally.
by the time I came along, and the best
places to take your film were those
where a good technician was on hand
to monitor the results and tweak the
process - better service than you'd get
at the local mall or one of those little
Fotomat booths you'd often find in
suburban parking lots.
In those days, serious photographers
had their own darkrooms equipped with
all the gear and chemicals necessary to
develop film. They didn't do this to save
money; rather, it was to get the results
they desired. Darkroom practitioners
worked their enlargers to optimize the
results while making their own prints.
In this all-analog context, the negative
was the RAW file, and the print was the
final, processed result.
Why the history lesson? Because in
many ways little has changed. Now,
the developing process is digital rather
than chemical. RAW data off the sensor
of your digital camera - just like a
film negative - needs to be processed
before you get the " pretty picture " you
want for posting online. Today, the local
drugstore and Fotomat booth have been
replaced by your smartphone and your
digital camera.
Computer algorithms analyze your
image and adjust the brightness and
color balance " automagically " just like
the old film-developing machine used
to do. When you shoot in JPEG mode,
what you get out of your camera or
smartphone is analogous to the snap56
OCTOBER 2024 * SKY & TELESCOPE
shot print of yesteryear. All the decisions
about color balance, contrast, and
so on have been made for you. But the
final result might be worlds away from
what you expect or desire.
That's why today's best photographers
and astrophotographers understand
that they need to be as skilled at
image processing as they are at using
their equipment. As the old saying goes,
" The more things change, the more they
stay the same. "
Dialing in the Light
If you want to see what unprocessed
RAW camera data look like, try reviewing
an image file in PixInsight before
applying any screen stretch or other
enhancements. What your monitor
will display is a dark scene with colors
that are often pretty awful. That's why
a RAW image file initially undergoes
a standard treatment when it's first
imported into photo-editing software
such as Camera Raw in Adobe Photoshop.
Usually, the result looks a lot like
what you might have gotten by shooting
in JPEG mode. So, why not just
shoot JPEGs in the first place? After
all, JPEG images can also be tweaked
and massaged with photo-editing
software. The difference is the degree
to which you can make changes before
the image starts to fall apart. Think of
RAW files as your digital negatives -
they're capable of revealing details not
initially apparent.
due to something called bit depth. JPEG
files are 8 bits deep while RAWs are 12,
14, or 16 bits depending on your camera
model. But why is that so important
for astrophotography? As a lifelong
software engineer, I'm going to resist
the urge to go overboard and teach you
all about binary math. But in practical
terms, the important thing to know
is that each pixel in an 8-bit image
can store up to 256 different intensity
levels. Compare that to a 16-bit pixel,
which can have up to 65,536 intensity
levels. That's a big difference. Most of
today's DSLR and mirrorless cameras
have 14-bit sensors, which are capable
of 16,384 brightness levels per pixel.
Some older cameras still in use have
12-bit sensors, which can produce
images with up to 4,096 intensity levels
- still vastly better than what an 8-bit
JPEG offers.
RAW Power
The main benefit of improved bit depth
is the ability to better resolve light
levels. This is a different type of resolution
than most people think of when
they hear that word. Normally when
we talk about a high-resolution image,
we're referring to the amount of fine
spatial detail present. With light-level
resolution, we're describing the ability
to display subtle differences in light
intensities when we have a greater bit
depth to work with.
This parameter is especially important
in astrophotography. When we
start stretching our data by using Levels
and Curves adjustments in Photoshop,
we want to be able to manipulate the
dynamic range of the dark areas to
reveal hidden details, while also preventing
the bright areas from saturating
and being " blown out. "
Here's a simple analogy. Let's say
you go into a store with a $100 bill. The
item you want to buy only costs $15, but
the store doesn't give change. So, since
you really need that item, you're effectively
forced into throwing away $85 to
get it. This is what it's like working with
8-bit data when you are trying to tease
faint details out of the darkness. With

Sky & Telescope - October 2024

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