Sky and Telescope - May 2018 - 31

These data are still very raw. Camera EDRs have all kinds
of artifacts, with which amateur astrophotographers are
painfully familiar. These include detector noise, dark current,
vignetting, varying light levels across an image, geometric
distortion, and other issues that get in the way of a pretty
picture and also in the way of science.
Spacecraft data can be calibrated or resampled to correct for some of these issues. For example, it's common for
spacecraft image data to be divided by a model spectrum of
incident sunlight to correct for there being different light levels at different times of day. It's also common to mathematically reproject images to correct for lens distortion or to turn
photographs of round worlds into ﬂat maps.
Resampled or calibrated data are called Reduced Data
Records (RDRs) to distinguish them from EDRs. For any
single EDR, there may be multiple RDRs that have had different types of processing applied to them - or there may be
no RDRs, only EDRs. Each camera team develops a set of PDS
deliverables that is unique to their instrument, which means
that enthusiasts who want to play with image data should
approach data sets one at a time.

Different Kinds of "Raw"
In 2004, Steve Squyres and Jim Bell of the Mars Exploration
Rover (MER) mission initiated a revolution in public participation in image processing. Their idea: Share images from
Spirit and Opportunity directly to the internet as soon as
they were received on Earth, without waiting for any proprietary period to lapse.
Squyres and Bell thought that sharing the images would
also share the excitement of participating in an active Mars
mission, while believing the risk of the key results being
"scooped" by outside researchers looking at the data was
slight. They were right, and their openness with the rover
data generated a booming online community of amateur
image processors. Cassini, New Horizons, and Curiosity later
chose to participate in the same tradition.
The MER, Cassini, and Curiosity raw-image websites use
the term "raw" to mean something different from its use at
the PDS. Raw-image websites share data that have not been
calibrated but have been processed.
When spacecraft data arrive on Earth, they pass through
an automated processing pipeline that branches out into
two directions. One branch sends EDRs to science teams for
inspection. The second produces images for the public that
are processed from the EDRs.
The processing is intended to make the images more accessible to the public, but it also (usually) degrades their quality
for scientiﬁc analysis. Image contrast is stretched to make the
brightest pixels white and the dimmest black. They are also
saved with lossy JPEG compression, dramatically reducing
their ﬁle size for easy sharing but also introducing artifacts.
(Whole subcultures on the internet have sprung up that seem
determined to interpret every JPEG artifact as evidence for
alien life.)

p DIGGING IN Each of the PDS's seven nodes is hosted in a different
location and appears very different. Shown here is the Cartography and
Imaging Sciences Node data portal, which hosts raw images from most
spacecraft cameras, imaging spectrometers, and imaging radars.
s k y a n d t e l e s c o p e . c o m * M AY 2 0 1 8

http://www.skyandtelescope.com

Sky and Telescope - May 2018

Table of Contents for the Digital Edition of Sky and Telescope - May 2018