ï ÆÏÔÏÇÒÁÆÉÑÈ Ó íÁÒÓÁ
Vasiliy Tomsinsky
Гиперссылки удалены.
____________________________
http://marsrovers.jpl.nasa.gov/spotlight/spirit/a12_20040128.html
Revealing Mars' True Colors: Part One
January 28, 2004
Difference in sky color in Spirit's first panoramic images, where frames show
different levels of darkness, depending on the weather when each frame was
taken (light dust conditions on the left, heavy dust on the right).
Browse Image
When you look across the dusty, rock-strewn landscape of Mars, gazing toward
the eastern hills on the horizon, it may seem as though Mars is right outside
your window. With a stretch of the imagination, you could almost step outside,
pick up a rock, and examine it for yourself. Scientists and engineers have
worked hard to provide us all with this human-scale view, because Spirit sees
things much differently from her robot perspective.
"Spirit doesn't see the whole vista around her all at once," says Jim Bell,
Professor of Astronomy at Cornell and Lead Scientist for the Panoramic Camera
(Pancam) Team. "It takes days for the rover to complete a full 360-degree
panorama of the surrounding landscape."
In an exacting sequence, the rover takes smaller pictures that are later
"stitched" together into one larger view, or "mosaic." That means that a single
mosaic may be built from images taken when there's more dust in the air or at a
different times of day. Such changes in environmental conditions and light
levels result in a patchwork quilt of varying lights and darks, and ultimately,
oranges, butterscotches, and browns.
Dr. Jim Bell
Lead Scientist for the Panoramic Camera (Pancam) Team
Getting the Colors Right Combines Art with Science
"Getting the colors right is not an exact science," says Bell. "Giving an
approximate view of what we'd see if we were there involves an artistic,
visionary element as well - after all, no one's ever been there before."
However, great pains are taken to be as accurate as possible, short of going
there ourselves.
To give people a sense of being on Mars, scientists combine views through
telescopes, data from past Mars missions, and new information from the current
mission to create a color-balanced, uniform scene. Color-corrected mosaics
simulate the view a person would see if all the images in the mosaic were taken
on the same day, at the same moment.
Dr. Eric De Jong,
Lead for the Solar System Visualization Team
In addition, the rovers can take three pictures in a row of the same surface
area on Mars using three different primary color filters - red, green, and blue
- to make one color image. "It works a little like an inkjet color printer,
which combines primary colors to create various shades on paper," explains Eric
De Jong, Lead for the Solar System Visualization Team at the Jet Propulsion
Laboratory. "Then, we can tweak the color just like you can adjust the color
balance on a TV screen at home."
So far, however, the images produced are only approximate martian colors.
That's because many of the pictures are taken with set of camera filters that
include near-infrared or ultraviolet wavelengths, which our eyes do not
perceive. Overall, there are 14 "geology" filters (two additional camera
filters were designed exclusively to observe the sun). Scientists find these
geology filters extremely useful because they provide maximum contrast for
analyzing some of the most interesting geological features at the landing site.
"We almost never choose to take the images in natural color, because that's not
as helpful to us scientifically," said Eric. "However, we're able to
approximate what humans might see because Jim's team lived and breathed with
this camera for many years, experimenting to get the colors in the camera
models just right."
Using the calibration target on each rover, scientists have a good knowledge of
how to adjust colors according to the circumstances found by the rover cameras
on Mars. Scientists saved half of the material for the silicon colored chips on
the calibration targets, so they can make comparisons and accurately measure
how the rings and chips on Earth are reflecting light on Mars.
Large Image
How accurate are the final colors
Before sending cameras to Mars, the team took more than 100,000 pictures in a
vacuum chamber on Earth that simulates martian conditions. They experimented
with different levels of light entering the lens, depending on the angle of sun
or amount of dust in the air. They then fine-tuned the cameras to respond
correctly to temperature changes or anything else that might cause the
instrument to vary.
"It's crazy how many pictures we took with the Pancam on Earth, but those
images were essential to finding the answers on Mars," Jim recounted. Using a
"calibration target" with green, red, blue, and yellow silicon swatches and
rings of varying shades of gray, the team monitored how those colors changed
under different lighting and environmental conditions in the laboratory.
Each rover carries its own calibration target, and is regularly instructed to
take pictures of it. With their earth-bound experience, scientists have a good
knowledge of how to adjust colors according to the circumstances found by the
rover cameras on Mars. They know how much sunlight the three rings of gray,
black, and white reflect on the calibration target in the different filters.
They also saved half of the material for the silicon colored chips on the
calibration targets, so they can make comparisons and accurately measure how
the rings and chips are reflecting light on Mars.
Through all of these color corrections, the unprocessed raw images are not
affected, and no science is compromised. In fact, there are many interesting
things that can be learned about the surface and atmosphere by studying how
their colors change during or between each day.
Revealing Mars' True Colors: Part Two
January 28, 2004
Painting by Number: From Ones and Zeros to Pictures of Mars
When Spirit sends pictures back from Mars, they aren't finished products by any
means. Each pixel in the image is coded in zeros and ones, and sent across 120
million miles of cold, dark space back to Earth. Once they are received by the
Deep Space Network antennas here, the zeros and ones are translated into pixel
color and brightness.
"We're basically doing a more sophisticated version of 'Paint by Numbers' when
we reconstruct the images," laughs Eric De Jong, who rarely leaves the glow of
his high-end computer screens. As one of the team members who is responsible
for processing the images from Mars, he has barely a moment to spare with all
of the data coming in.
A sequence of zeros and ones means a certain color/brightness, so it's just a
matter of translating each sequence into the right visual information. "It
works a lot like your digital camera, which registers pixels in a similar way,"
he relates. "It just takes a little longer to create the final picture since we
have to wait for the information to reach us from Mars."
The "black box" is an example of a temporary drop-out of data from a raw Pancam
image. Each rover can retransmit data to gain the information lost in the first
transmission due to communication hiccups.
Large Image
Black Boxes of Data
To complicate matters, the pieces of the picture don't always make it back to
Earth all at once. The images streaming down from Mars are sent in data
"packets" that are delivered either directly from the rovers or through their
sister spacecraft orbiting Mars - Mars Odyssey and Mars Global Surveyor.
Sometimes all of the data does not make it through the 120-million-mile journey
all at once, leaving a temporary black box or hole in the image.
"If there is rain in Spain near the Deep Space Network station, or some other
communications problem, a packet of information can get distorted mid-stream
and not show up at the Jet Propulsion Laboratory in Pasadena, California,"
explains Jim Bell, the lead scientist for the Panoramic Camera, which captures
360-degree views of the martian surface at a higher resolution than any camera
previously sent to the surface of another planet.
"Just like you can get "holes" in your conversation when talking on cell phones
that temporarily drop out of range, we can get holes in our pictures from
Mars," Jim says a bit ruefully, since he and his team are especially eager to
study the full data as soon as possible. When that happens, though, the mission
team quickly identifies the missing packets and asks Spirit to resend the
missing zeros and ones to complete the picture.
Historical photo of an engineer coloring by numbers sent down by the Mariner
spacecraft in the 1960s, when the first digital images were transmitted through
space from Mars.
Large Image
After a day or two, the missing packets are resent to complete the picture, and
the science team intensively pores over them, looking for unique rocks and
surfaces that might give clues about the history of water, wind, and other
processes at the landing site.
"My whole team spends all their waking hours gazing at Mars," Jim comments. "We
don't want to miss a thing!"
Eric couldn't agree more. "We've come such a long way," he says with great
enthusiasm and an appreciation for historical accomplishments. "Early
astronomers used sketches, paintings, and photographic plates to share their
visions of other worlds, and as late as the 1960s, engineers literally used a
handful of crayons to color in numbers that represented a few hues on Mars.
Today, we have the most realistic, clearest view of Mars ever."
This view of the "real Mars" is due to the modern, highly sophisticated "paint
by number" image processing techniques that Eric and his fellow
image-processing team members use. Racks of servers hum in the cool, darkened
environment of the Multi-Image Processing Laboratory where they work, which is
lit largely by plasma and other computer screens bearing the latest images from
Mars.
Unlike the laborious processes of the past, which could take as long as weeks,
images are in the hands of the science team within a minute after they hit the
ground at the Deep Space Network Station. "It's awesome!" Eric smiles. "We were
only expecting 50 megabits per downlink, but now we're receiving five times
that. It doesn't stop us at all-high speed computers do the work and shoot out
the data through the pipeline to everyone."
"It's exciting to have all of this technology - digital cameras, image
processing, and digital communications - at hand," says De Jong. "It gives us
an unprecedented opportunity to share the images with everyone on Earth. They
can share in our exploration of Mars the moment it is happening."
Best Regards.
.