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What is a webcam


Webcams are small digital video cameras that hook up to your computer at the USB ... Make small adjustments in blue and red level until the color of the object ... – PowerPoint PPT presentation

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Title: What is a webcam

(No Transcript)
What is a webcam?
  • Webcams are small digital video cameras that hook
    up to your computer at the USB or Firewire port
  • Some webcams are true CCD devices
    (like the Philips ToUcam)
  • The produce 320x240 pixel images
    (and other resolution modes as well)
  • They are lightweight Cheap, 100 or so
  • They produce color digital video files with sound
    (avi file format)

Many varieties of Webcams
  • Logitech QuickCam - one of the first to be used
    by amateur astronomers for Lunar and Planetary
  • Philips ToUcam has been a very popular and
    inexpensive webcam for astronomy and is the
    camera I use.
  • an excellent entry level webcam costing around
  • There are higher performance (and much more
    expensive) webcams used by advanced amateurs for
    astronomical purposes (Luminera, DMK21F04, Point
  • DMK 390 for the camera, 199 for filter wheel,
    285 for filter set.
  • Lumenera 995 for camera alone.
  • Point Grey Research has some nice fire-wire minis
    700 or so
  • Avoid currently available CMOS devices, they lack
    sensitivity compared to CCD based devices.
  • Color vs gray-scale - filter wheels vs deBayering.

Whats Inside a typical webcam?
Lens with NIR filter
CCD chip behind window
Video Circuit Board
USB connector and cord
How do you use it for Astronomy?(you are going
to void your warranty)
Add 1.25 adapter and NIR blocking filter
Remove lens and discard

Replace eyepiece with the webcam Plug webcam
into your laptop
3) 4)
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If youre lucky and persistent, you will get some
.avi video files of planets jiggling around with
fleeting glimpses of details on the edge of
visibility, a lot like what you see through the
You can also extract single frame snapshots from
the video, but they tend to be blurry and dont
show the detail you glimpse in the video.
Wouldnt it be great if we had some way to take
the information that we know is in the video, and
somehow put it all into one picture?
Well, thanks to a young Dutch amateur
astronomer/computer programmer named Cor
Berrevoets, we have a FREE downloadable program
named REGISTAX which does just exactly that
  • Heres what Registax does
  • Examines every frame of your video file
  • Does a critical evaluation of its quality.
  • Arranges frames in order of quality
  • Lets you pick a reference frame and how many
    of the best ones to keep.
  • Aligns each frame with the reference frame
  • Adds the frames digitally (stacking)
  • This gives an enormous improvement in signal to
    noise ratio (by vn).
  • Uses wavelet analysis to sharpen low contrast
    details in the image.

Believe it or not. This image came from the
video we saw in the previous slide! There are
some details we need to deal with before we start
getting pictures to rival the Hubble.
Critically Important Factor
Critical Details
  • To get good results we need to match the
    resolution of the telescope to the digital
    sampling ability of the webcam
  • We do this by amplfying the focal length of the
    telescope until the smallest resolved image
    details are big enough to be realistically
    sampled by the pixels of the webcam CCD
  • We determine how much magnification we need using
    the digital sampling theorem - also the basis for
    high fidelity digital music recording and the
    operation of cell phones.

The Digital Sampling Theorem
  • In 1927 Harry Nyquist, an engineer at the Bell
    Telephone Laboratory determined the following
    principle of digital sampling
  • When sampling a signal (e.g., converting from an
    analog signal to digital ), the sampling
    frequency must be at least twice the highest
    frequency present in the input signal if you want
    to reconstruct the original perfectly from the
    sampled version.
  • His work was later expanded by Claude Shannon and
    led to modern information theory.
  • For this reason the theorem is now known as the
    Nyquist-Shannon Sampling Theorem

What does this all have to do with webcam
  • The image made by the telescope optics is a two
    dimensional analog signal made up of spatial
  • A webcam is a digital sampling device
  • Lets re-state the sampling theorem in
    terms that relate to telescopic imaging using a

The sampling frequency implied by the pixel
spacing on the webcam CCD must be at least twice
the highest spatial frequency present in the
image to faithfully record the information in the
If you violate this rule its called
UNDERSAMPLING Undersampling is BAD
Effects of Undersampling
Alias signals - illusions, not really there
We can illustrate this with a digital scanner and
a radiating line pattern
60 dpi
13 dpi
302 dpi
23 dpi
Oversampling is ok Undersampling is not! How can
we avoid undersampling in our imaging?
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Here is a highly magnified view of a small
portion of the surface of the webcams CCD
The maximum spacing of 15.8 microns on the
diagonal determines the sampling frequency ns
1 sample/15.8 m 1000 samples/15.8 mm
63.28 samples/mm The Nyquist frequency is exactly
one half this value
nN ns/2
32 cycles/mm
Default mode 2x2 binned, 320x240
This is the maximum spatial frequency the webcam
can accurately sample in any telescope image.
Heres how the pixels are utilized in the
higher 640x480 resolution mode
The maximum spacing of 11.2 microns in green
pixels determines the sampling frequency for
green light ns 1 sample/11.2 m 1000
samples/11.2 mm 89.29 samples/mm The
Nyquist frequency is exactly one half this value
nN ns/2
640x480 mode No binning
45 cycles/mm
However, blue and red still have a Nyquist
frequency of 32 cycles/mm
Heres how the pixels are utilized in the 160x120
Pixels are binned into 4x4 pixel arrays with
vertical and horizontal spacings of 22.4 microns
and a diagonal spacing of 31.6 microns. The
minimum sampling frequency is ns 1 sample/31.6
m 31.64 samples/mm The Nyquist frequency is
4x4 binned 160x120 pixels
16 cycles/mm
nN ns/2
Now lets talk about the resolution of the
telescope. First, some optical
definitionsFocal length FAperture D
diameter of lens or mirrorFocal ratio
F/D(usually written f/ as in f/8 or f/2.5 or
referred to as f-number or f-stop)
Spatial Frequencies in the Telescope Image
Diffraction causes the image of a point source to
be spread out into a circular spot called the
Airy disk
  • The diameter of the disk, d, is dependant only
    upon the focal ratio (f) of the optical system
    and the wavelength, l, of the light used d
    2.44lf 1.34f
  • 1.34f (for green light

How do we convert this information into a spatial
d/2 1.22lf
Two points of light separated by the radius of
their Airy disks can just be perceived as two
Raleigh Limit for Resolution
Minimum Spatial Wavelength Based on Raleigh Limit
Imagine the images of many points of light lined
up in a row, each separated from the next by the
radius of their Airy disks
The sinusoidal wave resulting from adding all the
images can be used to define the minimum spatial
wavelengths present in the image lmin d/2 The
highest resolved spatial frequency, nmax 1/
lmin 2/d 1/1.22lf.
  • So, in the image from the telescope, we find that
    the maximum spatial frequency, nmax, is given by
    a simple formula
  • Maximum spatial frequency nmax 1/1.22lf
  • For green light, l 0.00055mm
  • At f/6, nmax 248 cycles/mm
  • At f/15, nmax 100 cycles/mm

Now that we know how to calculate this, we can
match the maximum spatial frequency with the
Nyquist frequency, nN , of our webcam.
Setting nmax nN and plugging it into the above
formula, we have nN 1/1.22lf
, which rearranges to f 1/1.22lnN
minimum focal ratio to avoid undersampling
f 1/(1.22 0.0005532) 46 This
applies to both the 320x240 mode and for red and
blue images in the 640x480 resolution mode.
  • An alternate expression for the maximum spatial
    frequency is given by the cutoff frequency where
    the MTF goes to zero contrast
  • Maximum spatial frequency nmax 1/lf
  • For green light, l 0.00055mm
  • At f/6, nmax 303 cycles/mm
  • At f/15, nmax 121 cycles/mm

Setting nmax nN and plugging it into the above
formula, we have nN 1/lf ,
which rearrange to f 1/lnN minimum
focal ratio to avoid undersampling
f 1/(0.0005532) 57 Again, this applies to
both the 320x240 mode and for red and blue images
in the 640x480 resolution mode. However, one
could argue that critical sampling at the cutoff
frequency is silly, since there is no information
available there.
  • Astronomers doing high resolution solar imaging
    routinely oversample by 50
  • This seems to result in higher contrast,
    particularly at high spatial frequencies

Digitally sampled point image
Result of 50 Oversampling
How do we get the magnifications we need?
  • Barlow Lens or Powermate
  • Microscope Objective Transfer Lens
  • Eyepiece Projection

A Barlow Lens is a good way to achieve
magnifications in the range of 2x to 3x and most
amateurs already have one in their eyepiece box.
Its not a good idea to try to use a Barlow lens
at a significantly higher power than its design
magnification. Spherical aber-ation is
introduced this way and can harm the image
quality. Stacking of two Barlows to get 4x works
better. Nagler sells Powermate image amplifiers
that work well in this application although they
are expensive. They are available in powers of
2x, 2.5x, 4x and 5x. They are used exactly like
a Barlow lens.
Microscope objectives are a convenient way to
gain high magnification with excellent optical
Typically, 5x, 10x, 20x and 40x are available.
The 5x and 10x would be useful for this purpose.
They are designed with a 160 mm back focal
length, and the front working distance to the
object being magnified is a little less than
160/M mm where M is the magnification. They are
designed to work at the stated magnification
(etched on the barrel of the lens) but can be
used at slightly higher magnifications because we
are not using their full numerical apertures with
an f/6 beam.
The third easy way to couple a webcam to the
telescope is using Eyepiece Projection. You need
to make a short extension tube that fits and
locks over the eye end of the eyepiece and which
accepts the webcam adapter on the other end.
A wide range of magnifications can be obtained by
this method which has a long history of use for
conventional astrophotography in the amateur
community. Magnification achieved and the quality
of the image obtained are dependant upon the
power and quality of the eyepiece. Plössl
eyepieces and orthoscopics should work well.
The atmosphere also affects the image
The effect of atmospheric turbulence is to blur
and bounce around the perfect Airy disk image
until it doesnt look so pretty any more

excellent good average
poor bad V IV
Various qualitative seeing scales
A quantitative measure of seeing is the Fried
Parameter, r0. This parameter is expressed as a
length and is the diameter of the largest
telescope that would be diffraction limited under
prevailing conditions. r0 varies less than 5 cm
under poor seeing conditions up to values as high
as 30 to 40 cm for excellent seeing at the best
Here is the statistics for how r0 varied at one
high altitude observatory site
Bad Poor Average Good
10 5 3 2 1 0.8
0.6 0.4 seconds
According to Cavadore, the probability of
getting a good image from a single exposure is
determined by the aperture size and the Fried
Parameter, r0 By good image he means an
exposure taken when the wavefront error across
the aperture is no more than l/6.28 (0.16
waves). 1/P is the number of exposures you have
to take to get a single good one.
Cyril Cavadore, Seeing and Turbulence,
Here is how this probability affects our chances
of getting usable images with the webcam
If we get lucky and seeing gets to be as good as
average, the 12.5 will give one good image for
every 100 frames that we take. To get 100 good
frames, we need to take 10000 frames!
Stop down when the seeing is unfavorable
  • In poor seeing conditions (what we have most of
    the time) it takes
  • About 20 frames to get one good image from a 5
  • More than 10,000 frames from a 12.5 aperture
  • Forget about it for a 24 aperture

Stopping Down
Note that when you stop down a telescope because
of bad seeing, you dont need to use as much
magnification to reach the critical sampling
focal ratios. For example, our 24 Cassegrain
telescope at Sperry has a focal ratio of f/11. If
it were to be used at full aperture, 4 to 5x
magnification would normally be needed to reach
the f/45 or so needed to avoid undersampling with
the webcam. Putting a 10 inch off axis stop on it
changes the focal ratio to f/26 so we need only a
2x Barlow to achieve critical sampling and at the
same time reduce the aperture to a more likely
match to New Jerseys seeing. Putting a 5 inch
off axis stop on this telescope changes the focal
ratio to f/44 without any amplification, just
about right for critical sampling and a good
match for poor to average seeing.
Length of Video
Planetary rotation imposes a limit on how many
frames you can take with your webcam. Emmanuele
Sordini has figured this out for us at
bloomingstars.com Here are his recommendations
for Mars, Jupiter and Saturn based on keeping
image blur smaller than the resolution of the
telescope and sampling ability of a webcam
Noah My night assistant
10 f/17.6 Newtonian. Barlow lens mounted on-axis
in front of small diagonal. Scope mounted on
Losmandy G11 Germain Equatorial. Later installed
in observatory. Used for Mars Opposition in 2003
and high resolution Jupiter pictures.
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Moon image made with small refractor at f/6.
Notice sampling artifacts.
Eratosthenes Region
Cassini Region
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Mosaic of Plato Region Taken with ToUcam coupled
to 10 f/6 Newtonian with Barlow lens.
EFL176 Stopped down to 4 aperture, f/44
Seeing good to excellent.
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Nov. 16 2004, 5x Powermate 12.5 f/6 Newtonian
April. 2 2003, 2. 5x Barflow Lens 10 f/6
Feb. 3 2006, 5x Powermate 12.5 f/6 Newtonian
Jan. 22 2005, 5x Powermate 12.5 f/6 Newtonian
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Price tag of observatory

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Coprates (Valles Marineris) image from Viking
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Images of Jupiter showing Oval BA
Red Spot Junior
Image taken 2/15/06 with 10 f/15 Refractor at
Sperry Observatory using 2x Barlow lens,
f/30 Seeing average.
First Light Image taken 6/18/07 with 7.25 f/14
Schupmann using 3x Barlow lens, f/42 Seeing poor
to average.
Thats all, folks
Helpful Hints
  • Use a 2x Barlow for your early experiments. It
    gets the focus outside the drawtube and into the
    webcam focal plane. You might not be able to
    focus without it and you really need some
    amplification no matter how lousy the seeing is.
    No problem if you have an SCT.
  • Parfocalize an eyepiece with your webcam. Life
    is much easier if you can prefocus before getting
    into the computer stuff. A motorized finder is
    very helpful. It is amazing how much image
    motion you get when you barely touch a manual
    focuser while youre working at f/45.
  • You must have a good finder. I doubt that even
    modern computerized GO-TO scopes are accurate
    enough for webcam purposes when you are using
    f/60 or higher. The 7-10x finder that came with
    your scope is probably not powerful enough. A
    second finder working at 25x or higher is a
    really good idea. I just attached a 3 f/10
    Newtonian on the side of my scope with a 12.5mm
    illuminated eyepiece giving 60x. This works
  • Start with the moon. It is bright and easy to
    find and rewards you with easy good results so
    you dont get discouraged.
  • If the seeing is poor to average, dont waste
    your time with full aperture if you have a 10 or
    12 scope. Stop down to 5 to 8 inches.

Computer stuff
  • The Computer
  • If you are setting up outside each time, the
    computer probably has to be a laptop. If you
    have a permanent observatory, a desktop is
  • You need a reasonably fast windows PC (no Macs,
    sorry), preferrably running 2000NT or XP. You
    can get by with ME, but it can be a painful
  • Buy as much RAM and hard drive space as you can
    possibly afford, 200 gigabyte hard drive space
    is not too much! Get a DVD writer to use for
    video file backup if you can.
  • You need a free USB port to plug the webcam into.
  • Webcam software All you need is the driver and
    it is probably already in your system. Look for
    a program called VRecord using the find function
    in your computer. Run it after plugging your
    webcam into the USB port. You dont need the
    stuff on the CDROM that came with your webcam.
  • Download Registax Its free! http//registax.astr
  • Registax tutorials http//www.threebuttes.com/Reg

Step by Step Procedure
  • Set up telescope and turn on drive.
  • Turn on your computer and make sure the date and
    time are set correctly.
  • Locate object in finder and insert amplifier and
    parfocalized eyepiece into the focuser.
  • Center and focus object.
  • Replace eyepiece with webcam
  • Plug USB cable from webcam into computer.
  • Launch VRecord. If you have centered the object
    properly, you will see something on the screen.
    It may even be in focus. If not, focus it.
  • In the options menu, make sure that the Set
    Preview box is checked and select Set Video
    Format and verify that it is set for the default
    320x240 pixel mode or whatever mode you wish to
  • In file menu select Set Capture File and name
    the file appropriately for the object you are
    observing. I like to use the name of the object
    followed by the date and a serial number, such
    as Mars 10 27 05 3. You dont need to put the
    time information in (or even the date) since that
    will be timestamped on the file when it is saved
    after taking the video.
  • In the capture window, select Set Time Limit.
    In the dialog box be sure to check the Use Time
    Limit box. Type in the video file length in
    seconds you wish to use and say ok. I use 120 or
    180 seconds for planetary work and 30 seconds for
    lunar work where I plan to take a whole lot of
    pictures to make a terminator strip.

Step by Step Procedure, cont.
  • In the options menu select Video Parameters,
    and under the Image Controls tab, turn off the
    full auto mode and select 15 frames per second
    and then click on the tab named Camera
    Controls. Put the exposure slider to 1/50th of
    a second and the gain at about 50 of full scale.
    If the image is too bright, select a shorter
    exposure, if it is too faint select a longer one
    or slide the gain control to a higher value. You
    also have controls for white level which may be
    difficult to use if nothing in the object is
    supposed to be white. Auto should be off. Make
    small adjustments in blue and red level until the
    color of the object is correct. Note you can
    make these adjustments during the daytime using a
    distant tree top as a subject. The automatic
    controls will work in this case and you can let
    it do its thing, then turn them off and save the
    settings for use at night. All you will have to
    change at night will be the exposure or gain
  • In the capture window, select start capture.
    This will give you another dialog box which you
    have to click on again to actually start the
    capture. At the end of the scheduled time, the
    file capture will stop automatically. You dont
    need to save the file since it is written to
    while you are taking the video.
  • Dont forget to create a new capture file for
    your next video. If you dont, the next video
    will overwrite the last one and you will be very
  • When you drop the file menu and select set
    capture file again. It will come up with the
    name of the last file as a default. You can
    either type in a new name, or if you use the
    serial number procedure I do, just change the
    last number.

Step by Step Procedure, cont.
  • If your computer has enough resouces (RAM,
    processor speed) you will able to run Registax on
    your .avi files as you accumulate them. If not,
    wait until you are done observing before trying
    to run Registax. I have had a number of computer
    crashes requiring restart because I had too many
    things going on at once (I was running Windows ME
    at the time, it is not a problem for later
    versions of Windows).
  • Launch Registax. Press Select button in upper
    left corner of startup screen. This will give
    you a file browser box which will allow you to
    point to the one of your .avi files you wish to
    process. When you click on the file name (even
    before you click the Open button) you will see
    the first frame of the video in the right side of
    the file browser box as well as in the Registax
    screen behind it. If this is the correct file,
    press Open.
  • When the file opens, if you move the cursor over
    the image you will see a square with a plus sign
    in the middle of it. If this square is too small
    (the 32 pixel one for example) processing will be
    fast, but the program might get lost. I like to
    pick the 128 pixel box when I am in the default
    video format of 320x240.

Step by Step Procedure, cont.
  • Move the slider at the bottom of the screen to
    step through the video file. When you find a
    good frame (one that is sharper, more
    symmetrical, shows detail) move the cursor over
    the image and center the square on the object and
    do one left click on the mouse.
  • At this point you will see a graph with a power
    spectrum of the selected frame. This is a red
    curve starting on the left near the top with high
    intensity of low spatial frequency components and
    dropping off rapidly to the right. You can also
    select a Quality Estimate method to use at this
    point. I really like the Gradient method.
    Select it if it is not already selected. You can
    also select the appropriate lowest quality
    parameter. I would pick 80 for a start.
  • You will also see a multicolored plot of the two
    dimensional Fourier transform of the image. You
    should see a reasonably symmetric plot with a red
    region in the center. If you do, you can leave
    it alone and press the button marked Align just
    below the blue tab also marked Align (if the red
    region is too broad, use the arrows by the FFT
    filter box to raise the number you see there,
    however, Cors defaults will usually work best).

Step by Step Procedure, cont.
  • When you press the Align button, you will see the
    image area start jiggling about madly with the
    alignment box moving with it. This is a first
    pass at roughly aligning all the frames, but most
    importantly, it is the time when the program
    measures the quality of each frame by the method
    you selected in a previous step (use the gradient
    method, it works best). When the quality
    estimation phase is finished, you will see a
    graph with a red curve and a blue curve. The red
    one is a plot of the quality of each frame as
    determined by the method you chose. Registax
    rearrange the frame order putting the best frames
    to the left, the worst ones to the right. The
    blue curve gives you the registration difference
    between each frame and the reference frame you
    picked at the beginning.
  • Now you get to pick how many of the best frames
    to keep for further processing and eventual
    stacking. Grab the slider at the bottom of the
    screen and move it to the left. The vertical
    green line will move to the left also. As you
    move it the frame number and the stack size
    number below slider will change. Notice that the
    frame numbers appear to be random now. When you
    have picked the stack size you wish to keep (the
    better the seeing, the more you get to keep)
    press the Limit button just below the Align

Step by Step Procedure, cont.
  • At this point you can go on and select Optimize
    and Stack (or just Optimize) and the program will
    go through the smaller set of frames and do a
    better job of aligning them up with the reference
    frame. A better choice at this point, however,
    is to press the Create button. This takes the
    fifty best frames (or whatever number you see in
    the box to the right of the button) and creates a
    much better reference frame from them to use to
    do the whole set of good frames you selected.
  • Lets say you press the Create button. When you
    do, the program will rapidly optimize this small
    set of the best frames, stack them and give you
    the chance to do some processing. When it is
    finished you will get a redundant dialog box
    telling you to enhance image and press continue
    and you have to say OK. When you do, you can
    play with the wavelet sliders to perk up the
    image. Dont mess around too much at this stage,
    otherwise the program might not recognize the
    unprocessed frames as being the same object as
    the tuned up reference frame. I would just move
    the slider under the checked 32 wavelet spot to
    about 50 and uncheck the first two boxes. At
    this point you probably are already pleased.
    Just wait.

Step by Step Procedure, cont.
  • Press the continue button, and the redundant OK
    button, and then press optimize and stack. The
    program now goes through the rest of the good
    frames and optimizes their alignment very
    precisely against the created reference frame.
  • When the program has finished this process, it
    digitally adds all the frames together, pixel by
    pixel to give the stacked image.
  • You can press the preview button by each wavelet
    to see the contribution it will make in the final
    image. Some look noisy or empty. Uncheck them.
    When you do, the noise in that particular wavelet
    component of the image will be eliminated from
    the final image. Sort of a smart low pass
    filter. Some look like they have interesting
    detail in them. Move their sliders to higher
    values to see how they can help the image. A
    good place to start is with the first wavelet
    unchecked, the second one at 25, the third and
    fourth at 50, the fifth at 25 and the sixth
    just left alone.
  • Play with the wavelets until you are satisfied
    and then you can check the box to hold the
    wavelet setting for processing the next video.

Step by Step Procedure, cont.
  • Look closely at the image. You may see a red
    fringe on one side and a blue one on the other.
    This is atmospheric dispersion and you can tune
    it out! Select the RGB shift tab to the right of
    the screen and you will see up/down/left/right
    buttons for shifting both the red and the blue
    components of the RGB image around relative to
    the green one. To help you do this, you can turn
    off any or all of the components as well. It
    usually only takes a few pixels shift of the red
    component in one direction and a few more of the
    blue component in the opposite direction to make
    a significant improvement. The fringes should go
    away, and small scale details may sharpen up
  • Now press the Final tab near the top and center
    of the screen. This stage of the process will
    allow you to rotate and or flip the image to
    correct for odd numbers of reflections (left and
    right reversed, like with a star diagonal) and
    make North or South up as you prefer.
  • You now save the final image as either a bitmap
    or a JPEG 8 bit file, or three 16 bit formats
    FITS, TIFF or PNG. The FIT option is the most
    accurate way to save the files as separate 16bit
    R, G and B components. TIFF is like jpeg and can
    be compressed, however it is a 16 bit file.
  • Now press Select to load another file and start
    all over again.
  • Enjoy the cool pictures!

Ed Grafton in Houston, Texas, imaged Mars on five
successive nights from October 16 to 21 while the
dust storm that began in Chryse pread southward
across Mare Erythraeum and Solis Lacus. On the
21st, the dust is the wide pale-yellowish veil
extending from the center partway down. Grafton
used a 14 SCT at f/39 with an SBIG ST-402 CCD
camera. North is up
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