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Image acquisition +
Processing
This site describes
the acquisition and processing of the avifiles for the images of our Moon
atlas.
All images were taken in a
standardized process. The avifiles were recorded with a Celestron SkyRis 445
Mono video module in the focus of a Celestron C14 telescope (built in 1999). A
Baader Planetarium IR pass filter was used for all
recordings to optimize the seeing conditions. |
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Videophotography of the Moon with Lucky
Imaging - a short introduction
The technique of video photography
of the moon, the sun and the planets is relatively new. Amateurs use this
technique called Lucky Imaging since about
2005. It comes - like many other things - from the professional astronomy. It
was used by the professionals to resolve close binary systems at the
theoretical resolution of the used telescopes.
The idea behind it is in
principle quite simple. Instead of one image there will be taken hundreds or
thousends of images with a very short integration time. Due to the short
exposure times, the air turbulence (seeing) will virtually be "frozen" at each
frame, because the seeing conditions do not change very much within several
milliseconds. More informations about the seeing can be found in two tutorials
here. |
As a
result of the short exposure times, the individual single images are very
noisy. If one has a software that filters out the sharpest of the many frames
and added these to a resulting image the noise will be greatly reduced and the
theoretical resolution of optics can be achieved (in the best case). With
amateur equipment, the lucky imaging works unfortunately only at bright
objects.
» The images on the right show the best
(left) and the worst single image (right) of an 1200-frame avifile. The images
differ not only in focus but also in contrast. The less sharp image shows a
significantly weaker contrast. Click
here
to load a larger version. |
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Monitor and Monitorcalibration
To present lunar images of
highest quality on a "foreign" monitor is a balancing act as we known from our
own experience. Other settings of contrast and brightness - with respect to the
image prcessing monitor - can lead to "hard" representations. Differences in
brightness can be too strong or too weak, black areas can be grayed out or
noisy. All images here were edited on two relatively high quality EIZO 21 inch
monitors and show up on the monitors of some friends approximately like on our
screens.
To view the images as good as possible, adjust your monitor so
that the gray scale from white to black appears well
graded. |
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Collimation
The
Nuts and Bolts to achieve
excellent rawavis is - in addition to the current seeing conditions - the best
possible collimation of the Schmidt-Cassegrain (SC) system. A decollimation is
immediately apparent in an unsymmetrical secondary mirror shadow at a defocused
star image. With this collimation, as shown in the image to the right, the
recording of high-resolution lunar and planetary images is impossible from the
outset. Instructions for collimating a SC system can be found numerously on the
web.
A perfect collimation of the SC system in the focus is shown in the
image on the right outside. |
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Focusing
Another important point
is the focusing. To avoid problems with the mirror shifting, the telescope was
equipped with an inexpensive Baader Crayford focuser. The pre-focusing was done
with the primary mirror focuser (always
counterclockwise) and the fine focusing with the Crayford
afterwards.
Why to focus counterclockwise?
While turning the focusing knob counterclockwise the primary mirror moves
against gravity (when the telescope looks into the sky) and it is fixed between
the focuser and the sky baffle. In this case the mirror can not tilt when the
telescope moves. In the case of clockwise focusing the primary mirror is "free"
and can tilt.
A further tip: If you need
to collimate the SC system, also focus in the counterclock direction. It should
be clear, that a loose position of the primary mirror during the collimation
leads to a slight de-collimation when the telescope moves afterwards.
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Image acquisition
All avi-files
were captured with a SkyRis 445 mono video module at a Celestron C14 with a
focal length of 3,900 mm (f/11). A Baader IR pass filter was used to calm down
the seeing. Other optical components, such as star diagonal or diagonal mirror
that could affect the image quality, were not used.
Depending on the
seeing, the length of the avi-files varies between 1,200 and 1,500 frames. The
image acquisition was controlled by the standard ICAP software which is
delivered with the video module.
The avi-files were captured in
8 bit mode (codec Y800) with a gain of +10 dB. The exposure times of
the individual images depend on the Moon phase and vary between 1/80 s at
the Terminator 1/500 s for brighter areas. Imaging in 8 bit
mode keep the file sizes in the region of 1 GB (with 1,200 to 1,500 frames) and
are still good to handle (regarding the computation time in the processing) and
saves a lot of hard disk memory. |
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A higher bit depth in the "stacked" image automatically results from the image
addition of the individual frames. The SkyRis camera can also be used in
12 bit mode but tests show that this doesn't improve the stacked image and
leads only to larger file sizes. The frame rate was at the maximum with 30
frames per second (1,280 x 960 pixels). |
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All
avi-files were processed with the software AviStack V 1.8 developed by Dr.
Michael Theusner. The software can be downloaded
as AviStack and is freeware.
The software dates back to 2008 and was approximately published at the same
time as Registax - a similar software package. The first video file processing
software from the German-speaking countries was GIOTTO (see the
links at the bottom of the page).
A few years ago
we have processed many avi-files with AviStack and RegiStax in parallel and
came to the conclusion that the |
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final results were still a little bit better when processing the
files with AviStack, although RegiStax works much faster. But AviStack can run
in batch mode overnight and the time factor is not very important for us - and
AviStack runs very stable. Software crashes are extremely rare.
Image
processing
In the following we
briefly describe the image processing of the avi-files with AviStack on the
example of the crater Clavius??. Because the programs work about the same, the
sequence can also be transferred to RegiStax. These steps can also be used for
solar images. All of the images can be enlarged by
clicking on it.
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After
reading the files into AviStack, two points are manually selected in the first
frame. This is the basis of the automatic determination of the relative
displacement between the frames and the following
alignment.
The
points should be structures that show a clear contrast (for example, small
crater rims between light and shadow). |
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After
completion of the first step the so-called threshold values are set. This is
generally done in a false-color mode. All areas without relevant imgae content
(black sky background and overexposed regions) are marked.
No reference points will be set in
this areas, which significantly reduces the processing time. |
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In the
third step AviStack automatically sets reference points for the following
processing. If you choose the default parameters there will be up to 2,500
reference points in an image of 1,280 x 960 pixels - depending on the image
content. Large highland regions of the moon results in a large number of
reference points, smooth mare areas require less reference
points.
If the
reference points are set, step 1 to 3 can be stored as a data file and can be
processed with many other files in the batch mode. |
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In
step 4, the image is divided into squared quality fields. We use a field size
of 64 x 64 pixels. AvisStack determines in every single image and each of the
squares the image quality.
The smaller the size of the quality fields, the longer the
processing time. |
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Now
follows the step which requires the longest processing time. In an 1,500 frame
avi-file of 1,280 x 960 pixels it can take up to 30 minutes, depending on the
computation power of your PC.
AviStack now computes the displacement of all the reference
points in the quality fields of each of the individual images and move these
points so that they are absolutely congruent at the end. |
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It
follows the penultimate step, the image addition. With the slider "quality
threshold", the user can specify how many images are added to suppress the
noise of the individual images. In our case, we add 12 to 13 percent of the
best frames.
At the beginning of
the lucky imaging, the software packages only added complete frames. Now the
current programs only use the best parts of the quality areas from ALL raw
images. They are superimposed in such a way that each segment is present in
equal numbers in the final image without brightness or contrast
differences.
Now
the stacked image can be saved in a 16 bit .fit or .tif file. In the batch mode
the images are automatically saved as .fit file. |
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In the
final processing step, the stacked image is sharpened. AviStack as well as
RegiStax provide the wavelet filtering. This is a very powerful sharpening
method where you have a fine control of the amount of sharpening depending on
the size of the structures (fine or coarse). In principle, one can imagine that
the image is devided into several layers with a different resolution and each
layer can be sharpened seperately. In general we sharpen at the 1st level to
the maximum amplitude of 200 with a sigma of 0.2. For images taken at excellent
seeing conditions the 2nd level will be also included.
Click here for a comparison between the initial stacked and
wavelet sharpened image. |
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The
final image processing done in Adobe Photoshop. Here each video photographer
should find its own way. We usually only correct something the "levels" and
"shadow and highlights".
The large mare areas tend to be a little bit
noisy after the sharpening. Here we apply a weak "Surface blur".
For
objects with a very high contrast such as the crater edges of Copernicus or
Aristarchus we use with two differently exposed avi-files and superimpose them
in Photoshop. In the final image the crater rims appear not overexposed and the
dark area surrounding the crater is not underexposed.
Finally, the image caption is inserted as in the case
of the images of volcanic structures. The final image is saved as an
uncompressed .png file. |
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Variable settings in
AviStack |
An
important parameters of AviStack is the setting of the correlation surface
radius. The default value is 24 pixels. In the case of non-optimal seeing
conditions it can be positive to reduce the default value, e.g., to 10 pixels.
The disadvantage is the increase of processing time. The two pictures on the
right illustrate the effect.
> Comparison between a correlation surface
radius of 10 and 24 pixels on a section of the Mare Nectaris, north of the
crater Fracastorius.
>> Comparison between different correlation
surface radius settings and the number of stacked images.
AviStack is named in the caption
Theusner / IDL, the images date back to the development phase of
AviStack. |
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The
more reference points, the sharper images? In principle, that's right.
AviStack automatically sets the reference points so it is sometimes necessary
to reduce the settings of the smoothing factor and/or the minimum distance
relative to the default values. This also increase the processing time
significantly. The picture on the left shows a comparison.
< Copernicus,
taken with a 6" Zeiss APQ with 2x Barlow lens (f = 2,400mm), left 1,200 and
right 2,200 reference points. The images are mirrored for a better
comparison. |
At this point we would like to thank our friend Wolfgang
Sorgenfrey. Since many, many years, he is one of the very "big" in the lunar
and planetary photography. In countless e-mail contacts we received from him
many valuable tips and informations that have allowed us to present these high
quality images of the Moon. A visit his website is recommended.
A visit of his
website is recommended. |
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Current software for editing video files
and online tutorials for videography of the moon and
planets |
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All Images and
all Content are © by Wolfgang Paech + Franz Hofmann |