GIGeoscientific Instrumentation, Methods and Data SystemsGIGeosci. Instrum. Method. Data Syst.2193-0864Copernicus PublicationsGöttingen, Germany10.5194/gi-6-9-2017Video cascade accumulation of the total solar eclipse on Svalbard 2015SigernesFredfreds@unis.noEllingsenPål GunnarPartamiesNooraSyrjäsuoMikkoBrekkePålEriksen HolmenSiljeDanielsenArneOlsenBerntChenXiangcaiDyrlandMargitBaddeleyLisaLorentzenDag ArneKrogtoftMarcus AleksanderDraglandTorsteinMortenssonHansSmistadLisbethHeinselmanCraig J.HabbalShadiaThe University Centre in Svalbard (UNIS), 9171 Longyearbyen, NorwayNorwegian Space Centre, Oslo, NorwayBrages 2, 1540 Vestby, Akershus, NorwayNorwegian Broadcasting Corporation (NRK) Troms and Finnmark, Tromsø, NorwayLufttransport AS, Longyearbyen, NorwayEISCAT Scientific Association, Kiruna, SwedenInstitute for Astronomy, University of Hawaii, Honolulu, USAThe Birkeland Centre for Space Science (BCSS), University of Bergen, Bergen, NorwayFred Sigernes (freds@unis.no)13January20176191417June20161September201622December201628December2016This work is licensed under a Creative Commons Attribution 3.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by/3.0/This article is available from https://gi.copernicus.org/articles/6/9/2017/gi-6-9-2017.htmlThe full text article is available as a PDF file from https://gi.copernicus.org/articles/6/9/2017/gi-6-9-2017.pdf
This work presents a novel image accumulation filter technique that
reveals small-scale features and details from intense luminosity or high
dynamic range (HDR) video recordings. It was discovered and developed from the
analyses of the Norwegian Broadcasting Corporation (NRK) film of the total
solar eclipse that occurred Friday 20 March 2015
in Longyearbyen (78∘ N, 15∘ E)
on Svalbard, Norway. The result of the filter is fused with a HDR image of the corona and the Solar Dynamic Observatory (SDO)
image of the solar disk.
Introduction
Stacking or accumulating camera frames is a well-known technique in
astrophysics (see Berry and Burnell, 2005). The track-and-stack technique is
an effective method to obtain long exposures from many short ones of faint
deep-sky objects while tracking. Accumulation will reduce noise and increase
the dynamical range. An inexpensive web camera sensor is capable of
capturing a large number of faint and noisy exposures that can be stacked
into sharp and clear images of deep-sky objects. Free software such as
RegiStax (2008) is widely used to align, stack, and process astronomical
images. Another benefit of high frame rate and short exposures is that it
can be used to minimize atmospheric effects such as seeing (Law et al.,
2006; Baldwin et al., 2008).
In this study we present a modified accumulation technique where the
target is not only faint but also intense. The intensities of a total
eclipse extend from faint background sky conditions in the outer corona to
several orders of magnitude brighter intensities close to the solar limb. In
order to image the full spatial extent of the event it was necessary to make
a sequence of variable exposures with two camera systems. Our filter focuses
on the bright chromosphere and the inner corona using data from a
professional video camera. In our approach, each individual video frame is
processed prior to accumulation to produce a final high dynamic range (HDR)
image. In addition, a custom-assembled telescope using a digital single-lens
reflex (DSLR) camera head is used to capture and stack images to produce a
HDR image of the outer corona.
Target: the total solar eclipse on Svalbard 2015
The total solar eclipse on Friday 20 March 2015 started in the
western Atlantic, 650 km west of Canada's Labrador coast and 450 km south of
the southern tip of Greenland. It then raced across the Atlantic Ocean
touching land at only two places: the Faroe Islands (between Scotland and
Iceland) and the Svalbard Archipelago.
In Longyearbyen (78∘ N,
15∘ E), Svalbard, the first contact, the start of the
partial eclipse, started at 09:11:53 UT. About 59 min
later, at 10:10:43 UT the second contact took place, marking the start of
totality. After a mere 2 min and 27 s, the third contact occurred at
10:13:10 UT, which marked the end of totality and the disappearance of the
corona. This was followed by a partial phase of about another 59 min
before fourth contact and the end of the solar eclipse at 11:12:21 UT.
Experimental setup
Two cameras were used to capture the totality. A professional
digital video camera captured 448 frames while a coronal telescope connected
to a DSLR camera head captured 20 images during
the totality.
The digital video camera
The event was filmed by the Norwegian Broadcasting Corporation (NRK)
at Nordlysstasjonen, the old auroral station in Adventdalen. The station is
located ∼ 4 km east of Longyearbyen. The film was broadcasted
in real time on the internet with close to 600 000 followers.
The NRK Sony camera model PXW-X500 was mounted on a tripod and the
Sun was tracked manually. The camera is shown in Fig. 1. The lens was a
Canon x 36 super zoom lens with the aperture set to f/5.6. Fully zoomed in,
the effective focal length was close to 1000 mm. An Baader AstroSolar safety film
was used as a protection filter prior to totality. The exposure time and
aperture were manually controlled by the operator.
Sony model PXW-X500 professional camcorder. Source:
(http://www.sony.com).
The coronal HDR telescope
In order to image the full extent of the corona a Nikon D7000 DSLR
camera mounted on a 400 mm focal length telescope was set up at
Nordlysstasjonen. Figure 2 shows the experimental setup. The telescope is a
triplet-lens apochromatic refractor with an aperture of 80 mm from the
company Sky-Watcher, model Esprit-80ED. A field flattener corrector is
installed between the triplet and the camera head to match and optimize the
illumination of the sensor chip. The telescope was mounted on an
azimuth-elevation tracker, from the company iOptron, model Minitower II. The
assembled system tracked the total eclipse with a nominal maximum angular
error of 0.1 arcsec.
The coronal high dynamic range (HDR) eclipse telescope.
(1) Astro-Baader solar protection filter (ND = 5), (2) Sky-Watcher
Esprit 80ED telescope, (3) tripod head, (4) azimuth-elevation tracker,
(5) Nikon D7000 DSLR camera, and (6) tracker controller.
Twenty images of the totality were taken with variable exposure time
ranging from 0.002 up to 2 s at ISO 100. After alignment of the
sequence, images with the same exposure were median filtered to reduce
noise. The open-source software Luminance HDR (2015) was used to produce a
final HDR image. Tone-mapping was used to improve the visual appearance
employing the methods developed by Debevec and Malic (1997) and Mantiuk et
al. (2006). The technique is well known in photography as
exposure bracketing.
The video filter algorithm
There are challenges in processing the video camera data. First it
is necessary to align each frame in the video to stabilize it. Secondly, a
method to sum the frames must be chosen that is independent of exposure and
gain variations of the camera.
Alignment of images
If we define I0(x,y) and Ij(x,y) as two frames in a sequence of j=0 to
(N-1) images, where (x,y) are
the pixel coordinates, the spatial shift between the frames can be
found using the Fourier shift theorem (Reddy and Chatterji, 1996). The
displacement between two images is found by locating the coordinates of the
maximum value in the real part of the inverse Fourier transform
(FT) of the ratio
R=F0⋅F¯j/F0⋅Fj,
where F0=FT(I0) and Fj=FT(Ij).
For local optimization at sub-pixel scale, the maximum shift correlation
between I0 and Ij is found by
sub-pixel displacements of 0.1 within a window of ±10 pixels
in both x and y
direction. The linear Pearson correlation coefficient function and the
float number shift routine by Lindler (1992) in IDL (Interactive Data
Language) are used.
Note that rotation and scale changes due to atmospheric turbulence
are neglected in the above calculations. Image I0
should be kept fixed as a reference shift frame. Otherwise, the
sequence will drift incrementally. In the following we assume that the
frames are aligned and denoted Ij(x,y) for simplicity.
Accumulation of video sequence
The video
frames have small intensity level changes due to both instrument effects and
manual adjustments by the camera crew. The target, the total eclipse, is
assumed to be stable in intensity during the time the video was captured.
Thus, we can use one of the images as the reference and mitigate the
instrument effects by taking advantage of the time series. A simple linear
model to estimate intensity changes between each color channel is defined as
I0(x,y)=αj⋅Ij(x,y)+βj.
Here αj and βj are defined
as the effective software gain and background level, respectively. If we
choose pixel values where the target is well defined in shape and stable in
intensity, such as the diagonal crossing the center of the Sun, the above
coefficients can be estimated for each frame by the least absolute deviation
(LAD) method. The IDL function LADFIT.PRO by Press et al. (1992) is used for
this purpose. The method is robust and fast. In this study, we have assumed
a linear response from the camera allowing a very trivial correction of
fluctuations in gain and background levels by using Eq. (2). If the true
intensity response curve is known, the intensities should first be converted
into a linear scale to minimize estimation errors. Table 1 shows that the
calculated values of αj and βj are
indeed fairly stable with low standard deviations for N=448 video
frames of the totality. This corresponds to a recording time of 17.92 s
of the totality where the camera was not moved by the operator. The
camera was fixed with the eclipse moving in the field of view only due to
the rotation of Earth.
Calculated average color channel software gain (α) and background values (β) according to Eq. (2)
for N=448 frames of the NRK video sequence. σα and σβ are
the corresponding standard deviation for α and β, respectively.
Color channelAverage αStandard deviation σαAverage βStandard deviation σβRed1.0060.007-25.580.48Green1.0040.007-23.270.27Blue1.0200.016-20.020.27
Next, the accumulated frames are now defined as
I(x,y)=∑j=0N-1αj⋅Ij(x,y)+βj=∑j=0N-1Ijs(x,y).
If j=0, then α0=1 and β0=0. The reference intensity I0 is then chosen to be
the first frame in the sequence, although it can also be any other frame.
Figure 3 shows the result of applying Eqs. (1)–(3). Note the
intense pink-colored bright features that rise out of the
background continuum, localized close to the solar limb. These emissions are
from hydrogen and helium and are associated with prominences. Coronal
streamers are also clearly detected, but structures close to the limb
appear too blurry to be identified.
Accumulated color video frames of the total eclipse on
Svalbard, 20 March 2015. A total of 448 frames of the NRK video are used.
High-pass cascading
Equation (3) may be modified to include a filter that enhances small-scale edges in each of the frames in the sequence. A high-frequency emphasis
filter, F, is known to enhance small-scale
features and edges can be detected by the use of a histogram-scale
Laplacian filter, S. The next step is to add the
filtered result to the original frame
Ijc=Ijs+S(F(Ijs))=Ijs+S(k⋅Ijs-L(Ijs)),
where L is a low-pass filter and
k=1.1 the amplification factor recommended by Gonzales and Woods
(1992). Higher values of k tend to increase the
background too much. In IDL the low-pass filter L
is the SMOOTH.PRO function. It uses the boxcar average of a specified pixel
width, w. SHARPEN.PRO by Fanning (2003) is the
Laplacian filter used. The IDL code for Eq. (4) then becomes
Ijc=Ijs+SHARPEN(1.1⋅Ijs-SMOOTH(Ijs,w)).
The net
accumulated result of Eq. (3) combined with Eq. (4) is named a high-pass
cascade emphasis filter:
I′(x,y)=∑j=0N-1Ijc(x,y).
Figure 4 shows an example of how the above filter works on a single
green channel frame for w=5.
The high-frequency emphasis filter increases noise and sharp intensity
transitions in the image. The latter is enhanced by the gradient filter.
The net result is added to the original frame. The detected transition
points appear in Fig. 4c to be randomly aligned with the small-scale features that are seen in Fig. 4b. The variation must be caused by
scintillation or any other high-frequency change in camera response and
noise. Accumulation of a large number of frames should solve this problem.
High-pass cascade emphasis filter applied to single green
channel NRK video frame. (a) is input video frame, (b) high-frequency
emphasis-filtered frame, (c) edge enhancement of (b), and (d) is (a) and
(c) added.
Results and discussion
The accumulated high-pass cascade emphasis filter images are
visualized in Fig. 5 as a function of boxcar width,
w. It is clear that the accumulation of filtered
frames makes the intense blurry chromospheric regions close to the solar
limb appear more structured. Chromospheric loops, spicules, plumes, and
prominences are now identified. These features have edges and abrupt changes
in intensity representing the high-frequency components of the image. The
choice of smoothing mask or boxcar size in the low-pass filter determines
the level of detail detected. The effect is seen as an increase of width of
the circular solar limb as the pixel width w
increases. The best results to identify loops and spots are obtained
with w∈[3,5]. Higher values tend to emphasize larger-scale
edges further out in the corona, with less detail in the chromosphere. It
should be noted that our technique could be improved by applying a Fourier
transform-based low-pass filter to reduce sharp intensity edge effects.
There is also a color shift from faint green-yellow to pink with increasing
boxcar width in Fig. 5 associated with change in color balance in the
composite images, indicating that our technique is not conservative.
Accumulated high-pass cascade-filtered color images.
Boxcar filter sizes in pixels are for panels (a)3×3, (b)5×5, (c)9×9, and
(d)12×12. A total of 448 color frames of the NRK video are used. The apparent
color changes are predominantly caused by processing the frames without a
rigorous camera calibration, which is required to measure absolute colors.
The use of wavelets has become a popular method to enhance details
in images (Berry and Burnell, 2005). By interactively recombining wavelet
transforms at different spatial scales and layers, small-scale features are
enhanced and large-scale shading effects are reduced. In order to compare
with our cascade filter, the wavelet module of the program RegiStax (2008)
is applied to the stacked frame of the aligned video sequence. Both the
cascade and the wavelet filter results are shown side by side in Fig. 6
for a boxcar width of w=5.
The spatial enhanced image is a combination of two layers of Gaussian
wavelets. It is clear, even though the image appears to be noisy, that the
same features are detected with the wavelet technique as with the cascade
filter. The identified loops, spicules, plumes, and prominences are
all known features that appear in the chromosphere. Also
note the difference in the colors. Each color channel is
independently processed in the cascade filter before the creation of the
composite RGB. The emphasized edges and features have spatially distinct
color difference as a function of color channel. The prominences appear to
be spatially multi-colored with a clear distinct blue core close to the
solar limb surrounded by a red plume with yellow or weak green outward
borders. The loops and spicules are more orange to red in appearance. The
blue and red colors are most likely associated with hydrogen Balmer line
emissions (Hγ,Hβ
and Hα). The yellow to green color is a mixture
of the Hα and the He
(D3) lines. These are the strongest line emissions
from prominences in the visible spectrum (see Slocum, 1912).
Two filter techniques side by side. (a) Accumulated
high-pass cascade-filtered color image. A total of 448 color frames of the NRK
video are used with a boxcar width of w= 5.
Identified features are (1) loops and
spicules region, (2) bright plumes, and (3) prominences. Two zoom windows
show the prominences magnified two times. (b) Wavelets enhanced image
using the RegiStax (2008) software.
It must be emphasized that the wavelet transform is a powerful tool
since it can produce an unlimited range of spatial frequencies and scales,
but it requires interactive user input to produce the final image. Our
attempt to reproduce the event might not be the optimum choice of wavelet
scheme. The cascade filter does not require any interactive feedback, except
for choice of boxcar size. On the other hand, a fused interactive
combination of cascading as a function of boxcar size could also be used to
emphasize features and structures with different size and scale. The novelty
in our simple technique is the accumulated effect of adding edge detection
to the high-frequency emphasis component of each individual frame in a
sequence of images. Only basic low- and high-pass filters are used, which
makes the filter easy to implement in any high-level program language such
as IDL or MATLAB.
Finally, it is now possible to compose an image of the Sun
using the video accumulation, the HDR image from the telescope, and the
Helium II Solar Dynamic Observatory (SDO) image at 304 nm. The last was
obtained at 10:24:20 UT, which is only 670 s after the end of the
totality in Longyearbyen. The result is shown in Fig. 7. The HDR image is
fused with the accumulated image in Fig. 3 and the filter image in Fig. 5a. The SDO image and the video accumulations are resized and
rotated to match the HDR image. The program Paint.net for Microsoft Windows
was used in the fusion. The matching criterion is based on aligning the two
prominences located in the upper left quadrant and the Helmet streamers
identified in both the video accumulation and in the coronal HDR image. The
SDO image is rotated 23∘ clockwise and rescaled to
match the diameter of the solar disk. As a result, the intense chromospheric
regions in the SDO image align up with the detected loop region in the video
accumulation.
Composite image of the total eclipse as seen from
Longyearbyen, 20 March 2015. The white light-colored corona is composed of 20
snapshots from a Nikon D7000 at ISO 100 with exposure times in the range
0.002–2 s. The inner yellow image with dark background is a video
accumulation of 448 frames by NRK applying a high-frequency emphasis cascade
filter. The solar disk is filled with a red-colored Helium II image at 304 nm
from the Solar Dynamics Observatory (SDO).
Note that if we knew the power of the cascade filter prior to the
eclipse, then the video sequence should have been recorded by the DSLR
camera of the coronal telescope instead of the NRK video camera. The video
frames will then match and align with the HDR image. No rotation between the
different camera systems would then be necessary.
The final composed image is an astonishing snapshot of the corona
and the outer chromosphere. Four large Helmet streamer belts are seen that
extended almost out of our field of view, close to five solar radii. The belts
are also structured with curved or twisted appearance, especially the upward-
and downward-directed ones. The high number of Helmet streamers and their
tortuous complex shapes are closely related to the solar activity cycle
(Low, 1996). The solar activity was in other words high during the total
eclipse on Svalbard in 2015.
Conclusions
The principal results obtained in the study can be summarized as
follows.
The total solar eclipse event on Svalbard on
20 March 2015 gave us a unique opportunity to
image the upper parts of the Sun's atmosphere. High dynamic range imaging
revealed four large structured coronal helmet streamer belts that fold
beyond our field of view, close to five solar radii.
A novel high-pass cascade emphasis filter technique is presented
that is capable of distinguishing multi-colored features such as loops,
spicules, plumes, and prominences from the intense and blurry regions of the
NRK video.
Based on our result, we encourage future eclipse photographers
to include – in addition to standard bracketing – a ∼ 15 s
high-frame-rate video recording of the event. This makes it possible
to apply our filter to enhance the high-luminosity regions of the inner
corona and chromosphere.
Data availability
The authors of the paper state that all the data are publicly available. The raw data for this paper can be downloaded here:
http://kho.unis.no/Eclipse/Rawdata/Rawdata.zip.
The authors declare that they have no conflict of interest.
Acknowledgements
We wish to thank the Solar Dynamics Observatory (SDO) for their
excellent webpage (http://sdo.gsfc.nasa.gov)
and free access to solar images. We also thank Kjellmar Oksavik from
the Birkeland Centre for Space Science (BCSS) for fruitful
discussions and media support during the eclipse.
Edited by: A. Benedetto
Reviewed by: R. Michell and three anonymous referees
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