Solar Total Eclipse (August 11th, 1999)
at Vouziers (Champagne-Ardennes, France)

Click here to get the French release, also here to have a report from ESO


The idea to achieve CCD imaging of a total solar eclipse is not new, nevertheless, for three CCD "freaks" who had never seen a total solar eclipse in their whole life, it was a real challenge ! We have prepared this experiment as best as possible by reading a lot of publication concerning this topic, by writing modules which are able to drive automatically CCD cameras by executing scripts (implemented in the PRiSM software). The use of this setup was easier for us, since this eclipse was passing over Europe (and close to our living places).
The goal of this page is to describe thoroughly our method and to share our unique CCD imaging total solar eclipse. (Best viewed using 1024x768 resolution)
If you want to use our images, please ask the authors, thanks !

This first paragraph shows all the results recorded with our telescope/lenses coupled to our CCD cameras, for more details concerning the way used to record these images, you are invited to read the text, hereafter the images.

Totality phase

at 10h27m32s GMT, CN212 Takahashi CCD2x2K maya CCD camera, 2 sec exposure, 1/10.000 filter transmission
Click over image to get a larger one

at 10h27m32s GMT, CN212 Takahashi CCD2x2K Maya CCD camera, 2 sec exposure, 1/10.000th filter transmission, different kind of processing compared to the previous image.
Click image

300 mm F16 lens and CCD_Kaf1600 Maya CCD camera, 50 ms exposure time,  polarized filter and 1/8 neutral filter transmission. Color combinaison of 2 images made of two orientations of  polarized filter (90°).

Some images recorded by  SOHO satellite at almost the same time :




195 Å

171 Å 

Click over images to get better resolution.
Other links to these images :

Partial Phase

at 10h07m30s  -  Filter 1/10000 + 1/100000  30msec de pose, 75% dynamic, CN212 CCD2K

at 09h31m46s  -  Filter 1/10.000 + 1/100.000  exposure 30sec, 75% dynamic, CN212 CCD2K


Observing place

Eclipse hunting is a tough task. Everything should be under control, organized, timed in order to avoid risks of failures. Several full moon weekends have been used to prepare this event. Only day after day, angst was increasing whilst getting closer to this event.

One week before the Eclipse,  we have to check out all weather forecast web site to find out the best place in Europe. A T-7 days, Normandy seemed to be the best place, also Hungary looked fine. Just one day before, Meteo-France foresaw a possible area between Reims and Metz. Rest of Europe did not seem to be better than what was predicted in north of France, and was even worst. All the team met at Chalons/Marne to choice the place of Vouziers (East of Reims). We arrived there at  Vouziers around 19h, the weather was awful : it was poring, wet and looked really hopeless. We started to seek for a suitable place to install our telescopes, the main concern was to tied our telescope/CCD cameras to power supply (220V). We were lucky, we found very quickly a very nice family willing to offer us their garden as an observing place and power supply ! It was really a wonderful way to welcome us !!!

Observing place

Click images to get higher resolution one.
Everything is ready, before partial phase, focusing of the Cameras needs to be carried out. 
One could notice the two PCs (and two displays) lying on weak camping tables. 
The sky lets appear some blue areas,nevertheless our mood is not as high as the sky wants to turn to. We were doubting about the forecast of MétéoFrance : this place was said to be the highest portability to watch the Eclipse ! Anyway, we cannot move anymore : a lot of stuff has been unpacked, it's too late.
Few seconds after 1st contact ! 
Background of this image, the whole team of  Pises Observatory (Montpellier) is visible. The two telescope are pointing toward the same point. Pressure is increasing also !! 
Some minutes after totality phase, the blue sky hole which had let us to watch the Eclipse, starts already to close . 
Despite our excitation, one should not forget to carry out flats-fields using the remainder of the blue sky which closes quickly !
The main investigators : 
From left to right : Boris Gaillard, Cyril Cavadore and Laurent Bernasconi. At that time our mood was good since our experiment was successful. The sky was (this picture) overcast and we didn't see a lot from the decreasing partial phase. Anyway, the most impressive part had taken place in front of our eyes !!

Seeing the satellite image of 12h00 UT, one could wonder how we were able to watch it , eclipse shadow is still visible over Iran !


The instruments

Two experiments have been settled. The overall weight was close to 150kg... it was difficult to move all the telescopes/cameras once it was unpacked !

Experiment #1 : solar corona imaging
- CCD Camera Maya 2k*2k :  CCD Thomson TH7899 (14µm pixels size)...home made camera.
- CN-212 takahasi telescope, focale length : 840 mm, 212mm diameter.
- Field corrector.
- Full aperture filter 1/100 000 + 1/10 000 transmission to allow us to focus using partial phase.
- Full aperture filter 1/10 000 just for corona pictures.
- NJP160 takahasi mount with MCMT.
- Old desktop PC (we don't have laptops).

Experiment #2 : solar corona polarization
- CCD camera Maya Kodak-Kaf1600... (here again an homemade CCD camera).
- Russian 300mm Photo lens.
- Polarized filter.
- Green filter and KG3 to block IR light.
- Super-polaris mount.
- Desktop PC

Each PC controls his own camera/telescope using Prism V4  beta 15 software.
We decided that all these experiment should be done automatically. No way to be front of our screens during totality phase !!!
Thus, programming script were written to ensure automated image recording, full moon was necessary to debug and test these scripts (see next section).

Nevertheless, the list above miss to provide you a good overview of all the hardware we used... these picture will give you a better idea.


Image acquisition method (CCD 2x2K + CN212)

The basic idea (and according to Francois Colas advice) was to avoid to enter any commands to the PC's keyboard whilst the short totality phase, and thus to use an automatic acquisition system through the Prism Software. The CCD La camera was based on a 2x2K Thomson TH7899M CCD, and read out by a controller sending datas thru the parallel port in EPP mode. The conversion gain was 8e-/ADU. This system, though nice for deep sky application, showed its weaknesses for this application because of the readout time of 71sec (binning1x1) and 22sec using binning 2x2. Considering that the eclipse duration was 2m17sec, it was out of the scope of this project to use the 1x1 binning for the whole acquisition process, but to keep this setting for the last imago before the end of the totality phase. The sampling was, with binning 2x2, using the TAKAHASHI CN212, 6.9" per 2 pixels of 14µm and the field of view of 1°49'x1°49'degrees.
I think it worth to say that processionals are using a 1Kx1K CCD camera, digitization performed with 12 bits, which allows almost a video frame rate of many images per seconds. These cameras are not performing well in term of noise, and one will notice that it could be a limiting factor to reach the faint external extensions.
This image shows the CCD 2x2K front side view, shutter opened. A 60mm PRONTOR shutter hd to be installed to avoid any voignetting problems and make this experiment automated. The small box contains the coil to command the shutter. The ring to attach any optical devices is a CANON EOS EF type. 
The CCD is visible (28x28mm sensitive surface)
Overview of the whole system is shown in this image : 
  • CCD camera itself.
  • The CCD controller inside the black box, based on PIC microcontroller, 14bits digitized, // port EPP.
  • The camera power supply (with a fan on top)
  • The shutter controller (the small gray box)
The ruler besides the camera is a 200 mm length.

To find out the exposure time and which kind of filter to use was a tough problem to solve. We, have given up the idea to use radial filter [Ref.3], which is beyond our reach. In that context, we had to use a full aperture filter to overcome the high brightness of this event. After reading papers  [Ref.1 ,4 et .2], we realized that the best test was to be done with the full moon with the same setup. The goal was to get a signal reaching 95% of the CCD dynamic, in the area close to the sun/moon's edge, otherwise all the useful information could be lost either around the saturated parts or by the spread of a CCD blooming tail. Tests have been done with a close full moon (June 99), using exactly the same setup we intend to use for the solar eclipse.
Nearly full moon, 5sec exposure time sampled at 6.9"/pixel (bin2x2) and 1/100.000 full aperture filter, the signal swing is 11000 downto 4500 ADUs. A 1/10.000 transmission aluminum filter should be better, because 5 seconds of exposure time is too long. 1 sec exposure will be a good starting exposure time. Literature states that full moon brightness match to the one from a solar eclipse border. Tests had been also carried out with a off axis hole and neutral wratten filter : it was not successful.

This preparation allowed us to know which exposure time to apply as a starting point, to refine it, an automated setup has to be found to get the optimal exposure time : to reach this point a special set of scripts has been written. An arbitrary window is set to compute the optimal exposure time, the maximum value is computed out, and the exposure time too. If the exposure time is greater than 2sec, it will be thresholded. Afterwards, 4 images are acquired using binning 1x1, all the images are saved to the disk automatically.
We have hesitated to vary the exposure time between the 2nd ,3rd and 4th frames and the 1st one, but safety reasons prevented us to do that ... (I think now 20/04/2000, that is was a great mistake....)
Those scripts have been tested with full moon and they fulfilled completely our requirements, this is how they look like Prism (v4):

test de recherche de temps de Pose
Next iter. -> tempsDePose=50
Max= 10630
Next iter. -> tempsDePose=73
Max= 13084
Next iter. -> tempsDePose=86
Max= 14443
temps de pose trouve !!
tempsDePose FINAL=86ms , iter= 3
Lecture image 1
Lecture image 2
Lecture image 3
Lecture image 4
Lecture image 5   -> 2 minutes 11 sec

The timing is also Ok....
The test has been done also with the PC we intend to use for the event : old Pentium 133, RAM 32Mb, nothing outstanding at that level...

Once arrived into the observing place at Vouzier, just during totality phase, the sun/moon needed to be properly centered into the frame and to launch the script.

We were lucky, the sky was clear, a nice blue sky appeared washed by a full night of rain !
During these 2minutes and 17sec, we have fully gotten advantage of the show : OUTSTANDING !
I did not have a look to the PC's screen, if something fails, I would have not done anything during the totality phase.

Once the event was over, this is what I could read from the log file :

test de recherche de temps de Pose
Next iter. -> tempsDePose=50ms
Max= 972
Next iter. -> tempsDePose=1150ms
Max= 6017
temps de pose trunque au max!!
tempsDePose FINAL=2000ms , iter= 2
Lecture image 1   -> 11/08/1999 - 10h25m54s
Lecture image 2   -> 11/08/1999 - 10h26m27s
Lecture image 3   -> 11/08/1999 - 10h27m00s
Lecture image 4   -> 11/08/1999 - 10h27m32s
Lecture image 5   -> 11/08/1999 - 10h28m05s

The image rate was one image every 32secondes, CCD readout time is 22secondes, 10 secondes have been lost :
to display the image, save the file, launch the next exposure, wipe the CCD, apply an exposure time of 2sec.

Under excitement, I save the output log and we cheers each others for this success, the images looks nice.
Once after I came back home, I've noticed the following points:

Despite all the small failures everything went fine...

Last image at 10h28m05s, the 3rd contact has taken place at 10h27m46s, this image had been recorded 20 too late, see the result !

Sum of the 1st and last image , the moon's edge had moved during 1m40s of 5 pixels, yielding to 35 arcsec.

Raw images analyzis

Raw image of the totality phase, at 10h27m32s TU

The SAME raw image with different display cuts ! This shows are large is the CCD dynamic

Again the same image, jets are visible, due to the CCD large resolution  !!

Again the same raw image, a display log scale has been applied .

Comments about exposure time and corona photometry

A close inspection of the images shows that the jets are almost all saturated (14 bits CCD camera= 16383 ADUs).
The following plots shows various radial profiles (ADUs) : Profile #1 includes a jet, profile #2 is a mean profile and profile #3 is taken accordingly to the Sun's magnetic poles

This LOG scale plots show the wide dynamic of intensity throughout all profiles. One can notice a jet saturation (protuberance) and the full moon's sees intensity matches to the corona intensity nearby the Sun
The useful signal for processing is extending up to 1.5 Solar radius, intrinsic CCD camera dynamic is about 12.6 bits. 
This is strange because literature foreseen that a 13 bits dynamic should be able to reach the corona up to 3 Solar radius !
One solar radii yield 138 pixels, and the largest corona feature is visible up to 300 pixels (SNR =3) which is 2.2 Solar radii. Sky background brightness is about 57 ADUs, which means we are more limited by CCD readout noise than intrinsic sky background photon noise.
To gain more fainter features inside the corona, a fast 16 bits low noise (5e-) CCD camera would have been great, may be next time !

Above, same plots as previous, but linear scale, the three plots intensity variations are emphasized.

What about the faint inner moon light ? :
Unfortunately, any image shows the faint inner moon light, event by tightening the display cuts, a weak gradient light is also contaminating the inner part of the eclipse. Sky background is 15ADUs in this area. All the optical parts were cleaned before and the sky was pretty pure. Radial filter would have helped.
However digital image processing can make fun of it....

Eclipse CCD image, cuts (brightness/contrast) has been set in such a way to check whether moon light was visible in the center (note, instead, diffusion toward the center)  !
Click over image

"Faking" made of a full moon combined with eclipse image ! easy, no ?
Click over image

Have a look here to an image made of overexposed films

Some polarimetry

Using a 300mm photo-lens and a CCD camera, we wanted to measure has far the corona was polarized.The experiment consisted in recording automatically solar images thru a polarized filter. The only manual action to be done was to rotate the filter manually. This sounds easy in a normal situation, it was not the case : due to the high excitation atmosphere during totality phase, we, at some point, missed to forget to rotate it.

image_polarisant.JPG (12439 bytes)
Dual image composition, for each image the filter has been rotated of 90°

These results shows that the corona is highly polarized, which indicates that most of the light is emitted by dust specular reflection. This is a well known phenomena that can be demonstrated with inexpensive amateur means.

Image Processing method

sbrute.JPG (6147 bytes)
Raw Image

Image display improving :

The goal here, is to take advantage of the high dynamic of the CCD, trying to show in a same image the protuberance (jets) area close to the bright Sun's edges and the very weak large scale features. In that way, several methods can be applied :

- "Log scale", or histogram equalization, .... which allows to see at the same time bright and faint areas. The main problem using these method is related to the fact that it kills the image contrast, increase the noise, the fine structures will also disappear.

slog.JPG (8379 bytes)
Image after "log" processing

- Rotational gradient, it consists in performing the difference between the original image and the same rotated of a given amount. This method is not really suitable in the sense that it filters/cancel out all details/features which the frequency is related   to rotation angle. To improve things, one can combine images that had been processed with rotational gradient having different angles, but in any case interesting features are lost, such as jets and the outcoming image does not look really realistic.

srotationnel.JPG (9374 bytes)
Image after rotational gradient

- A most interesting method is to remove the radial component by doing the median or the mean of many images. We aim at subtracting the continuous corona light decrease outward the center. For each pixel having a given distance form Sun's center, its value is set according to a median coming from all the same pixels at the same radial distance. The initial image is divided by this model gotten from the previous algorithm.
This method provides good results, but "stairs like" features (or quantification) problems appears. If one applies a low pass filter; thinks goes better. It looks attractive, but another means need to be found.

- Computed model of the Corona : this method consist of performing a computed model of the corona by mathematical functions. The functions parameters has been fitted to the real image according to a less-square method. We tried many set of functions such as exponential, exponential multiplied by a polynom, etc....
The resulting curve is smooth and continuous and furthermore, this set of functions fits properly the corona. This is finally the method we used to remove the radial gradient, making easy the unsharp masking process.
modeles.JPG (4319 bytes)
sbrute-modele.JPG (17505 bytes)
Corona model
Raw image, divided by corona model


wpe1.jpg (33120 bytes)
Corona model cross section

Final image sharpness improvement :

In order to improve image sharpness, unsharp masking filter could be applied. The Signal to Noise Ratio being higher when getting closer to the Sun's edges, in this case the strength of the unsharp-masking process can be decreased according to the distance to the Sun's edges. This is, in this case, radial unsharp-mask. Once, having computed previously the model of the corona intensity, this model can be used to fed the unsharp mask strength according to the image pixel position. This one will be proportional to the square root of the local intensity (or flux), and thus to the Signal to Noise ratio.

Finally, it looks clear that a set of image processing features must be coded/written to take the best advantage of the specific shape/signal from a Total Solar Eclipse image and had consisted hereafter as  :
 - Corona model coding
 - Radial unsharp masking coding, according to the radial corona intensity.


A large combinaison of factor helped to get a successful experiment :

Finally, it is clear that things can be improved, like making this experiment more portable over the world. The use of radial filter coupled with a CCD could be extremely efficient. In any case, this is a very exiting experiment and that's really a shame that total solar eclipse are so short and so rare...

Back to my homepage

References :

1 : C.Buil email available in the AUDE-mailing list archive about CCD experiment on Solar Total Eclipse in Guadeloupe.
2 : F.Clette paper issued in "Ciel et Terre" vol 115 p 92-100
3 : S.Koutchmy paper issued in "Ciel et Terre" vol 115 p 82-91
4: Radial filter use in photography

December 1999, by L.Bernasconi, C.Cavadore and Boris Gaillard