Home page
Solar Archives
  • Year :
  • Synoptic maps :
  • Videos
  • Maunder's Diagram
  • Cycle 23 in images
  • Venus Transit 2004
Photos album
Spectroheliograms processing

 

Spectroheliograms Hα

In a general way spectroheliograms are degraded by several kinds of disturbances. They require some process to restore the image geometry, amplifiy the poor contrasted details and obtain a more aesthetic result. I thus ended to a little bit empirical method which answers globally the problem. These modifications do not respect the photometry of the image.

Here are the stages of the processing :

  1. Raw image. (Spectroheliogram on 15 september 2003)


  2.  
  3. Hi-pass filter (5x5 matrix)


  4.  
  5. Calculation and extraction as images of the horizontal and vertical defects :
    We calculate the mean value of the pixels for each line (white curve) then the polynomial of degree 4 the best fitted to this profile. The difference between these 2 profiles gives the orange curve, which is used for creating the image of the defects in a direction (A). The operation is also made for columns (B).


  6.  
  7. The sum of these 2 images (A & B) multiplied by the filtered image gives the result below. Horizontal lines have several causes : irregularities and dusts on the slit of the spectro, dusts on the CCD sensor, irregular response of photosites... Vertical lines result from the electronics or from the variations of transparency of the atmosphere. In 2 cases, the presence of contrasted structures (spots, etc.) can generate over--corrections.

  8.  
    (
    +
    )
    x
    = ...

     
  9. This image which contains essentially the amplified defects (a kind of synthetic "flat") is then balanced (% of transparency) and subtracted from the filtered image. The adjustment is visually made up to maximal reduction of streaks.


     
  10. After histogram adjustment, the spectroheliogram look like this :


  11.  
  12. The following stage consists in circularize the solar disk. Indeed, there is always a distortion caused by the speed of sampling which is not necessarily synchronous with the speed of scanning of the Sun.


  13.  
  14. It still remains to delete the "sky background" which is more marked in the center of the disk in the direction of the CCD lines. An synthetic image is generally used to make this correction. As for the stage of "straks cleaning", this image is balanced and subtracted from the previous image.


  15.  
  16. Here is the result of this "sky background" correction. The image in the center of the Hα line - except filaments and active regions - is rather flat.


  17.  
  18. The last stage aims at increase the contrasts on disk and prominences. For that purpose, I select only the disk (muliplication by itself) to lower the luminosity of the disk and strengthening the contrasts. Then the histogram of all the image is adjusted to clear up and harmonize at the same moment disk and prominences.


  19.  
  20. Optional process when prominences are interesting : the "coronograph-like" image is obtained by placing a circular mask on the image of the stage 10. Colorization and intensity adjustment with image processing software.


  21.  

Spectroheliogram in Helium D3 line

The Helium D3 spectral line (587,8 nm), quite visible in emission on the prominences, is difficult to perceive on the solar disc where it can be in emission or absorption. After having tried some spectroheliograms with this wavelength, I could note the presence of absorbing structures but with extremely weak contrast. After reading an article of B.J. Labonte (1), I tried to improve the appearance of these absorbing structures. The method consists in taking one image in the helium line, another image near this wavelength in the continuum, and to make the difference of the 2 images. (Ideally, it would be necessary to make average images for each wavelength but, the Sun rotate quickly and it becomes impossible to superimpose images taken with 1/2 h interval). I thus took 2 images with 4 minutes interval, one at 587.6 nm and the other to approximately at 587.3 nm. (Images taken on 14 september 2002).

The first step consists in superimposing the 2 images. This is not perfectly done because turbulence causes small random displacements. I thus " smooth " a little bit the images as a preliminary and I eliminate the background of the sky, source of unaesthetic artifacts. The operation "Helium image - continuum image + constant" can be then processed. One obtains a gray image, rather "flat". Second step constists in modification of contrast and brightness to make details more visible. Many artifacts are also increased but it is interesting to compare this image to those taken in Hα or Ca-K lines. Below, left : He amplified and right : Hα.


 

Below : Direct imaging with spectroheliograph and webcam. NOAA 0775 and 0776 on 14-06-2005


 

(1) Labonte B.J., 1977, Solar Physics, 53, 369-374

haut de page