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 :
Raw image. (Spectroheliogram on 15
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).
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
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.
After histogram adjustment, the spectroheliogram
look like this :
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.
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
generally used to make this correction. As for the stage of "straks
cleaning", this image is balanced and subtracted
from the previous image.
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.
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.
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.
Spectroheliogram in Helium D3
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