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Creating monochromatic solar images
with spectroheliography
Eline Steijlen
6271391
July 23, 2014
Supervisor: Prof. Dr. Huib Henrichs
Second reader: Prof. Dr. Alex de Koter
Verslag van Bachelorproject Natuur- en Sterrenkunde, omvang 15 EC,
uitgevoerd tussen 01-04-2014 en 18-07-2014
Anton Pannekoek Observatory
Faculty of Science
University of Amsterdam
Samenvatting
De atmosfeer van de zon bestaat uit drie lagen. De fotosfeer is de binnenste laag, daarboven ligt de chromosfeer
en de corona is de buitenste laag. De fotosfeer is het gedeelte waar het zichtbare licht van de zon vandaan komt. De
andere twee lagen kunnen ook gezien worden vanaf de aarde, maar alleen tijdens een zonsverduistering. De gehele
atmosfeer kan daarentegen wel worden waargenomen en daarmee kan de activiteit van de zon worden weergegeven. Dit
is mogelijk door afbeeldingen van het oppervlak van de zon te maken. De wisselende temperatuur op verschillende delen
van het oppervlak is gekoppeld aan de activiteit van de zon in die gebieden. Donkere en lichte plekken corresponderen
met respectievelijk koude en warme gebieden. Zonnevlekken zijn een voorbeeld van deze koude gebieden, waar de
temperatuurverlaging wordt veroorzaakt door een sterke magnetische activiteit. De afbeeldingen van de zon kunnen
gemaakt worden in één bepaalde golflengte van het zonlicht, waardoor de activiteit van de zon alleen te zien is in die
bepaalde golflengte. De zon is hieronder afgebeeld in Hα, gelegen op 6563 Å.
Deze afbeeldingen kunnen gemaakt worden met een methode genaamd spectroheliografie. Een moderne spectroheliograaf bestaat uit een telescoop, een spectrograaf en een CCD camera. Het licht van de zon valt eerst op de telescoop en
wordt op de spleet van de spectrograaf geprojecteerd. Daar wordt het gescheiden in verschillende golflengtes, zodat het
spectrum van het zonlicht waargenomen kan worden. Van het spectrum wordt een smal golflengtegebied geselecteerd
rond de gewenste golflengte en met de CCD camera vastgelegd. Door nu een scan van het zonsbeeld te maken wordt
telkens een spectrum van een verticale strook van de zon gemaakt. Ieder spectrum wordt zo verkregen van een klein
stukje van de zon, dus voor een afbeelding van de hele zon zijn veel spectra nodig. Uit al deze spectra wordt alleen het
stukje van de gekozen golflengte gehaald, waarmee het totale monochramatische zonsbeeld wordt gereconstrueerd.
Voor dit bachelorproject zijn er afbeeldingen van de zon gemaakt met behulp van spectroheliografie. Daarvoor is er een
spectroheliograaf op het Anton Pannekoek Observatorium(APO) op de Universiteit van Amsterdam gebouwd. Dit was
een groot experiment, omdat de eigenschappen van alle instrumentele delen zeer belangrijk zijn om de kwaliteit van
de spectrograaf te optimaliseren. Daarbij ontstonden er tijdens het bouwen veel technische en mechanische problemen.
Toch is het gelukt om een spectroheliograaf te bouwen op het APO en om een afbeelding van de zon te maken. De
beste afbeelding, gemaakt in helium(5875 Å), bevat een aantal zonnevlekken dat overeen komt met de hoeveelheid zonnevlekken op een gevonden afbeelding van de zon,gemaakt op dezelfde datum. De afbeelding is daarentegen niet scherp
genoeg om nog andere details te kunnen zien, dus er zijn nog veel verbeteringen mogelijk.
Astronomy & Astrophysics manuscript no. Spectroheliography
July 23, 2014
c
ESO
2014
Creating monochromatic solar images with spectroheliography ?
E.D. Steijlen
Anton Pannekoek Institute for Astronomy, University of Amsterdam, Science Park 904, 1098 XH Amsterdam, The Netherlands
July 23, 2014
ABSTRACT
Context. Among the many phenomena visible at the solar surface are sunspots, prominences, filaments and faculae, which represent
the always changing activity of the Sun. These features can be observed by displaying the Sun at different wavelengths. With the
technique of spectroheliography, monochromatic images of the Sun can be constructed, rather than using expensive narrow-band
filters.
Aims. With the high-resolution spectrograph at the Anton Pannekoek Observatory (APO) attached to a refractor, spectroheliographic
images of the Sun were aimed to obtain at various wavelengths.
Methods. After several experimental setups, the final setup was achieved with a 70 mm telescope, the LHIRES slit spectrograph (R =
17000) and with an Atik 460EX CCD detector. Series of spectra were taken around Hα and He i λ5875, from which a monochromatic
solar image could be constructed.
Results. A working spectroheliograph was obtained, with which several usable images around Hα and He i λ5875 have been recorded.
It was found that focusing of the telescope is the most critical parameter.
Conclusions. This experiment turned out to be much more difficult than anticipated. The choice of refractor, focal length and CCD
properties had to be tuned carefully. The conclusion is that spectroheliography is feasible at APO.
1. Introduction
1.1. Physical properties of the Sun
The Sun consists of a core, a radiation zone, a convection zone
and the atmosphere, see Fig. 1. The core has a temperature of
about 15 million K. In the radiation zone energy from the core
is transported outwards. In the convection zone large currents of
hot plasma, or ionized gas, carry heat upward by convection1 .
The atmosphere, the observable surface of the Sun, is divided in
three layers, with a surface temperature of 5800 K. The lowest
layer is called the photosphere, the visible layer of the surface
of the Sun, from which most of the radiation reaches the Earth.
The cell structure is the solar granulation, which can be seen
in the photosphere. This bubbly look is caused by changes of
the temperature2 . Just above this region lies the chromosphere
and the upper layer is called the corona. These two layers
also emit visible light, but they can only be seen during a
solar eclipse. The upper chromosphere is a region where
the density of the matter decreases and where the magnetic
pressure dominates the pressure of the gas. The plasma here
is displayed by the magnetic field lines, so the structure of the
upper chromosphere is governed by the magnetic fields3 . In the
corona charged particles flow outwards, carried away by the
solar wind. A measure of the activity of the Sun is the amount
of sunspots and faculae. Sunspots are dark spots with respect
to the photosphere that can be seen at the surface of the Sun.
They are associated with strong local magnetic activity. This
magnetic activity inhibits convection whereby the temperature
in these areas will be cooler than their surroundings. Faculae
are bright regions on the surface of the Sun, which seem to
appear before the development of sunspots and they persist after
?
Based on observations obtained at the Anton Pannekoek Observatory of the Faculty of Science of the University of Amsterdam, located
at Science Park, Amsterdam, and operated by the Anton Pannekoek Institute.
sunspots disappeared (Aller1953). The chromospheric part of
a facula is called a plage, where the magnetic field has a 10 to
100 times larger density than the surroundings. Irregular dark
strings are called filaments and mark opposite polarity on the
surface, where prominences are non-uniform filaments seen on
the edge of the solar disk (de Koter). Solar flares, sudden intense
brightenings in the corona, are usually seen in the vicinity of
sunspot groups4,1 .
Fig. 1.
Schematic overview of the Sun. From the inner layer: the
core, radiation zone, convection zone, photosphere, chromosphere and
the corona
1.2. Spectroheliography
In principle, the best way to create monochromatic images is
with narrow-band filters. These give the highest quality images. However, the prices of these filters vary between 4.000
Article number, page 3 of 11
and 14.000$. A practical disadvantage is that for each different
wavelength, a different filter has to be used. As an alternative,
the technique of spectroheliography allows to create a 2D image
of the Sun, by using a high-resolution slit spectrograph. With
this instrument, monochromatic images can be obtained at arbitrary wavelengths5 .
The first spectroheliograph was invented in 1890 by Henri Deslandres and George E. Hale, see Fig. 2. They built this instrument at Meudon Observatory in Paris, see Fig. 3. With the spectroheliograph, solar prominences could for the very first time
be photographed without the need for a total eclipse of the Sun2
(Deslandres 1894). A modern spectroheliograph mainly consists
of a spectrograph with an entrance slit to obtain spectra of the solar disk, which passes over the slit. Afterwards software is used
to create a monochromatic image from these series of spectra6,7 .
2. Theoretical background
2.1. Principle of spectroheliography
Spectroheliography is a method to create monochromatic images
of the Sun at different wavelengths. The most common way is to
orient the slit perpendicular to the daily motion of the Sun and
let the Sun move across the slit while taking spectra around the
chosen wavelength, see Fig. 4. Since only a narrow vertical slice
of the Sun is projected on the slit, the height of the spectra of
different parts of the Sun will not have the same size. The length
of the spectral lines at the limb of the Sun will be smaller than
the lines in the middle of the Sun, see Fig. 5 and Fig. 6 respectively. The length of the spectral lines will increase towards the
middle of the Sun and will decrease again towards limb, so that a
circular disk will be restored. Then, a solar disk image can be restored from slices of the spectra by collecting the same columns
of pixels of all spectra and by putting these together.
Sunspots will appear as dark lines that cross all columns of the
spectra, see Fig. 7. Slit impurities will appear as horizontal lines,
perpendicular to the dispersion.
Fig. 2. Left panel: Deslandres, right panel: George Hale.
Fig. 4. The orientation of the slit perpendicular to the daily motion of
the Sun.
Fig. 5. Spectrum obtained around He i λ5875 at the limb of the Sun
with the height of the slit on the horizontal axis and the width of the Sun
on the vertical axis.
2.1.1. Spectral lines
Fig. 3. A drawing of the first spectroheliograph built in Paris (1890).
From the top: telescope, spectrograph, prism that disperse the sunlight
and the place where the spectrum is displayed.
Article number, page 4 of 11
Some interesting spectral lines to observe are Hα, Hβ,
He i λ5875 and the Calcium II K line near 3934 Å, from which
6563 Å and 4861 Å are the corresponding wavelengths of Hα
and Hβ. The Calcium II line is unfortunately outside the sensitivity range of the CCD.
Images obtained at Hα show the upper chromosphere. The
magnetic fields reveal the structure of this part of the atmosphere,
Steijlen: Spectroheliography
spectral line is not present in the photosphere, because no helium
lines fall within the visible spectral range at 6000 K (de Koter).
Images of the observable surface of the Sun at different
wavelengths depend on the depth of the photosphere. The
outermost layer of the Sun consists mostly of hydrogen. This
means that light from the inner layers will be absorbed if the
wavelengths correspond to the energies of hydrogen. Hence,
at the line center of Hα the only part that can be seen is the
outermost layer, because of the absorption of the light of the
inner layers. That is why a strong absorption line appears at
the center wavelength of Hα. Absorption lines around Hα are
weak, because light from the inner layers of the photosphere
will not be absorbed and information of these inner layers
will be gained. Fig. 8 shows the dependence of depth and
wavelength. Prominences, which stick out of the disk, will
appear as emission lines3,5 .
Fig. 6. Spectrum obtained around He i λ5875 in the middle of the Sun
with the height of the slit on the horizontal axis and the width of the Sun
on the vertical axis. .
Fig. 7. Spectrum obtained around He i λ5875 with the height of the
slit on the horizontal axis and the width of the Sun on the vertical axis.
A sunspot appear as the dark line crossing all columns of the spectrum.
and prominences, filaments, vortices, faculae etc. can be seen.
Images obtained near the core of Hα show a lot of detail of the
Sun. Notably granulation will be visible, of which the structure
changes near sunspots because of the lower temperature, including solar vortices. Sunspots are still visible at this wavelength.
This exhibits the middle layer of the chromosphere. Granulation
and solar vortices are less visible in images created at the center of the Hα line and small sunspots seem erased. Solar flares
can be observed better at this wavelength and prominences at
the limb of the Sun are visible. The chromospheric emission
of plages can also be observed in Hα, provided that the spatial
resolution is not too high (de Koter). Filaments are visible in
emission and absorption (Aller1953). The Hβ line also shows
filaments and prominences in the upper chromosphere, but with
lower contrast. It also shows broad dark areas around active areas and a dark chromospheric network. The Calcium K lines
show a profile of absorption and emission, which results from
various levels of the chromosphere. The emissive part of this
line reflects the temperature and shows faculae, which can particularly be noticed in the vicinity of sunspots. Faculae can also be
seen without sunspots. This probably means that new sunspots
appear or that sunspots disappear in this area. Other bright points
mark the outlines of the cells of granulation. He i λ5875 is visible in emission in the prominences, in emission and absorption
in the filaments and shows sunspots as well3 (Aller1953). This
Fig. 8. The depth information around Hα. The arrows show that a
strong absorption line, like the center of Hα, gives information of the
outermost layers of the photosphere and a weak absorption line gives
information of the innermost layers.
3. Experiment
3.1. Experimental setup at APO
The observations were carried out at the Anton Pannekoek
Observatory, Amsterdam. Firstly, the observations were attempted on the roof of the faculty building at Science Park.
This did not work, as the Gemini mount could not follow the
Sun automatically with the necessary relative drift. In addition,
no filter was used, which damaged the slit because of the heat
excess. (The Shelyak company was so kind to replace this slit
free of charge.) Therefore, for all subsequent observations the
spectroheliograph was moved to the solar dome of APO, which
contains a very well controllable 10micron GM4000 mount, and
a protected environment against wind and excess of solar stray
light.
Several experimental setups were attempt to build a spectroheliograph. To optimize its quality, all properties of all instrumental
parts are important. The available telescopes are listed in table 1.
The CCD cameras available at APO are listed in table 2.
The angular resolution gives the smallest detail that can
be seen. This is given by the Rayleigh criterion: θ = 1.22λ /D,
which is around 200 at Hα for a 104 mm diameter telescope.
In practice however, the angular resolution is limited by the
Article number, page 5 of 11
seeing conditions, which is typically around 300 . This implies a
pixel size of 7 µm in the vertical direction along the slit at the
focal plane. According to the Nyquist criterion the minimum
sampling is 2 pixels per resolution element, which requires a
pixel size of 3.5 µm. The solar disk measures 180000 , implying
that at least 600 resolution elements in the vertical direction
along the slit are needed. According to the Nyquist criterion this
corresponds to 1200 pixels in the ideal case. To obtain the same
resolution in the horizontal direction, 1200 spectra are needed
to cover the solar disk.
3.1.1. Spectrograph
To obtain a spectrum of the Sun, the LHIRES III spectrograph
was used. LHIRES III is a high resolution spectrograph with
a resolution of R = 17000, or 0.47Å at Hα, designed for amateur and educational astronomy. Incoming light passes a slit,
with height 7 mm, and will be send through the collimator lens
via a mirror. Thereby, the spectrograph has switchable grating modules of 2400 lines/mm in standard, 1200 lines/mm, 600
lines/mm, 300 lines/mm and 150 lines/mm. For this observation
the grating module of 2400 lines/mm is used, so that a very small
part of the spectrum could be observed. An important thing to
mind is that LHIRES III is build for an f /10 instrument. This
means that for a telescope with a diameter of 70 mm, the focal
length should be 700 mm to avoid light losses8,9 . See Fig. 9 for
a schematic overview of the LHIRES.
Focusing the telescope was done by pointing the telescope
to a point considered to be at infinity. Then, the telescope was
pointed to the Sun and was focused again at sunspots and on
the edge of the Sun. To obtain spectra, the slit was put on the
West side of the Sun, because of the daily motion from East to
West, with a slit orientation from North to South. In front of the
objective of the telescope a solar filter of neutral density ND = 5
was placed, giving a 105 reduction of the intensity of the light to
enable a safe vision by the human eye to look at the Sun. This
filter is used for placing the Sun near the slit by looking through
an eyepiece.
The daily motion of the Sun is 360◦ in 24 hours. The diameter of the Sun is 0.5◦ , which is observable in 2 minutes.
This means that the daily motion of the Sun is 0.25◦ or 90000
per minute. The motion of the telescope was therefore set at
10000 per minute, i.e. slower than the daily motion of the Sun.
The program Autostar Envisage was used to record the spectra.
The single exposure time was chosen to be 4 seconds to obtain
sufficient signal. The waiting time is chosen to be 0 seconds,
because the observation had to be continuous. With this setup
the total time appeared to be 20 minutes, giving 300 frames,
which is actually insufficient because 1200 frames were needed.
A Mathematica (Wolfram) program was used to extract a
slice, consisting of a chosen wavelength range with one or more
columns of each spectrum. Then, these columns were collected
and an image of the Sun was created with all extracted slices
along the x-axis. The range on the x-axis consisting of Hα is
chosen to be 10 pixels. This gives a chosen wavelength range of
4.7 Å with a resolution of 0.47 Å per pixel.
The first result is shown in Fig. 10. It appeared that the
full disk did not fit in the slit, which means that the focal
length was too long. Second, a lot of stray light made parts of
the image unusable. Third, this camera could not be cooled,
causing a very significant background noise, which could not be
removed by dark frames. So the first required improvement had
to be replacing the CCD camera.
4.2. Second setup
Fig. 9. A schematic overview of the LHIRES. From the top: Incoming
light that passes a f /10 telescope, passes the slit in the spectrograph and
goes via the mirror and the grating module to the CCD camera.
4. Observations
4.1. First setup
For the first experimental setup the LHIRES III spectrograph
was attached to the Robtics refractor. The used detector was the
Meade Deep Sky Imager PRO II (DSI). This refractor gives a
solar image of 7 mm diameter, which appeared just too small to
fit in the slit height. In addition, the minimum pixel size had to
be 3.5 µm (see above), which means that the pixel size of this
camera is actually too big for a good resolution. However, a first
observation was done to create an image at Hα.
Article number, page 6 of 11
For the second observation the Deep Sky Imager Pro II was replaced by the Atik 460 EX CCD camera. Its pixel size of 4.5 µm
comes close to 3.5 µm, which is required for the seeing limited
resolution of 300 . This time, the fact that LHIRES is build for a
f /10 telescope was taken into account by putting a diaphragm
on the telescope to reduce the diameter from 104 mm to 70 mm.
The program Maxim DL was used to record the spectra of the
Sun. With this program a sequence of observations can be automated. The exposure time needed to be to be 4 seconds. Since
the readout time is 6 seconds, the total time was too long to give
a sufficient amount of spectra during the crossing time of the
Sun. Therefore the binning at readout was adapted to 2×1 for a
shorter readout time.
The second result is shown in Fig. 11, a result after two observations. Also with this observation not the whole size of the
Sun could fit within the slit, because of the focal length of the
telescope and the size of the CCD chip. A f /6.3 focal reducer
was attempted to be inserted in the beam, but no focus could be
achieved due to the limited backfocus of the telescope. Also,
Steijlen: Spectroheliography
Table 1. Telescopes suitable for spectroheliography available at APO
Robtics refractor
William Optics
Diameter
mm
104
70
Focal length
mm
700
420
Solar image
mm
7
4.2
Angular resolution at Hα
pixels
765 × 510
1600 × 1200
752 × 582
2750 × 2200
pixel size(µm)
9
7.4
8.3 × 8.6
4.54
00
2.1
1.7
Table 2. Available CCD cameras at APO suitable for spectroheliography
SBIG ST7
SBIG ST2000
Meade DSI
ATIK 460 EX
CCD size(mm2 )
6.9 × 4.6
11.8 × 8.9
5.6 × 4.7
12.5 × 10
Fig. 10.
The created image of the Sun in Hα with a wavelength
range of 4.7 Å after the first observation with the 100 mm refractor and
the DSI camera on 25/04/2014. On the horizontal axis: the amount of
pixels. On the vertical axis: the size of the Sun.
still an insufficient amount of spectra could be obtained with this
setup.
Fig. 11.
The created image of the Sun in Hα with a range of 100
pixels (370-470) on the x-axis, which gives a wavelength range of 47 Å
and the image is normalized at spectrum number 184. On the horizontal
axis: the amount of pixels. On the vertical axis: the size of the Sun. The
image is created with the final setup: the 104 mm telescope and the Atik
460EX CCD camera on 05/05/2014.
4.3. Third setup
With a total exposure time of 20 minutes the amount of spectra
that could be obtained was 250, which is still less than the 1200
needed.
For the third observation the Robtics refractor was replaced by
the William Optics telescope, which came with the just installed
VU Meade telescope. On this telescope a solar filter of neutral density ND = 3.8 was placed, with reduction factor 103.8 .
This filter is especially made for photographic purposes, and was
needed to shorten the exposure time to allow for more spectra in
the same amount of time. In addition a grey neutral density filter
that reduces sunlight to 18 % of the initial intensity was improvised and placed before the eyepiece. With these two filters the
intensity was reduced with a factor of 100000, which is safe for
a human eye to look at the Sun. Fig. 12 shows this final experimental setup.
The program Maxim DL was used to record spectra of the
Sun. The exposure time was chosen to be 0.2 s with a binning of 2×1 for a shorter readout time, which became 4 seconds.
The first created solar image with this experimental setup is
an image created at Hα, see Fig. 13. The Sun is clearly not in focus, and has an oval shape (which could be adjusted by choosing
a different aspect ratio). There were clouds during the observation, visible as black vertical lines. The vague horizontal line
represents dust on the slit. Before the observations done to create this solar image at Hα, eight observations were done earlier.
Recommended improvements are focusing the telescope better
and optimize to 1200 spectra by reducing the readout time and
get a circular shape. For the first improvement, in Maxim DL
a subframe can be used, where only the spectra of the Sun will
be downloaded, not including space above or below the spectra.
To obtain a circular shape the aspectratio between the x-axis and
y-axis can be adapted or the slit orientation has to be corrected
to obtain an orientation exactly from North to South.
Article number, page 7 of 11
motion of the Sun again. Most of the correction is done in the
Mathematica (Wolfram) program where this time the range on
the x-axis consisting of He i λ5875 is chosen to be 50 pixels for
collecting the columns. This corresponds to a wavelength range
of 23.5 Å centered at 5875 Å. Creating a solar image at Calcium
II is unfortunately not possible with the final setup, because this
line lies outside the sensitivity range of the CCD.
If there were more images created like the one in Fig. 15 within
two months, one will see that the location of the sunspots on the
solar disk will change. With such a series of images, the spin
rate of the Sun can be determined. Unfortunately, these series
could not be made because of time limitation.
Fig. 12. The final experimental setup. The spectroheliograph consists
of the William Optics telescope with solar filter, the LHIRES III spectrograph, the ATIK 460 EX CCD camera and an eyepiece with 18 %
filter.
Fig. 14. A small slice of the solar disk at He i λ5875. The dark spot
is a sunspot on the solar disk. On the horizontal axis: the amount of
pixels. On the vertical axis: the size of the Sun.
Fig. 13. The image of the Sun in Hα with a range of 10 pixels (690700) on the x-axis, which gives a wavelength range of 4.7 Å. On the
horizontal axis: the amount of pixels. On the vertical axis: the size
of the Sun. The image is created with the final setup: the 70 mm f /6
refractor and Atik 460EX CCD camera on 23/06/2014.
4.4. Further observations at APO
In the He i λ5875 line the sunspots are prominent, which are the
easiest to focus on. So, spectra were recorded around this spectral line. In practice it is difficult to judge through the eyepiece
if the telescope is in focus. So, at first, images of small slices
of the solar disk were created, to see if sunspots were in focus,
see Fig. 14. Then, with the Sun at best focus, nine images were
created at this wavelength were the last and the best image can
be seen in Fig. 15. The Sun is much better in focus and at least
6 sunspots can be seen. This image can be compared to an image created at Calcium II by Ph. Rousselle on the same date,
see Fig. 16 10 . All sunspots in Fig. 15 seem to accord with the
sunspots in Fig. 16. In Fig. 17 a small part of both images is enlarged to see the sunspots in detail. The dark spot on the image
of APO seems just one sunspot, but in the image of Ph. Rousselle can be seen that it contains two separate sunspots. Therefore the sunspots in Fig. 15 are less focused. The faculae visible
in the image of Rousselle are not visible in the image of APO,
but these will not be visible in an image created at He i λ5875.
There is also corrected for the oval shape of the Sun, although
the slit orientation is probably not exactly perpendicular to the
Article number, page 8 of 11
Fig. 15. A monochromatic image of the Sun created at He i λ5875 with
a range of 50 pixels (350-400) on the x-axis, which gives a wavelength
range of 23.5Å. On the horizontal axis: the amount of pixels. On the
vertical axis: the size of the Sun. The image is created with the 70 mm
f /6 refractor and Atik 460EX CCD camera on 03/07/2014.
Steijlen: Spectroheliography
Fig. 18. A monochromatic image of the solar disk at Helium where
the East side is located on the right, created by Jean-Jacques Poupeau
on 20/03/2014.
Fig. 16. A monochromatic image of the Sun created at Calcium II,
created by Ph. Rousselle on 03/07/2014. Here, the East side of the Sun
is located on the left.
Fig. 17. On the left: an enlargement of a dark spot seen in the image
at He i λ5875 created at APO. On the right: an enlargement of a dark
spot seen in the image at He i λ5875 created by Ph. Rousselle.
5. Conclusions
The main conclusion is that spectroheliography is feasible at
APO. All optical elements have been fine tuned to obtain
monochromatic images. This required considerable mechanical efforts. On the image created at APO, the sunspots that appear on 03/07/2014 accord to the sunspots in the image of Ph.
Rousselle on the same date. There is, however, still considerable
room for improvement. See for instance images created near
Paris by the amateur Jean-Jacques Poupeau, see Fig. 18, 19 and
20, all created on the same date with his own spectroheliograph,
see Fig. 21. A next step would be to obtain sunspots, filaments,
prominences and faculae like in the images of Jean-Jacques Poupeau. To see more detail at the surface of the Sun and thereby to
see more activity of the Sun, clearly more observations have to
be done.
Specifically the following recommendations can be made:
(1) improvement of the focusing procedure, by using a webcam
instead of an eyepiece
(2) shorten the readout time by further binning
(3) slowing down the drift rate of the sun across the slit. This
would require a different setting of the 10micron mount, which
may be available in a future firmware update.
Fig. 19. A monochromatic image of the solar disk at Hα where the
East side is located on the right, created by Jean-Jacques Poupeau on
20/03/2014.
Fig. 20. A monochromatic image of the solar disk at Calcium II where
the East side is located on the right, created by Jean-Jacques Poupeau
on 20/03/2014.
Article number, page 9 of 11
Fig. 21. The spectroheliograph built by the amateur astronomer JeanJacques Poupeau. It consists of a 120 mm Newton telescope with a focal
length of 1200 mm protected by a tube. An Ebert-Fastie spectrograph
was attached to this telescope with a Skynyx camera and an eyepiece.
This whole setup is motorized to follow the motion of the Sun11,12 .
Article number, page 10 of 11
Steijlen: Spectroheliography
6. References
Alex de Koter, Stellar Atmospheres and Radiative Transfer, 222
Henri Deslandres, 1894, Mémoires et observations, Recherches photographiques sur les flammes de
l’atmosphère solaire, 55-74
Lawrence H, Aller, 1953, Astrophysics, The Atmospheres of the Sun and Stars, 355.
356
(1) http://www.britannica.com/EBchecked/topic/573494/Sun
(2) http://solar-center.stanford.edu/hidden-pic/photosphere.html
(3) http://www.astrosurf.com/spectrohelio/observation-shg-en.php
(4) http://science.nationalgeographic.com/science/space/solar-system/sun-article/
(5) http://www.astrosurf.com/spectrohelio/shg_video-en.php
(6) http://www.astrosurf.com/cieldelabrie/sphelio.en.htm
(7) http://www.astrosurf.com/spectrohelio/shg1-en.php
(8) http://www.shelyak.com/rubrique.php?id_rubrique=6
(9) http://www.astrosurf.com/thizy/lhires3/e_optique.html
(10)http://www.astrosurf.com/spectrohelio/archives.php?an=2014
(11)http://www.catchersofthelight.com/catchers/post/2012/06/08/Solar-Astrophotography.aspx
(12)http://www.astrosurf.com/ubb/Forum2/HTML/033220.html
Acknowledgements. I would like to thank Prof. Dr. H.F. Henrichs for his support during this experimental project.
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