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
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
spectroheli-ograaf bestaat uit een telescoop, een spectrspectroheli-ograaf 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
zon-nevlekken 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 ESO 2014c July 23, 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
Observa-tory of the Faculty of Science of the University of Amsterdam, located at Science Park, Amsterdam, and operated by the Anton Pannekoek In-stitute.
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 im-ages. However, the prices of these filters vary between 4.000
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 arbi-trary wavelengths5.
The first spectroheliograph was invented in 1890 by Henri Des-landres and George E. Hale, see Fig. 2. They built this instru-ment at Meudon Observatory in Paris, see Fig. 3. With the spec-troheliograph, 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 so-lar disk, which passes over the slit. Afterwards software is used to create a monochromatic image from these series of spectra6,7.
Fig. 2. Left panel: Deslandres, right panel: George Hale.
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.
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 respec-tively. 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 re-stored 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. 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
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 sensi-tivity 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
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, includ-ing 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 cen-ter 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 ar-eas 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 partic-ularly 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 vis-ible in emission in the prominences, in emission and absorption in the filaments and shows sunspots as well3(Aller1953). This
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. 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 at-tempted 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 spectroheli-ograph. 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
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 am-ateur 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 grat-ing 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.
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.
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 105reduction 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
di-ameter 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
For the second observation the Deep Sky Imager Pro II was re-placed 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 auto-mated. 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 ob-servations. 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
Diameter Focal length Solar image Angular resolution at Hα
mm mm mm 00
Robtics refractor 104 700 7 2.1
William Optics 70 420 4.2 1.7
Table 2. Available CCD cameras at APO suitable for spectroheliography
CCD size(mm2) pixels pixel size(µm) SBIG ST7 6.9 × 4.6 765 × 510 9 SBIG ST2000 11.8 × 8.9 1600 × 1200 7.4
Meade DSI 5.6 × 4.7 752 × 582 8.3 × 8.6 ATIK 460 EX 12.5 × 10 2750 × 2200 4.54
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.
4.3. Third setup
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 neu-tral 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 impro-vised 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 experi-mental 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 bin-ning of 2×1 for a shorter readout time, which became 4 seconds.
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.
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.
The first created solar image with this experimental setup is an image created at Hα, see Fig. 13. The Sun is clearly not in fo-cus, and has an oval shape (which could be adjusted by choosing a different aspect ratio). There were clouds during the observa-tion, visible as black vertical lines. The vague horizontal line represents dust on the slit. Before the observations done to cre-ate 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.
Fig. 12. The final experimental setup. The spectroheliograph consists of the William Optics telescope with solar filter, the LHIRES III spec-trograph, the ATIK 460 EX CCD camera and an eyepiece with 18 % filter.
Fig. 13. The image of the Sun in Hα with a range of 10 pixels (690-700) 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 spec-tral 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 im-age created at Calcium II by Ph. Rousselle on the same date, see Fig. 1610. All sunspots in Fig. 15 seem to accord with the
sunspots in Fig. 16. In Fig. 17 a small part of both images is en-larged to see the sunspots in detail. The dark spot on the image of APO seems just one sunspot, but in the image of Ph. Rous-selle can be seen that it contains two separate sunspots. There-fore 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
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. 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. 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. 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 mechani-cal efforts. On the image created at APO, the sunspots that ap-pear 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 Pou-peau. 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. 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. 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.
Fig. 21. The spectroheliograph built by the amateur astronomer Jean-Jacques 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.
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
