The Solar Explorer   -   September 23, 2021

Observation

Part 1: Select the imager instrument

Because of its principle and the shooting method ("spectroheliograph" mode), the focal length should preferably be between 200 and 1200 mm. Below a focal length of 200 mm, the smallness of the image of the sun disk at the focus prevents obtaining a very detailed view. Above 1200 mm of focal length, the image delivered is very detailed, but the time exposure must be reduced, which induces an increase in noise. The volume of data acquired also rises very quickly, the slightest scan can reach 2 GB or more (size of the SER file), a situation which becomes easily unmanageable in the long run (unless you resign yourself to scan only restricted areas of the disc).


The focal length of the imaging device should not exceed 420-450mm to capture the image of the sun disk in a single scan pass (a "scan", see "Theory" section). Above, you can perfectly take images, but you will only have a fraction of the disk, which in itself is not at all prohibitive, and will oblige to assemble the result of several "scans" in a single image under desire a global view, which takes a little more work.


As a general rule, telescope using mirror optics cannot be fine with Sol'Ex. It remains the astronomical glasses and the photographic objectives (telephoto lenses). The ideal aperture ratio (focal length to diameter ratio) for Sol'Ex is between 6 and 10.


The views below show a number of perfectly exploitable associations with Sol’Ex:

200 mm telephoto lens

EVOGUIDE SkyWatcher refractor, D=50 mm, F=232 mm.

Canon de 400 mm téléphoto lens + 2x extender (800 mm final focal length) 

TS refractor, 65 mm for the diameter, 420 mm for the focal length.

Astrograph FSQ85ED Takahashi (F = 450 mm)

FS128 Takahashi (D=128 mm, F=1040 mm).

The two pictures below taken with Sol'Ex illustrate the aspect of the solar disk in the H-alpha line for the two extreme configuration of this enumeration:

Image taken with a Canon 200mm telephoto lens at f/ 5.6. Despite the instrument's modesty, this document reveals many details in the Sun's atmosphere (there was no activity visible on the surface in white light at the same time). This light and economical configuration is ideal for beginners and to have fun over time.

Image taken with an FS128 refractor (1040mm focal length). The configuration is much heavier, but reveals fine details if the degree of atmospheric turbulence allows it.

Simultaneous observation of the Sun with two Sol'Ex, one operated in the foreground by Valérie Desnoux at the focus of a 120 mm AstroPhysics refractor, the other operated in the background by the author at the focus of a 128  mm Takahashi refractor.

The quality of the focusing system is essential to obtain sharp images of the Sun. Positioning of the Sol'Ex entry slit to within +/- 0.010 mm is sometimes necessary, or even better. Adding a measuring device is very useful, such as a mechanic's micrometric probe (a dial gauge). This element is fixed to the body of the telescope, and the measurement is made in relation to the mobile part which supports Sol'Ex via a gauge. Many economical models exist, mechanical or digital (I like the mechanical needle, because more readable in all positions).


Focusing with a photographic lens requires skill. If the mechanics are generally very good (helical system without clearance for the right material), on the other hand, you need a helping hand to succeed and ideally have a direct view of the acquisition screen to judge the result in real time (see a video later on this).


To estimate the correct focus of the image of the Sun on the Sol’Ex slit when acquiring the "scans", you have at least three criteria:





(1) the sharp edges at the end of slit image.

 

(2) a jagged H-alpha line structure, which changes quickly as you scroll the disc across the slit. This structure reflects the variations in gas velocity in the chromosphere that Sol'Ex can perfectly detect (Doppler-Fizeau effect).


(3) if a sun spot is present on the disc and you have centered it on the slit, the black line that it forms all along the dispersion axis is all the more contrasted the better the image is focused.


This procedure is concretely explained in a video that you can view by clicking on the following image:

Note that the capture is always done by orienting the camera so that the dispersion axis is vertical (parallel to the columns of the detector), and therefore the horizontal lines. This arrangement greatly optimizes the CMOS reading frequency.


A question arises as to the orientation of Sol'Ex relative to the the telescope mount, in order to either perform a scan in right ascension or in declination. There are several schools, but mine is to scan in right ascension, without a shadow of a doubt. This is more natural (this is what you are led to if you scan with the telescope's drive motor off). Often the mechanical quality of the mounts and the motorization is of better quality in right ascension compared to the declination axis. If you only capture fractions of the solar disk in one pass, because the focal length of the telescope is long, it also makes astrophysical sense to work along an axis close to the equatorial plane of the solar disk, with the equatorial belt often being the richest. in detail.

Aspect of the chromosphere when the slit tangents the solar edge. If you do  right ascension scan - which is recommended - the observed solar point is quite close to the solar equator.

Part 2: The camera

Aside from the refractor (or telephoto), the main financial investment in using Sol’Ex is purchasing the shooting camera if you don't already have one. I broached the subject of the camera in part 4 of the "Construction" section.


I just recall that a small pixel CMOS camera is generally preferable, like the ASI178MM (2.4 micron pixels, ideal) or the ASI2890MM / Mini (2.9 micron pixels). If you own a ZWO ASI174MM camera (or the equivalent from QHY), don't worry, you can use it; you will get beautiful images of the Sun despite a pixel size of 5.86 microns, without spending a penny, and knowing that small pixel models are quite often used with 2x2 (software) binning.


On the other hand, I do not recommend the use of color cameras, if it is not for an educational purpose (showing the spectrum in color always does its little effect!).


You need software to read the camera. You can find free ones like SharpCap or FireCapture, which can produce SER files at high frame rates (the file is a series of individual images). You must tell the application to acquire data in 16 bits (not 8 bits). Familiarize yourself with the software before acquiring your first solar data.


For the camera to operate at high frame rates - frequencies above 100 frames per second are often required - you must have a computer with a USB3 interface. In addition, the connection between the camera and the computer will be short and direct, without going through a HUB, which generally does not provide the required reading frequency. Here again do some testing to ensure performance using the camera outside of Sol’Ex. One more point: this high speed should not lead to the use of too high a gain for the camera. Normally, a gain of 100 should not be exceeded (in the ZWO repository). Above it, the noise starts to be annoying.

Part 3: The mount

We know that the principle of observation consists in letting the image of the Sun scroll in front of the slit while making regular acquisitions at high speed, so as to correctly sample the solar disk over time, with good regularity. The images thus acquired are saved in a single SER file.


An equatorial mount is essential for this operation in order to maintain over time the orthogonality between the direction of the scan and the long axis of the slit. 


The speed of the disc image on the slit has a strong impact on performance and user comfort. A natural scroll, at sidereal speed (15 ° / hour) corresponds to stopping the right ascension drive motor. It is quite possible to do this, but the east-west scan of the entire sun disk takes more than 2 minutes. This affects interactivity. If the average movement is by definition very regular, on the other hand, the agitation of the images caused by atmospheric turbulence is felt over such a long time. If you have a mount that allows 8X, 16X… sidereal speed, the scan will be much faster and there is a greater chance of falling into a “hole” of atmospheric turbulence  The scanning is then said to be “forced”. Here, the acquisition chain must make it possible to acquire images at high speed (several hundred images per second).


Be careful, at high scanning speed, some frames are victims of oscillation (resonant phenomenon) which have a dramatic effect on the quality of solar images. The result depends on the direction of the scan (east or west) and the balance of the mount. Check the evenness of the solar edge (apart from turbulence) to make sure all is well.


At the top, the balancing is such that it generates clearly visible oscillations on the solar limb when scanning the disk at 4X sidereal speed with an FS128 telescope. At the bottom, the mount is balanced differently and the oscillations have completely disappeared. The image of the Sun presented is the result of processing a Sol'Ex acquisition outside of any line, which produces a classic view of the solar photosphere,

Part 4: Sol’Ex thermal protection

If you use Sol’Ex as shown in the photograph to the right, you are exposing a black case to the Sun, and the temperature will rise very quickly. This causes thermo-elastic deformations of the plastic from which Sol'Ex is made, with the visible effect of more or less random shifts of the spectrum in the plane of the sensor. It is not possible to work this way, as conditions may have changed during scan!


It is therefore imperative (I stress the term "imperative") to keep Sol’Ex at a temperature close to room temperature, which means interposing a screen between the Sun and Sol’Ex. I use a really simple technique, to the detriment of aesthetics, effective, which does not weigh down Sol'Ex and which does not add wind resistance: wrap Sol'Ex in kitchen aluminum foil ( use the double-layered model to limit tearing problems). This is what the image on the left shows…. guaranteed effective!

Opposite, aluminum foil protection against temperature rise when Sol’Ex is mounted on the focus of a Takahashi FSQ85ED telescope.

Part 5: Filter the solar flux

The shooting instrument (refractor or photographic lens) concentrates a strong energy at the focus in a small area. The temperature is high there if you do not use an element that absorbs or reflects this energy. It is precisely here that the Sol'Ex entrance slit is positioned, a very thin glass blade covered with chrome (inclined at 30° relative to the optical axis), a material that is both reflective and absorbant, on which is drawn the pattern of the slit (a transparent line 4.5 mm long and 10 microns wide). The glass is glued to another piece, made of aluminum, serving as a support. Both of these can withstand the energy contained in the focused image of the Sun, but the differential expansion between glass and metal can lead to mechanical stress and breakage. This is what you see in the image opposite; a slit that is broken because it is subjected to intense focused solar radiation for a long time and without any attenuation.


It is therefore important to attenuate the solar flux to a certain extent to secure the operation of the instrument. To verify that there is sufficient attenuation, place the palm of your hand in the focus of your telescope. If you can hold your hand for at least 15-20 seconds, you can assume that it will be okay while using Sol'Ex.


The simplest solution to reducing the intensity of the solar flux is to position a neutral absorbent filter at the entrance of the refractor or photographic lens. Filters are available from HOYA or B + W brands with various densities at fairly reasonable prices up to a diameter of 82 mm. The correct density values for us range between 0.6 and 1.2 depending on the instrument used. For example, with a 400 mm telephoto lens used at f/5.6 associated with a focal doubler (final aperture at f / 11.2 - see the films below) I use a particularly clear filter, of density 0.6. On the other hand with a 200 mm lens at f / 5.6 the value is ND = 1.2


HOYA offers a larger density staging than B + W. In addition, after various tests, the surfacing quality of the HOYA filters is such that it does not degrade the quality of the images delivered by Sol'Ex, which is essential. The HOYA filters are also less expensive and do not present fringe phenomenon, which is not the case with the B + W filters tested, which is troublesome. In summary, the right source is the HOYA PROND filter series.


At right, fringes observed by using B+W filter, left, the HOYA filter. 

These filters, designed for photography or video, are of course easily adapted to all photographic lenses. For a refracting telescope, it will be necessary to manufacture a 3D printed interface.


They prove to be effective over a wide wavelength range, certainly for the visible, but also for the infrared. In case of doubt, you can add an infrared cut-off filter (IR-cut) screwed onto the 31.75 mm interface - but not necessary with the filters indicated. For information, solar energy below 400 nm represents 3% of the total energy, between 400 and 700 nm we find 42% of the total energy, between 700 and 1100 nm we have 34% of the total, between 1100 and 1700 nm, 15% of the total, and beyond to infrared, 6%. The exact proportion depends on atmospheric transmission, but we can see that the infrared part, not visible to the naked eye, is not negligible.

The other way to filter the intense solar flux is to use a Hershel helioscope, an accessory well known to solar observers. This looks like a telescope 90° folder. The principle is simple: the solar flux captured by the telescope meets a little in front of the focus a strip of bare glass (without surface treatment), most often inclined at 45°. Naturally, a bare glass surface reflects about 5% of the incident flux and transmits the rest, or 95% (the internal absorption of the glass is almost zero). The subsequent attenuation eliminates any risk of material breakage as a result of the helioscope (but viewing with the naked eye without further filtering is still prohibited). The second side of the blade forms an angle with the first in such a way as to avoid the appearance of a ghost image.


The following photograph shows some helioscopes available on the market for 31.75 mm interface:

After evaluation, the use of the TS model turns out to be disappointing in the specific context of the Sol'Ex application, a particularly absorbing polarizing treatment on the glass causing the whole to lose a lot of luminosity, to the point that it happens that there is a lack of light in certain cases, which is a shame when observing the Sun (the TS helioscope behaves like a density of approximately 1.8). The LUNT, HERCULE, or even BAADER models, which use an untreated blade, however, do the job perfectly. The LACERTA model exploits an untreated blade at a particular angle of incidence, known as Brewster, which maximizes the linear polarization induced by this blade. This property, and the fact that Sol'Ex itself polarizes the light, to more than 75% in the red (it is the diffraction grating which is responsible), that the overall transmission is maximized, which is turns out to be an advantage. Despite its massive side and high optical draw that will not fit all scopes (make sure you can focus), the LACERTA is the ideal helioscope for use with Sol'Ex. How to operate a helioscope will be explained in a video, the link of which will be given a little further on.


Occasionally, a neutral density filter is fitted in the helioscope when it is delivered (see right). If this is the case, this filter must be removed, along with any polarizing filter offered by the manufacturer.


The photographs below show how to orient the Hershel helioscope in relation to the frame (in the case of a right ascension scan, the most common) and how to orient Sol'Ex in relation to the helioscope. The way to put these elements together is not intuitive, but it is crucial to respect it. The origin of this arrangement is linked to the joint polarization of the helioscope and Sol'Ex, in order to maximize the optical flux transmitted in the system.

One last remark to close this part. The glass slit is mounted on a piece of black anodized aluminum. This metal part absorbs more heat than the glass part. To make sure that the temperature does not rise when observing with a large instrument, you can make this absorbent part of the slit holder reflective by placing strips of aluminum tape on the affected parts, as shown in the photograph. on the right (but above all, do not stick anything on the glass part!).

Part6: Observation by using 200 mm telephoto objective and a neutral density filter

To get started with Sol’Ex do not hesitate to use a modest instrument, this is the best school. For example, in the videos that will follow, I will show you how to use a simple 200 mm photographic lens of 200 mm, already allowing to obtain interesting and demonstrative images of the solar surface in the red line of hydrogen (H -alpha), but also many other lines.


The videos in this section show the basics of what you need to know to use Sol'Ex.


In the first one below (click on the image), I explain step by step how to mount the lens in question on Sol’Ex, how to use a density filter, how to approach the focus of the disc, etc:

The following video shows how to prepare Sol'Ex for the actual observation, by addressing the subjects of pointing the Sun, focusing the spectrum, focusing the solar image (see also part 1 of this page) and the orientation of the instrument:

The last video in this section shows how to perform a scan, how to use the SharpCap capture software, and how to use the INTI software for fast scan processing and solar disk visualization:

Part 7: Observation by using 800 mm telephoto lens and neutral density

In this part I describe how to use an objective with a larger focal length. This is a 400mm telephoto lens combined with a focal extender (doubler). The images produced by this system are much more detailed (but do not show the whole disc in one pass):

The following video, how to use these setup :

Part 8: How to use a Hershel helioscope

The following video explains how to set up a Hershel helioscope associated with a telescope 65 mm in diameter and 420 mm in focal length. In particular, we stress the important question of the Sol'Ex orientation in relation to the helioscope:





Partie 9: How to observe solar spots

Certainly, Sol'Ex does a great job of delivering images of the Sun with light coming from narrow spectral lines, but don't forget that you can also perfectly generate an image of the photosphere and sunspots, by using the light of the spectral continuum. The following video (click on the image), shows how to do this, in particular using the INTI software:

Part 10: The impact of binning and focal length

The operation of binning consists of agglomerating adjacent pixels to form a single one. For example, 2x2 binning groups together the signal of the pixels forming a square with two pixels on each side. With CMOS sensors, binning is achieved by a simple arithmetic sum, which is less efficient than with CCD sensors which perform analog summation. Software like SharpCap offers this possibility of binning on acquisition.


For us, the purpose of binning is threefold. First, some reduction in noise upon viewing. Then, a very significant reduction in the size of the files. At constant scanning speed, the volume of a SER file acquired in 1x1 binning is 8 times larger than a file acquired in 2x2 binning, which is a considerable difference.


Conversely, 1x1 binning samples the spectrum and the solar disk more finely (along the spatial axis) than 2x2 binning. So the picture, is potentially more resolved. For example, the two images of protuberances below were taken in continuity with 2x2 binning and 1x1 binning (scaled afterwards):

Takahashi FSQ85ED - binning 2x2.

Takahashi FSQ85ED - binning 1x1.

Careful examination of these images actually shows a gain in resolution using 1x1 binning. The theory of Sol’Ex is well verified. But we must put this gain in relation to the size of the images, which degrades the flexibility of use and interactivity.


Be careful, 2x2 binning degrades the spectral resolution compared to 1x1 binning. If you need to make precise measurements on the spectrum or work on very fine lines, 1x1 binning is essential.


The other lever to increase the sharpness of the images is to use a bigger shooting instrument, which more focal length, of course the cost of the operation is much higher than the simple fact of choosing the binning factor, but the performance climbs much faster; as shown in the following video:

Partie 11: Observation of H & K lines (Ca II)

One of the interests of Sol’Ex is to deliver images of the Sun in the spectral line of its choice, without having to buy a specialized filter for it. It suffices to select this line by turning the grating wheel. Here are some examples of images at various wavelengths taken with a small telescope:

In this part we will focus on ionized calcium (Ca II) lines located in the ultraviolet (UV). They are easy to recognize because they are very wide.


But two difficulties arise. First in the UV the signal decreases rapidly in strength. Often, it will be necessary to push the gain of the camera to see the details. Then, the chromaticism of the optics (that of the telescope, and especially of Sol'Ex) becomes severe. This last point requires certain manipulations: refocusing of the spectrum on the detector to find very fine lines in the image of the spectrum, refocusing of the telescope to have a sharp image of the disc on the slit. The following video discusses these topics and delivers some tips:

A question addressed in this video is the choice of the line, the so-called K line or the so-called H line. On the astrophysical level, they are equivalent. Traditionally; the K line is preferred because it is well isolated, as shown in the spectrum below with a slit tangent to the solar limb. The H line is interesting however, because it is located in a more intense part of the recorded spectrum, but unfortunately polluted by the neighboring H-epsilon line. When observing with the 2400 lines / mm grating, the spectral resolution is sufficient to separate these two lines well during processing, and therefore my preference is more on the H line, which offers less noisy images.

Inversion phenomenon of H and K lines at the solar limb, but also of neighboring hydrogen lines. Image taken with the 2400 lines / mm grating of the basic version of Sol’Ex.

Part 12: Observation of D3 helium line (D3)

It may happen that the spectral lines with which we want to produce a spectroheliogram are almost invisible in the continuum. It's a complication. The typical case is that of the yellow line of helium (He I) at the wavelength of 5875.65 A, called the D3 line. Its intensity is only on the order of 1% of the intensity of the continuum. Suffice to say that she is discreet. To see it properly, it is necessary to observe very close to the solar limb, where it is revealed as a "bright" line in the vicinity of the famous sodium doublet, lines D1 and D2, respectively at wavelengths of 5889.97 A and 5895.94 A:

The images made in the light of helium are particular, with a contrast often reversed compared to what one sees in the H-alpha, and also reveal many other details (note, the yellow coloring is artificial):

It is not possible to obtain an image of this type directly. The method consists in subtracting from an image of the disc produced in the light of the D3 line another image taken in the light of the continuum. The procedure is illustrated by the following video (click in the image to display the film):

Note: Helium was discovered in sunlight on August 18, 1868 by astronomer Jules Janssen. Using the same technique, we invite you to relive this beautiful discovery

Part 13: The Doppler-Fizeau effect

The deformations of the red H-alpha line of hydrogen along the slit are the manifestation of the Doppler-Fizeau effect:

Depending on whether the light-emitting material approaches or moves away from the observation point, the line shifts towards blue or towards red. The result is spectacular, because we have a way to measure a field of (radial) velocities on the surface of the Sun using the core of the line or these two edges. At the same time, we probe the atmosphere of the Sun in its thickness: the light emitted in the wing of the H-alpha line coming from regions deeper than that coming from the heart. In the example below, which concerns an active area around a spot (use of an FS128 telescope) we see a filament appear when we move 0.876 A from the center of the line (it is invisible if we use the light from the core of the hydrogen line):

The other example below focuses on a filament (a protuberance). When we move about 5 A from the line, we reach the photospheric surface, and we no longer see the protuberances and the chromospheric structure (that of the solar atmosphere):

The images shifted spectrally, in addition and in less, compared to the center of the H-alpha lines (or others) can be associated to achieve a colored composition, as below (observation dated May 26, 2021, by using a Takahashi FS128 refractor):

Below is another colorful composition of images synthesized from the red and blue wings of the H-alpha line. Depending on the color, red or blue, the material moves away or approaches us. We synthesize such a color document by attributing to the red calculation the image taken in the blue wing of the line, to the blue channel, the image taken in the red wing, to the green channel, the average of the two previous images:

It is even possible, thanks to the Doppler effect, to measure the rotation of the Sun on itself, following the same principle, the east and west parts approaching or moving away from the observer, hence a shift spectral differential between the opposite edges in the vicinity of the equator:

Part 14: How to observe E corona

This is surely the most difficult observation, that of the solar corona. But… it seems possible with Sol’Ex. It should be understood that the solar corona is extremely pale in comparison to the dazzling photosphere. Here is some difficulty.


The following image is taken with Sol'Ex mounted at the focus of a 60 mm refractor (Takahashi FS60) at the Fe XIV line wavelength at 5302.2 A on June 13, 2021 (average of several scans) while the sky was far enough from being coronal:

We can see a glow around the disc. Is it the E corona (from the emission of radiation from highly ionized atoms)?


Here is the image taken at approximately the same time from Hawaii, taken at the Mauna Loa Solar Observatory (MLSO):




The correlation is strong between the two images. It is supported by other observations on other dates. It seems possible to detect the solar corona with Sol’Ex, a rather mythical objective for the author, especially as my observation is made by the sea (Côte d'Azur).


The spectral extracts to follow give the localization of the coronal green line Fe XIV 5302.86 A, as well as that of the red line Fe X 6374.56 A (of course, impossible to detect at the level of the photosphere given their large relative weakness of brightness, it is necessary to aim at judgment during the synthesis of the solar disk):

Probably very more difficult, the observation, by using coronal lines  Fe XI at 7891,89, Fe XV  at 7059,59 A, Fe XIII at 3388,10 A,.

Part 15: How to observe magnetic field

With the observation of the corona, here is another subject at the limit of Sol’Ex's capacities: the detection of the magnetic field through the Zeeman effect. This consists of a doubling of the lines in the presence of a magnetic field, but in the case of the Sun, where the magnetism remains relatively modest, this doubling is really discrete. It can only be "easily" directly demonstrated in solar activity centers. The amplitude of the doubling is proportional to the strength of the magnetic field.The use of a polarimeter which isolates the circular polarization of the components of the Zeeman effect facilitates this measurement for the longitudinal field and makes it possible to record the sign of the field (orientation of the lines of force),


Below are some brief explanations of the observation principle:

The following document shows a Sol'Ex measurement of the magnetic field on the Sun's surface on June 29, 2021, compared with a measurement of the HMI instrument mounted on the SDO satellite. The more the representation is in an intense black or white, stronger is the magnetic field. Black and white indicate sign of the longitudinal magnetic field, directed towards the observer or away.

From this confrontation we conclude that Sol’Ex is able to observe the solar magnetic field. That this is possible with such a compact and inexpensive instrument is a real achievement. But of course a lot of care is needed when measuring.

Part 16: The « cinematrographic » observation

We don't forget that the Sun is a dynamic star, with changes that can be very rapid. This is the opportunity to make videos of these phenomena. Below, an animation of the first observations made with Sol’ex (May 5, 2021) - 36 images taken with a time spacing of 5 minutes:

Part 17: A solar spectrum atlas

If a spectrographic neophyte can successfully use Sol’Ex, it must be admitted that the first contact with a star's spectrum can be confusing if one has never been confronted with this kind of situation. To help you find your way around, to understand what a star spectrum is, this "bar code", which is the physical signature of the star, I suggest you download, by clicking on the image opposite , an atlas of the solar spectrum produced by Olivier Garde. In the form of a single image and in visual colors, you will have a global view of the solar spectrum, with the additional identification of the chemical element responsible for a particular line. This tremendous work by Olivier was carried out with an ESP scale type spectrograph (Shelyak Instruments) at very high resolution (R = 30,000, this means that the fineness of the details in wavelength is equal to the wavelength divided by 30,000. ).


A very good exercise is to compare Olivier's spectral map to the real data that comes from Sol’Ex. You can explore the spectral domain by moving the grating gnob, in either direction (note: you may need to retouch the camera focus to ensure sharpness of the observed spectrum area. due to the residual chromatism of the lenses used in Sol'Ex). A small difficulty is that the spectrum returned by the camera is in gray level, while the Olivier spectrum is in color. The exercise amounts to recognizing at the end of the code the patterns of the barcode between the two. You will be surprised to get there very quickly, as if you were reading a road map. The specter of the Sun will become your territory!


See also the guided tour in the form of a video in part 14 of the "Construction" section of this site, or the extracts of Sol'Ex spectra below.


Some region of the solar spectrum. The H&K lines (UV spectral domain):


H-beta line (hydrogen):

Magnesium triplet (green part):

Sodium Na I (D1 & D2 lines):

Red H-alpha line:

In summary, this is how the global view of the solar spectrum looks in its visible part:

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