Solar observation

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Under solar observation is the observation of the sun and the phenomena caused by it - especially sunspots and solar flares , as well as net radiation and radio interference - understood sporadically flares , magnetic interference and the shadow or direction measurement with the sun.

Visual observation

Warning of dangerous optical radiation

You must be very careful when observing the sun visually, because the enormous brightness of the sun can easily lead to permanent eye damage. In particular, we must never with a pair of binoculars or a telescope without optical sunscreen look at the sun, because the burning glass effect of the device, the retina can destroy the eye within a split second. With modern go-to telescope mounts, it can be dangerous if the motor drives past the sun when moving and you accidentally look at the eyepiece.

The solar observations include:

  1. Observing sunrise or sunset and their accompanying phenomena ( sunset glow , lunar illusion , anomalies of refraction , etc.)
  2. The observation of sunspots and flares , with special instruments also of protuberances and solar flares
  3. The location of sunspots ( heliographic coordinates ) and the sun's rotation
  4. The navigation and the localization of elevation angle measurement to the sun
  5. The measurement of a solar azimuth for a survey network
  6. Direction measurements with a shadow stick or by alignment on buildings
  7. The measurement of solar radiation with various thermometers, with heliographs or by means of the radiation balance
  8. The measurement of particle flows, magnetic or radio interference as a result of solar activity .

The above warning mainly relates to aspects 2–5, but must also be observed for No. 1 and the other types.

The following statements concern above all astronomical-astrophysical observations of the photosphere . For the other types of solar observation, see also Astronomical Phenomenology , Aeronomy , Astronavigation and Meteorology .

Observation with the naked eye

Sunset over the Aegean Sea. Flight altitude 11 km, image section enlarged about 10 ×. The left edge of the sun shows a small refraction anomaly when clicking the image to enlarge it.

The sun is often observed with open eyes as it rises and sets , for example to prepare photographs on the horizon. Measurements of the shadow cast - for example on a gnomon or for architectural purposes - should also be listed here. On the other hand, details on the sun can only be seen in extremely large sunspots (3–4 times in 2014–15), occasionally with strong solar flares (places around 1000 ° hotter) or during a passage through Venus such as in June 2012.

In the past, sooted glasses were often recommended for this purpose, but it is not advisable for longer observations, as the soot does not adequately filter out infrared and UV rays, which are also dangerous to the eye . The use of rescue foils for this purpose is therefore not recommended. Only astronomical sun filters and protective goggles with Mylar films or very special welding glasses according to DIN EN 169 and at least filter level 14 are safe . Other welding goggles are not suitable.

The free-eyed observation of the sun, which is very low or is strongly weakened by haze and clouds, without filters and optical aids is usually safe because the eyelids usually close in time if the light is too bright. Since sunlight with UV and infrared radiation contains invisible components that can damage the eye, open - eye observation should not go beyond a few minutes.

Observation with the telescope

Astronomical observations

For telescopic solar observation - especially sunspots - you have to use suitable solar filters, ideally as a lens filter in front of the telescope lens . A light attenuation in the vicinity of the focal point can also take place by means of a Herschel wedge , Brandt 's sun prism or a pentaprism . In the latter case, however, the residual light must also be attenuated with gray filters or polarization filters, otherwise it is still too bright for the eye.

Even with moderate cloud cover you can see all sunspots in a few minutes.

Basically, the sun filter should always be attached in front of the lens, as the sun's rays do not even get into the telescope and the tube stays cooler. It is useful and inexpensive to stretch a filter sheet in front of the lens. We strongly advise against using eyepiece filters made of glass or plastic, which are screwed into the eyepiece behind the telescope . The bundled sunbeams generate so much heat here that glass filters can burst and plastic filters can melt. In this case, the reaction time of the eyelid is not sufficient to close the eye in time.

Measurements with the theodolite

The telescopic solar observations also include the measurement of geodetic solar azimuths with a theodolite . Only a few manufacturers offer suitable sun eyepieces for this; mostly you have to rely on the self-construction of a filter. According to experience at technical universities , exposed photographic films that are mounted in a cardboard tube and placed in front of the lens prove to be the best. Due to the theodolites, which are common with an aperture of only 4–5 cm , their light attenuation is usually sufficient.

In the 1950s , the Dutchman Brar Roelofs developed a solar prism with which, in addition to the filter effect, the solar disk was also split into 4 images for more precise centering . However, these accessories are no longer produced today.

Older theodolites had their own small eyepiece filters for the purpose of solar azimuths - but with a very small exit pupil or eye diaphragm. The filters were therefore less dangerous than for the astronomical observations mentioned above - also because of the smaller lens opening of the geodetic instruments.

Low sun

A filter must also always be used for observing the sun, which is low or has been weakened by haze , because the telescope also bundles the invisible UV and infrared rays which can damage the eye. The problem here is sometimes (as with the low position of the sun in winter) fine haze or cloud veils, through which the sun image is strongly attenuated and the contrast is too weak for a sharp observation of the sunspots. Some solar observatories use something weaker absorbing solar filter in such cases.

Projection method

An alternative method, which Galileo Galilei already knew, is the projection of the sun image onto a paper attached behind the eyepiece, the so-called projection method . You don't need a filter for this, and the telescope can also be aimed at it by casting shadows - without the dangerous view of the sun. However, achromatic eyepieces can be damaged because of the high temperature. With a lens about 8-10 cm aperture should therefore shrink by a senior panel, as are reflecting telescopes applies. In addition to the eyepiece, the secondary mirror can heat up here. If dimming is undesirable (e.g. because it reduces the resolution), the observation time should not exceed a few minutes or should be interrupted by pauses.

The projection process is not only harmless, but you can also easily draw the sunspots directly on the paper. It also allows multiple people to see the details of the Sun at the same time — which is also useful for solar eclipses . When guiding a star tour in a group, however, one should be careful so that visitors (especially children) do not reach into the beam path or even look into it. The finderscope is covered so that none of the observers burns a hole in their clothing.

Special solar telescopes have also been developed for observing the sun , which are mainly designed as tower telescopes to avoid air turbulence close to the ground . The heating of the air in the tube is avoided by evacuating it .

See also:

Sun photography

Sun with sunspots on October 28, 2003, image taken with a 4 "Maksutov telescope and ND 4 filter film

Sun photography is a part of astrophotography . No filters are required to photograph the low sun. For photography of the higher, blinding sun, either Mylar foils must be attached in front of the lens or neutral density filters with a density of ND 4.0 must be used. Since the latter filters do not adequately suppress invisible infrared and UV rays, they should only be used to observe the sun for a short time, for example to focus the camera or to track the sun. Newer special solar filter foils are vapor-coated on both sides and can be used visually and photographically without hesitation.

See also


  • Rudolf Brandt , The telescope of the star friend . Kosmos-Verlag, approx. 1960/1975

Web links

Wiktionary: Sun observation  - explanations of meanings, word origins, synonyms, translations

Individual evidence

  1. SAG: amateur astronaut. Do-it-yourself telescope for advanced users . Rascher-Verlag, Zurich 1962