The following sun photograph show a nice sunspot group, which much larger than the diameter of our Earth. The evolution of sunspots and groups is clearly visible within 1 day interval. The relative position of sunspots changes continuously due to the differential rotation of the sun (the angular velocity is latitude dependent).
· Sun photograph taken on March 11, 1989
Instrumentation : Newton telescope with f = 1000 mm, f/d = 6.7
Sun filter : Chromium filter (handmade) (density approx. 5), optical diameter 72 mm (off-axis), f/d = 13.9 (effective)
Film : Ektachrome 100 Asa
Time : March 11, 1989, 11H00 (UT), from Saignelégier (Switzerland), © M. Willemin
The limb darkening is noticeable on this sun image. When the sun is viewed in the white light, which is approximatively the case here, radiation essentially emerges from a point in the photosphere whose height depends on its agular distance from the sun's center; at disc center, radiation emerges from the photosphere's base (high temperature), and at the limb, it emerges from several hundred kilometres above the photosphere's base, near the temperature minimum. According to the Stefan-Boltzmann and Wien laws, the radiation from the limb must be both less intense and somewhat redder. The slight color change is not detectable on this photograph due to the dominance of the red.
· Sunset on March 17, 1989
The sunspot group on the right part of the sun is the same as shown of the previous photograph. Note the flattening and distorsion of the low sun. Contrarily to the expectation of a lot of people not really familiar with atmospheric effects, the lens effect of the atmosphere does not exist. For low sun as well as for low moon, the horizontal apparent diameter is the same as for a zenith observation. Only the vertical apparent diameter is reduced close to the horizon, which is called flattening. The apparent larger size of the low sun or low moon is only an artifact of the human brain.
Instrumentation : Newton telescope with f = 1000 mm, f/d = 6.7 and Barlow lens 2x
Resulting focal length : 2000 mm, with an effective f/d = 13.4
Sun filter : WITHOUT SUN FILTER !
Film : Ektachrome 100 Asa
Time : March 17, 1989, sunset, from Saignelégier (Switzerland), © M. Willemin
If the atmospheric absorption is sufficient, it is possible to take such a sun picture with a telescope, but without specific filters. This only possible and relatively safe during a sunrise or a sunset. I do not recommend this technique to the non familiar sun observers. The risk of this kind of solar photography is not negligible and great care should be accorded to the eye protection.
Gold rule for solar observation and photography :
WARNING : Never view the sun through optical instruments like telescopes
or binoculars without reducing light intensity with appropriate filters.
Irreversible eye damage may result !
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