Posted on Apr 21, 2011, 11:16 pmAuthor: Jen Winter
Solar Observing seeing concerns vary greatly from Nighttime viewing.
During the daytime, radiant heating from the sun affects seeing significantly. Characterized by turbulence or shimmering as seen over a hot street, seeing can cause significant impact on quality of solar observations.
- Bad seeing is caused by air of different temperatures mixing. This typically happens within the lowest 10 feet of air. It occurs most often over pavement, dark objects, rooftops and sometimes trees.
- High cirrus clouds or “scuz” will cause scattering of sunlight in the high atmosphere which often makes for bad viewing conditions. A classic sign of high cirrus clouds is the inability to achieve focus, or the need to “chase focus”.
- A jet-stream moving overhead can also hurt seeing conditions even on a clear day.
DayStar Filters are high power viewing platforms and this high resolution can be succeptable to seeing issues. Solar Observers using high powered, high resolution telescopes and DayStar filters should heed daytime seeing. While many of these conditions are beyond our control, observing in an area with ideal conditions without pavement in the direction of viewing and on days with no high cirrus will offer best results. Grass is the best environment for daytime seeing stability.
Each observing location offers different behavior for daytime seeing cells at different times of the day, as the air through which one views changes with movement of the sun. Some locations benefit from best seeing in the morning, while many have best seeing in the afternoon. Because most heat variation between air and ground surfaces occurs within the first 10 feet above the ground, often a high observing platform offer superior seeing. This might include a second story deck which overlooks grass. Rooftop observing rarely offers better seeing, as roofs themselves are culprits of heat gain themselves.
A longstanding debate exists about the what maximum aperture can be used in Solar Observing considering terrestrial seeing concerns. Until very recently, the academic standard was 90mm for photographic purposes. However, this figure was reached due to the limitations of still photography prior to the mid 1990's. Seeing cells are constantly and rapidly moving and changing like bubbles in the atmosphere. Occasionally and very briefly a single seeing cell can align in which the view of the sun is more clear. These fluctuations in seeing are so brief that still photography suffers the opportunity to catch them in a still frame. The human eye is able to naturally interpolate those moments of exceptional seeing, so to the naked eye, exceeding 90mm is not the same issue as for still photography.
The matter still remains as to what the actual maximum aperture can be for Solar Observing in concern to Seeing Limitations. Numerous professional observatories perform solar observing and imaging in excess of 1 meter in aperture. However, those facilities are placed on well researched mountaintops and in island environments. The average observer must consider his or her local environment in the decision of aperture. Visually and photographically, we at DayStar hear regular reports of good observing experiences in apertures of 100mm, 130mm or 150mm. However, most experiments in apertures exceeding 150mm have reported difficult
ies in achieving focus on a limb which cannot be effectively resolved. DayStar suggests that in urban and suburban environments, a 130mm maximum aperture should provide regular good resolution visually and photographically if using fast frame photography. (see technical note on fast frame solar photography) Urban and suburban environments may occasionally offer seeing which supports 150mm aperture, but one should not expect good clarity in 150mm telescopes during every observing experience in urban and suburban environments.
Over-magnification of the image will have resolution limitations in which features can not be resolved. Also note that the sun’s limb in hydrogen alpha is soft, with a fuzzy layer of chromosphere surrounding the surface. The same computations for figuring Dawes' Limit apply to solar telescopes as for nighttime telescopes. Because most DayStar applications result in an F/30 beam, wide field eyepieces are always suggested. Exceeding Dawes limit with higher powered eyepieces will result in poor focus and clarity performance. DayStar recommends Tele Vue brand Plossl eyepieces of 32mm, 40mm and 55mm for best results.