Atmospheric Seeing Conditions
Contributed by Tom Fleming (FLET)
Reports and observations submitted to the AAVSO use the following ratings for atmospheric turbulence (Seeing): Poor, Fair, Good and Excellent.
Table 1 shows the same sunspot group as it would appear under these four conditions. Of course a still image on a printed page cannot represent accurately the rippling turbulence of the atmosphere when conditions merit a ‘Poor' rating. So, the images for Poor Seeing and Fair Seeing represent an image averaged over several seconds.
|Perception of Sunspots as Seeing Conditions Vary|
Seeing:Poor 6 Sunspots
Seeing:Fair 7 Sunspots
Seeing:Good 12 Sunspots
Seeing:Excellent 16 Sunspots
The Poor Seeing image shows three sunspots within the large penumbral area. Three more sunspots are to the right. There are hints of a few more but turbulence will prevent these from being verified. As conditions improve to 'Fair', another sunspot just to the right of the large penumbral area is revealed. Under 'Good' Conditions several smaller spots are revealed - four in the large penumbral area (the largest spot has resolved into two as there is a complete break - any bridging of that gap and the sunspot would be counted as a single instead of two) and lastly another near the smaller sunspot with the penumbra. Finally, under excellent conditions all is revealed.
In general, Poor and Excellent seeing conditions are less frequent with Fair and Good conditions making up the majority of your observing experience. Observing the rippling of the limb of the sun is a common barometer for judging the quality of the seeing. If you are inexperienced, you may wonder how you will recognize Excellent seeing conditions. The experience is similar to peering at a quarter through 4 feet of swimming pool water. You can see it there but the ripples prevent you from seeing the whether it is head or tails. Excellent seeing is akin to putting on a face mask and breaking through the surface. Not only will you see the face on the coin you can read its date and see the reflections off of the scratches. The detail in the penumbral regions of large sunspots, for example, will be amazing. An extended sunspot under average conditions may show itself to be 3 or more individual spots under excellent conditions and so on.
In the adjoining section you will find a detailed discussion of conditions that impact seeing and how to optimize them.
A Detailed Discussion of Seeing Conditions
Seeing turbulence is the result of volumes of atmosphere of unequal temperature mixing. The conditions that cause unequal temperature are many and varied. Some are within your control and others totally beyond your ability to control.
Turbulence may occur within the light path of your telescope. Before observing, allow time for your telescope to adjust to the local temperature. Select your observing location with the following factors in mind: Beware of nearby walls and fences, these vertical faces will receive maximum heating from the sun when the sun is near the horizon (this is generally a favored time to observe). Avoid observing over roof lines or pavement when possible. In general, nearby areas with trees or grass will help stabilize the air in your light path. Observing over a body of water generally provides the most stable air (note Big Bear Observatory). If your location is at an altitude in excess of 5000 ft this is also a big help. Frontal passages regularly bring turbulence as existing warm air is replaced by cool or cold air. However, there is a brief time shortly after the front has passed as the last clouds no longer interfere with your view of the sun where you may experience good seeing before conditions degrade. The window of opportunity is about 10 to 15 minutes.
Observing the limb of the sun will give you some clues as to the type of seeing you will encounter for your observing session. Large scale ripples that traverse the sun in fractions of a seconds can be attributed to local conditions - these indicate unfavorable conditions at your selected site that you may be able to adjust. Rippling evident along the limb that exhibits a more random motion is due to turbulence at higher altitudes. If you are observing the sun at a low angle this type of turbulence occasionally improves as the sun rises above a layer of disturbed air. However, daytime heating of the Earth's surface and adjoining air is your biggest enemy. This is why observing the sun at low altitude rather than near local noontime is recommended. For this same reason, it is important to be ready to observe as clouds clear your area before surface heating begins to affect the air.
More than once in this manual you will see references to recommended methods of observing the Sun and collecting your data. Multiple scans at different magnifications, for example, is recommended. Experienced observers have noted remarkable variations in seeing conditions over time spans lasting from a few seconds or even a few minutes. A patient observer who is ready to exploit improved seeing conditions will be rewarded with better quality data.