M

1. 0; the worst possible seeing, the image is completely unsteady and no fine detail can be made out.

2. 2-4; the whole disk moves with little detail made out.

3. 5; the planet's disk is stationary but boils, as though viewing the image through a moving liquid (Benton). Some fine detail can be intermittently seen.

4. 6-9; the planetary disk is stationary with little scintillation and fine detail can be made out.

5. 10; perfect seeing. The image is perfectly still. Smaller features normally not seen are easily made out.

Normally, a serious observation under high magnification should not be attempted unless the seeing is at least "5" or better.

Although poor seeing may be difficult to overcome, there are steps we can take to make sure we avoid conditions that would make seeing worse. We have just discussed that as heat rises, currents are put into motion that cause seeing to deteriorate. We want to avoid observing from locations that contribute to this problem. Keep in mind that stone, concrete, brick walls, and patios are all structures that act like a heat sink, storing up heat during a sunny day and radiating it away long into the night. Therefore, you should avoid setting up a telescope on a brick or concrete patio, or next to a building or street. Certainly do not set up a telescope on a concrete driveway or sidewalk, and try to avoid observing over rooftops or fireplace chimneys. As you see, with a little thought, some of this becomes common sense. If observing from inside a shelter, such as an observatory dome or roll-off roof facility, the temperature inside the shelter should not be different from the temperature outside. If observing from a dome in which the air inside the dome is warm from the day's heating, air will rise through the dome opening for hours until equalized. Have you ever gazed over a barrel of burning leaves at something in the distance? Remember how wavy everything was in your field of view. The heated dome will produce the same effect. Shelters should always be opened early enough prior to observing so the temperature inside can equalize with the outside air (Fig. 8.1). Large fans to move air during the day may be needed to help with this problem. Even outdoors, we often find that seeing improves as the night wears on and things cool down. I have often found the best seeing for observing Jupiter occurs after 2:00 a.m.

And so, we are not only concerned with the observing environment up high in Earth's atmosphere, but also the immediate environment around our telescope. Even telescope design can play an important part in "seeing". A refractor or a closed tube reflector like a Schmidt-cassegrain that has been stored indoors during the day will take considerably more time to cool down than a Newtonian reflector whose optical tube is an open truss system. With the closed tube, wavy air currents inside the tube can result in poor seeing until the tube cools down. Poor seeing not only affects visual observations, but photography and imaging as well, which we will discuss later.

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