Observation Tid-Bit #1 - “Oh say can you See?”
- by Aaron Clevenson and Henry Norton
Well the sky is clear, not a cloud to be seen, and the equipment is all set. You are looking at your notes and blank forms with great anticipation of filling in as many as possible. The sun is quickly dropping towards the horizon… Hold on, hold on… What’s this? What is this field that I am supposed to fill in called “Seeing”. Looking I understand. As in “It is looking pretty good.” But Seeing? Of course I am seeing!
Well maybe, maybe not. Seeing, as it is used in astronomy, refers to how well you are actually seeing the things you are looking at. For example; have you ever looked at Jupiter when it is low on the horizon? Have you noticed that none of the details are sharp but they seem to be jumping and boiling? That is poor Seeing.
Seeing affects many things that you look at. It is caused by turbulence in the atmosphere. Some problem areas are:
- viewing low on the horizon, more atmosphere to be moving.
- large buildings nearby.
- large parking lots nearby.
- weather changes.
What it does to your observing is it reduces detail. This could be detail in a nebula, on Jupiter or the Moon. It could mean the difference between seeing the split between a close binary pair. Ultimately it determines your telescopes ability to resolve things.
So, what can you do about it, short of coming back another night? Not a thing! But what do you fill in on that blank line? Seeing is a rather subjective measure, but here is a breakdown that might help you fill in that blank on the form.
1 - Severely disturbed skies: Even low power* views are uselessly shaky. Go read a good book.
2 - Poor seeing: Low power images are pretty steady, but medium powers are not.
3 - Good seeing: You can use about half the useful magnification of your scope. High powers* produce fidgety planets.
4 - Excellent seeing: Medium-powers are crisp and stable. High-powers are good, but a little soft.
5 - Superb seeing: Any power produces a good crisp image.
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