Seeing versus Transparency - a simple explanation

Introduction

When you first get started in astronomy you might hear terms like seeing and transparency.   The lingo is often confusing until you finally "get it" and it clicks in your head.  I'll try and give a simple explanation to explain the differences between the two.  Other sites are already out there that go into the details.

Seeing

Seeing is a term that describes how stable the air is.  The more stable the air, the finer details you can see in your telescope.   Have you ever seen a mirage?   The shimmering air over a hot road causes the road behind it to blur.  This is the same thing that happens in your telescope, but since it is magnified many many times, it doesn't take as much of a difference in temperature to show.   Differences in air temperature in the different layers of the atmosphere cause small "mirages".  Even looking over a warm rooftop can greatly affect your seeing!

Viewing the planets are where seeing is the most critical.   To get the fine details on Jupiter's belt or Mar's surface or the rings of Saturn you need the nice stable air.  

Splitting double stars and resolving stars in globular clusters are other areas that need good seeing.   Looking at galaxies, nebula and other objects are less critical, so on nights of "bad seeing" those are better targets of interest.

If the stars are twinkling a lot, that is an indication of poor seeing.   Often they twinkle near the horizon, but are steady overhead.  You are looking through more air when going near the horizon, so there's more opportunities for the air to deflect the light.  If the stars are clear and steady near the horizon, it's a night of excellent seeing.  Get out the scope and look at whatever planets are up!

When you are looking through the telescope and the seeing is bad, keep looking.  Often you'll get little patches of good seeing and the view will clear up for a couple seconds.  Be patient and you'll be rewarded.   Even on nights of good seeing, you'll get small patches of excellent seeing!

Transparency

Transparency is a little more obvious as it is a measure of how much light makes it through the atmosphere.   The worst days of transparency are cloudy days!  Many things affect transparency like dust, humidity and pollution.  On bright objects like Jupiter transparency won't affect your view much (unless it is really horrible).  In fact even a little bit of fine haze can help on the planets.   

But on other objects like galaxies and nebula transparency can be the difference between find an object or not even seeing it all!  Some nebula from my home (with moderate light pollution) disappear completely when the transparency is bad.

An Analogy 

Imagine you are the bottom of a pool looking up at the blue diving board.  The water is crisp, clear and still.  Both the seeing and transparency are excellent.  The diving board is clear and it's easy to see the color.

One child jumps in the pool and makes waves all over the pool.   The seeing has just gotten much worse and the view of the diving board shakes back and forth.  It's hard to make out the edges of the diving board and it looks more like a shifting blob.   This is the same thing that will happen to the view in the telescope.  The air is shifting slightly and causing the light to bend back and forth just like the waves in the pool.

Another child who was in the sandpit and covered with sand now jumps in to wash off, then gets out.   The sand goes everywhere and is suspended in the water.  The water gets murky and hard to see through.  Now the diving board is shaking back and forth like above, but it's also dim and hard to see. You may not even know what color it is.  Both the seeing and the transparency are bad.  

Once the water has settled down (seeing improved again) you can make out the shape of the diving board again, but it's still very dim.   All the sand in the water is blocking some of the light.  Now the seeing is good, but the transparency is poor.

Telescope "seeing"

If you don't let your scope cool long enough, then your telescope itself can affect your seeing.   Basically you have warm air coming off your lens/mirror which is making a localized air current in the scope causing the light to shimmer and shake.  The effect is usually much worse than atmospheric seeing, so it's important to let your scope cool to the air temperature by leaving it out for a while before you use it (at least for planetary or other high magnification work).

Generally the larger the scope, the longer it needs to cool.  Many of the bigger scopes actually have cooling fans for just that reason.  If you can, set out the telescope as soon as the sun sets.  If you set it out while the sun is still up, then you'll heat up the scope, which is counter-productive!

Smaller scopes ( less than 4") take less than half an hour to cool.  Many you can use right away with very little degradation.  

Medium sized scope (4" to 10" or so) take about an hour to cool.   

Larger scopes (over 10") can take 2 hours or more depending on the scope type.   Some larger scopes never fully cool, especially when the air temperature keeps dropping as the night goes one.  They "chase" the ambient temperature.

You can use a larger scope before it's fully cooled, but just use it for lower magnification work.  I take out my large 18" scope and start using it immediately, but only at 60x magnification.  Sometimes it's neat to look at Jupiter at the beginning of the session to see how bad it looks, then I have another look a couple hours later before I put the scope away.  The difference is astonishing in how much detail you see.