- What does "good seeing" mean?
- The atmosphere is a complex and ever changing mass of air which can
drastically affect how well you can see with your telescope. To the
naked eye, on what would appear to be
- What does "good seeing" mean?
- The atmosphere is a complex and ever changing mass of air which can
drastically affect how well you can see with your telescope. To the
naked eye, on what would appear to be a clear night, stars and planets
might look just fine. But through a telescope, focusing may actually be
next to impossible.
- Observing planets, planetary nebulae or any celestial object with
details at high power requires excellent seeing conditions. The seeing
is the term used in astronomy to quantify the steadiness or the
turbulence of the atmosphere. Seeing should not be confused with sky
transparency, which is the terminology used to qualify the darkness of
the sky. When we look at planets, we need high power to see all the fine
details but most of the time we are limited by turbulence occurring in
the telescope (local seeing) and/or in the atmosphere. During a night of
bad seeing we are usually limited to see only two bands on the Jupiter
disc and we can hardly use power over 100-150x. On excellent seeing
conditions we can use high power and see many bands, white spots,
festoons and details in the great red spot. Excellent seeing with high
quality telescopes can also show details on the largest moon of Jupiter,
Ganymede. What we are seeking is the best nights where we can boost our
telescopes to their limits… which reach as high as 50X per inch
diameter for quality telescopes… which means 500x for a quality
10-inch ( 25cm ) instrument.
- A night of exceptionally good seeing, a night where the detail seen on
Jupiter causes observers to swoon and swear, is thought to be rare. It
would be boon to a know in advance when good and bad seeing might occur.
- see Mars for an
example of effects of seeing
- twinkling; irregular changes in the brightness of a star caused by
atmospheric turbulence. A star will appear to wander around its
average position in a telescope since the image is being disturbed
by the atmosphere.
- contrary to popular belief, crisp, clear Winter nights, with the
stars twinkling like Christmas lights, are the worst possible for
- factors affecting seeing:
- haze can be due to a number of different factors, but most
commonly is pollution. For example, if no wind has
blown, smoke from cars and factories accumulates close to
ground level. Haze can also be due to an environmental effect called
inversion. That's when cooler air (along with air born
pollutants) is trapped below a layer of warmer air. This obviously
affects seeing by reducing detail and brightness of objects. Expect
only the planets and brightest of stars to show through during these
times. Light Pollution also increases due to haze or smog in
- Light pollution
- The Jet Stream/Upper Level Winds:
- if the jet stream is overhead or just the fact that upper level
winds are acting up (winds several kilometres up that cannot be felt
at ground level), it'll look as if everything was under flowing
water when you look through your telescope.
- keep in mind that your telescope doesn't just magnify the objects
you want to see, it also magnifies the atmosphere as well. That's
why the Hubble
telescope is in space and can get those remarkable images. Focusing
at higher magnifications will be next to impossible to accomplish
during these times. And unless you have sophisticated technology,
like that found on the Keck
telescopes on Mauna Kea in Hawaii, there's nothing you can do about
it, but wait for a better day.
- local air disturbances:
- "surface layer seeing":
- lower atmosphere temperature differentials and wind shear
effects especially in the lowest 200m above ground level
- in general, the higher up in altitude, the better the seeing
because there is less atmosphere to see through. This is one
reason professional observatories are usually located high up on
mountain tops. However, some sea level locations (southern
Florida for example) can be nearly as good at certain times and
some locations such as the Midwestern U.S. are nearly always
- telescopes with top of opening at least 10m above ground level
can be expected to perform much better than those at ground
- rising heat:
- try to use a position
which is not looking at the planets over the top of the house
especially if the house heater is on. The heat fluctuations
coming off the top of the house get between you and the planets.
- bad seeing caused by local effects, like a hot driveway, is
properly called ground seeing.
- "dome seeing"
- poor seeing 