Skip to content
$9.99 flat-rate Priority Mail shipping in U.S.
Search our store.
OTAs & TELESCOPES
EXPLORE SCIENTIFIC EYEPIECES
BAADER PLANETARIUM EYEPIECES
APM / LUNT EYEPIECES
EXPLORE SCIENTIFIC BARLOWS
BAADER PLANETARIUM BARLOWS
EXPLORE SCIENTIFIC FILTERS
BAADER PLANETARIUM FILTERS
DGM OPTICS FILTERS
FARPOINT COLLIMATION TOOLS
ASTROSYSTEMS COLLIMATION TOOLS
GLATTER LASER COLLIMATORS
GLATTER COLLIMATION ACCESS.
EXPLORE SCIENTIFIC ACCESSORIES
BAADER PLANETARIUM ACCESSORIES
GLATTER MISC. ACCESS.
NEXUS DIGITAL SETTING CIRCLES
SNAPZOOM CELL PHONE ADAPTER
SKY QUALITY METERS
ROR LENS CLEANER
OTAs and TELESCOPES
TELEVUE OTAs & TELESCOPES
ESSAY: Seeing and Transparency
See Extended Information for Full Description
See "Extended Info" for Text
Seeing and Transparency Explained
Seeing, Transparency, Clarity, and Darkness described as sky attributes
HOW ASTRONOMERS RATE THE QUALITY OF THE ATMOSPHERE
(understanding how the air influences the view through a telescope)
The atmosphere interferes with the telescope’s ability to see. Every beginner learns that axiom very quickly. First, we blame the scope: “I just didn’t spend enough money to get a good one.” But soon we talk with other amateurs or telescope shop employees and we quickly learn we’re all in the same boat. Young or old, big scope or small, we are all limited by this thick soup of an atmosphere we look through.
Though there are many factors at play in our abilities to penetrate that thick soup, there are two definitions that stand out in nearly every beginner’s text on the use of a telescope:
Seeing and Transparency.
Seeing refers to the steadiness of the atmosphere. It’s the “Great Equalizer” that makes the big, expensive scopes perform like smaller scopes, and limits even the biggest observatory scopes’ resolution to quantities similar to an amateur’s small scope.
It’s as if we were viewing the pebbles on the bottom of the shallows in a small lake—the quieter and steadier the water, the sharper and easier the view of those pebbles. But let a small amount of turbulence enter into the water above the pebbles and it becomes hard to tell what’s even there. So it is with the atmosphere—just a little turbulence in the air between your scope and space and the star images dance around and send out spikes in every direction until they become large fuzzy balls, denying us the clear view of the skies we paid our money for when we purchased our telescopes in the first place.
Seeing limits the magnification we can use--sometimes to quite low powers of 50X to 60X. Don’t worry—it’s probably not the telescope. Just be persistent in your observing and those moments when you can use higher powers will come.
The Seeing differences have been scaled by various observers in the past. A good example of a Seeing Scale (of horrors) can be seen in the Pickering scale of Seeing, a ten point scale that allows you to estimate how good your Seeing is.
It can be found on the web at: http://www.damianpeach.com/pickering.htm
Unfortunately, Pickering 1 is all-too-common, and Pickering 10 is quite rare.
Probably, more than anything else, this is what limits our abilities to see detail on the planets and the Moon. The good news is that out of every minute of horrible Seeing, there will be a few seconds of relative calm and the images will sharpen up, revealing to us that we did indeed buy decent scopes and that it’s just the crummy atmosphere preventing those scopes from seeing more.
Patience.! Keep at it, and sooner or later you will have one of those mystical moments when the Seeing steadies and allows us to see the Universe through a veritable Hole in the Atmosphere. Those of us out West and those of us in Florida see more of those nights than the people who live east of the Mississippi, but all of us are eventually treated to one of those nights of especially good Seeing.
One clue: the Earth’s atmosphere steadies, in most places, more completely between the hours of midnight and dawn. If your interests in planets, Moon, or double stars makes you seek the higher powers, these are your best hours to observe.
Transparency is the other atmospheric issue for us amateur astronomers, according to the books. It is usually defined as the darkness of the night sky. The farther you are from civilization, the more Transparent is the sky, or so it is said.
I want to deviate from the common definitions, here, and refer to Transparency as being actually a combination of two atmospheric characteristics, which I will define as Darkness and Clarity. I’d use the same term, transparency, for the second characteristic, but this might be confusing.
Darkness is what is often referred to as transparency. But it doesn’t take very many trips to a dark site to notice that the stars don’t always appear equally bright—especially toward the horizon. The stars overhead appear just as impressive, but the fine details in the Milky Way seem to be less obvious and the sky slightly less impressive.
I’ve used a sky brightness meter to actually measure the brightness of the night sky, and these nights of seemingly lower clarity can measure just as dark as the nights when the Milky Way looks like big, cumulus, clouds in the sky. An obvious case in point is that if you are at a site well away from civilization and it’s overcast, the sky is quite dark, yet the atmospheric Clarity is terrible.
So, while Darkness is an important thing to seek out—it determines how “deep” our telescopes reach, both in magnitude and into the Universe—it becomes obvious to the sky observer that it has to accompany atmospheric Clarity to really be called Transparent.
Clarity is probably best explained as the absence of aerosols and dust in the atmosphere. When water vapor or wind-blown dust is present between your scope and space, you will just not see the faint details in deep-sky objects that a clear atmosphere allows. There will be more Extinction (the reduction in your ability to see faint stars with lower altitudes) in the air, and more light scatter. At its worst, the sky seems to take on a silver sheen and the Milky Way’s details are lost.
These two characteristics can be exclusive. Here in LA, we often get strong winds that blow all water vapor and dust and smog out to sea, and the visibility of distant mountains and buildings is superb—as if they were just next door instead of many miles away. We have wonderful Clarity. Yet, we still have the worst light pollution in the nation, and the skies are not dark at all. It is true during these times that somewhat fainter stars can be seen, but the dimmest are several magnitudes brighter than what can be seen in a dark site, and the Milky Way is never visible.
So the stargazer has to accept that conditions will not always be perfect when he/she heads for the dark place to observe the sky. But is there a way we can help ourselves experience better Seeing, Darkness, or Clarity?
Yes there is.
Seeing is often better in those places where the wind, if it blows at all, blows in a smooth manner without turbulence. You can look for sites near a body of water, or in a saddle-shaped mountain valley. There’s a site in LA famous for its wonderful Seeing—Mount Wilson. You may recall they built a big observatory there over a hundred years ago.
Darkness is usually better the farther from cities you are. State or National Parks and Monuments excel in this regard. Look at the information for your local areas on the website, www.cleardarksky.com to find the nearest dark site. Be prepared to drive a bit if, like me, you live in a big city.
Clarity is best at high altitudes (one of the advantages we Westerners have), so if you live at low altitudes, the days just after a front passes may be the best days to observe the fainter objects. Seeing can be turbulent at these times, but there is usually a day or two after the front comes through when the air has not yet “steamed up” again, yet the air is also not strongly turbulent. That’s the day to go out.
I hope my simple explanation will help you understand what you are seeing when you take your telescope out under the stars. This is a hobby that rewards perseverance—observe often and the magical combination of good Seeing, Darkness, AND Clarity will be encountered a lot more often.
Good luck, and clear skies to all,
Seeing, Transparency, Clarity, and Darkness described as sky attributes
ESSAY: Newtonian Collimation v.4
ESSAY: Telescope Accessories
ESSAY: CONTRAST IN EYEPIECES
ESSAY: Laser Collimation
ESSAY: Using a Telescope
ESSAY: Eyepiece Parameters and Aberrations
ESSAY: DSO Visibility Index
ESSAY: Choosing a Nebula Filter
Share your knowledge of this product.
Be the first to write a review »
Back to top