...What's a dark and "still" site?

   The Earth is shrouded in a thin bubble that we call an "atmosphere". The bubble is comprised of a soupy mix of air, moisture and pollutants, which is constantly bombarded and illuminated by the photons of our own artificial light. The bubble is not still or smooth, but is full of tides and ripples that are always in motion. As the ripples pass our field of view, celestial objects are alternately hidden and revealed in a strobing rapid cadence that we call turbulence. This is why stars appear to twinkle, and why it's so difficult to see objects in deep space with ground-based telescopes. The best observing sites are where the bubble is very dark, and very "still".

Left: NASA photo

    There are a few indicators you can use to tell how dark and still your location is. First, the more "still" the air is, the less stars will appear to twinkle. Second, the darker the site is, the blacker the sky will appear (especially in the eyepiece!). And third, the darker the site is, the more you can see. For instance, at a fairly dark site, you can count 5 stars in the little dipper with the naked eye. You may even see a little of the Milky Way overhead. At a very dark site, you can make out 7 stars in the little dipper, and you should see "structure" in the Milky Way (i.e: - two shades of gray, and a hint of the carpet of light associated with all those billions of stars). If you live in an urban neighborhood, the background sky in your telescope's eyepiece will appear anywhere from dark gray to pink. Sky backgrounds that are pitch-black are extremely rare, reserved for those "dark sites" that most of us only read about -- the desert Southwest, the Florida Keys, a mountain observatory...

 
 
EXAMPLE:
This is how the galaxy M-104 might appear in the eyepiece of a 10-inch SCT at a very dark site.
This is how it would appear with the same telescope, but from a light-polluted urban location. Note that the sky background is not as "black". The lack of contrast causes fainter details to blend-in and wash out, and so the galaxy appears "smaller". 

Most good observing locations fall somewhere between these two extremes.

Drawings © The Belmont Society

 

 

...A Chart for Resolving-Power

Telescope aperture size versus resolving power, dawes limit, and maximum power.
Use this chart to determine your aperture's ability to split close double stars.

(a) Go to your aperture size in the left two colums. 
(b) Find Limiting magnitude, Dawes limit, and Maximum power to the right.

 Aperture Size
Limiting Magnitude
Dawes Limit
Maximum Power
MM
Inch
50
2.0
11.2
2.3"
100x
60
2.4
11.6
1.9"
120x
75
3.0
12.1
1.5"
150x
100
4.0
12.6
1.2"
200x
112
4.5
13.0
1.0"
225x
125
5.0
13.2
0.9"
250x
150
6.0
13.6
0.8"
300x
200
8.0
14.2
0.6"
400x
250
10.0
14.7
0.5"
500x
300
12.0
15.1
0.4"
600x

 

Notes:
Limiting magnitude refers to the faintest star visible.
Dawes limit refers to the minimum separation resolvable.
Maximum practical power for any given telescope is typically 2 times the aperture in millimeters (or 50 times the aperture in inches) although powers over about 400x are rarely practical, and only even POSSIBLE under very-very DARK skies.

 

 

...Seeing Guide for Viewing Double Stars . . .


. . . a gauge of how good (or bad) conditions are for viewing doubles.

 

On a scale of 1 to 10 (10 being the best; 1 being the worst):
NOTE: The 3rd condition (7 to 8) is the minimum qualification for a "dark site".

1 to 3 (Urban, polluted) = Even "easy" doubles are difficult to split. Colors are not discernable (white and yellow stars look alike). The Trapezium only reveals 3 resolvable components.
4 to 6 = Wide doubles are splitable, but it's difficult to use more than 60 power. Some colors are discernable (the difference between white.and.yellow stars is noticeable, but barely).
7 to 8 = Trapezium resolves at 40 power; 28 to 30x per inch of aperture possible. White.and.yellow stars are easily discernable.
9 to 10 (Very dark site, rare) = Stars display perfect Airy disks with clearly defined (textbook) diffraction rings. White.and.yellow stars are clearly discernable. 70 to 75x per inch of aperture possible on good achromatic refractors. 100x (and more) per inch on APO refractors. Stacking of Barlows is possible. Antares splits at 250x.

 

Beware! The dangers of  TV Shopping Networks, Big Box and Department Stores
Telescope Recommendations: Affordable Scopes
Telescope Recommendations: Ultra-High Quality Scopes
Telescope Recommendations: Mid-Range Scopes
Mount Recommendations
Binoculars - the portable solution
The Mars Hoax strikes again!

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