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Frequently Asked Questions 

Seeing and Transparency Scales
Magnitude and Brightness Scale
Filters for Visual Observation

Use the following scales for astronomical seeing and transparency when filling out your observing logs.

ASTRONOMICAL SEEING

  • LEVEL 1 - Severely disturbed skies: Even low power* views are uselessly shaky. Go read a good book.
  • LEVEL 2 -  Poor seeing: Low power images are pretty steady, but medium powers are not.
  • LEVEL 3 - Good seeing: You can use about half the useful magnification of your scope. High powers* produce fidgety planets.
  • LEVEL 4 -  Excellent seeing: Medium-powers are crisp and stable. High-powers are good, but a little soft.
  • LEVEL 5 - Superb seeing: Any power eyepiece produces a good crisp image.

* The PRACTICAL LOWEST power magnification for any telescope is approximately 7 times for each inch of aperture. Example: 28X for a 4-inch (100mm) diameter telescope.
* 'The
PRACTICAL HIGHEST power magnification for any telescope is approximately 50 times for each inch of aperture. Example: 200X for a 4-inch (100mm) diameter telescope.

TRANSPARENCY SCALE

0. Do Not Observe - Completely cloudy or precipitating.
1. Very Poor - Mostly Cloudy.
2. Poor - Partly cloudy or heavy haze. 1 or 2 Little Dipper stars visible.
3. Somewhat Clear - Cirrus or moderate haze. 3 or 4 Little Dipper stars visible.
4. Partly Clear - Slight haze. 4 or 5 Little Dipper stars visible.
5. Clear - No clouds. Milky Way visible with averted vision. 6 Little Dipper stars visible.
6. Very Clear - Milky Way and M31 visible. 7 Little Dipper stars visible.
7. Extremely Clear - M33 and/or M81 visible.

BRIGHTNESS SCALE

Magnitude*

Comments

very bright stars

-4

  Venus at its brightest

-3

 Jupiter at its brightest

-2

 Sirius in Canis Major, the brightest star in the sky

-1

 Betelgeuse in Orion

0

 Vega in Lyra

+1

 Spica in Virgo, Deneb in Cygnus, Pollux in Gemini

+2

 Polaris, the North Star

+3

 Megrez, the faintest star in the Big Dipper

+4

+5

 Probable naked eye limit in the suburbs

+6

 Probable naked eye limit in the country

+7

+8

 Neptune

+9

 Approximate limit of typical binoculars  (Comments: assume as

+10

 Approximate limit of a 60-mm telescope  dark a sky as possible.)

+11

 Approximate limit of a 3-inch telescope 

+12

 Approximate limit of a 4-inch telescope 

+13

 Approximate limit of a 6-inch telescope 

+14

 Approximate limit of an 8-inch telescope 

very dim stars

* Magnitudes are approximate
The difference in brightness between any successive two number is a ratio of two and one half (2.5) times. 

FILTERS FOR VISUAL OBSERVATION

A good resource at the telescope is a set of colored filters. Filters can be acquired from various sources. Consult your astronomy magazines. Kodakís Wratten series can be purchased in over a hundred colors and densities, and can be mounted in slide mounts and simply be held between the eyepiece and the eye. For longer observations, as when sketching, screw-in filters are available for both 1-1/4 and 2-inch eyepieces. You donít need to choose between dozens of colors, though; only a few will do.

Filters can reduce glare, improve image definition, and enhance tonal contrast. Here are some suggestions.

  • A BLUE filter, such as a Wratten #44A, 47B or 80A, can be used for the detection of high altitude clouds on Mars, white ovals and spots in the belts of Jupiter, and the zones of the clouds of Saturn. It can also be used to cut down glare on a bright Moon.
  • A GREEN filter, such as a Wratten #58, allows you to see more clearly the edges of the Martian polar caps, and enhances the belts and Great Red Spot in the clouds of Jupiter.
  • A YELLOW filter, such as a Wratten #8, 12, or 15, can improve markings in the clouds of Venus and enhance Martian dust storms.
  • An ORANGE filter, such as a Wratten #21, is one of the more useful ones you can have. It is used for bringing out detail on Mars, and enhancing some of the zonal detail on Jupiter. An orange filter also darkens the blue sky, so daytime observations of Jupiter, Venus, and the Moon are much improved.
  • A RED filter, such as a Wratten #23A, 25, or 25A, can also be used to enhance contrast on Mars, Jupiter, and Saturn. A red filter, however, is fairly dark, so it works best on larger aperture telescopes which give brighter images. Flipping back and forth between red and blue filters can sometimes bring out subtle colorations on the Moon.
  • A POLARIZING filter can cut down glare when observing a nearly full Moon, making it easier to see ray structure. It will also cut down day-time glare.
  • LIGHT POLLUTION and O-III filters are good for planetary and emission nebulae.

Q. Will a light pollution filter be worth the $80 or so to improve my view?  Also, I see there is a 'filter A' and a 'filter B'.  What would be the difference?

A. Yes and no. Not all nebula respond equally to any given filter. Additionally, the performance is variable. Certainly natural air glow affects the view, but it seems that ambient temperature also affects filter performance and this effect seems more pronounced with the narrow band LPR filters. Sometimes filter "A" works well on object "X", but sometimes filter "B" works best on Object "X".  In my experience, the wide band LPR filters provide more consistent performance even though the visual effect usually isn't as dramatic as with a narrow band filter. The one wide band filter I have gets used twice as often (2/3rds of the time) as the two narrow band filters do.

The wide band filter is probably the best "bang for the buck". However, since your site, your scope, and your eyes are different, you may have to get both types of filter and do some serious observing to decide which filter works best for you. 

Doug Kniffen

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