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Observing the Planets with Color Filters (1)

  Scope City Learning Center
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By: Jeff Beish, Former A.L.P.O Senior Mars Recorder

INTRODUCTION

A set of photo-visual color filters is an important observing aid that every planetary astronomer should have. Color filters help overcome image deterioration caused by atmospheric scattering of light, permits separation of light from different levels in a planetary atmosphere, increases hue contrast between areas of differing color, and reduces irradiation within the observer's eye. All of these factors increase the sharpness of details in the atmospheres that are seen on the planets Venus, Mars, Jupiter, Saturn, Uranus and Neptune, and on the surface of the Moon and Mars.

Planetary observers work endlessly to improve the definition of telescopic images of the Moon and planets. While using filters will not eliminate optical defects in the telescope they will help improve image definition even in a bad system.

IMAGE definition in a telescope is dependent upon resolving power, contrast, and sharpness. Resolving power is primarily a function of aperture; however, optical quality, collimation, tube currents, etc., can have great effects. IMAGE sharpness is a factor of "astronomical seeing," atmospheric scattering, irradiation in the eye, and the condition of observer's eye. Contrast is the difference in brightness between areas of an image. Each of these factors can be improved upon by the use of color filters at the telescope.

The Moon and some planets are brighter than deep sky objects and surface details are often lost in the glare. Filters cut down the intensity of the brighter planets and increase the contrast of the features in the process. For example, the Moon is very bright, so, a neutral density filter used in conjunction with a green filter or "Moon" filter will cut down the tremendous glare and greatly increase contrast in Lunar surface details.

In addition to image contrast, color contrast is important to the planetary observer. Differences in color hues between features on a planet can lead to strange perceptions and confusion about true nature of the planet in study. This is primarily a function of the human eye; however, some optical systems with chromatic aberrations shift certain colors too. To help explain this we must look at some of the properties of the human eye and how we perceive colors. Color Contrast is affected by sharpness of boundaries and by differences in color and shade.

SOME EFFECT OF COLORS IN THE HUMAN EYE

The human eye contains two light sensing elements or nerve ends: cones and rods. Rods respond to different intensities of light and not to color stimuli. Three types of color sensations are produced by a composite response of red, green, and blue color-sensitive cones. The smaller cones are 0.0015 mm in diameter and are called fovea. In order to stimulate two cone nerve ends the subtended diameter of the light beam has to be larger than 12.4 seconds of arc. The eye is sensitive to wavelengths ranging from deep violet 390 nanometers (nm) to 710nm (deep red). Maximum sensitivity is around 550nm at normal illumination. With decreasing light levels this sensitivity shifts toward the blue. Cones do not function at light levels below 0.03-candle power per square meter (cd/mē):

Scotopic (night) vision and photopic vision (below 0.03 cd/mē ) are subject to Purkinje effect, which causes objects to appear bluer to us in very low light conditions.

In some lighting conditions yellow-green or reddish-orange objects appear more yellow than they really are. The size, or angular extent, of the object may even effect our color perception, i.e., very small reddish features on Mars may appear gray or blue-green. Brightness ranges from 0.5 to 50 cd/mē, above the Purkinje effect, referred to as the Bezold-Brucke phenomenon. This renders red, yellow, green, and blue light to remain the same hue, with decreasing intensities the yellow- greens colors begin to look more yellow and violet and blue-green appear bluer. At high surface brightness colors tend to lose saturation. While increasing the angular extent of an object colors begin to increase in saturation, especially with violet, blue, and green.

While increasing the angular extent of an object colors begin to increase saturation, especially with violet, blue, and green. Tritanomalous vision is when violet and yellow-green colors begin to appear gray and other colors look more reddish- orange or blue-green [Dobbins et al, 1987].

If we consider the color of Mars is predominately RED, with a mix of features displaying dark gray-orange and brown hues, it becomes interesting when attempting to describe Martian dust clouds as "yellow." When we observe bright Mars against the dark nighttime sky, the planet's color hues are often perceived as complementary to the dark background sky. This effect is known as "simultaneous contrast" [Hartmann, 1989].

After-images are seen as a ghost image in your eye after staring at some object for a long time. The after image takes on the complimentary color from the object, that is, it will appear as a ghost image but is of the opposite color of the image you stare at.

IMAGE CONTRAST

Contrast, as measured by our eye, is the difference in brightness or intensity between various parts of the telescopic image, i.e., a star against the background sky. In planetary work contrast efficiency of our telescopes is very important because a planet's surface or atmosphere is composed of various materials that reflect different levels of Sunlight. IMAGE contrast can effect our color perceptions, especially while observing the planets using larger aperture telescopes.

If we scatter stray light throughout the image it makes the dark areas of the object brighter and the bright areas darker, so, we loose contrast. Mars might display very fine surface details during perfect seeing in a telescope with 12% secondary obstruction and barely any detail in one with 35% obstruction, even though the limbs (edges) of the planet are sharp and well defined in both scopes. A quality refractor is an example of a high contrast instrument, but suffers from other problems, such as chromatic aberration, not found in reflecting telescopes. We will try to design our Newtonian to deliver near refractor like contrast without the color problems.

This also applies to extended deep sky objects such as galaxies or nebula. These images are made up of varying intensities from bright wisps with dark lanes to dim fuzzy globs. Contrast is calculated by a simple formula:

c = (b2 - b1) / b2

where b1 and b2 are the brightness of each of two areas of the object and c is the contrast. If we measure brightness in candle power/meter squared (cd/mē) the Earth's daylight sky brightness is about 8000 cd/mē.

For example, Jupiter has a surface brightness of around 600 cd/mē for light areas. If we compare a dark belt of 300 cd/mē, then the contrast between these areas would be:

c = (600 - 300)/600 = 0.5 or 50%

If we scatter light from the bright area, say 50 cd/mē and add it to the dark belt then the contrast between the two becomes:

c = (550 - 350)/550 or 0.36 or 36%

A relatively small amount of scatter may cause a significant decrease in image contrast.

Figure 1. Two images of Mars taken with same CCD camera but with different contrast levels. The image on the left was taken without a red filter and right image with red filter. Contrast difference is readily apparent. Images by D.C. Parker.

Next >> Observing the Planets with Color Filters (2)

 


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