Chapter 7 -- EQUIPMENT TO OBSERVE MARS
The planetary astronomer has a wide variety of telescopic equipment available to the planetary astronomer in these modern times. You may wish to purchase commercially made telescopes or build one of your own design. You may even want to ask for time on an instrument at an established observatory. In any case, one needs to consider several important factors about telescopes before getting started.
In recent times, telescope optical performance has come under more critical scrutiny, and while it is not the intention of this handbook to recommend or not recommend certain commercial products, a word of caution about obtaining optics may be in order. If possible before purchasing, test the optics on a planet, not the moon or a galaxy, nor on a double star. Join a group of observers during a star party when a planet is visible and compare the different types of optics.
The most common telescopes in use today are: the Refractor achromatic or apochromatic lens system, the Newtonian reflector all mirror optical system, the Classical Cassegrain reflector folded mirror optical system, and the Schmidt-Cassegrain, or Catadioptric, which combines elements of both the refractor and the reflector. Telescopes are also classified for different uses according to the focal length of their optical systems.
The most important consideration for an astronomical telescope is to have quality optics throughout, including eyepieces, and a sturdy equatorial mount. For the planetary observer, consider that all of the above mentioned types of telescopes have certain strengths and certain weaknesses.
Refractor Telescopes. Refractors usually have f/15 or f/16 focal ratios, are the most light-efficient optical design, give maximum image contrast at high powers, and are simple to use, and require little cleaning and maintenance. However, moderate size (10 inches or more) refractors are expensive per inch aperture, difficult to house, not portable, and usually not completely achromatic, thus requiring a new focus each time a different color filter is used for visual observations or photography.
A new lens system, the Apochromatic refractor, reduces chromatic aberration to the point one is hard pressed to notice. The cost of these instruments has come down substantially in recent years and is affordable to the amateur planetary observer.
Newtonian Telescopes. Used for Lunar and planetary work should have focal ratios from f/6 to f/12 with small diameter secondary mirrors. Image contrast is a function of the area of the central obstruction caused by the secondary mirror, its mirror holder, and the spider support system. The simple rule is: the smaller the diameter of the secondary with respect to the primary mirror, the better the apparent contrast. An obstruction ratio between 10% and 15% will make an excellent planetary Newtonian telescope. The Newtonian reflector is completely achromatic and gives sharp images through all color filters.
Classical Cassegrainian Telescopes. The Classical Cassegrainian reflecting telescope has focal ratios from f/15 to f/60 with small secondary mirrors. They are comfortable to use, have folded, compact optical systems, and require medium size mounts, and, in modest instruments, are portable. The optical design is completely achromatic for all color filter work, gives excellent image contrast, and is a stable scope for photographic patrol programs. Focal ratios of below f/20 require relatively large secondaries that reduce contrast significantly.
Schmidt-Cassegrain Telescopes. Schmidt-Cassegrains are usually a compromise between a planetary type and a deep-sky system, having f/10 or f/11 focal ratios. This type of telescope is extremely compact, easy to use, light in weight and portable. Little maintenance is required because of the closed tube. Image quality may vary from poor to satisfactory in commercially produced models. It is advisable to thoroughly test the quality of the optical system before the warranty expires. With the use of a Barlow Lens, this type of instrument can become a fair planetary telescope by increasing the focal ratio two- or three-fold, to f/20 or f/30. A systematic photographic patrol of the planets is also possible by use of eyepiece projection.
Richest Field Telescopes (RFT). RFT’s have short focal ratios of f/3 to f/5 and are used for wide-field observations, deep-sky objects, and comet studies and are excellent fast photographic systems. This type of instrument is not suited for planetary studies because of inherent low contrast images caused by a large secondary mirror usually installed and the small image scale produced by its short focal length. Also, fast optics are difficult to produce with a sufficient figure quality required for planetary work.
The above is a very brief description of the various telescopes used in planetary observing. See AMATEUR TELESCOPE OPTICS for a more complete and detailed account of the astronomical telescope. For those who are ATM’ers/electronic hobbyists join this interesting automation group: SGL Observatory Automation and the ephemeris computers go join the ALPO Computing Group ALPO Computing Section.
With the advancing technology in astronomical equipment and resources found on Internet this section can only be a guide for beginners. Technology has advanced so fast that attempting to keep up with it here is futile; so astrophotography, CCD and webcam imaging will not be discussed and left to the volumes of web sites devoted to those interests.
PRACTICAL LIMITS OF THE PLANETARY TELESCOPE
The following is a list of possible Martian studies for various telescope apertures:
Less than 6 inches: Observations are limited by resolution to the study of large surface features, bright clouds, limb brightenings, extensive yellow dust clouds as indicated by low contrasts of gross features, polar region conditions, and blue-clearing within 2 or 3 months of opposition. The routine use of color filters, which allow high transmission of light, permits the observer to collect a useful set of observations showing seasonal changes. (Blue Clearing is a phenomenon that enables observers to see quite clearly Martian surface details that are usually blocked out in violet light).
6 to 10 inches: Observational studies of conspicuous surface detail are possible within 3 or 4 months on either side of opposition. Variations of surface features, seasonal cloud activity, and blue-clearing can be successfully studied in a routine manner with the aid of a full set of color filters. Positional micrometric work on the retreat of polar cap boundaries and on dark feature boundaries showing change can be done. Much needed planetary photography with medium to fine grain films (Tech. Pan 2415, Kodachrome 64, Kodachrome 200, and 200-Ektachrome) using large f-ratios f/150 to f/200 is possible.
12 inches and larger: All of the observations mentioned above can be done with a higher standard of accuracy and ease. Critical visual and quality photography at f/60 to f/200 of surface variations and atmospheric meteorology is possible on a professional level. It is possible to detect subtle color changes with these larger apertures. Quality color slides and multi-color black and white photography in red, yellow, green, blue, and violet light is practical. A professional type synoptic color filter observational program is most useful for Martian research.
The quality of an ocular, or eyepiece, is just as important to the telescope’s optical system as the primary mirror or objective lens. Good contrast images can only be obtained with high quality multi-coated oculars. There are many types and brands on the market today with focal lengths from 4 mm to 75 mm. A basic set of 3 to 6 oculars is a good investment that will last a lifetime. The Plossl and orthoscopic type eyepieces with standard 1 1/4" diameter barrels and multi-coatings will give excellent planetary images. Old symmetrical-design eyepieces work well with refractors and some Cassegrains. Oculars should be threaded on the field lens end to accept standard color filters and polarizers.
Of course when choosing a set of oculars, we must consider the type of telescope employed. For the long focal length Refractor or Cassegrain telescopes, one should consider longer focal length oculars that provide greater eye relief and reduced color aberration. Oculars with focal lengths from 10 mm to 28 mm are excellent for high magnifications and 32 mm to 45 mm focal lengths for medium-power work. For the shorter focal length Newtonians, short (4 mm or 5 mm) to medium (7 mm to 18 mm) focal length oculars are best for the high-power work required for planetary observing.
A Barlow Lens
A quality Barlow Lens, or telenegative amplifier lens, is a most useful optical device for the planetary astronomer and astrophotographer. It can double or triple the effective focal length (efl) of your telescope. Its effect is to double or triple the power of each ocular used, which expands the choice of focal lengths of your set of eyepieces.
Furthermore, the Barlow Lens allows greater eye-relief because a longer focal length ocular can be used to obtain an equivalent magnification. This also allows the more convenient placement a color filter between the eyepiece and your eye. A quality Barlow Lens is achromatic and multi-coated, and has all optical elements centered on, and perpendicular to, the optical axis. The lens must be mechanically well constructed.
Figure 7-1. The Barlow lens. a = b / M, b = f(M - 1), M = b / f + 1
Most Barlows have a set amplification factor of -2X or -3X. There is available a research grade Barlow that is multi-coated and achromatic and offers a continuously variable amplification between -2X and -3X with a calibrated scale. This one important optical tool can expand your effective set of oculars many times. (see Figure 7-1).
COLORS OF MARS
What color is Mars through a telescope? This question has been asked by astronomers for at least three centuries and is a subject of debate even today. Observers even report certain dark features on Mars grow darker and even change color during seasonal transitions. This has led to some startling conclusions, some of which has brought the wrath of the scientific community down on a few very prominent astronomers.
Confusion over the colors of Mars is nothing new. Reports of green or even blue features on Mars are common from ground-based observers. In the early 20th century, some astronomers saw the apparent greening of Martian maria, during spring and early summer, as proof that vegetation it was the cause. We have since found that the human eye is subject to a variety of illusionary perceptions, one being the inability for us to correctly identify colors in low light conditions. We may have a good idea of what the average human response to a particular color might be on Mars; however, observers often describe the planet’s colors completely different even while using the same telescope.
A layer of volcanic ash and rock covers the surface of Mars, at least in the smooth areas where the United States landed two spacecraft during the 1970’s. Using the robotics arms on each of the two Viking Landers the nearby surface was sampled and the results suggested a surface of ash-like material was saturated by water vapor. When similar materials on Earth are saturated to water vapor it tends to darken and/or change color hues. Also, during the colder Martian seasons its surface has been observed covered with frost or snow like condensates that tend to brighten some areas and make adjacent dark areas appear darker that they really are.
Hoar frost in Earth’s surface tends to clump ash-like and sandy materials into mounds or irregular piles, which will appear darker, especially when accompanied with long shadows. However, when viewed from certain incident angles, these same piles may appear brighter.
Using the proper color filters one can determine colors of Martian features, usually red; however, we have found that certain atmospheric clouds display blue-to-blue white color at times [ Beish, J.D . and C.F. Capen , The Mars Observer’s Handbook, Publishers: The Planetary Society, (May 1988), 65 N. Catalena Ave, Pasadena, Ca. 91106. Second printing: Astronomical League National Books]. Even without filters there are a few clouds that appear bluish, such as the "Capen Blue Syrtis Cloud." This cloud shows up often during Martian northern spring (southern autumn) and will appear blue-white visually. In photographs taken with color film and tri-color CCD images this cloud is a vivid blue. To prove this the observer can use a yellow filter on Mars to see this particular blue cloud turn greenish in the filtered image. It is usually brighter in blue color, that is while observing Mars with a blue filter, and darker in red light.
Dust clouds often appear bright yellow in the telescope without observing with filters. They are usually brightest in red light, but can be also bright in yellow light. Dust clouds will be blurry or hazy in yellow light. A blue filter will not brighten the dust cloud and often will make the dust cloud appear to vanish. However, we have found that white-yellowish clouds can accompany dust clouds so the observer should watch for these phenomena. Look for a dusty polar cap following any dust clouds they show up in red and green light.
Since Mars is red in color it will be brightest in red or orange filters. While observing Mars using a deep blue or violet filter the surface features will most often disappear and only a dull bluish haze will been seen. Occasionally surface features will appear dark in deep blue light, a phenomenon not well understood.
Observing Mars with Color Filters
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, increase 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 atmosphere and the surface that are seen on 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.
In additions 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 a 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/m2).
Scotcptic (night) vision and photoptic vision (below 0.03 (cd/m2) 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/m2, 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, Thomas A., Donald C. Parker, and Charles F. Capen, Introduction to Observing and Photographing the Solar System , 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, William K., "What’s New on Mars?", Sky & Telescope, pp. 471-475, May 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.
An interesting web article on eye sensitivity to colors. While the article deals mainly with deep sky object (DSO) color sensitivity us planet watchers also note a difference in color perception when observing planets, especially using color filters.
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/(cd/m2) the Earth’s daylight sky brightness is about 8000 (cd/m2.For example, Jupiter has a surface brightness of around 600 (cd/m2 for light areas. If we compare a dark belt of 300 cd/(cd/m2, 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/m2; 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 7-2.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.
THE ATMOSPHERIC AND PHYSICAL EFFECTS
Several atmospheric conditions and physical effects are modified by the use of color filters at the telescope:
Scattering interposes a luminous veil between the observer and his/her subject. Scientists have shown that for particles in a planet’s atmosphere of a given size, the scattering is inversely proportional to the fourth power of the wavelength of the light. Hence, violet light of 400 nm is scattered about 16 times more than deep red light of 800 nm; Earth’s daytime sky is blue as a result of this property. The Martian atmosphere scatters light in the same manner and thus allows us to observe Martian aerosols at the different relative atmospheric depths.
Prismatic dispersion by our atmosphere is most evident when a star or planet is seen near the horizon. It results from refraction being less for the longer wavelengths where the red appears nearer the horizon and violet toward the zenith.
Color Contrast is controlled to some extent by filters. Light yellow and orange filters are useful in judging the colors of the low-hue cloud belts and zones of Jupiter and Saturn. To bring out a white area on a reddish background, a green filter is useful.
Atmospheric penetration. To explore an atmosphere similar to Earth’s to various depths, molecular scattering can be exploited. Since the shorter wave lengths are scattered more, it follows that ultra violet light scarcely penetrates an atmosphere at all, violet light penetrates to some depth, blue still deeper, while blue-green may reach the solid surface.
Irradiation occurs between adjoining areas of unequal brightness. The amount the bright area appears to encroach upon the fainter one is approximately proportional to their intensity difference. This is evidently a physiological effect, originating within the eye itself. A deep red or orange filter reduces this effect while observing.
EFFECTS OF FILTERS ON MARS
In general, "astronomical seeing" is improved by using filters. Red filters improve seeing the greatest and is followed by orange, then yellow, and so on. Each color filter will pass their characteristic color of light and block their complimentary colors. Red objects will appear very dark in a blue or green filter and bright in a red filter. Green features will be bright in green light, dark in blue or red light. Blue is bright in blue light and dark in red or green and yellow. Many A.L.P.O. members recommend Eastman Kodak Wratten Color Filter for observing the planets. Listed below are the filter reactions on Mars:
at night and in twilight hours. Difficult to observe due to
bright surface. Mars is similar to Earth because the surface
and its atmosphere can been seen.
NEAT FILTER HOLDER
An excellent method to use color filters while observing at the telescope it to hold them between the eye and the eyepiece. You can use the glass screw-in types that fit into the eyepiece barrel or tape several different colored gelatin sheets into a slide holder or frame. It is best to select a film slide frame with plastic windows and use transparent tape to attach the ends to the inside frame and hand held between the eye and the eyepiece. (See Figure 7-3 below).
Using the same technique as described above a person can view images of Mars on a PC screen to reveal surface and atmospheric details; just like we see them in the eyepiece. The same general rules for telescopic viewing a planet using color filters applies to viewing it on a PC screen. This author has found that an old Lumicon "Deep Sky Filter" can highlight atmospheric aerosols and enhance surface features at the telescope and PC screen as well.
A.L.P.O. observers usually obtain slide frames or holders with a clear plastic window and avoid those holders with the so-called "anti-Newton Ring" glass windows.
Those who use CCD cameras often employ filters designed for the spectral response of their cameras. If this is the case, it is necessary to provide information about what filters were used, so that those who receive the images will know the wavelengths involved. It is also suggested that, when using infrared or ultraviolet filters, the spectral range or the "bandwidth half-maximum" (BWHM) be provided. This information is usually readily available from the filters manufacturer.
Advanced Reading List for Theorists
Amateur Astronomer’s Handbook, by: J.B. Sidgwick, Dover Publications, Inc., New York ISBN 0-486-24034-7, 1971, p445 - 470.
Descriptive Micrometeorology, by R.E. Munn, Advanced in Geophysics, supplement 1, 1966. LCCCN 65-26406, Academic Press, 111 Fifth Ave., New York 10003.
Elements of Meteorology, By: Miller and Thompson, Charles E. Merrill Publishing Company, Columbus, OH. ISBN 0-675-09554-9.
Handbook for Planet Observers, Gunter D. Roth, Van Norsrand Reinhold Company, 420 West 33rd St., New York, NY, 1970.
Manual for Advanced Celestial Photography, by: Brad D. Wallis and Robert W. Provin, "Chapter 12, High Resolution Photography: Seeing," Cambridge University Press, New York, ISBN 0-521-255553 8, pp 257-266. 1988
Observing the Moon, Planets, and Comets, Clark Chapman and Dale Cruikshank, Association of Lunar and Planetary Observers (A.L.P.O.).
Introduction to Observing and Photographing the Solar System, Dobbins, Parker, and Capen, Willman-Bell.
The Saturn Handbook, Julius Benton, Association of Lunar and Planetary Observers (A.L.P.O.).
The Solar System, Volume III: Planets and Satellites, Audouin Dollfus (Observatoire de Paris), Chapter 15 - Visual and Photographic Studies of Planets at the Pic du Midi, University of Chicago, 1961.
Through the Telescope, Michael R. Porcellino,
Tab Books, Inc., ISBN 0-8306-1459-1