Instruments and observing techniques
Mars is not particularly accommodating to the observer, for its oppositions occur at intervals of over two years. In contrast to the other outer planets, the appreciable ellipticity of Mars’ orbit dictates that not all oppositions are equally favourable as regards the apparent angular disk size. Also, whilst Jupiter and Saturn can be satisfactorily observed for a considerable part of each year, Mars presents an acceptably large disk for only a few weeks about opposition, at least to small-telescope users. Perihelic oppositions occur in August and September, at which time Mars may have an apparent diameter of nearly 26 arcsec. Unfortunately for observers in the UK, the planet’s considerable southern declination at such times gives it an undesirably low altitude. Because the summer solstice of the southern hemisphere of Mars occurs soon after perihelion, this hemisphere is tilted towards us at these ‘favourable’ oppositions. Aphelic oppositions, when Mars may subtend only about 14 arcsec, occur in February and March, but its northern declination compensates for the reduced image size. At conjunction, the disk may shrink to only 3.5 arcsec diameter.
Useful observations of Mars are certainly possible when the disk exceeds a diameter of around 6 arcsec, but those observers with larger apertures have found it possible to draw or image details on a disk as small as 4 arcsec, greatly extending the period over which the planet can be watched. Thus a martian apparition can last for much more than a terrestrial year if one is prepared to develop the necessary skill in observing and imaging the small disk.
Successive oppositions occur during different portions of the martian year so that a full picture of the seasonal changes can only be built up by observing the planet over seven or eight apparitions: a ‘cycle’ of 15 or 17 terrestrial years. The terms areocentric longitude, martian date and heliocentric longitude are all means of describing the position of Mars in its orbit. Areocentric longitude is abbreviated to ‘Ls’ in the BAA Handbook, and the northern hemisphere seasons begin at the following values of Ls: spring, 0°; summer, 90°; autumn, 180°; winter, 270°. These naturally correspond to southern hemisphere autumn, winter, spring and summer. A martian year consists of 687 Earth days or 669 Sols (martian days).
The close similarity in rotation periods of the Earth and Mars has a restrictive effect upon the range of martian longitudes that may be sampled by a fixed observer on a given night. The range of longitudes will be different for observers at different terrestrial longitudes, so the need for international cooperation is immediately obvious. Thus we have especially good cooperation with observers in continental Europe, Australia, Japan and the United States. Since Mars rotates in 24h 37m, any given martian CM longitude recurs on successive nights 37m later, giving rise to an illusory ‘reverse rotation’ effect with a period of 5.5 weeks, when the planet is observed at the same hour of the night.
The objectives of the Mars Section are as follows. 1) To detect any changes in the shapes and intensities of the classical dark albedo markings, and the presence of new features. These changes are related to the occurrence of dust storms (‘yellow’ clouds). 2) To map the extent and shape of the polar caps, to note their brightness and definition, and to observe any interior details or detached portions. In particular we are interested in the rate of shrinkage of the caps in springtime. 3) To chart the positions and movements of martian dust storms, and to document the occurrence of seasonal or topographic white clouds.
Effective participation in the programme will require instruments of the order of 200 mm for reflectors and 150 mm for refractors. Apertures of 300 mm and above are better still. Magnification depends on personal choice, but generally speaking at least 200x is desirable. With his 41-cm Cassegrain the Director finds that with a power of more than 400x he can still make useful observations with the disk diameter below 6 arcsec.
The Mars Section visual report form uses drawing disks 50 mm in diameter. Data from the BAA Handbook allows a blank disk to be prepared with the correct phase and orientation before observing. Some observers prefer to be ignorant of the CM longitude when commencing work, but this is an old-fashioned habit if the observer is to be on the lookout for specific features. Nevertheless, even when the region is well-known to the observer, he or she must not forget that the outlines and intensities of the dark markings are subject to change in a manner that cannot be predicted easily. At the telescope, the observer might first sketch in the polar cap or hood visible at each pole, then the larger dark makings, and finally any clouds or other brighter patches. The time of completion of this outline must be noted, after which fine details can be filled in. Is there a dark fringe to the polar cap? Is its outline sharp? Are there any irregularities at the terminator due to projecting cloud?
CCD detectors and electronic cameras, generally being red-sensitive, are excellent for recording the surface features and dust storms when used in conjunction with a red filter such as a Wratten 25. The most modern ones also work well in the blue end of the spectrum (for example, with the blue–violet W47) to record white clouds, but they then need longer exposure times. Webcams are generally best of all for the Red Planet (and other planets), for they enable many images (usually a short section of video) to be combined and stacked to offset the effects of atmospheric turbulence. Several types of software exist to help the observer obtain the most information from images, and derotation is another recently developed technique that can improve the appearance of an image.
With apertures of 150 mm or more, examine Mars visually with colour filters of known transmission characteristics. Filters reveal details of the structure of the martian atmosphere unobtainable by observation in integrated (white) light alone. Some of these filters are fairly narrow-cut and dense, and so have low transmittance. The blue–violet W47 requires at least 200 mm aperture. When employing filters, always avoid ‘threshold’ observations with a dim image, for these will only prove to be misleading and useless. The W25 red and W15 yellow (and any orange filter) will be of help in intensifying the dark markings and in enhancing the visibility of faint desert details. Furthermore, they assist the observer in recognizing the discrete yellow dust clouds as the latter appear brighter through these filters: strictly speaking, only the W25 can be used diagnostically here, for some white patches also appear bright in yellow light. Dust clouds are more difficult to detect when they lie wholly over desert regions. The W44A blue, 58 green and 47 blue–violet filters enhance the limb brightening and the outlines of limb and terminator clouds and bright patches. The dark markings will be visible, though subdued, with the W44A filter. Surface details are rarely seen at all with the W47, but on rare occasions they may become more or less visible. White clouds are better seen in blue–violet light, and the Equatorial Cloud Band described elsewhere stands out much more clearly at this wavelength. The cones of the human retina are not very sensitive to this waveband, and the W47 (or its Schott or other equivalent) is much better when used with a suitable CCD or webcam (not all such devices show a very good sensitivity at this end of the spectrum). Because the filter transmits some infrared, to which imaging devices are very sensitive, it should be used in conjunction with an infrared-rejection filter (such as the Baader UV/IR filter) when working in this waveband. Near-infrared images are also interesting, though the laws of Physics dictate that they will have a lower resolution than visible waveband images.
Until the recent past, the assignment of numerical intensity estimates to features formed another line of observation. The black night-sky background is rated at intensity 10 and the brightest scale-point is 0, representing the normal brightness of the polar caps. As an intermediate guide, most of the desert regions would normally be 2. Estimates should be made in white light only. Their main use today is in assisting the observer to recognise veiling by dust, or other short-term changes. Unprocessed images would be even more valuable in such analyses, particularly if the observer takes a series with precisely the same equipment over a number of apparitions.