← A single frame of Mars (1/15th sec exposure) showing South Polar Cap
A comparison star (1/6th sec exposure)→
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← A single frame of Mars (1/15th sec exposure) showing South Polar Cap
A comparison star (1/6th sec exposure)→
The Earth and Mars race around the sun in roughly concentric circular orbits, the Earth going somewhat faster. Mars is normally far away and a very small telescopic target, but approximately every two years the Earth catches up with it and as it passes between the Sun and Mars the two are briefly in opposite directions in the sky. At this point of "opposition" Mars is closest to Earth. It was especially close at its most recent opposition reaching a maximum apparent diameter of 25 arc-seconds on August 27, 2003. The following pictures, taken slightly before and after that event, show that, in such circumstances, a telescope of the sort used by Galileo is capable of resolving surface features on Mars. Although atmospheric conditions were mediocre at best and the target was very low in the sky, these were the only two opportunities that presented themselves for photographing Mars during the entire period of opposition.
To aid in recognition of the features being observed, the Galilean photos have been enhanced using the Registax and Photoshop image processing programs, and are shown accompanied by nearly-simultaneous photos taken through the Celestron 8-inch telescope on which the Galilean replica is mounted. All pictures shown on this website are represented as they appeared through the Galilean telescope: North is at the top and the Earth's Easterly direction to the left. This is the same orientation as when the sky is viewed with binoculars or the naked eye. Mars rotates from left to right when viewed in this way, which differs from the usual convention of reproducing planetary pictures with South at the top. The Celestron pictures (which are affected by several mirror reflections) have been inverted east-west to match the Galilean view. The Galilean images are shown at twice the original pixel scale (see note on processing, below); while the Celestron images have been reduced to 2/3rds their original size.
Left: A single 1/25th-sec afocal exposure through a 15 mm eyepiece on the 8-inch aperture Celestron Schmidt-Cassegrain taken, as all the photos on this website, at ISO100. Mars was 22 arc-seconds in diameter and at an elevation of 27 degrees.
Center: An enhanced composite of fourteen such exposures taken from 1:53 to 2:03 PDT (8:53 to 9:03 UT -- Mars' Central Meridian = 359°).
Right: An enhanced composite of ten 1/15th-sec exposures through the Galilean replica from 2:10 to 2:12 PDT (9:10 to 9:12 UT -- Central Meridian = 3°). The most obvious features visible in the Galilean image are the South polar cap and the crescent of light on the eastern limb, with a prominent brightening in the upper left, corresponding to the region known as Tharsis. The prominent crescent-shaped dark feature in the right half of the Celestron image does not show well in the Galilean photo. Note that in the un-enhanced Celestron image it is smaller and less prominent than Syrtis Major (see below). The portion of it on the central meridian is known as Sinus Meridiani.
Left: A single 1/30th-sec exposure through the Celestron. Mars was 24 arc-seconds in diameter and at an elevation of 28 degrees.
Center: An enhanced composite of 10 Celestron exposures taken from 0:02 to 0:04 PDT (7:02 to 7:04 UT -- Central Meridian = 291°). Although Mars was larger than in the previous series, the seeing was worse, leading to the lumpier limb. The prominent dark feature extending up towards the North pole along the central meridian is Syrtis Major.
Right: An enhanced composite of thirty-two 1/15th-sec exposures taken through the Galilean replica from 23:36 to 23:39 PDT (6:36 to 6:39 UT -- Central Meridian = 285°). Syrtis Major appears to be clearly visible, somewhat rotated to the left relative to the Celestron image, as would be expected from the difference in timing. Note also the clear shift in illuminated limb from left to right that is visible in this Galilean image as compared with the previous one. The Elysium region is crossing the limb at upper right in these views. The Galilean image shows the South polar cap to be less prominent than in July, as confirmed by the Celestron.
It is quite difficult to render photographically the full sharpness of the images visible through the Galilean telescope. Although the pictures of the Sun and Moon given on this website are single frames subject to no enhancement other than possibly a slight adjustment of contrast, the planetary images benefit from a certain amount of processing. Such processing generally involves making a composite of several frames to reduce noise, and then an enhancement of features at certain spatial frequencies. The following shows the sequence used to produce an image similar to that shown above for September 13th.
The result resembles more closely what could be seen through the telescope by a person with extremely acute vision. Although this detail is truly present in the image, few of us could perceive it at the 20 power magnification used by Galileo. At that power, even a 24 arc-second Mars is a mere 8 arc-minutes in apparent diameter (1/4 the size of the moon as seen by the naked eye), and the subtle surface features are below the resolution of most eyes. In addition, at opposition it is extremely bright, which makes seeing the detail even more difficult. Despite these difficulties, Galileo was well aware of Mars' changing apparent diameter, and thought he may have detected some slight non-roundness at certain phases of illumination.
Because of Mar's low angle in the sky, the effect of atmospheric dispersion is quite evident in the raw Celestron frames, with an arc of blue color at the top and red at the bottom. This has been removed in the processed images. Registax 2 provides a mechanism for sliding the RGB (red-green-blue) color layers relative to one another after aligning and stacking the frames. Alternatively, the raw frames can be separated into their RGB components and Registax used to process the whole batch (three times the original number of pictures) as if they were separate independent pictures. When the seeing is so bad that the different colored images move independently of one another in the sky, the later approach produces slightly better results.
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Images © Tom Pope and Jim Mosher
Last modified: March 14, 2006