Mars Rotation

Mars has other webcam advantages, too. It has a relatively small disc compared to giant Jupiter and it rotates on its axis in 24 hours and 37 minutes (compare that with Jupiter's day of just less than 10 hours). This small size and slow rotation gives the webcam imager more time to capture images before resolvable detail in the center of the planet has smeared enough to be noticeable. How do you calculate the smearing rate? Well, let us pick a maximum allowable drift of the features on a planet's equator and meridian (i.e., the dead center of the disc) as 0.5 arc-seconds. Admittedly, under perfect seeing conditions, higher resolutions might be achievable but, 0.5 is a reasonable value to start with. The formula we want is:

time window = drift limit / ((n x planet diameter) / rotation period)

The units for time window and rotation period must be the same, e.g., minutes. The units for drift limit and planet diameter must be the same, too, e.g., arc-seconds.

Let us examine a few worked examples. For a 0.5 arc-second drift limit, the time window for a big, 25 arc-second Mars, rotating in 1477 minutes is: 0.5/((3.14 x 25) / 1477) = 9.4 minutes to collect the images. For a 0.5 arc-second drift and a smaller 15 arc-second Mars we get: 0.5/((3.14 x 15) / 1477) = 15.7 minutes to collect the images.

Contrast this with Jupiter at opposition with, say, a diameter of 45 arc-seconds and a rotation period of 590 minutes: 0.5/((3.14 x 45)/590) = a scant 2.1 minutes to collect the images.

With so much time available, the webcam imager can obtain very sharp results just by using a simple ToUcam webcam and a deep red filter. Also, Mars is so bright that a monochrome webcam and a complete filter set are unnecessary. I obtained some nice images of Mars in 2003 by constructing an LRGB image with just one normal color webcam AVI file and an extra red-filtered AVI file with the same webcam. There was enough time to take the deep red filtered webcam image and then remove the webcam/Barlow unit and replace the red filter with a UV-IR reject filter. I ended up with a blurry color image and a high-contrast, deep red filtered image. By splitting the color image into its RGB components and using the red filtered image as the "L" or luminance monochrome component, the transformation was dramatic. All by using one filter and exploiting Mars' slow rotation and small size.

We can see, compared to imaging Jupiter, imaging Mars is an almost leisurely pursuit! However, the slow rotation of Mars can be frustrating, too.

If there is one planet that needs a worldwide network of amateur observers, it is Mars. As we have seen, the Martian day is only 37 minutes longer than our own, so astronomers around the world can only view one hemisphere at a time. By the time the other Martian side has rotated into view the planet has set and it is daytime. To see an entire Martian rotation from one site you have to observe the planet for five weeks! However, this does mean that a few cloudy nights will not be a problem: you will not miss much. To the novice, observing Mars through an astronomical telescope, things can be quite confusing. In an inverting telescope, with south at the top, Mars will slowly rotate from right to left, with the morning terminator on the right and the evening terminator on the left. Look at Mars a day later, at the same time and you will see 10 degrees further as features "emerge" from the left-hand evening terminator, as if the planet was rotating backwards! It is an illusion of course: you are simply seeing features on Mars slightly earlier in the Martian day, but it can be very confusing.

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