Frequently Asked Questions

Since the beginning of the Phoenix mission the Telltale team has received many questions about the Telltale. This is an addition to the information given on the rest of the website.

Send e-mail to Haraldur Páll Gunnlaugsson (hpg@phys.au.dk) to add questions and answers to this list.

Q: How are images of the Telltale transformed into wind information?

The first step is to analyze the images, and find out where the Kapton part of the Telltale is in 3 dimensional space relative to the orientation marker, mirror and suspension point. Then, using calibration data from the Aarhus wind tunnel, we find the wind speed necessary to put the Kapton part of the Telltale into this position. In detail, this is done by comparison to the Earth calibration data, but taking into account the reduced gravity of Mars and density of the Martian atmosphere, which can be derived from temperature and pressure data available from the mission.

Q: Why do the images in some cases appear blurred?

Preparations for landing operations (Operational Readiness Tests). The Telltale team is working hard on determining the optimal way of delivering wind information from the surface of Mars.

Sequence of images taken roughly minute apart on Sol 7 showing increased blurring.

This blurring contains important information, as it can tell us about the wind turbulence conditions during image exposure. The Telltale Kapton part has a natural frequency of 3 Hz (swings back and forth every 0.33 seconds when excited), and the exposure times were chosen to be longer than that so we would not get an accidental image where the Telltale is untypically in maximum position of a single swing.

The images obtained so far suggest relatively calm wind situations, perhaps due to the flatness of the landing site.

Q: Why does the Telltale line have a kink on it?

The suspension structure of the Telltale line is made from braided Kevlar fibers (same material as used in bullet-proof west’s), and this material has internal tension and bending that could not be avoided in the construction. Pictures below show the unit before launch and after landing.

Left: Picture of the Telltale taken on Mars, Right: Picture of the Telltale taken on Earth

The bend was also observed on Earth before launch. If anything, it seem more pronounced on Mars, and this is due to the reduced gravity on Mars, where there is not the same force pulling the string.

Q: Was the Telltale damaged during launch?

We have not found any damage that would change the characteristics of the instrument during the transport to Mars. However, close inspection reveals that one fiber has loosened.

Image taken of the Telltale on Sol 6. The arrow points to a loose fiber.

It was on the other hand known that this might happen, and this does not compromise the use of the instrument whatsoever. There is about 50 threads in the Telltale suspension and loosing one does not significantly change its properties.

Q: Why is the mirror image sometimes so dark?

The Mirror reflects about 80% of the light that hits it. However, the Zenith on Mars is almost black, and illumination from the horizon dominates. Below are shown two different cases

Images taken of the Telltale on Sol 3 (left) and Sol 5 (right).  

On Sol 3, the atmosphere was very clear (dust free), and the mirror image consequently very dark. On Sol 5, a dust storm had passed the landing site, and the mirror image was consequently much brighter compared to the horizon sky in the background.

The camera (the Surface Stereo Imager, SSI) has many more gray-tones than usual commercial cameras (4096 while commercial digital cameras have usually 256), and this allows us to scale the image for detailed analysis of the mirror image.

Q: Is the Telltale offset due to the kink?

Taking all factors into account such as the tilt of the lander, tension of the Kevlar fibers and misalignment of the mirror etc., the rest position (equilibrium position in zero wind) of the Telltale is slightly shifted relative to the centre of the orientation marker. The rest position of the Kapton part is approximately 3 mm below and 1 mm to the left relative to the centre of the orientation marker in the mirror image.