Emily Lakdawalla continues her excellent blog coverage in this article, which does a great job of explaining some of the reasons why this image was especially difficult to take. Along the way, she includes a tutorial on TDI (Time-Delay-Integration), written by one of the engineers that helped build the instrument. TDI is the method HiRISE uses to gather lots of light into its CCDs, and it’s one of the reasons we get such high signal-to-noise in our images. It’s a complicated concept, but it’s an important one for understanding HiRISE’s incredible imaging abilities, as well as its limitations.

From her blog post:

This is a fascinating story showing how necessary it sometimes is to have a deep understanding of an instrument in order to understand the data that comes from it. …It can be dangerous to read too much into space images until you have studied how the cameras really work.

It’s a great post – she deserves a cookie!

View Images

There are, as always, many magnificent images here. Some of the noteworthy observations are:

PSP_001521_2025 and PSP_001501_2280: On the HiRISE web site you can see diagrams made by Tim Parker show the locations of various parts (lander, backshell, heatshield or parachute) for Viking Lander 1 and Viking Lander 2. It’s possible they aren’t in the color strip (I haven’t found them)!

PSP_001508_1245 and PSP_001510_2195: These two exhibit a “glow” pattern of saturated pixels due to high TDI (Time Delay Integration) settings on the blue-green CCDs. (All of the exposure settings are chosen for each observation based on a photometric model of the scene).

PSP_001538_2035: This is a rim-to-rim section across a crater called Tooting that is about 30 kilometers in diameter. It’s also interesting to note how the altitude of the rims, when combined with the large off-nadir roll angle (23 degrees), leads to an oddly bowed geometric projection. But it is correct; as the terrain rose, fell, and rose again from HiRISE’s angled point of view, the center of the ground track deviated slightly east or west from a true great-circle line.

PSP_001558_1325 and PSP_001593_2635: These dune fields are striking, forming incredible patterns.

PSP_001582_2245: Looking like a super-sized area of dried mud, the polygonal cracks in this image are amazing.

Updated (2008-Apr-10)