29 August 2013

Why does the $2.5 billion Curiosity use a 2-megapixel camera?

It’s not often these days that people get excited about 256×256 images or that anyone considers 2 megapixels to be “high resolution,” but when the imaging device in question is between 34 and 249 million miles away the story changes significantly.

The Mars rover Curiosity, despite being one of the more impressive human achievements of the last decade and costing tax payers around $2.5 billion, uses a 2-megapixel sensor in its main camera array. That detail might have raised eyebrows with anyone familiar with digital imaging as it’s surprisingly low, especially considering that today’s affordable digicams have at least 10 megapixels and “expensive” models pack upwards of 40.

The sensor used on Curiosity was the KAI-2020 from TrueSense Imaging. It shoots at 1600×1200 and is an interline CCD (as opposed to full-frame CCD, frame transfer CCD, or CMOS). Interline sensors are characterized by small pixels, the use of microlenses to aid the pixels, snappy imaging times, and the use of an electric shutter as opposed to a mechanical one.

So why go with just 2MP? The choice was made for a number of reasons, some of which might not be immediately obvious. First and foremost is the distance over which data must be transmitted. The rover sends data to two satellites that orbit Mars, which then relay it back to Earth. This stream of data is quite limited, to something like 256 megabits (32MB) a day. And images aren’t the only thing that Curiosity is sending back — there are all sorts of other recordings and measurements that needs to be transmitted.

The next hurdle to more modern equipment is simple logistics: Curiosity has been in the works since 2004. At that time, a 2-megapixel sensor was fairly bleeding-edge, but it’s obviously been eclipsed since then. But design and testing takes time and components can’t just be swapped out like on your computer. Parts need to be able to stand up to the rigors of the Martian environment, and ensuring that they can takes time.

There were other logistical reasons as well, such as the fact that the 2MP sensor isn’t only used in one camera - it’s in four on the MSL. These include the Mars Hand Lens Imager (MAHLI), Mars Descent Imager (MARDI), and both MastCams. By using the same sensor NASA was able to keep certification time to a minimum and not have to learn the ins-and-outs of each sensor (such as the effect of radiation on the individual pixels).

The final reason why a 2MP sensor made the cut is also the most practical: nothing on Mars is moving. That means that multiple images can be easily stitched together into panoramas. This will have the same effect as a higher resolution camera and none of the drawbacks.
And one perk of the KAI-2020 is that is can take video. The chip can handle 720p HD (1280×720) and up to 6 fps if called upon to do so.

For reference, you can check out this panorama of 817 stitched images taken by the Opportunity rover from December 2011 to May 2012. The shots were taken on Greeley Haven, on the western edge of Mars’ Endeavor Crater.


Read more: Hubble discovers the first blue planet outside the Solar System.


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