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(I have two questions.)

I was looking at some pictures taken by the rover Perseverance, on Mars, and it got me thinking about the sensor it uses.

Turns out that it uses a, about, 2MP sensor in its "Mastcam-z" camera.

This does not seem to me like a lot of resolution by today's standards, and it's not like NASA didn't have the budget to get a top of the line sensor with a high resolution. Especially considering that, let's just say that you sort of have to plan what you're sending carefully because you're not doing it twice...

I am assuming that there is some explanation as to why they decided to send this sensor, with these specs as opposed to another one. I understand that MP aren't everything, but 2 MP does seem a bit low.

So, my first question is this: Why the seemingly low resolution of the sensor? Is there some kind of trade off, I'm not understanding? Robustness when landing on Mars? Maybe even politics?

I've seen some higher-res pictures from the rover, and it seems like its able to take pictures with greater resolution then 2MP. Now, maybe it's using a different camera. Or...

My second question: Is it possible for Perseverance to send back images with a greater resolution than its sensor can contain? I think I've seen some consumer grade cameras that can up their the resolution of their images by moving the sensor, I think. Would it be how it's doing it?

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  • \$\begingroup\$ I don't know enough about it to post as an answer, but they stitch images together. I just downloaded a 427 megapixel image from Nasa, 36952x11570px \$\endgroup\$
    – Tetsujin
    Apr 25, 2021 at 17:20
  • \$\begingroup\$ Petapixel did a report on the camera - petapixel.com/2021/03/01/… - & this is a much-reduced version of the 427mp shot, showing a zoomed section - i.sstatic.net/RrFBm.jpg \$\endgroup\$
    – Tetsujin
    Apr 25, 2021 at 17:45
  • \$\begingroup\$ Thanks! I did see that report, but it doesn't seem to really explain the choice of sensor... So far I'm thinking that the main reason is that, if they can stitch the images anyways, it's probably not worth it to get an insanely high resolution sensor (which will likely come with some compromises regarding low-light performance, durability, etc.). \$\endgroup\$
    – poupou
    Apr 25, 2021 at 18:06
  • \$\begingroup\$ I believe the Mastcam-z is a stereoscopic video camera. The 1600x1200 resolution seems reasonable for video particularly considering the need for onboard processing and interplanetary transmissions. mars.nasa.gov/mars2020/spacecraft/instruments/mastcam-z \$\endgroup\$ Apr 26, 2021 at 1:09
  • \$\begingroup\$ The MastCam-Z wikipedia entry shows how a big image has been obtained. The camera has a 135-400mm equivalent zoom. \$\endgroup\$
    – xenoid
    Apr 26, 2021 at 6:49

3 Answers 3

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Notice the order in which the specs of the cameras are given by NASA here:

  • Function
  • Location on the rover
  • Mass (which is always critical on a space probe)
  • Power (energy consumption)
  • Size
  • Data return (the amount of data generated by the device)
  • Color quality
  • Image resolution

Function obviously includes an expectation of long term reliability. Mass, energy consumption, and data generated are also listed before the measurable performance in terms of color and resolution. The same list of attributes is probably followed in the same order for the descriptions of the other sub-systems that are part of Perseverance.

In the end, following mass, which is most critical in terms of being able to place a thing somewhere other than Earth where one wants to use it due to the energy costs involved in getting it there, almost all design decisions about interplanetary devices like Perseverance, whose life expectancy depends upon solar panels and rechargeable batteries, comes down to energy management vs. acceptable performance.

  • The ultimate durability of the entire system (i.e. the entire rover) is dependent on preserving the energy supply system for as long as possible.
  • The less energy each subsystem uses, the less stress it places on the energy supply.
  • The less energy each subsystem uses, the greater the number of total subsystems that can be used for the same power budget, increasing the overall capability of the total system.
  • If two competing sub-system designs can get approximately the same results and have the same durability, the one that consumes less energy will get the nod.
  • In addition to consideration for how much energy it takes to capture a photo, with a camera on the surface of Mars there must also be consideration for how much energy it takes to transmit the data collected within the photo off the surface of Mars and back to Earth.

The last point above is the key one here.

Using a lower resolution sensor allows for lower energy consumption when lower resolution results are satisfactory. When higher resolution results are desired various techniques such as slight movement of the sensor or of the optics in front of the sensor in successive exposures can be used. Many of the cameras placed on similar probes in the past have the ability to shift the sensor slightly so that greater detail can be obtained using multiple exposures. The mast cameras have lenses with the capability to zoom in and out.¹ If high detail of a distant object is required the lenses increase focal length to make the object larger as projected onto the sensors of the stereo camera. This ability to zoom removes the need for a very high resolution sensor so that details of more distant objects can be seen in very small parts of the total image.

The higher computing power, and thus higher energy consumption, required to process the results of multi-exposure images can be terrestrially based after the results of each sensor actuation has been received on Earth. Ditto for the computing power needed to stich images together to form larger panoramic views. Ditto for any advanced color processing algorithms to produce "true color" results.

An image sensor of a given size isn't affected much in terms of mass by how many photosites it is divided into. That is, I'd be very surprised if a 24MP sensor with the same total surface area as a 2MP sensor using the same generation of technology has any increased mass compared to the 2MP sensor. Where it does differ is in the greater amount of data generated per photo. With the cameras most of us use while holding them in our hands and then taking the results home to transfer the results to our computers, the more data we collect the better, right? That's because the energy cost to us from using a 24MP camera instead of a 2MP camera is trivial compared to what the energy cost means to a designer of a Mars rover who needs to transmit that data via radio from the surface of Mars to Earth. The radio relay satellites orbiting Mars also have energy budgets that are affected by the amount of data per image.

Beyond the energy cost per photo:

When very high tolerance manufacturing is involved, as it is for any interplanetary space probe, a sensor with larger photosites also allows for fewer potential defects due to the increasing challenges of manufacturing chips using smaller and smaller die sizes which are needed to produce sensors with smaller and smaller photosites. Smaller die sizes do allow for lower power consumption, though, or at least they do with computing chips which don't suffer performance-wise from smaller total area. Ultimately the designers will choose a point where the miniscule gain in energy efficiency no longer outweighs the advantage of fewer defects in manufacturing. The ability to have a high reject rate probably also comes into play. When the design and engineering costs far outweigh the manufacturing cost for such a very limited number of sensors, the cost of making several orders of magnitude more examples and testing to find the one(s) with the fewest defects doesn't add that much to the total cost of the product. What's an extra $10-20K in the cost of production when you've already spent a couple of million designing the camera and it's going to cost a few tens or hundreds of millions more dollars to place the thing on the surface of Mars where you plan to use it?

Larger photosites may also make the sensor more resistant to the environmental challenges encountered by a space probe. Even though Perseverance is now safely on the surface of Mars and protected to a degree by the very thin Martian atmosphere, it spent months in the near total vacuum of space while journeying from Earth to Mars. If cosmic rays, for instance, strike a sensor it can affect the surface of the area where the particles impact it. If such a strike affects only 5% of a given photosite's total area, it would probably have less impact on that photosite's performance than if the same amount of surface area made up 50% of a smaller photosite's total area. Extremes in temperatures will also stress micro-electronics. Larger photosites and larger die sizes should be more resistant to such thermal stresses. Vibration resistance should also be better for larger photosites than smaller ones.

¹ The actual focal length range is 28-100mm, according to NASA. The sensor has a 4:3 aspect ratio with a diagonal of 14.8mm, giving it a roughly 3X crop ratio compared to FF. So 85-300mm FF equivalent. Thanks to BobT for the links!

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    \$\begingroup\$ Nice summary... For a more detailed look into the optics and electronics of the Mastcam you can look here. According to the site the sensor is a Truesense (Kodak) KAI-2020 CM interline transfer CCD with 1600x1200photoactive pixels. Datasheet here. \$\endgroup\$
    – BobT
    Apr 26, 2021 at 14:39
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There is a very long lead time between procuring equipment (sensor in this case) and actually using it (on Mars).

It has to be obtained and tested against extreme reliability parameters of cold, heat, radiation, vacuum, vibration. Then it can be years before it actually flies. A newer sensor cannot be trusted as a substitute without undergoing all of the same tests from scratch.

Many spacecraft sensors can perform resolution boosts by moving the sensor. In fact some have a line sensor that rotates to create a virtual huge round sensor. I don't know what sensor capabilities are on Perseverance.

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    \$\begingroup\$ There were plenty of 20+ MP sensors available in a wide variety of size formats in the consumer market over a decade ago. There were even greater resolution sensors available to advanced project managers for things like defense projects and such. I would highly doubt that at the time the sensors for the mast camera were in development 2MP was the absolute largest available size. The limitation was not the maximum available resolution at the beginning of the project. After all, even older space probes have had higher resolution sensors. \$\endgroup\$
    – Michael C
    Apr 25, 2021 at 21:14
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Another aspect of selecting chips for space duty is radiation. Spacecraft that venture beyond low earth orbit are subjected to much higher levels of radiation which cause all sorts of problems. This limits the use of many high performance IC's.

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