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Pioneer 10 and 11 launched in 1972 and 1973, respectively, whereas both Voyager probes launched in 1977. I do expect the imaging quality to improve in 5 years but the difference is a bit too much.

Pioneer 11 image of Jupiter:

a

Voyager 2 image of Jupiter:

b

Pioneer 11 image of Saturn:

c

Voyager 2 image of Saturn:

d

The images were obviously processed to show more detail and make the images clearer and brighter, but still, the difference is stark. Why are the Voyager images so much better than the Pioneer ones?

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The 1960s and 1970s were a period of rapid technological development, so it's not actually surprising that the relatively new field of electronic imaging advanced so far in that five-year period. Especially for spacecraft applications, where you have severe power and weight constraints and a harsh operating environment, compromises had to be made in quality - resolution and signal-to-noise ratios.

Resolution of the Voyager cameras was 800x800 pixels; the Pioneer imaging was done with just a pair of detectors, one for blue and one for red, scanned line-by-line for (if I did the math right) about 500x500 pixels. The registration of scan lines in the Pioneer images seems reasonably good, and the Pioneer 11 came much closer to both Jupiter (43,000 km) and Saturn (21,000 km) than Voyager 2 did (570,000 km, 101,000 km), so raw resolution doesn't seem to have been the problem; it must be that the quality of the light detectors just improved vastly from Pioneer to Voyager.

Note that the Pioneer imaging system is billed as a photopolarimeter rather than a camera; it wasn't primarily intended to produce nice pictures.

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    $\begingroup$ Re The 1970s were a period of rapid technological development -- Better said, the late 1960s to early 1970s were a period of rapid technological development. As the Pioneer spacecraft were launched in the early 1970s, their instruments were almost certainly a product of mid 1960s technology. That was an era where slide rules still ruled and mainframes were larger than a house but had less computational power than your toaster oven. $\endgroup$ – David Hammen Aug 1 at 0:37
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    $\begingroup$ +1 I wonder if the focal length and FOV were different as well? Did Voyager have a higher magnification (more pixels per degree) while Pioneer had a wider angle lens? Could the Voyager images published be mosaics perhaps? $\endgroup$ – uhoh Aug 1 at 1:47
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It is not only the progress in imaging over that period. Voyager was a more ambitious and expensive mission in general. The mass of Pioneer 11 was 259 kg, while that of Voyager was 825.5. That extra mass included a proper camera with multiple lenses on a steerable platform. This is different from the Pioneer spacecraft, which were spin-stabilized. The scanning that Russell Borogove's answer mentions was actually based on the rotation of the spacecraft sweeping the detectors past the planet. That wasn't because that was the only way they knew to do it - they could have sent a better camera, but there was a budget they needed to meet. With Voyager, they had both better technology and more money.

Really, though, this should be no surprise. According to a quick Google search, the first (American, apparently) cell phones with cameras came out in 2002, thirty years after the first Pioneer launch. The first iPhone came out in 2007, thirty years after Voyager. If you compare the pictures you get from phones at both ends of that span, I think the difference is at least as great.

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    $\begingroup$ Slight correction: the first cell phone with a camera was released already in 1999 in Japan. It had a 0.1Megapixel resolution. $\endgroup$ – Michael Borgwardt Aug 2 at 20:35
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    $\begingroup$ Thanks. Edited to fix in a way that keeps the delayed 5-year span. $\endgroup$ – Mark Foskey Aug 2 at 22:05

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