The Lunar Orbiters, LO were launched 1966 and 1967. The Lunar Reconnaissance Orbiter, LRO was launched more than 40 years later in 2009.

This paper has some information about the narrow angle camera NAC of LO. The focal length was 610 mm and the angle resolution 4.4 arcseconds. Images were stored on 70 mm film and developed and scanned later.

The NAC of LRO has an effective focal length of 700 mm and 10 micro-radians per pixel. The electronic image sensor has 5064 pixel.

4.4 arcseconds are 21 µradians. But the 4.4 arcseconds are based on photographic images resolution given in line pairs, not pixels. 2 pixels are needed for one pair of lines. The resolution of LO is therefore 2.2 arcseconds or 10.5 µradians per pixel, very close to 10 µrad.

At 46 km height, the Moon's surface resolution of LO was 1 m (line pairs). The resolution of LRO was 0.5 m per pixel at 50 km. Again the resolution of both orbiters is very close.

The very sophisticated electronic image sensors of LRO are an enormous progress to the photographic image system of LO needing a film supply and chemical development. But am I right there was no progress in optical image resolution?

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    $\begingroup$ double-checking, are you asking if over four decades the resolution of imaging of the Moon's surface, as expressed in line-pairs in meters, hasn't really improved from the initial LO spacecraft in the 1960's until today, when considering all the subsequent lunar orbit imaging missions? $\endgroup$
    – uhoh
    Sep 18 '18 at 4:18
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    $\begingroup$ The photographic system of the LO was the very best available for its time (derived from military photo reconnaissance systems), using 70 mm film (which has very high resolution). $\endgroup$
    – Hobbes
    Sep 18 '18 at 10:09
  • $\begingroup$ @Hobbes: for professional aerial photography (of Earth), film formats of 120 x 120 and 230 x 230 mm have been used too. $\endgroup$
    – Uwe
    Sep 18 '18 at 10:16
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    $\begingroup$ @uhoh: Yes, I did compare the optical image resolution of LO amd LRO and found no difference. Therefore the question if there was no progress at all? $\endgroup$
    – Uwe
    Sep 18 '18 at 10:22

The maximum resolution of all optics is limited by its aperture, see wikipedia for the Airy disk.

enter image description here

The wavelength λ is 560 nm, the mean of visible wavelengths.

For LO, the diameter d is 610 mm and the calculated resolution Θ is 1.12 µrad.
For LRO, the diameter d is 700 mm and the calculated resolution Θ is 0.98 µrad.

The difference is very small. To acheive a fairly better resolution, increasing the focal length of the telescope alone would not help. The diameter of the optics should be increased to get a better resolution. But a telescope with a diameter of more than 1 m was too heavy for a lunar orbiter.

During 40+ years, there was a huge improvement of the electronics of lunar orbiters, but the physics of optical telescopes did not change.


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