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This answer links to history.nasa.gov's SP-404 Skylab's Astronomy and Space Sciences. In what looks like chapter 2, page 14 there is mention of Skylab's electrographic camera, shown below.

In the image I see what looks like a Cassegrain optical telescope except that there are also electron trajectories shown and a magnetic field.

Question: How did Skylab's electrographic camera work? How does the magnetic field contribute to the operation, and why is that curved surface that looks just like a Cassegrain hyperbolic secondary mirror actually curved?

Skylab electrographic camera

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    $\begingroup$ The answer is there in the link to chapter 2 page 14. The magnetic field is used as a lens for the electrons to build an image of electrons on the film as it is done in an electron microscope. The UV light is translated into electrons by a photocathode on the second smaler mirror. $\endgroup$ – Uwe Dec 1 '18 at 23:10
  • $\begingroup$ Is the photocathode in front of the secondary, or on it? Why is there a secondary mirror at all? There is a puzzle here. Magnetic lenses have strong chromatic aberration, how does the system achieve good electron image quality in this case? $\endgroup$ – uhoh Dec 1 '18 at 23:16
  • $\begingroup$ The photocathode should be on the secondary mirror, the UV photons are absorbed in the photocathode and the electrons are emitted. The vertical arrow is only to indicate the optical or electron image. Poor quality of the magnetic lens is compensated by the very short wavelength of the electrons. $\endgroup$ – Uwe Dec 2 '18 at 0:09
  • $\begingroup$ @Uwe there is no such compensation, that doesn't make sense. Electron microscopes work extremely hard to fight the huge chromatic aberration inherent in magnetic lenses. $\endgroup$ – uhoh Dec 2 '18 at 0:19
  • $\begingroup$ Okay, I am wrong about compensation. But chromatic aberration requires different electron wavelengths or energies. If the acceleration voltage is constant and the UV wavelength bandwidth is small, the electrons wavelength variation is small and thus chromatic aberration too. $\endgroup$ – Uwe Dec 2 '18 at 15:15
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Similarly to night vision devices, the light sensitive part is the photocathode, which releases electrons when hit by photons. The electrons at the photocathode are accelerated by the -25 kilovolt bias, which allows them to be focused with good resolution onto a film surface using the magnetic field.

"Electrographic cameras for the vacuum ultraviolet" by Carruthers, G. R. in "Electrography and astronomical applications; Proceedings of the Conference", Austin, Tex., March 11, 12, 1974. (A75-23926 09-89) Austin, University of Texas, 1974, p. 93-113; Discussion, p. 114-116.

Citing the article:

[...] we have been developing a series of magnetically focused electrographic cameras utilizing front-surface alkali-halide photocathode [...] In these devices, the photocathode is mounted at the focus of an optical system which is partially contained within the imaging device.

diagram of an electrographic Schmidt camera

So, apparently, the secondary mirror does not actually work as a mirror (the optical path ends here) and its shape only corrects the field curvature. See Schmidt camera.

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