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.
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.