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Scrolling through the Roscosmos twitter feed I saw this image of a dish antenna in a recent tweet. Google translate shows that the post says:

Новый измерительный пункт будет построен на Сахалине для космодрома Восточный — https://www.roscosmos.ru/25478/

A new measuring point will be built on Sakhalin for the Vostochny cosmodrome https://www.roscosmos.ru/25478/

I noticed that in front of the dish there is both a convex secondary mirror (currently tilted away from the axis and presumably unused at the moment) and some microwave feed horns.

What puzzles me is that the distances between these two items and the dish don't make sense. The secondary looks to be roughly 10% of the diameter of the primary, which means that to intercept the beam nicely, the focus of the primary should be about 10% farther behind where the secondary would be if the feed horns were removed and the secondary swiveled into place (I believe I see a hinge there).

However, the feed horns seem to be far in front of that point — far too close to intercept a signal focused by the primary, that's the reason why the secondary is as large as it is.

Does the stage holding both the secondary and the horns translate axially to bring them to to the focus? How does this work?

See answers to How does this strangely-shaped horn at Honeysuckle Creek Tracking Station work? for more on hyperbolic secondary mirror placement in dish antennas.

below x2: From Roscosmos tweet.

Roscosmos dish antenna, Vostochny cosmodrome, Sakhalin

Roscosmos dish antenna, Vostochny cosmodrome, Sakhalin

below: From figure 6.4 on page 49 of Practical Conic Sections: The Geometrical Properties of Ellipses, Parabolas and Hyperbolas by J. W. Downs, Dover. 1993 also used in this answer

hyperbolic secondary Practical Conic Sections J. W. Downs, Dover. 1993

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  • $\begingroup$ I've added the russia tag in hopes of getting this double-checked by someone who speaks Russian, thanks! $\endgroup$
    – uhoh
    Sep 17, 2018 at 7:27

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It can be a replaceable feed for two ranges/bands. A horn (feed antenna) and a sub-reflector which are mechanically changed at dish's focus.

enter image description here

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  • $\begingroup$ Thanks. So the reflector can rotate and swing in front of the feed horn i.stack.imgur.com/Jtwap.png I'm surprised that the reflector's diameter is so large compared to the diameter of the feed horn, even though they appear to close together. $\endgroup$
    – uhoh
    Dec 11, 2018 at 0:18
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    $\begingroup$ A horn for the transmitter is in focus of the satellite dish mirror. Like this transmitter kik-sssr.ru/Pluton/i0241rp.jpg A horn for the receiver is in focus of subreflector and is located on the dish mirror. And we do not see the horn from this angle. Like this receiver - kik-sssr.ru/Pluton/i0185rp.jpg $\endgroup$
    – A. Rumlin
    Dec 12, 2018 at 15:59
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    $\begingroup$ I finally figured out how this works; I am sure it's obvious to most people, but I included the sketch to illustrate the mechanism. The focus will be down inside the horn which is well behind where the sub-reflector ends up. $\endgroup$
    – uhoh
    Feb 6, 2021 at 6:23

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