I have written about this lens before, not long time ago. The Russian mirror telephoto lens Maksutov 3M-6A 500/6.3 which I got from Per-Olof back in 1992 (for 300 SEK / 30 Euro in 1992' prices)

It is actually not bad at all. But you have to work it well, with modern tools in post processing. Naturally, since it is a manual focus lens - you have to be utmost careful to set it at the right point of desired focus in order to get sharp - so you can enhance it further.

Then you might be surprised how much goodies come out of it.

But as I said, you need to work carefully in the post process in order to get out both fine sharpness and an overall freshness of the colors ! The images do not come out of the camera that fresh.

40% coverage of the sun in that moment

When I took the main photo above during the ongoing total eclipse in Türkiye back in March 2006 - it was 40 minutes before totality. So, at the moment of the image, the moon had covered around 40% of the sun.

At that point the light had already dropped slightly - but you don't perceive that with your naked eye, unless you have a reference, or measurement - or are extremely sensitive.

Don't underestimate the Maksutov 3M-6A 500/6.3

Yes, in the digital era, this lens is remarkably good. Much better than most mirror lenses. Especially compared to cheap Chinese knockoffs. The Russian telephoto lens is among the best (i would say). If you have the skills in post processing, then you can get remarkably sharp images, and fresh colors.

I used it at several occasions with my Olympus OM-1 camera - which turns the 500mm lens, into an 1000 mm ƒ 6.3 equivalent lens (!) like in the below images. It isn't a perfect sharp lens at 100% - but damn close after post processing. It is not easy to handle - but it does have potential if you know how to work it. I can't say that i ever seen images with other mirror lenses to be worth it. Perhaps one of the newest which have been introduced might come close ? I don't know.

Ergo

I think the images speak for themselves what the lens is capable of if you combine careful work, tripod together with good skills in Photoshop and AI sharpening.