[bojan]

Well.. I do not think there is anything like this, especially for 50mm..
Smaller F number the better :-) But not too small, because the lens faults will show (CA, coma in corners etc). I am using F2.8 - 4, this is giving me the good star images in corners of frame (50mm Canon FD, F1.4)
And I am using ISO 1600. Noise is dealt with by stacking (5-10 frames).
However, all this applies to dark sites, away from city lights.

In the city (at my home), well, LP almost saturates sensor after 30 sec at F4, ISO1600.
I (still) do not have any filters.. but for 50mm I think it does not make much sense to take pics from polluted areas anyway. 50km away and you are much better off, and camera with that lens (with mount, like barndoor or small equatorial) is not a problem to carry and set up on site.

However, for stars, I was using my MTO1000-A (1000mm, F10.5) at ISO1600 and at 30s exposure LP background was pretty much OK.
I could reach <13m without a problem, even for reliable photometry.