If you are not aware, the Pentax LX (film SLR) is the only camera ever made - by any manufacturer, that can aperture priority auto expose a scene for as long as it takes while monitoring the scene for changes in lighting and adjusting exposure accordingly. Since it takes it's exposure off the film plane, it doesn't require a cover for the viewfinder and stray light entering there does not influence the meter.
I have successfully conducted low light shots with very long exposures as shown below.
This one is on Kodak Ektar 100 for about 40 minutes . . .
This one on Kodak Gold 100-7 for more than 3 hours . . .