Last night, for the first time, a sub exposure displayed horizontal lines. I thought it was caused by clouds. Well, this morning I was shooting flats and was astonished to see these horizontal streaks again. Some flats had no streaks and others had varying amounts of streaking. I inspected the camera and the filter and couldn't see anything unusual.
This is what it looks like.
https://bydawnlight.zenfolio.com/p51053098/h612f92c8#h612f92c8
Since I as using a light panel indoors, it couldn't be meteorites or aliens!
