Originally posted by UncleVanya Try the 0.5cm x 0.5cm square of lens cleaning tissue on your rear element.
That would be something rather large, even a .5mm X .5mm bit would be fairly large and I could easily see either causing issues. A few spots from condensation or finger print less so, unless really pusing the lens's capabilities. Specks of dust aren't a real big concern in astro images as they cover only a tiny portion of the area and one is typically using telephotos so they become less of a concern. Dust on the sensor is a nightmare though with astro shots. What I saw with the dried condensation spots was what appeared to be some bad coma in a set of astro shots.