FILM AND EXPOSURES
The ideal subjects for piggy-back photos are constellationg Milky Way panoramas, larger nebulas and galaxies, and comets. These are best shot with an ISO 400 or 800 film and lenses set to an aperture f/2.8 to f/4.
Lenses with settings are available, but most perform poorly when used "wide open" at f/1.4 or f/2. Stars appear bloated, especially at the corners of the frame.
Exposure times depend on sky conditions. In a dark sky,
10 to 20 minutes at f/2.8 with
ISO 800 film records an astonishing amount of nebulosity.