Astrophotography Exposure Table

This is a table for determining the approximate exposure times for deep space astronomical objects depending on the f/ratio of the optical system used. The table is based on experience of exposure times from a semi-dark site that result in near maximum acceptable sky fog. Hence the figures should be close to optimum, however they should be used with caution since significantly different times may apply depending on film sensitivity and sky conditions.

Look up the f/ratio you will use in the left column and read off the exposure time in the columns to the right, depending on the elevation of the object and whether or not a filter will be used.

• Use of a modern colour negative film such as PPF, PJM-2, PJ400, PJ800, PMZ, SuperG 800, etc. It is expected that the table will be applicable to most modern colour films, however it has only been tested with fast colour negative films. It is not expected to apply to Kodak Technical Pan.
• A semi-dark site. As a rough guide, this means that the sky should be essentially dark within a 50 degree radius of the zenith. The Milky Way should be clear and bright. There may be a few sources of distant light pollution, however the glow from any light pollution should not extend by more than 30 degrees above the horizon. There may be a few local sources of light pollution such as street lights or security lights, however they should be very few in number and not in close proximity.

Low Elevation: 20 – 30 degrees above horizon Med Elevation: 30 – 50 degrees above horizon High Elevation: 50 – 90 degrees above horizonThese definitions can only be a rough guide. If shooting at low or medium elevation you have to make a decision on how to factor the exposure based on the transparency of the air and the proximity of the object to any sources of light pollution.

Assumes use of a Lumicon Deep Sky filter. The filter factor used in the above calculations is 2.4. This is a rough guide only - it means that a filtered exposure may be between 2 and 3 times longer than an unfiltered exposure for a similar level of sky fog. The use of a filter does not imply that the film must be exposed for the longer period, only that it may be, for equivalent sky fog.