Why is the maximum exposure time important?
The brightest astronomical targets e.g. the moon, major planets, (and the sun using a suitable filter) easily fall within the normal exposure range of a digicam. The equivalent ISO rating (speed) of a digicam may be adjustable up to 800 or even 1600ASA if the camera offers control over its’ electronic gain. For astronomy, most deep-sky targets are so faint that they cannot be recorded within the maximum exposure of a digicam. However, new and ever more sensitive models are being produced, and as the technology advances then so more targets come within range. Recent top-of-the-range models (e.g. Canon D60) extend the maximum exposure time to a minute or more whilst still retaining low image noise, this extends their target list to the brighter deep-sky objects (e.g. Nebulae such as M42, bright globular clusters).
