The limiting magnitude achievable with photographic plates depends on several factors including plate scale, sky brightness, seeing, and granularity and contrast of the emulsion. As an approximate guide, at the AAT prime focus in 1" seeing, the limiting B stellar magnitudes are:
B = 24.5 for IIIa J emulsion, in a typical exposure time of 80 minutes, and
B = 23.5 for IIa O emulsion, in a typical exposure time of 25 minutes.
The corresponding limiting magnitudes at the f/8 Cassegrain focus are:
B = 25.4 for IIIa J, 6 hour exposure, and
B = 24.5 for IIa O, 2.5 hour exposure.
With the IIIa J emulsion the effective pixel size is 0.3" at the prime focus and 0.1" at the Cassegrain focus.
The practical detection limit for extended objects using photographic amplification techniques is approximately 5.5 magnitudes below the night sky brightness on sky-limited III or IV-N emulsions, i.e. B=28 mag/arcsec2.
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