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At a 1000mm f/l a consistent 30 seconds is not bad, and even then some
subs will be unusable due to periodic error. The majority of high volume
production mounts will not do much better, including HEQ5s, NEQ6s etc.
Of course, the greater the declination of your target, the longer your exposures can be. At Dec 60 I got 95% usable subs @ 30secs, around
60% usable @ 60 secs, and 40% usable @ 90 secs, around 25% @ 120 secs,
and the odd usable one @ 150 secs.
raymo
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