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Hi Stephen,
I measured the distance from Rigel to Bellatrix (14.8 degrees seperation) as 182mm on my screen and then measured the distance that Mintaka (0* dec) had travelled - 61mm. Stars at the equator travel 15 degrees in one hour. 182 / 61 = ~3. 60 minutes / 3 = 20. Therefore the exposure time is pretty much bang on 20 minutes - unless my logic is flawed...
Cheers
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