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With an ST-4, the fainter the star the better, that said, I normally have the Exposure set to 1 second and the Gain set to 2, with my 80mm guidescope that equates to a guidestar around Mag 8 -9, the ST-4 will quite happily guide on that all night with an intensity of around 20 - 30.
The ST-4 has been out of production since the early 90's but it is still one of the most desired autoguiders around. The ST-4 has now been well and truly superceded by the more modern cameras with USB 2.0 and direct coupling to the guide ports.
I have never used an OAG but I would imagine that if you can get a star on the CCD chip (which is quite small) you "should" be able to guide on it. The ST-4 has a gain of 4 steps and can pull in quite faint stars when set to 4.
Hope that helps a little.
Cheers
JohnG
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