Focal length has nothing to do with how long to do unguided. Unguided depends on how well your model is made and how much drift you are willing to accept in your image.
Rolando, I'm not sure I'm getting this right. I'm probably missing something about the modeling part here... Please enlighten me.
I thought the "amount of drift I'm willing to accept" was the amount of recorded signal (pixels) that is "offset" in the frame due to the mount's (in)ability to accurately track an object,(isn't it why we usually guide ?) which is also related to the image scale, which in turn relates to the focal length of the telescope...
So I'm not sure how two scopes with different focal lengths would lead to the same amount of drift in an unguided sub of the same exposure time (all other parameters being equal). Seems to me that the more focal length, the more drift (pixel offsets over time) I get, wether it's caused by PE (which I understand is not an issue with AE) or bad PA.
Sorry if my question seems silly, I'm a kind of "advanced newbie in astrophotography" as I like to call myself... 😉