The Fig shows the sun during TOV June 6 2012 (local date). The lines of solar latitude and longitude are invisible, sadly . So how to tell where the disc of Venus will first touch the sun?
Timing first contact is hard - unless you know where on the disc it will happen. Prep for this before TOV. You will need to know where North and West are on the sun. West is easiest - it's where the sun goes out of the FOV first when the 'scope's not driven. North is found by (gently) nudging the scope in the direction of local north - new sky comes into the FOV from the north. Right! Now insert your wide-field cross-hair eyepiece (What? Good luck finding one). Align this with E-W drift. With the ep aligned and locked we move to the sun's northern hemisphere and transit the cross hair across the disc (with drive OFF). Where a 92 second chord (a in Fig)touches the following (ie east) limb is the point of first contact. Repeat a few times to be sure of your timing.
Do this days before for practice and just before TOV (if theres time) - there may be a feature on the disc, a sunspot perhaps, that will mark the place. If not keep your eye on the contact 1 point - with a tracking scope - and leave the cross-hair ep in place (I will do so) and wait for Venus to appear. What could be simpler? ( It worked in 2004). Chord b in Fig predicts where Venus will leave the sun's disc, that is contact 4.
Caution: You must use a Certified solar filter to observe the sun. Also note that star diagonals flip and rotate the field in unpredictable ways. I wont be using one, and my view will match the diagram. If you use one (or a newtonian say) you will need to flip the Fig to match. Identify N and W in your 'scope well before the event.
CAUTION: the diagram was prepared from published data, some of which disagreed on times by up to ten seconds! No warrantee for accuracy is offered. Best of luck. Any comments?