Its the mechanism of the top hung version, the centre pivot is a lot simpler and basically works on balance. Once you have the frame in put the sash in and test it for square all round, best way to do this is to open the skylight a few inches with the bottom handle and you should be able to see if the gap is the same at each side, fully close it and again look and see if the gaps between the sash and the frame are the same. My advise is not to bang the frame about! it will be perfect out of the box.
One other question that popped into my head on all this. I've included a couple of photos of where I am at with this so far.
You will see the horizontal trimmers installed that I will now move in light of this discussion.
In terms of the rafters you will see from the white line I have added to the left of the second image indicates that the window width (and Velux recommended gap) will be narrower than the rafter gap by approx 50mm. Am I correct in thinking that I don't need install an additional rafter timber to meet this measurement - it would just sit on the horizontal battens? My understanding is that the battens take the contact of the window - therefore the window could sit anywhere between the 2 rafters? (note that the right hand side rafter is yet to be doubled up)
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