This widget allows you to offset the telescope. ... Offset type : one of "Object Arc", "Object Arc XY" or "Boresight". ... This is motion on the sky or instrument, so don't worry about cos(dec). ... If you want to move that point (e.g. center your object along a different bit of the slit), then use a boresight offset. ... Note that a +X object xy offset will move the sky in the -X direction on the focal plane, because you are centering something in the +X direction on the boresight. ...