FAST slit sky orientation
Created 09/27/2019 by EF
Warren Brown verified the orientation on the sky of the FAST slit.
At default PA=90, objects move to the top of the slit (high pixel
values) when I slew E.
During a short window with clear sky the night of 09/26/19, he viewed
a star on the guider at rotator PA 90 and 0, as he slewed E and N.
top is the W side of the slit, and the bottom the E side of the slit.
At PA=0, objects move to the top (high pixel values) when I slew
Thus, the top is the S side of the slit, and the bottom the N
side of the slit.