This calculator can be used to estimate the beta angle and the slit position angle on a given date for a given look direction (coordinate) on the sky. The beta angle refers to the distance between the anti-solar point and the direction of the target and is nominally in the range 30-85 degrees for FUSE on-orbit operations. (In certain special cases, this range can be opened up to 15 - 105 degrees, but with alignment difficulties.) The position angle increases by rotating the slits from north through east. A position angle of 0 degrees is defined as the orientation in which the 30" LWRS aperture is due north of the 1.25" HIRS aperture. Thus, the position angle provides information on the angle of the long dimension of the FUSE rectangular apertures on the sky.
Note: Targets with declination coordinates below 45 degrees (absolute value) have periods where they are not observable due to the orbital plane (Ram) avoidance zone. For such targets, also run the CVZ/Ram Calculator to verify visibility at the desired slit position angle times!!