We claim the ornamental design for a calibration apparatus, as shown and described.
FIG. 1 is a front perspective view of a calibration apparatus, showing our new design;
FIG. 2 is a back perspective view thereof;
FIG. 3 is a front elevation view thereof;
FIG. 4 is a right side elevation view thereof;
FIG. 5 is a back elevation view thereof;
FIG. 6 is a left side elevation view thereof;
FIG. 7 is a top plan view thereof; and,
FIG. 8 is a bottom plan view thereof.
The broken lines immediately adjacent to the claim, represent the bounds of the claim while all other broken lines are directed to environment; the broken lines form no part of the claimed design.