It's a pretty basic skill, and also pretty easy. The instructions that come with typical "orienteering" compasses have all the information needed for direction-finding. I don't recall ever actually using the sighting mirror for its intended purpose.
And the weight is insignificant. I just weighed mine. The Silva Landmark 27 weighs 0.7oz., and the Suunto (don't know the model, but has larger 2"x2" mirror), weighs 1.1oz. Neither was expensive
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Always remember that you are absolutely unique, just like everybody else. -Margaret Mead