The dictator does not enjoy the recognition, support and respect for his authority. His power cannot be really described as authority because he gets the power to rule through illegitimate means and exercises power which is devoid of legitimacy.
When a democratically elected leader starts using the power of the state in an arbitrary or coercive way, his legitimacy also gets reduced. When PM Indira Gandhi imposed emergency in June 1975 and began making an excessive, discretionary and even arbitrary use of her authority, her legitimacy got considerably reduced. The use of power of her government continued to be legal; nevertheless it lacked essential legitimacy and tantamount to a misuse of authority.
Authority means legitimate power i.e. power based on law and characterised by legitimacy. Legitimacy is an essential quality of authority. On the contrary, the use of power can be legal but at the same time devoid of legitimacy.
Authority is a recognised legal power which enjoys legitimacy and is willingly obeyed by the people. Power is the legal capacity to get things done by the use of force or threat of use of force.