Morningstar Inc., best known for its analysis of mutual funds, said it is acquiring privately held credit-rating firm Realpoint LLC for $52 million in cash and stock to build on its entry into corporate credit ratings. Morningstar Chairman and Chief Executive Joe Mansueto cited strong demand for “unbiased ratings and research in the structured credit market” as a reason for the deal. “Together, we want to restore credibility to the credit-ratings business and be a positive force in rating structured products,” he added. Entrenched credit-ratings firms came under fire during the financial crisis for how they rated billions of dollars in mortgage-related securities, which turned toxic as the subprime loans that helped fuel the U.S. housing bubble started to default. Horsham, Pa.-based Realpoint, which rates only commercial mortgage-backed securities, had been looking to expand into rating residential mortgage-backed and consumer loan-backed deals.