An interferometer (100, 144, 220, 250, 260, 300) comprising an optical assembly, the optical assembly comprising: first and second partially reflective surfaces (118, 122) positioned along an optical axis and oriented at different non-normal angles (α, 2α) to the optical axis, wherein the second partially reflective surface is configured to: i) receive light transmitted through the first partially reflective surface along the optical path; ii) transmit a portion of the received light to a test object to define measurement light for the interferometer; and iii) reflect another portion of the received light back towards the first partially reflective surface to define reference light for the interferometer, wherein the second and first partially reflective surfaces (118, 122) face each other such that the reference light is reflected from the second partially reflective surface to the first partially reflective surface, then from the first partially reflective surface back to the second partially reflective surface at least once, wherein the second partially reflective surface is configured to combine the measurement light, after the measurement light reflects from the test object back to the second partially reflective surface, with the reference light, after the reference light travels from the second partially reflective surface to the first partially reflective surface, then from the first partially reflective surface back to the second partially reflective surface at least once. first and second partially reflective surfaces (118, 122) positioned along an optical axis and oriented at different non-normal angles (α, 2α) to the optical axis, wherein the second partially reflective surface is configured to: i) receive light transmitted through the first partially reflective surface along the optical path; ii) transmit a portion of the received light to a test object to define measurement light for the interferometer; and iii) reflect another portion of the received light back towards the first partially reflective surface to define reference light for the interferometer, wherein the second and first partially reflective surfaces (118, 122) face each other such that the reference light is reflected from the second partially reflective surface to the first partially reflective surface, then from the first partially reflective surface back to the second partially reflective surface at least once, wherein the second partially reflective surface is configured to combine the measurement light, after the measurement light reflects from the test object back to the second partially reflective surface, with the reference light, after the reference light travels from the second partially reflective surface to the first partially reflective surface, then from the first partially reflective surface back to the second partially reflective surface at least once. i) receive light transmitted through the first partially reflective surface along the optical path; ii) transmit a portion of the received light to a test object to define measurement light for the interferometer; and iii) reflect another portion of the received light back towards the first partially reflective surface to define reference light for the interferometer, wherein the second and first partially reflective surfaces (118, 122) face each other such that the reference light is reflected from the second partially reflective surface to the first partially reflective surface, then from the first partially reflective surface back to the second partially reflective surface at least once, wherein the second partially reflective surface is configured to combine the measurement light, after the measurement light reflects from the test object back to the second partially reflective surface, with the reference light, after the reference light travels from the second partially reflective surface to the first partially reflective surface, then from the first partially reflective surface back to the second partially reflective surface at least once.