In most cases, conventional interferometric methods can be used to test the figure of large spherical and flat optical components. There are, however, certain types of unconventional surfaces, such as those used in grazing incidence x-ray applications, that are nearly impossible to test by conventional means. These cylindrical aspheres are usually tested by some type of scanning optical profiler. We discuss the use of a versatile slope measuring scanning interferometer system, the Long Trace Profiler (LTP), in measuring the figure error of large surfaces, particularly those that have extremely long radii of curvature in the tangential direction. Use of this instrument in different configurations has permitted measurements to be made on cylindrical asphere segments that are over one meter long, on water-cooled high heat load mirrors in ultra high vacuum synchrotron beam lines under actual operating conditions, and on complete x-ray telescope mirror shells and mandrels that are mounted in a vertical configuration to minimize gravity sag errors. Each of these different configurations has its own particular advantages and shortcomings. The ultimate performance of the LTP depends upon the thermal stability of the local environment. We illustrate the effects that temperature variations on the order of plus or minus 0.1 degrees Celsius have on the errors in the measurement of a long radius sphere.