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Hi Stefan,
To test a convex surface at 6770 my first thought is a concave test plate, polished spherical and test the two by putting them almost in contact (use 3 pieces of paper as spacers) and counting interference fringes (newtons rings will be visible) under a diffuse monochromatic light source. For that to work though you'll have to partially polish the surface being tested.
Alternatively make an auto-coliimator setup with say an accurate larger parabolic mirror say f/5, and adjusting the position of the light source to create a beam converging on the centre of curvature of the convex surface to be tested. Reflect this off the convex surface back to the mirror and examine the resulting focus.
There will be some spherical aberration in this due to the converging beam and umyou could either calculate the expected amount and figure the lens surface accordingly, or if you want a null test, insert a lens near the focal plane that will give the right amount to correct it. Shouldn't be hard to calculate the required lens, similar to a Ross null test.
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