Model 4VPS shown below, is an example of the Vertical Fizeau interferometer
with integral Tip/Tilt Table whose height can be adjusted to accommodate
larger assemblies, or to minimize the air gap that can result in drifting
While requiring fine pitch screws to facilitate accurate adjustment of the test piece, in a vertical interferometer, the Tip/Tilt table must also be ruggedly built since it may be required to rigidly support heavy components or assemblies.
All interferometers require a Tip/Tilt device in at lease one position in the optical path.
In vertical format interferometers such as the Model 4VPS at the left or the smaller Model 2VP PHASE MITETM shown below, this device is usually configured as a table which supports the test piece.
In horizontal format interferometers such as the Model 339ir Infrared Interferometer shown below, it usually takes the form of a fixture or bracket mounted on the optical table.
If only Plano work is involved, only two axes of adjustment are required, Tip and Tilt.
If Spherical surfaces are to be measured, however, X, Y and Z axes are also required.
When doing Plano work, the Tip/Tilt Table is used to bring the test surface parallel to the reference surface of the interferometer. Centration is not too important since plano surfaces do not really have an optical axis.
When a spherical test piece is being measured, a 5-axis fixture is required to adjust of the orientation and position of the spherical surface so that:
The image at the left shows changes in the interferogram of of a test piece as the Tip /Tilt Table is adjusted. At the beginning of the cycle there are many fringes, and as the Tip/Tilt is adjusted to bring the test surface parallel to the reference surface, the number of fringes steadily decreases, until at the last frame of the sequence, one fringe pretty well covers the entire interferogram.
This technique often called "fluffing out" the fringes is one that is performed routinely in the day to day use of the interferometer.