**3.2.3 Penta-prism LTP**

84 Modern Metrology Concerns

Some recent versions of the LTP use a single probe beam instead of the dual beam, eliminating the need for the initial beam splitting optics. The probe beam is focused to a single spot with no internal interference structure. The peak or centriod of the intensity distribution on the sensor determines the angle of the surface seen by the beam footprint. In this respect, the LTP operates very much like an autocollimator. Various algorithms are used

Over the years, many improvements have been made to the original LTP-I design by many collaborators involved in synchrotron and x-ray telescope optics metrology. The original LTP system utilized an external electronic autocollimator to measure the pitch angle error of the optical head as it moved along the air bearing. The autocollimator was replaced by the internally-generated reference beam, shown in Fig. 3, by Irick, et al. (Irick et al., 1992) This allowed for simple correction of the measured profile for mechanical pitch errors by adding the two signals together. The relative intensity between the test and reference beams could now be adjusted easily by the use of polarizers and wave plates added to the optical system (not shown). The commercial version of the instrument, the LTP-II, produced by Continental Optical Corp. for several years, incorporated the internal reference beam and used a dualarray linear photodiode sensor as the detector. The dual array detector allowed the reference beam to be aligned nearly along the optical axis of the system instead of at an extreme angle, which places the spot at the end of the sensor. Having the reference beam centered in the lens aperture minimizes the introduction of phase shifts by glass inhomogeneities that translate into beam spot location variations with lateral movement of the beam across the aperture during a long scan. Bresloff added a Dove prism into the reference beam to change the phase of the laser pointing direction drift to be the same as the mechanical pitch angle error, allowing for correction of both error signals simultaneously by addition of the two

Fig. 4. The LTP II optics board layout showing the 10 mrad surface slope acceptance angle optics in place. A 4-mirror arrangement folded the beams from the 1.25 m focal length lens with 7 reflections onto the detector mounted in back of the plane of the figure. The Dove prism mounted on the optics board in the REF arm inverted the phase of the pitch error

signal to allow for simultaneous correction of pitch and laser drift

to extract the centroid location of the spot with high precision.

signals (Takacs & Bresloff, 1986; Takacs et al., 1999).

The LTP II uses a scanning optical head (OH) in the tilted reference mode. Qian et al. developed the scanning penta-prism mode LTP (PPLTP) in 1995 at Sincrotrone Trieste, Italy (Qian, 1995), an evolution that extends applications and improves the accuracy of tests for plane- and near plane-mirrors because a tilted reference beam is unnecessary. The main characteristic of the penta-prism is: the angle between incident and output beams of the pentaprism will always be equal to 2 is angle between two reflection surfaces of the pentaprism, nominal angle is 45°), even if the penta-prism is tilted. So for a scanning penta-prism, the slide pitch error will not influence output beam direction. This measurement method has been successfully adopted by many SR metrology laboratories. In addition to accuracy improvement, the penta-prism LTP has enabled the testing of in situ heat load distortion of mirrors in vacuum chambers and the testing of small diameter aspheric surfaces of astronomy telescopes by use of a derivative of the penta-prism LTP: the in-situ LTP and vertical scan LTP (VSLTP). These will be described later. Also, the NOM employs a scanning penta-prism mode to enhance accuracy to the nano-radian level in small test angles.
