**3.2.4 The multiple functions LTP (LTP-MF)**

Qian has developed a Multiple Functions LTP (LTP-MF) that incorporates two compact LTP optical heads into various configurations that allow for self-correction of scan-induced errors with subsequent improvements in measurement accuracy (Qian et al., 2005; Qian & Takacs, 2007). Some important facts of the LTP-MF, related to the approach to nanoaccuracy, are briefly described here. The LTP-MF can operate in the mode of scanning optical head with non-tilted reference by use of a second optical head to ensure higher accuracy (Fig. 5). If a high quality air-bearing system is used and 0.01°C temperature stability can be maintained, the LTP-MF can achieve 0.1rad rms accuracy in testing plane mirror surfaces. The LTP-MF can also operate in the penta-prism scanning mode for testing nearly plane mirrors with high accuracy.

Fig. 5. The LTP-MF collaborated between BNL (USA) and NSRL (China). The first optical head (OH) acts as scanning OH with non-tilted reference by use of the second OH. The second OH can also act as scanning penta-prism LTP

The schematic of the LTP-MF optical head is shown in Fig. 6. An unstabilized diode laser (DL) with a 633nm wavelength with power of 1-5mW is used as the light source. The optical fiber is used as a controllable beam transport tool. There are several advantages to using an optical fiber: a) to achieve a compact and convenient package; b) to isolate the thermal source from the optical system, which is helpful for achieving nano-accuracy; c) to minimize laser beam pointing error; d) to change the light source wavelength with ease and to replace the source without distorting the beam direction; e) to insert optical fiber devices in order to perform various new functions, for example, adding a fiber attenuator for controlling intensity. The beam is collimated to 1 mm diameter spot by a fiber collimator (FC). Then it passes through a monolithic wave-front splitting beam splitter (WSBS) and becomes two half-beam spots with a phase-shift so that when it is focused on the CCD by lens (FT), it will produce an interference fringe with the shape of valley-at-center for low noise fitting. The LTP-MF uses an equal optical path WSBS for the purposes of nano-accuracy and compactness. Otherwise, the large frequency shifts of diode laser will produce the fringe position shift, which will degrade the nano-accuracy. A pair of microscope cover plates are constructed as an adjustable phase shift WSBS, which is the easiest and lowest cost solution (Qian & Takacs, 2004; 2003)

The 20 x 20 mm PBS splits the beam into sample and reference arms, then they are reflected back from the mirror under test and reference mirror to the FT lens which has a focal length of 400 mm and a 28 mm aperture. The return beams are then focused onto a linear array CCD of 14 m pixel size. Two quarter wave-plates (QWP) are for isolating unwanted reflected beams, the half wave plate (HWP) is for adjusting the intensity ratio between the sample beam and reference beam. However, in the case of using the scanning OH with non-

Fig. 5. The LTP-MF collaborated between BNL (USA) and NSRL (China). The first optical head (OH) acts as scanning OH with non-tilted reference by use of the second OH. The

The schematic of the LTP-MF optical head is shown in Fig. 6. An unstabilized diode laser (DL) with a 633nm wavelength with power of 1-5mW is used as the light source. The optical fiber is used as a controllable beam transport tool. There are several advantages to using an optical fiber: a) to achieve a compact and convenient package; b) to isolate the thermal source from the optical system, which is helpful for achieving nano-accuracy; c) to minimize laser beam pointing error; d) to change the light source wavelength with ease and to replace the source without distorting the beam direction; e) to insert optical fiber devices in order to perform various new functions, for example, adding a fiber attenuator for controlling intensity. The beam is collimated to 1 mm diameter spot by a fiber collimator (FC). Then it passes through a monolithic wave-front splitting beam splitter (WSBS) and becomes two half-beam spots with a phase-shift so that when it is focused on the CCD by lens (FT), it will produce an interference fringe with the shape of valley-at-center for low noise fitting. The LTP-MF uses an equal optical path WSBS for the purposes of nano-accuracy and compactness. Otherwise, the large frequency shifts of diode laser will produce the fringe position shift, which will degrade the nano-accuracy. A pair of microscope cover plates are constructed as an adjustable phase shift WSBS, which is the easiest and lowest cost solution

The 20 x 20 mm PBS splits the beam into sample and reference arms, then they are reflected back from the mirror under test and reference mirror to the FT lens which has a focal length of 400 mm and a 28 mm aperture. The return beams are then focused onto a linear array CCD of 14 m pixel size. Two quarter wave-plates (QWP) are for isolating unwanted reflected beams, the half wave plate (HWP) is for adjusting the intensity ratio between the sample beam and reference beam. However, in the case of using the scanning OH with non-

second OH can also act as scanning penta-prism LTP

(Qian & Takacs, 2004; 2003)

tilted reference, the HWP is unnecessary. Polarizer (P) is for adjusting the beam intensity, but now it is easily replaced by a fiber attenuator. Two folding mirrors are for reducing the overall mechanical length, but for new nano-accuracy profile they should be removed in order to reduce systematic error. The FT lens is designed to keep aberrations below 1 µrad for two scanning modes conditions. The PBS needs to be of extremely high quality, including each surface, angles, and material uniformity to insure LTP precision. Good alignment of the optical system will ensure measurement accuracy when the angular test range is large.

It is conceptually easy to enlarge the LTP measurement range by reducing the focal length of the FT lens, by increasing the CCD size, by using a high resolution CCD, or by enlarging the PBS and FT aperture size. However, if nano-accuracy is required over the entire angular test range, then the improvement task becomes extremely difficult.

### **3.2.5 The vertical scanning LTP (VSLTP)**

The mirrors used for X- ray telescopes are all based upon the use of grazing incidence optics in various configurations. Wolter Type I systems use a combination of paraboloidal and hyperboloidal surfaces; a Wolter-Schwarzchild Type I system consists of a small figure modification of a Wolter I system; the foil cone is an approximation of a Wolter I system, and the Kirkpatrick-Baez (K-B) system consists of two sets of orthogonal spheres or parabolic cylinders, produced by bending thin plates. The surface figures are generally conical in shape with very small sag deviations, on the order of microns, from the best-fit conical surface. These optics are ideally suited for measurements with the LTP. The ideal configuration for measuring x-ray telescope optics, however, is to measure the object while it is oriented with its symmetry axis in the vertical direction. This minimizes the effects of gravity-induced distortion on the surface figure, especially on thin shells or foil surfaces. A vertical scanning LTP (VSLTP), a modification of the scanning penta-prism LTP, was developed for testing X-ray telescope mirrors in the vertical orientation and is shown in operation in Fig. 7 (Li et al., 1996,1997). The benefit of the VSLTP is that a small penta-prism can be scanned inside a small diameter x-ray telescope mirror.

Fig. 7. Vertical Scan LTP (VSLTP) at Marshall Space Flight Center set up for measuring x-ray telescope mirrors and mandrels in the vertical orientation

In 25 years of development, there have been a number of other applications of the LTP: insitu heat load test, measurement at machine shop, calibration of the profiler, thermal shift treatment, 2D detector development and so on. Some will be described in the following sections.
