Defocus

The laminography method was developed initially for medical applications as a non-computer layer-by-layer visualization of the human body [1,2]. In this case an inclined initial X-ray beam projects an image of a specific layer of the object to the detector surface with defocusing of the other layers during a synchronous rotation of the object and the detector (Fig. 1). 

Figure Bl.17.5. Examples of CTFs for a typical TEM (spherical aberration = 2.7 mm, 120 keV electron energy). In (a) and (b) the idealistic case of no signal decreasing envelope fimctions [77] are shown, (a) Pure phase contrast object, i.e. no amplitude contrast two different defocus values are shown (Scherzer focus of 120 mn imderfocus (solid curve), 500 mn underfocus (dashed curve)) (b) pure amplitude object (Scherzer focus of 120 mn underfocus) (c) realistic case mcluding envelope fimctions and a mixed weak...

Figure Bl.17.9. A CoSi grain boundary as visualized in a spherical-aberration-corrected TEM (Haider et a/ 1998). (a) Individual images recorded at different defocus with and without correction of C(b) CTFs in the case of the uncorrected TEM at higher defocus (c) CTF for the corrected TEM at only 14 nm underfocus. Pictures by courtesy of M Haider and Elsevier.

Toyoshima C and Unwin P N T 1988 Contrast transfer for frozen-hydrated specimens determination from pairs of defocus images Uitramicroscopy 25 279-92... 

Some solid materials are very intractable to analysis by standard methods and cannot be easily vaporized or dissolved in common solvents. Glass, bone, dried paint, and archaeological samples are common examples. These materials would now be examined by laser ablation, a technique that produces an aerosol of particulate matter. The laser can be used in its defocused mode for surface profiling or in its focused mode for depth profiling. Interestingly, lasers can be used to vaporize even thermally labile materials through use of the matrix-assisted laser desorption ionization (MALDI) method variant. 

The laser can be used as a finely focused beam that, with each pulse, drills deeper and deeper into the specimen to give a depth profile. Alternatively, the beam can be defocused and moved over an area at lower power to explore only surface features of a specimen. 

Because the laser beam is focused on the sample surface the laser power is dissipated in a very smaU area which may cause sample heating if the sample is absorbing and may cause break-down if the sample is susceptible to photodecomposition. This problem sometimes may be avoided simply by using the minimum laser power needed to observe the spectrum. If that fails, the sample can be mounted on a motor shaft and spun so that the power is dissipated over a larger area. Spinners must be adjusted carefully to avoid defocusing the laser or shifting the focal spot off the optic axis of the monochromator system. 

In real curvature sensors, a vibrating membrane mirror is placed at the telescope focus, followed by a collimating lens, and a lens array. At the extremes of the membrane throw, the lens array is conjugate to the required planes. The defocus distance can be chosen by adjusting the vibration amplitude. The advantage of the collimated beam is that the beam size does not depend on the defocus distance. Optical fibers are attached to the individual lenses of the lens array, and each fiber leads to an avalanche photodiode (APD). These detectors are employed because they have zero readout noise. This wavefront sensor is practically insensitive to errors in the wavefront amplitude (by virtue of normahzing the intensity difference). 

Under 1 arcsec seeing, the LGS FWHM was 1.4 arcsec. Images taken with the Keck primary mirror defocused (Fig. 24) demonstrated the level of distortion expected for an off-axis projection system. Low-level scattering was also minimal, consistent with expectations and seeing conditions. 

Figure 10. Eigenvalues of a 4 LGSs 3D mapping system with a /O field. Neither piston nor tilts are measured from the LGSs. Dashed line without natural guide star. Solid line with a NGS to measure tilts, defocus and astigmatisms (Le Louam and Tallon, 2002).

When neither the piston nor the tilts are measured from the LGSs, there are 5 more 0 eigenvalues the two even tilt modes, and the odd defocus and astigmatism modes. These last modes result from combinations of not measured tilts in front of the LGSs. 

Spatial resolution is a function of altitude of the backscattering region Hi, since the size of the defocus image writes D Hj — Hi)/ HfHi). With Hf 25km and Hi =11 km and if the spot is observed with a resolution of 1 arcsec, then when projected to the pupil the resolution is 5 x Hf Hi)/ Hf — Hi) 0.26m. At a 32 m, the defocus image is 2 arcmin wide. 

Van Dam, M.A., Lane, R.G., 2002, Wave-front sensing from defocused images by use of wave-front slopes, Appl. Opt. 26, 5497... 

Roddier, C., Roddier, R, 1993, Wave-front reconstruction from defocused images and the testing of ground-based optical telescopes, JOSA.A 10, 2277 (1993). 

Our SIMS data is taken using a VGQ8 quadrupole with unit mass resolution between 0-300 amu and a differentially pumped argon ion gun used in a defocused mode. A current of l-2x 10 A in a 0.S cm spot area is used. 

For the high resolution case, the phase-contrast effects are automatically introduced owing to the combined effect of defocus and spherical aberration, which gives rise to an image of a structure complicated by the fact that also the amplitude term, resulting from the propagation process, interacts in a non-linear way with the phase term [16,89,90,96]. 

---