Diffraction-limited system

A useful measure of imaging quality is the Strehl ratio. It is defined as the ratio of the on-axis intensity in the image of a point source, to that given by the diffraction limited system with the same aperture. A phase aberration (p x) will result in a Strehl ratio of... 

According to the Rayleigh criterion, a diffraction-limited system can distinguish two equally intense points provided that their geometrical images are separated by at least one Airy disk radius. That distance is known as the Rayleigh limit and defines the resolution limit RL of a diffraction-limited system. For a diffraction-limited lens... 

Diffraction-limited system Optical system in which... 

In the above derivations we neglected the effect of the stmcture necessary to hold the secondary mirror in place. The impact of diffraction effects by these stmctures has been examined in detail by Harvey Ftaclas (1988). If the primary is imaged onto the detector, as is often done to obtain a uniform field of view, the sensitivity pattern across the detector element influences the effective diffraction pattern of the system. The integrations of Eq. (5.3.3) should then include the detector sensitivity pattern as a weighting function under the integral. If the sensitivity pattern has pronounced peaks or voids, the effect can be substantial for a diffraction limited system. 

Mt. Wilson Observatory. The UnISIS excimer laser system is deployed on the 2.5 m telescope at Mt. Wilson Observatory (Thompson and Castle, 1992). A schematic of the system layout is shown in Fig. 11. The30W, 351 nm excimer laser is located in the coude room. The laser has a 20 ns pulse length, with a repetition rate of 167 or 333 Hz. The laser light is projected from the 2.5 m mirror and focused at 18 km. A fast gating scheme isolates the focused waist. A NGS is needed to guide a tip-tilt mirror. Even with relatively poor seeing, UnISIS has been able to correct a star to the diffraction limit. 

Such a pinhole density test was performed on the AZ/PMMA two-layer deep-UV PCM system (26). The result is shown in Table IX where a pinhole density of 8 and 6 per cm was obtained for the capped (A) and uncapped (B) systems. Because only three wafers were used for each test, the result should be taken only qualitatively and the numerical difference between 6 and 8 pinholes/cm should be taken as being indicative of measurement fluctuations only. It should not be attributed to the use of different developers or O2 plasma because in the subsequent tests of batches C and D in which the DUV exposure was omitted, the numbers were 0 and 1 pinhole/cm with the capped system giving the smaller pinhole density. The low pinhole density in batch E in which the AZ development step was omitted suggests that the pinholes arise during the development of the AZ layer. Presumably, a small portion of the AZ base resin molecules were not linked up with the photoactive compound and therefore still exhibited their intrinsic high solubility in the AZ developer. After development, these high solubility spots became pinholes. These pinholes are apparently larger than the diffraction - limited sizes so that they can be transferred into the PMMA film by deep-UV exposure. 

In order to determine the spatial resolution of the system, various sized polystyrene beads were imaged at a Raman shift of 2850 cm-. This experimental condition was achieved by choosing a signal-idler pair at wavelengths of 924 nm and 1254 nm. The characteristic lateral (xy) and longitudinal (z) resolutions were found to be diffraction limited to approximately 420 nm and -1.1 J,m (FWHM), respectively. 

If we define resolution as the shortest distance corresponding to the formation of one interference fringe, Fig. 5 represents the resolution limit of any possible optical system using the corresponding geometric configuration. The diffraction limited resolution and the interference limited resolution are represented by the separation of the hyperboloids and the ellipsoids, respectively. 

Further developments are also expected in imaging applications with faster imaging methods with higher spatial resolution becoming available (e.g. sub-diffraction-limited spatial resolution). Advanced non-linear techniques such as CARS and specialist methods such as ROA will broaden their respective application areas, as instruments become more compact and more systems become commercially available. 

Figure 3.20 A schematic idea for an imaging IR microspectrometer system using the synchrotron source and FPA detector. The FPA detector serves to spatially oversample a diffraction-limited region of the specimen for subsequent PSF deconvolution and resolution enhancement.

Consider a bolometric radiometer with an 18 GHz at 72 GHz with a diffraction-limited throughput, Ail = A2. A background limited (BLIP) system would have a temperature sensitivity of 19 /jK in 1 second. This is already 6 times better than the 107 /iK in one second needed to reach the point of diminishing... 

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