Astigmatism Of Objective Lens Research Articles

Defocus and magnification dependent variation of TEM image astigmatism

Daily alignment of the microscope is a prerequisite to reaching optimal lens conditions for high resolution imaging in cryo-EM. In this study, we have investigated how image astigmatism varies with the imaging conditions (e.g. defocus, magnification). We have found that the large change of defocus/magnification between visual correction of astigmatism and subsequent data collection tasks, or during data collection, will inevitably result in undesirable astigmatism in the final images. The dependence of astigmatism on the imaging conditions varies significantly from time to time, so that it cannot be reliably compensated by pre-calibration of the microscope. Based on these findings, we recommend that the same magnification and the median defocus of the intended defocus range for final data collection are used in the objective lens astigmatism correction task during microscope alignment and in the focus mode of the iterative low-dose imaging. It is also desirable to develop a fast, accurate method that can perform dynamic correction of the astigmatism for different intended defocuses during automated imaging. Our findings also suggest that the slope of astigmatism changes caused by varying defocuses can be used as a convenient measurement of objective lens rotation symmetry and potentially an acceptance test of new electron microscopes.

Real-time detection and single-pass minimization of TEM objective lens astigmatism

Minimization of the astigmatism of the objective lens is a critical daily instrument alignment task essential for high resolution TEM imaging. Fast and sensitive detection of astigmatism is needed to provide real-time feedback and adjust the stigmators to efficiently reduce astigmatism. Currently the method used by many microscopists is to visually examine the roundness of a diffractogram (Thon rings) and iteratively adjust the stigmators to make the Thon rings circular. This subjective method is limited by poor sensitivity and potentially biased by the astigmatism of human eyes. In this study, an s2 power spectra based method, s2stigmator, was developed to allow fast and sensitive detection of the astigmatism in TEM live images. The "radar"-style display provides real-time feedback to guide the adjustment of the objective lens stigmators. Such unique capability allowed us to discover the mapping of the two stigmators to the astigmatism amplitude and angle, which led us to develop a single-pass tuning strategy capable of significantly quicker minimization of the objective lens astigmatism.

A rapid method to correct objective lens astigmatism in a TEM

This work describes a rapid method to correct the two-fold astigmatism of transmission electron microscope (TEM) objective lens employing caustic curve when no objective aperture is inserted. The method makes use of rounding the caustic curve via the objective lens stigmators after the condenser lens astigmatism has been corrected. It has many advantages over other methods, it is fast, straightforward, and does not need holes or an amorphous material.

Fabrication of three-dimensional calixarene polymer waveguides using two-photon assisted polymerization

Low-loss three-dimensional (3D) polymer optical waveguide was fabricated using a two-photon assisted polymerization method. Calixarene derivatives were selected as core material with good thermal stability and transparency. To fabricate 3D waveguide, photopolymerizable polymers composed of two types of monomers were used. Core shape was controlled by adjusting an astigmatism of objective lens. Refractive index difference between a core and a cladding was formed by exclusion effect of one monomer. Propagation loss at 1.3μm wavelength was low, 0.72dB∕cm, without using deuterium or fluorine but using hydrogen. Using this technique, 3D 1×3 splitter polymer waveguide was fabricated.

Astigmatic intensity equation for electron microscopy based phase retrieval

Phase retrieval, in principle, can be performed in a transmission electron microscope (TEM) using arbitrary aberrations of electron waves; provided that the aberrations are well-characterised and known. For example, the transport of intensity equation (TIE) can be used to infer the phase from a through-focus series of images. In this work an “astigmatic intensity equation” (AIE) is considered, which relates phase gradients to intensity variations caused by TEM objective lens focus and astigmatism variations. Within the paraxial approximation, it is shown that an exact solution of the AIE for the phase can be obtained using efficient Fourier transform methods. Experimental requirements for using the AIE are the measurement of a through-focus derivative and another intensity derivative, which is taken with respect to objective lens astigmatism variation. Two quasi-experimental investigations are conducted to test the validity of the solution.

Effects of an asymmetrically molded plastic objective lens on the push–pull tracking-error signal in an optical disk drive

The effects of a plastic objective lens's astigmatism on the push-pull tracking-error signal (TES) of an optical disk data storage system were investigated theoretically and experimentally. Astigmatism of plastic objective lenses arises commonly from the asymmetric deviation from their designed shape during the molding process. By carefully studying the aberration characteristics of the objective lens and including the astigmatism of the laser diode in the analysis, we can calculate the combined effects of astigmatism of these two components on the push-pull TES. It is found, from both the simulations and the experiments, that, by rotation of the objective lens about the optical axis, the peak-to-peak value of the push-pull TES varies with the lens's rotation angle, and a change as great as 340% in its value was observed in a given optical pickup.

An “on-line” correction method of defocus and astigmatism in HAADF-STEM

The imaging characteristics of high-angle annular detector dark field scanning transmission electron microscopy (HAADF-STEM) and the Fourier transform patterns of the images are formulated under the projected potential approximation. By using the Fourier transform patterns, it is clarified from the present theoretical and experimental studies that the defocus and astigmatism of objective lens in STEM are corrected “on-line”, which is very useful for finally tuning HAADF-STEM in high-resolution observation.

Practical autoalignment of transmission electron microscopes

We present a working method for automatic correction of beam misalignment, defocus and objective lens astigmatism based upon computer-induced beam tilts which result in image shifts that are measured and used to adjust the microscope. The performance of the method has been tested on 100 and 400 keV microscopes, and it proves to work reliably over a wide range of magnifications for both weak phase and amplitude contrast specimens. The performance of the alignment procedure is instrumentation, specimen and magnification dependent. The reproducibility for alignment on the coma-free axis was measured to be less then 0.1 mrad at magnification M = 500 000 and less then 2 mrad at M = 10 000. The reproducibility for focusing and stigmation was measured to be less then 3 nm at M = 500 000 and less then 50 nm at M = 10 000.

The use of histograms for transmission electron microscopy: Defocus and astigmatism correction at high resolution

AbstractA real‐time method for optimizing the defocus of a conventional transmission electron microscope in the phase contrast imaging mode has been investigated using image histogram data. This method can also be used to minimize the objective lens astigmatism. It will be shown both theoretically and empirically, using a digital television frame store, that a histogram will give the largest peak when an image has a broad and flat contrast transfer function. This method has distinct advantages of speed and minimal computational requirements over obtaining the power spectrum of an image.

A systematic method for the correction of astigmatism in the Siemens Elmiskop I

A systematic but simple operational procedure for the correction of objective lens astigmatism in the Siemens Elmiskop I is described. Asymmetry in the Fresnel fringe at the edge of the image of a hole is expressed as a vector (the astigmatism vector). The direction of this vector is such that the angle it makes with a fixed reference line is twice the angular displacement of the fringe asymmetry from the same reference line; the magnitude is conveniently taken as the number of clicks of the fine focus controller over which incomplete fringes are visible. Astigmatism vectors defined in this way are shown to obey the law of vector addition and this leads to the procedure described; this procedure enables the required degree of correction to be achieved rapidly and with the minimum of effort. Other practical aspects of astigmatism correction are considered.