Reflective optics do not produce chromatic aberrations. Therefore, reflective objectives must correct only for spherical aberration. Using one positive and one negative reflective element, spherical aberration can be corrected, so reflective objectives are inherently simple devices. However, due to the high surface quality and tight positional requirements of reflective elements, the use of custom optics is impractical. The Schwarzschild objective is a readily-available off-the-shelf design that incorporates an on-axis configuration. Light enters the objective through a hole in the secondary mirror. Due to the on-axis geometry, source light hitting the central portion of the primary reflecting surface is reflected back through the opening and is lost, causing a reduction of image contrast (transmission efficiency). This effect is known as central obscuration.
Schwarzschild reflective objective
The modulation transfer function (MTF) is a measure of optical transmission efficiency as a function of spatial frequency (assuming that the primary mirror is uniformly illuminated). Lower spatial frequencies correspond to larger feature sizes. The dip in the MTF curve (as shown in C in the following chart) indicates lower optical contrast at larger feature sizes as numerical aperture increases. Therefore, the larger the numerical aperture, the greater the loss through the central obscuration. H
Modulation transfer function of Schwarzschild objective
However, large numerical apertures are used for higher contrast at high spatial frequencies, indicating good resolution of small features. Translated into a laser illumination scheme, this simply means that a reflective objective with a large numerical aperture is required to achieve high feature resolution, but at the expense of optical transmission efficiency.
As previously stated, the MTF curve assumes uniform illumination on the primary reflector. However, with UV excimer laser radiation, the beam can be directed off-axis at the reflector to miss the central obscuration, resulting in good beam transmission efficiency with minimal sacrifice of resolution. Concentrating the beam at localized spots on the primary mirror can result in optical damage. The optical damage threshold limits power transmission through the optics.