A Simple Method for Reducing Astigmatism from Off-Axis Concave Spherical Mirrors
The advantages of mirror optics for spectrographic systems are well known, namely, that all wave lengths focus at the same place and that the reflection losses can be made very small. Most modern spectrographs use mirror optics. Some are able to use spherical mirrors1 while others must use off-axis paraboloidal mirrors2 to achieve the desired resolution. Inexpensive concave spherical mirrors are ideal for imaging the source on a spectrograph slit, except for the problem of astigmatism. Expensive off-axis corrected mirrors can be obtained, but they must then be used only at the designed off-axis angle. We have found a simple adjustable way to reduce the astigmatism from inexpensive spherical mirrors used at moderate off-axis angles.
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