The MS257 is designed around an asymmetric in-plane Czerny-Turner configuration, with an input F-number of 3.9 to provide highly efficient throughput. The input and exit focal lengths are 220.0 and 257.4 mm respectively. The dispersion is twice that of a 1/8m spectrograph. Toroidal optics are utilized to produce accurate images of the input slit in the flat output plane. The MS257 is designed to eliminate stray light caused by reflections from a camera window or optics internal to the instrument. This is critical to achieving high performance, as stray light could potentially be re-diffracted and affect the spectral purity of the output. The reflectors inside the MS257 are coated with AlMgF2 to maximize UV efficiency. For gold-coated reflectors, which are used to enhance IR efficiency, contact a Newport sales engineer for more information.

In summary, the design was carefully optimized to:

• Minimize coma throughout the working wavelength range
• Eliminate re-diffracted light from either of the two main mirrors
• Eliminate re-diffracted light from reflections off the window or surface of a focal plane detector
• Provide the best flat focal field for the entire working wavelength range
• Provide the best imaging using corrected optics (toroidal reflectors) over the entire working wavelength range

In order to obtain the best optical performance, a fine focus adjustment is included. This knob is set at the factory for best focus on an exit slit; if you are using MS257 as a spectrograph, with a CCD other than our Linespec, you may use this adjustment in the field to translate the exit mirror of the MS257. This can optimize the location of the image onto a focal plane detector such as a diode array or CCD.

The term “imaging” has different meanings depending upon the field of application. In this context, the term describes the point-to-point replication of the input slit at the output plane of the spectrograph. This concept is best understood by comparison; for simplicity the source will be monochromatic radiation, focused on the entrance slit at the instrument’s F/#. In a non-imaging spectrograph, this source radiation is focused in a narrow, vertical line on the exit plane of the instrument, at the appropriate position for that wavelength. If one masks half of the input slit on a conventional spectrograph, a decrease in intensity will be evident at the output plane with very little spatial difference in the focused line. By contrast, the imaging spectrograph focuses the source radiation on the exit plane so that the focused spot parallels the shape and distribution of the input slit. Because of this spatial relationship between input and output, imaging systems are valuable tools for measuring multiple samples simultaneously.

Multi-track Spectroscopy is the recording of spectra from multiple sources simultaneously. Other terms such as “imaging spectroscopy,” or “multi-source spectroscopy” are also used. A two dimensional CCD and a high spatial resolution, imaging spectrograph are needed in order to resolve the various “tracks”. Some spectrographs are designed with spherical mirrors and are optimized for the best spectral resolution at the expense of vertical spatial resolution. The astigmatism in the final image is usually so great that even a point source appears as a tall, thin line several millimeters high.

The corrected optics of the MS257 were specially designed for point to point spectrographic imaging. The mirrors correct for the astigmatism and enable many sources to be simultaneously diffracted and resolved as separate spectra on a CCD. The aberration limited, spatial resolution of an Imaging MS257 is approximately 40 µm. Theoretically this would allow well over 100 separate point sources to be imaged onto a 7 mm CCD. However, there are a number of other practical considerations. Newport has demonstrated spectroscopic imaging using as many as nine separate channels (100 and 200 µm fibers at the input) with very good results.

Newport installs the shutter inside the MS257 input port. Two shutters are available, depending on the application. Both shutters are normally closed. They can be controlled through the MS257 command language, utility software, optional hand controller, TracQ™ Basic Data Acquisition and Radiometry Software or via an externally applied TTL signal. The shutter can be closed for dark scans and dark current measurements. The 77755 is a solenoid driven shutter. This shutter automatically closes when changing gratings and filters in order to protect the detector from possible high light intensities at various grating angles - particularly the zero order white light. For high repetition rate, short exposure applications, choose the 77717 fast shutter. This shutter may be run in short bursts at repetition rates as high as 40 Hz, with exposure times as short as 5 ms. A synchronizing output signal is available from the fast shutter for very precise timing of external events. The 77717 fast shutter does not automatically react to grating changes.