Holds Up To Four Diffraction Gratings

 Newport offers the very popular, patented model 77708 quadruple grating turret assembly for use with the MS257 configurations. The 77708 may hold between one and four gratings and allows users to perform scans over a broader wavelength range than other spectrographs. If less than four gratings are installed in the 77708, the unused mounting locations may be utilized later without the need to purchase another grating turret. Newport offers a wide selection of MS257 gratings to cover the UV, VIS and NIR wavelength regions. The gratings are installed and aligned on the turret by Newport, followed by calibration of the MS257. For custom grating designs, or special coatings such as gold or AlMgF2, please contact Newport Sales for more information. The MS257 uses a high speed stepper motor drive, with a precision worm gear to ensure excellent repeatability and stability. Grating positioning accuracy is typically better than 0.15 nm, and wavelength repeatability is 0.06 nm when using a 1200 line/mm grating. Each MS257 is meticulously calibrated by "mapping" the grating drive using an interferometer. Newport measures hundreds of data points for each instrument and uses correction algorithms loaded into the microprocessor in order to meet specifications.

Quadruple Grating Turret Installation

In order to protect the fragile optical components, the quadruple grating turret ships separately when ordered with a MS257 monochromator. A simple installation procedure can be done in minutes, as demonstrated in the video.

Optical Configuration

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.

Imaging Spectrograph

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.

Configurable Input and Output Ports

The MS257 is designed with one input port and up to two output ports, designated as axial and lateral ports. The axial port provides the greatest throughput and does not require additional internal accessories to utilize it. The lateral output port allows the greatest flexibility when mounting a detector or other components outside of the instrument. Using both output ports is very convenient, where different detectors can be simultaneously for extended range scans without needing to break down and reconfigure the setup. Newport installs the necessary internal reflector and individually calibrates all output ports to be used with the instrument. To use only the lateral output port, the 77718 side exit mirror is required inside the instrument. To use both output ports, Newport will install the 77716 motorized output mirror into the MS257. The highly reproducible mirror position is controlled by software, an optional hand controller or low level commands. The output port can be selected at any time, or can automatically be chosen for a specific wavelength range. Newport’s motorized beam steerer, model 77765 , may be coupled to the input port of the MS257. This allows two light sources to be utilized with the instrument, complementing the extended range capabilities of this instrument. As an example, a quartz tungsten halogen (QTH) source may be used with a deuterium source for UV light. A TTL signal provided by the MS257 controls when to change the mirror position.

Controls Up to Two Filter Wheels

The MS257 controls up to two optional filter wheels, model 77737 . Each filter wheel holds up to five 1-inch [25.4 mm] diameter filters, with a 0.92-inch [23 mm] clear aperture. Wavelength changeover points can be loaded into MS257 so the appropriate filter is automatically selected for any wavelength. This is of particular importance during a broad spectral scan, to block multiple orders and residual stray light. Newport’s Colored-Glass Alternative Filters are an excellent choice when selecting order-sorting filters. The filter wheels are externally mounted to the MS257.

Fixed Slits for High Accuracy and Repeatability

Fixed slits should be used for the utmost accuracy and repeatability, especially at high resolution. Fixed slits are the best option when only a few slit sizes are required, as they are less expensive. The fixed slits slide into the model 77721 holders located at the input and output ports. The width and height cannot be adjusted, but may be individually replaced with other slit sizes. Individual fixed slits are required and sold separately to allow customized choices based on the needs of the application (see accessory section). Order two identical fixed slits and holders for single output port models, and three fixed slits with holders for dual output port instruments. The slit width is one of the factors determining the resolution and throughput of the instrument, with a narrow slit providing higher resolution and lower throughput.

Micrometer Adjustable Slits for Maximum Flexibility

The 74002 micrometer adjustable slit offers flexibility and high throughput, but is not as accurate or repeatable as a fixed slit. The micrometer adjustable slit assembly is continuously variable from fully closed to 3 mm width. A height adjustment slide allows variation in the height from 2 to 15 mm. This type of slit is designed primarily for versatility and convenience in changing resolution and throughput. Order two micrometer adjustable slits for single output port models, and three slits for dual output port instruments.

Motorized Slits for Automatic Control

Two motorized slit designs are available for use with the MS257. Order two micrometer adjustable slits for single output port models, and three slits for dual output port instruments. The slit widths can be set using software, low-level commands or optional hand controller in order to maintain a consistent bandpass over the entire operating range of each grating. The MS257 adjusts the slit widths for each data point. The model 77722 motorized slit assembly uses a micro-stepper motor to control the slit width from 4 µm to 2.0 mm, in 2 µm increments. Slit height is fixed at 15 mm. The model 77738 motorized slit assembly provides improved throughput and resolution and a 20 mm slit height. This is achieved by designing the 77738 using curved slits to compensate for the inherent bending of the entrance-slit image.

Communication Interfaces

The MS257 is equipped with both a USB and RS232 port. The model 77793 external IEEE-488 interface, ordered separately, is required for GPIB communication. Auxiliary input and output connectors are located on the back panel of the instrument for TTL signal control of select functions. LabVIEW™ based utility software is included at no extra cost with the MS257 to control both the spectrograph and filter wheel accessory. An intuitive command set is provided for those wishing to create their own programming. Newport’s optional TracQ™ Basic Data Acquisition and Radiometry Software coordinates the operation of the MS257 and popular detector instruments. For those who do not wish to use a computer, for either convenience or security concerns, Newport offers the optional model 77709 hand controller. The backlit LCD display provides information on the grating selection, active filter position, current wavelength, shutter status and more. Using the hand controller is intuitive and provides access to nearly all the functionality of the MS257.

Instrument Mounting

The optional 77775 mounting plate can be used to secure the MS257 to an inch or metric optical table. The ability to mount the instrument simplifies setup and alignment of the optical system. The mounting plate also helps ensure consistent results over time, as the instrument cannot be accidentally moved out of position. The plate adds 0.38 inch [9.6 mm] to the optical height.

Two Built-In Shutter Options

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.

Long Life Instrument

These instruments continue to function for many years. With proper care, they only require periodic maintenance to ensure gears are lubricated, spring tension is correct, etc. It is suggested to send the instrument to Newport annually for recalibration, in order to meet its performance specifications. During recalibration, the instrument is thoroughly examined and routine maintenance services are performed. In addition to this service, Newport is also able to reconfigure the grating selection, slits, shutter and output mirrors if desired. Contact Newport Sales for more information.