MS257™ 1/4 m Monochromator and Imaging Spectrograph

The MS257™ is a completely automated, efficient 1/4 m instrument, with enough versatility to satisfy most spectroscopy applications. It is an F/3.9 instrument with a focal length of 257.4 mm, usable as a monochromator or true flat field imaging spectrograph. Stray light is negligible, and there is no re-entrant spectra. The MS257 has 2 output ports, 1 input port (with optional accessory can support 2 inputs), and supports 4 gratings. USB and RS232 are standard. GPIB (IEEE488) communication is available using an external USB to GPIB converter sold separately.

High Efficiency

The low F/number of 3.9 (a numerical aperture of 0.128) combined with slits of up to 5 mm wide x 20 mm high, gives an efficient throughput. The dispersion is similar to that of our other 1/4 m monochromators, and twice that of our 1/8 m systems. The mirrors and gratings in MS257 were designed to further enhance throughput. Each of the optics is uses an aluminum and a magnesium fluoride coating which improves their UV efficiency. (Contact a Sales Engineer for gold coated optics.) The 77708 Multiple Grating Turret allows you to choose up to four gratings to cover your wavelength range.

Optical Configuration

MS257 is designed around an asymmetric in-plane Czerny-Turner configuration. The input F/# is 3.9, and the input and exit focal lengths are unequal, 220.0 and 257.4 mm respectively.

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 (toroids) over the entire working wavelength range

Input and Output Ports

MS257 has one entrance port and two output ports (see Fig. 2). Normally, the axial output port is used since this does not require a turning mirror and therefore prevents additional light losses at the extra reflecting surface. The in-line configuration of the lateral port offers greater detector mounting flexibility. To use just the lateral output port you’ll need the 77718 Side Exit Mirror. This mirror is pre-aligned and can be inserted in the field.

If you intend to use both output ports, and regularly switch between them, you should use the 77716 Motorized Flip Mirror. You can pre-program MS257 to automatically change the output port at a particular wavelength. The mirror drive has excellent positioning reproducibility of better than 0.05 nm (with a 1200 l/mm grating). This precision is essential to obtaining reproducible scans when changing detectors in mid scan.

An Auxiliary Input Port, or Motorized Beam Steerer (model 77713 110V or 77765 220V), is available to convert MS257 to a four port monochromator or spectrograph. This is an external accessory and has a motor driven mirror which is similar to the output mirror drive of the 77716 . It enables two sources to be alternately switched into the input port, for example a deuterium lamp and quartz halogen lamp (See Fig. 3). The mirror can be controlled through the MS257 command language; the Hand Controller does not control the 77713 / 77765 . The optical path length of this Beam Steerer is 6.5 inches (165 mm).

Corrected Optics for Spectral Imaging

We offer a choice of optics for the MS257. The high quality spherical optics ensure excellent resolution over the full spectral range; choose these when MS257 is used for non-imaging applications. For imaging, we offer the 77782 MS257, which uses computer optimized toroidal mirrors. These produce accurate images of the input slit in the flat output plane.

What is “Imaging?”

The term “imaging” has different meanings depending upon the field of application. In this context we use the term to describe 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 our source will be monochromatic radiation, focused on the entrance slit at the instrument’s F/#. In a conventional 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 you mask half of the input slit on a conventional monochromator, you’ll see a decrease in intensity 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 the focused spot parallels the shape and distribution of the input slit. In fact, with an imaging system, if you mask the top half of your input slit, half of your output image disappears. Because of this spatial relationship between input and output, imaging systems are valuable tools for measuring multiple samples simultaneously. Fig. 4 illustrates the results of two fibers carrying monochromatic radiation, placed at the input plane of a conventional spectrograph and the same two fibers placed at the input plane of an imaging spectrograph, such as our MS257™.

Negligible Stray Light

Many factors influence the stray light at the output. The grating used, the operating wavelength and the scattering of light inside the monochromator are usually the most significant. The stray light signal also depends strongly on such external factors as the intensity and spectral distribution of the input source and detector's spectral response.

Great attention has been paid to eliminating stray light in MS257. The wide open optical layout and exaggerated tilt of the exit focal plane ensure this. While other instrument designs may be more compact, it is inevitable that they will suffer from “re-entrant” spectra at certain grating angles. It is important that diffracted light is not allowed to reflect from the mirrors, or from the face of a focal plane, back onto the grating. This light could then be re-diffracted and cause ghost images or stray light at the detector. MS257 was designed to remove this possibility for CCD detectors even as large as 28 mm x 28 mm. MS257 has superb stray light rejection; just look at the two measurements below.

Measurement 1

With a Deuterium Source, a UV-VIS photomultiplier, model 77340 , and a 1200 l/mm Holographic Grating blazed at 250 nm, model 77741 , measurements were made with and without a glass plate in the beam. The glass blocks all radiation below 320 nm. At 250 nm, the stray light signal was only 3 x 10^-4 of the unblocked signal.

Measurement 2

With the same grating and a 632.8 nm Helium Neon laser focused on the slit in an F/4 cone, the average of the stray radiation measurements at 612.8 and 652.8 nm was 1.5 x 10^-5 of the 632.8 nm reading.

Wavelength Drive

MS257 uses a high speed stepper motor drive, with a precision worm gear to ensure excellent repeatability and stability. Our worm drive is much better than most sine drive mechanisms; grating positioning accuracy is typically better than 0.15 nm, and wavelength repeatability is 0.06 nm (for a 1200 l/mm grating). Each MS257 is meticulously calibrated by “mapping” the grating drive using an interferometer. We measure hundreds of data points for each instrument and use correction algorithms loaded into the microprocessor in order to meet our tight specifications. Data points can be automatically set so that the wavelength intervals between data points are equal, no matter what your choice of units; nanometers, microns, or even wavenumbers. And, MS257 can take control of a strip chart recorder, or other recording device for linear recording of spectra from the UV to the IR, skipping all the pauses for grating, filter and detector changes!

Filter Wheel for MS257

MS257 automatically 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 clear aperture of 22 mm. Even when both wheels are attached to MS257, the system F/# is still F/3.9.

Automatic Filter Changes

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 so that multiple orders and residual stray light are eliminated.

External Mounting for Best Optical Performance

The filter wheels are mounted externally in front of the input slit. While it may seem logical, and easier, to place the filter wheels inside the housing there are good optical reasons not to do so. The refractive index and thickness of the filters would significantly affect the focal distance to the collimating mirror. This would be different for every wavelength and every filter, and would therefore adversely affect resolution and imaging. When multi-track fibers are used for point to point imaging, the practical focal distance must remain constant in order to maintain good resolution at the edges of the field. MS257 is designed and manufactured to have errors of only a fraction of a millimeter in the focal distance. Mounting the filters before the input slit only slightly affects the light throughput. Since the slit acts as a secondary source the focal distance is not affected.

Shutters for MS257

We offer two internal shutters for MS257. Choose the one most suited to your application.

Background Shutter

The 77755 is a solenoid driven shutter which can be closed for dark scans and dark current measurements. You can also automatically close the shutter during grating, filter, and optical port changes to protect the detector from exposure to abnormally high intensity levels. When using MS257 as an Imaging Spectrograph, trigger the 77755 Shutter to open in order to expose the CCD detector. The 77755 is controlled through the MS257 command language, the 77709 Hand Controller, directly via an externally applied TTL signal, or through the optional TRACQ or InstaSpec Pro software.

Specifications for 77755

Fast Shutter

For high rep 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. The exposure may be set and triggered from within the MS257 command language. A TTL output signal is also provided for precise synchronizing of external events. This shutter is ideal for the 77782 Imaging MS257 when using our InstaSpec™ CCD detectors at high rep rates in either the fully binned mode or with only a few channels.

Specifications for 77717

Fine Focus Adjustment

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 InstaSpec, 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.

Mounting

MS257 weighs ~40 lbs, and can therefore support optical accessories mounted to its ports. However, if you wish to fix it to an optical table use the 77775 Mounting Kit. The 77775 attaches the MS257 to an inch (1/4-20) or metric (M6) optical table and allows ±0.38 inch (±9.6 mm) horizontal adjustment. It nominally raises the optical axis of the MS257 by 0.38 inches (9.6 mm), to 5.38 inches (136.7 mm) total height.

Gratings

We offer a single grating mount and a multiple grating turret which holds 1, 2, 3 or 4 gratings. The turret lets you switch between gratings at the touch of a button on the Hand Controller, or automatically, during a scan. The 77708 Multiple Grating Turret offers an unequaled degree of automation with no compromise in optical performance. This high precision turret scans about the face of the selected grating, as do traditional single grating monochromators, thus offering the best resolution and light throughput over the grating scan range. The grating selection mechanism uses a second drive perpendicular to the scan axis, so it does not interfere with the accurate wavelength positioning of the grating. Grating changes take only about ~0.5 s, and the grating is locked into place with arc second accuracy.

Table 1 Grating Specifications (The array bandpass assumes a 25 mm wide spectrographic field.)

Slit Assemblies for MS257

We offer a variety of precision fixed slits and micrometer or motor driven variable slit assemblies. All have our standard Oriel 1.5 Inch Series male flange so you can directly couple optical accessories such as fiber optics, diffusers and focusing lens assemblies.

Fixed Slits

• Most economical for single slit size
• Highest accuracy and precision

Fixed slits are a good choice when only one or two widths, or superior accuracy, are required. The narrow fixed slits are also the best choice for high resolution applications. Each of the fixed slits is mounted in a precision machined slide which is inserted into the 77721 Fixed Slit Housing.

Micrometer Driven Variable Slit Assembly

• Continuously variable from 4 µm to 3.0 mm
• ± 10 µm repeatability
• ± 10 µm accuracy from 4 µm to 250 µm; ±5% from 250 µm to 3 mm

The 74002 uses a precision micrometer to continuously vary the slit width from 4 µm to 3.0 mm; slit height is also variable from 2 mm to 15 mm.

Motor Driven Variable Slit Assembly

• 2 µm steps from 4 µm to 2.0 mm
• ±5 µm repeatability
• ±10 µm accuracy

The 77722 Motor Driven Slit Assembly uses a microstepper motor. Slit widths range from 4 µm to 2.0 mm; slit height is 15 mm. The slit width can be set using the hand controller, by computer, or automatically by MS257 in order to have a predetermined and constant bandpass. Entire scans can be taken with the slit widths automatically adjusted by MS257 at each data point.

Up to three motorized slits (one for each port) can be controlled. A special motor driven, high throughput, curved slit assembly (model 77738 ) is also available. This assembly has 20 mm high slits which are slightly curved to compensate for the inherent bending of the entrance slit image. Slit widths range from 4 µm to 2.0 mm, as with the 77722 Assembly (repeatability and accuracy are also the same as the 77722 ). The 77738 gives better throughput and better resolution, despite the larger, 20 mm height.

Multi-track Spectroscopy

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”.

Standard 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 77782 Imaging 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. We have demonstrated spectroscopic imaging using as many as nine separate channels (100 and 200 µm fibers at the input) with very good results.

Communicating with the MS257

A utility program provides the fundamentals for control of the MS257; you only need to setup your experiment and start the process. The simple utility program allows instrumental configurations to be saved to and loaded from computer files. These files can be loaded when a variety of experiments need to be conducted over time.

There are three ways to communicate with MS257:
1. Via the TTL level Input/Output BNCs on the back panel.
2. Via the 77709 Hand Controller.
3. Via an external computer using either USB, RS-232 or IEEE-488 protocols.

The 77781 or 77782 MS257 models comes configured for TTL, the hand controller, and USB/ RS-232. The optional external 77793 IEEE-488 Interface is required for GPIB communications.

The MS257 TTL Interface

MS257 has binary Input/Output communication facilities. These enable the scanning process to be closely integrated with data acquisition by providing the following synchronizing signals (assuming optional accessories are installed):

• A gate TTL signal is output during the time that the drive is scanning, from the start wavelength to the end wavelength, [SCAN]
• A gate TTL signal is output when the drive is stationary at each data point, [WAIT]
• A TTL signal is output when the optional fast shutter is >90% open, [SHUTTER OUT]
• A TTL signal is output when the motorized output mirror is set to Port B, [PORT]
• Scans can be started and moved from one data point to the next by an external TTL signal, [GO]
• The shutter can be triggered by an external TTL signal, [SHUTTER IN]

Hand Controller

For easy access to the most frequently used MS257 features, there is no substitute for the hand controller. This small device packs a lot of power into its 40 keys and 4 line, backlit, LCD display. Unlike other hand-held controllers, the 77709 was designed specifically for controlling MS257. Therefore, the command keys are clearly labeled with intuitive titles. The hand controller allows immediate access to most of the system functions.

Software Options for MS257

The software options for MS257 are:

• MS257™ Command Set (included with instrument)
• Configuration Program (included with instrument)
• LabView™ VIs (optional at no charge)
• TRACQ Basic™ or InstaSpec Pro Data Acquisition Software (optional)

MS257 Basic

The many features and accessories for the MS257 can be controlled through a series of easily recognizable commands. Every instrument leaves Oriel with the MS257 Programming Manual which contains detailed descriptions of each of the instrument's features and the associated commands. MS257 Basic commands may be sent through the USB, RS-232, or optional external IEEE-488 (GPIB) interface.

Utility Program

Included with every MS257 is a simple program that lets you check the communication between your computer and MS257. Moreover, this program can be used to easily view all the MS257 systems parameters, backup those parameters to a disk, and restore a previous system configuration.

VIs for Labview

For those who wish to integrate MS257 with other LabView controlled instruments, we offer VIs. This library enables you to create your own interface and application for the MS257, in LabView.

TRACQ Basic™ Software

TRACQ Basic™ is an integrated radiometric data acquisition and control package for the MS257 and select Oriel Detection Systems. Through TRACQ Basic you command the MS257 and detector, and acquire, process and display the data in real time.

MS257 Double Monochromator (only functions with RS-232 communication)

Table 2 Comparison of Additive and Subtractive Double MS257 and Single MS257

Additive Configuration

The resolution limit of an additive double monochromator is slightly better than a single monochromator. In this case, the output spectral band from the first monochromator is dispersed again by the second monochromator. Ignoring all aberrations, an additive double monochromator should provide twice the dispersion of each single monochromator. That is to say, a double 1/4 m monochromator should have dispersion equivalent to a 1/2 m monochromator using the same grating. In practice this is never quite possible because system aberrations, alignment, and synchronization slightly deteriorate the resolution of a double monochromator.

Subtractive Configuration

Subtractive dispersion refers to the homogenizing of the spectrum which occurs within the second monochromator. Usually, the output band of a monochromator is a narrow slice of spectrum, changing slightly in wavelength from one side of the band to the other. In a subtractive dispersion configuration the spectrum passed through the central slit by the first monochromator is mixed at the exit slit of the double system. Theoretically, double monochromator instruments in subtractive configuration have only the dispersion of the first single monochromator. Again, there is some deterioration from this theoretical limit due to system aberrations, alignment, and synchronization. The resolution limit of a subtractive double monochromator is slightly worse than a single monochromator.

Stray Light With Double MS257

The most significant advantage of using a double monochromator system (in either additive or subtractive configuration) is low stray light. Stray light is any radiation, other than the wavelengths of primary interest, which passes through the monochromator to the detection system. Although the MS257 has very low stray light compared to other 1/4 m monochromators, some applications may require even tighter specifications. The second monochromator in a double MS257 system accepts the output of the first monochromator, both the signal and any stray light, then acts as a very narrow variable filter, further rejecting whatever stray light was present. Since any radiation which enters the second monochromator may be scattered again and contribute to stray light, the center slit should be as narrow as possible. A theoretical performance limit is reached when the center slit matches the input and output slits of the system; beyond this point stray light may decrease but throughput suffers dramatically. To avoid these problems we suggest using a center slit that is at least 2X wider than the entrance and exit slits for an additive system and an exit slit that is at least 2X wider than the entrance and center slits, for a subtractive system.

Double Monochromator Synchronization and Control

The MS257 is an intelligent monochromator; the internal microprocessor controls scanning and accessories such as filter wheels and grating turrets. In a double monochromator system the two units independently control their own operation according to the commands you send. There is no need for mechanical or electrical fixturing because the two units are identically configured. Instead, you simply place the monochromators in handshaking mode and send the same command to both units. This can only be accomplished using two RS232 configured MS257 units and a splitter cable. Please note that the TRACQ Software does not control a double MS257 system. Please contact an Oriel Sales Engineer for details on the available options for controlling a Double-MS257 system.

What Do I Need to Build a Double MS257?

Only the RS-232 version of the MS-257 can be configured and operated as a double monochromator. The double monochromator kits, either additive or subtractive, hold two MS257 monochromators in tandem. A mechanical assembly provides light-tight connection of the two optical ports while a 27 x 27 inch (686 x 686 mm) baseplate holds the two units in stable alignment. To build a complete MS257 Double Monochromator System you need:

• Two MS257 Monochromators (RS-232 only)
• One 77719 Additive or 77715 Subtractive Dispersion Double Monochromator Kit
• At least one 77718 Side Exit Mirror for the 77715 , two for 77719 set-up
• Three Slit Assemblies: one input, one center, and one output
• Two Grating Turrets with identical gratings

• Interferometrically mapped drive
• Factory default referenced to internal optomechanical home
• Individual grating calibration factors

• Central control by on board CPU
• External control by hand controller
• Programmed control by external computer

• 40 key hand held keypad
• 4 line, 20 character, backlit supertwist LCD
• 5 status lights
• Remote/local switching
• 14 ft (4.26 m) long cable

Specifications

All specifications are determined with a 77700, 1200 l/mm grating and 10 µm x 2 mm slit, at 546 nm.

System

Optical Field

Imaging

Ports

Gratings

Wavelengths

Calibration

System Control

Handheld Controller

Background Shutter (Model 77755)

Fast Shutter (Model 77717)

Filter Wheel

Auxiliary Input Port (Models 77713 & 77765)

Newport Resource e-Catalogs *

• Newport Resource e-Catalog Page (English)
• Newport Resource e-Catalog, pages 1291-1300 (English)
• Newport Resource e-Catalog Page (French)
• Newport Resource e-Catalog, pages 1291-1300 (French)
• Newport Resource e-Catalog Page (German)
• Newport Resource e-Catalog, pages 1291-1300 (German)