Imaging Spectroscopy and Detection Systems
The emergence of CCD detectors has brought the ability to measure spatial as well as spectral features to many spectroscopic techniques. Spectrometers with corrective optics are more frequently requested. The typical spectrometer has employed spherical mirrors for collimation and focusing. On scanning systems the astigmatism present in spherical mirror systems does not effect resolution. The dispersion plane was in excellent focus and therefore resolution was obtained. All the energy reached the (large) photomultiplier photocathode or other solid state detector.
Spherical mirrors are ideal for use in commercial scanning instrumentation because their shape permits the use of master polished optics. Featuring at least 1/4 wave surfaces (measured at 632.8), 40/20 scratch dig specifications and 2.5-nm RMS these mirrors provide high throughput, low scatter in the deep UV and excellent image formation in the dispersion plane.
CCD detectors are capable of measuring both the dispersion plane and the spatial plane, or slit height, simultaneously. Development and use of novel optics in McPherson instrumentation reduces the astigmatism in spectrometers to better suit these detectors. Follow the links for details:
- Mirror Types
- McPherson Approach to Imaging Spectrometers
- Selecting Optics for Imaging Spectrometers
- Some UV-VIS-IR Imaging Systems
- Some Vacuum Ultraviolet Imaging Systems
Mirror Types used in Spectrometers - Spherical Aberrations vs. Astigmatism
First surface optical systems contain four major aberrations: spherical aberration, astigmatism, coma, and line curvature. The first two are the most significant in spectrometers and correcting for either one means aggravating another. Balancing aberrations to improve astigmatism generally incurs a deterioration of resolution.
Parabolic mirrors are ideal for imaging when used on axis, when moving off axis however (dispersion or spatial plane) it is not uncommon to see a factor 10 or more, reduction in their imaging performance. Toroidal mirrors will typically provide the least astigmatism across the exit plane but will not provide optimum resolution or spatial information - spherical aberration and coma remain unchanged. Parabolic and toroidal mirrors can both be obtained commercially and are prohibitively expensive.
McPherson Approach to Imaging Spectrometers
Our instruments use only high quality, master spherical optics for collimating and focusing as they produce the best system resolution and line shape. For customers interested in imaging we introduce a master cylinder correction mirror at one of the side port mirror positions. If the corrective cylinder is placed at the entrance
side port, it serves both exit ports with imaging spectra. The exit side port selector serves one detector position with imaging spectra and the other with optimum resolution. The McPherson approach frees the user to utilize the instrument(s) to it's full potential rather than select one fixed application.
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Above, ray traced spot diagrams graphically depict the difference between systems optimized for resolution, imaging on the dispersion plane, and those compromising resolution and spatial imaging using corrective optic(s) for best 2-D readout. Below, the point spread function is shown.
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The ray trace analysis at 600 nm using a 2400 G/mm grating in the McPherson Model 207, 0.67 meter focal length, f/4.7 system with 100 um input image.
Selecting Optics for Imaging Spectrometers
The ray trace spot diagrams are intended to provide a general overview of performance and a means of selecting between optics schemes. 9 spot diagrams are shown for each mirror type. There is one on-axis and an array at ± 10 mm spatially, ± 13 mm on the dispersion axis. Individual spots are shown at 20X magnification and are based on an input spot size of 100 um.
| Spherical mirrors with corrective cylinder
The images are somewhat irregular because the optics are designed to balance best resolution with minimum astigmatism. This method keeps the instrument flexible and useful for imaging or scanning applications and provides best optical performance for cost.
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Parabolic mirrors
Superior on-axis images over a limited field of view. Even these 1 meter focal length parabolas degrade significantly when moving off axis in dispersion of spatial direction. Possibly the ideal monochromator mirrors for small slit height not a good choice for imaging.
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| Toroidal mirrors
Fair image formation, excellent correction for astigmatism but equivalent reduction in resolution. Prohibitively expensive to obtain at qualities equivalent to spheres. Lower cost mirrors, e.g. diamond turned or replicated may not be available in qualities suitable for UV-VIS spectroscopy.
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Some UV-VIS-IR Imaging Systems
Several McPherson Models are suitable for use in imaging applications. All feature 2 input, and 2 output ports for experimental versatility. All use master spherical optics combined with master cylindrical mirror(s), with excellent surface finishes, for imaging. Some feature multiple grating turrets and all operate with any and all existing McPherson accessories.
| Model | Focal Length | f/number | Resolution* |
| 2035 | 350-mm | 4.8 | 0.05-nm |
| 207 | 670-mm | 4.7 | 0.03-nm |
| 2061 | 1000-mm | 7 | 0.017-nm |
*with 1200 G/mm grating and 10 um x 4 mm entrance slit.
Some Vacuum Ultraviolet Imaging Systems
Many McPherson vacuum UV instruments are suitable for use with special array detectors, open microchannel plate intensifiers or scintillated arrays. Most however, are not imaging instruments as defined in the preceding pages. Instrument optical geometry in the vacuum UV prohibits multiple reflective surfaces and therefore the addition of corrective optics. One exception useful in the wavelength region > 105 nm in the 500 mm focal length Model 219.
| Model | Focal Length | f/number | Resolution* |
| 219 | 500-mm | 8.7 | 0.04-nm |
*with 1200 G/mm grating and 10 um x 4 mm entrance slit.
Many other vacuum UV spectrometers can be used with array detectors for multichannel detection.
| Vacuum UV Multichannel Detection The simplest form of using a multichannel detector in the vacuum UV is to use a fiber windowed scintillator with lens coupling to an array device. This simple solution can be used on any instrument. Due to the multiple interfaces resolution is not as good as with proximity coupled devices or devices which have no scintillator. |  |
This universal method permits users of existing instruments, e.g. Model 218, to add array detection capability to what was previously a scanning monochromator. It also offers the new buyer the opportunity to explore multichannel detection in the vacuum UV without spending a fortune. This device mates to the output focal plane of most vacuum UV instruments. In focus, on the focal plane, a scintillator layer converts the impinging VUV energy into a visible emission at ~440 nm. (The scintillator has ~ 65% QE from 300 - 3,400 Å.) This visible emission is relayed by imaging optics, to the multichannel detector of choice.
| Model | Focal Length | f/number | Resolution* |
| 234/302 | 200-mm | 4.5 | 0.1-nm |
| 218 | 300-mm | 5.3 | 0.05-nm |
| 235 | 500-mm | 11.4 | 0.05-nm |
| 231 | 1000-mm | 22.9 | 0.025-nm |
| 2061V | 1000-mm | 8.6 | 0.017-nm |
| 225 | 1000-mm | 10.4 | 0.015-nm |
*with 1200 G/mm grating and 10 um x 4 mm entrance slit.