Vik dhillon: phy217 - instruments - spectrographs - blazes and grisms
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`Page 1 of 3
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`blazes and grisms
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`BBKIEIDDJI
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`blazes
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`The fact that diffraction gratings produce multiple spectra, or orders, each
`with a different dispersion, makes them very versatile but also very
`inefficient. Most of the light is directed to the zeroth order, which is
`undispersed and hence useless for spectroscopy. Only ~10% of the light is
`directed to the first order, which is the most commonly used order in
`astronomical spectrographs. Some means of directing all of the light into
`the order of interest would therefore be highly desirable, as illustrated in
`figure 96.
`
`figure 96: Top: the diffraction pattern produced by a white light source
`incident on an unblazed (top) and blazed (bottom) diffraction
`grating, where only the orders n = 0, +1, +2, +3 are shown.
`Note how the spectra get fainter as the order increases in the
`unblazed case, and how most of the light is directed into the
`order of interest (/7 = +1) in the blazed case.
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`
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`The solution is to employ the fact that gratings are most efficient when the
`rays emerge from the grating as if by direct reflection. By creating tilted
`facets, or a blaze, in the grating, as shown in figure 97, it is possible to
`direct the majority of the light (~70%) into the order of interest.
`
`figure 97: Left: a microscopic view of the tilted facets in a typical blazed
`diffraction grating. Right: an illustration of the saw-tooth profile
`of a blazed reflection grating. The tilt of the facet ensures that
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`http ://www.Vikdhillon. staff. shef.ac.uk/teaching/phy2 1 7/instruments/phy2 1 7_inst_b1aze.htm1
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`2/10/2015
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`F N C 1 035
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`Vik dhillon: phy217 - instruments - spectrographs - blazes and grisms
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`Page 2 of 3
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`most of the light is diffracted to the first order rather than the
`zeroth order.
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`facet normal
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`grating normal
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`first order
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`incident radiation
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`1
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`zeroth order
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`Having covered gratings, dispersion, resolution and blaze, you should now
`be able to understand a typical table from a spectrograph user manual,
`such as that shown in table 3. The first column gives the grating name,
`where "R" refers to a diamond ruled grating, "H" to a holographic-etched
`grating, "1200" (for example) to the number of grooves per mm and "B" to
`the fact that the grating is optimised for blue light. The second column gives
`the blaze wavelength of the grating in A, Le. the wavelength of peak
`grating efficiency. The third and fourth columns give the dispersion of the
`grating in units of A/mm and A/pixel, where each detector pixel is 13.5 pm
`in size. The fifth column gives the wavelength range of the resulting
`spectrum, obtained by multiplying the dispersion in A/pixel by the number
`of CCD pixels in the dispersion direction (4096). Only 3500 of these 4096
`pixels are usable, so the sixth column lists the usable wavelength range.
`The final two columns list the slit widths in arcseconds that must be used to
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`obtain a projected slit width at the detector of 54 pm (i.e. 4 pixels) and 27
`um (i.e. 2 pixels). The latter case corresponds to sampling at the Nyquist
`critical frequency, and the spectral resolution is then given by twice the
`dispersion. Using a slit which projects to less than 2 pixels will not improve
`the spectral resolution, as the slit will then be undersampled by the CCD
`pixels. The 54 um projected slit width is well matched to the typical seeing
`on La Palma, resulting in good throughput but oversampling by the CCD
`pixels; the spectral resolution in this case will then be defined by the slit,
`not by the pixels.
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`table 3: Details of the gratings available in the blue arm of the ISIS
`spectrograph on the 4.2 m William Herschel Telescope on La
`Palma.
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`http ://www.Vikdhillon. staff. shef.ac.uk/teaching/phy2 1 7/instruments/phy2 1 7_inst_b1aze.htm1
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`2/10/2015
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`Vik dhillon: phy217 - instruments - spectrographs - blazes and grisms
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`Page 3 of 3
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`ISIS wavelength coverage and resolution with EEV12
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`Blaze Dlsper5|on DlsperSIon Spectral
`
`Unvignetted
`range
`d
`(3500
`etector etector
`pixels)
`(In
`(In
`arcsecs) arcsecs)
`|R1588 “3600 ”120
`”1.62
`”6635
`“5670
`”0.8
`”0.4
`
`d
`
`|
`
`
`
`R6008 l3900 33
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`l0.45
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`|1825
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`|1560
`
`0.9
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`l0.45
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`grisms
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`removed from course
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`©Vik Dhi/Ion, 18th September 2012
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`http ://www.Vikdhillon. staff. shef.ac.uk/teaching/phy2 1 7/instruments/phy2 1 7_inst_b1aze.htm1
`
`2/10/2015
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