02Echelle Spectrographs: instruments covering a Wide Wavelength Range with High Spectral Resolution
Spectroscopic observations enable us to investigate light divided into many wavelengths using a prism or a grating. The light divided into a narrow wavelength range passes through a camera and is recorded by a detector. When the light is dispersed more finely (in short, higher spectral resolution), the spectra are distributed over a wider area on the detector. This means that one who wants higher spectral resolution must give up wavelength coverage, because the sizes of detector and of focal plane are limited.
However, an echelle spectrograph is capable both of high resolution and wide wavelength coverage. The HIDES (HIgh Dispersion Echelle Spectrograph) at Okayama Astrophysical Observatory is this type of spectrograph. Image 1 shows the Sun's spectrum taken by HIDES. (To be truthful, this data was taken for the bright sky). Each horizontal streaks are spectra for different wavelength ranges. The solar spectrum is folded onto the detector array to obtain data over a wide wavelength range (note).
The spectral resolution is very high and the accuracy is equivalent to the width of the spectral lines formed in atmospheres of stars such as the Sun. In other words, the spectral resolution of HIDES has reached a level at which higher spectral resolution is meaningless. The Echelle Spectrograph provides us with an efficient tool to simultaneously obtain spectra covering most of the optical range width a high resolving power.
: Estimate of the Formation Temperature of Ammonia Ice in Comet LINEAR
Let's look at the data of Comet LINEAR taken by Subaru Telescope's HDS (High Dispersion Spectrograph), which is also an echelle spectrograph. Image 2 displays the image taken by Subaru's CISCO (Cooled Infrared Spectrograph and Camera for OH-Airglow Suppressor). Image 3 shows part of the raw data of the spectrum of Comet LINEAR observed with HDS. The light dispersed into many wavelengths continuously extends horizontally in this image. This means that the comet emits the light consisting of most of the optical wavelength range. This is because the coma of the comet reflect sunlight. You can see several bright vertical lines in the streaks. These are emission lines originating in certain atoms and molecules in the comet. These emission lines tell us the chemical composition of the comet. Researchers analyzed the data and found that C2 and NH2 molecules in Comet LINEAR. The NH2 molecules originated in ammonia (NH3) and detailed analysis enables us to estimate the formation temperature of the comet.
A grating is an optical component with a periodic structure, which splits and diffracts light into several beams traveling in different directions. Usually this pattern is a corrugated surface of grooves. A grating disperses an incident beam on it. The directions of these beams depend on the spacing of the grating and the wavelength of the light. This makes the grating act as a dispersive element. Light of a single wavelength in a standard grating at normal incidence is diffracted to the central zero order and successive higher orders at specific angles, defined by the grating density/wavelength ratio. For multiple wavelengths the same is true; however, it is possible that longer wavelengths of a higher order might overlap with the next order(s) of a shorter wavelength, which usually is an unwanted side effect. In echelle gratings, however, this behavior is deliberately used and the blaze is optimized for multiple overlapping higher orders. Since this overlap is not directly useful, a second, perpendicularly mounted dispersive element (grating or prism) is inserted as an "order separator" or a "cross disperser" into the beam path.