Spectrographs comes in a large variety. Let us start with an example of the format of the spectrum observed with a modern high resolution instrument FIES at the Nordic Telescope. The spectrograph is a cross-dispersed spectrograph, which means that the spectrum is divided into small pieces stacked one over the other:
If we take out a small section and turn it 90 degrees, it becomes easier to see a large number of spectral lines, which comes from the absorption of a large set of different atoms in the atmosphere of the star
All the small dots comes from a Thorium-Argon spectrum interlaced with the stellar spectrum. This is used for a very precise wavelength calibration.
Applying a procedure for extracting the spectrum one gets a table F(lambda) for each slice (order) of the spectrum, where much more details are visible
The technique behind these results are discussed next.
Most larger telescopes are equipped with one or more spectrographs. The basic principle in a spectrograph is illustrated in the following figure:
The dispersive element D can be one of several elements:
The principle behind a grating is shown in the next figure, which leads to the grating equation derived in the lecture notes and shown here.
The width of the slits in the grating gives rise to a broad function like an envelope. The peak corresponding to a given wavelength is much narrower. The dispersion increases with the order number as visible in the next figure. It is also apparent, that the intensity drops severely for a transmission grating with increasing order due to the falloff og the envelope function.
To repair for this inefficient behaviour at large order numbers m, one can produce an Echelle grating, which have maximum refelction at a given wavelength for a selected angle, which is chosen as high as feasible without making the grating too long. The concept is shown next
The equations are slightly modified due to the tilted grating. The grating is used in very high orders like 50-100. An Echelle grating needs a filter or additional element to choose among overlying orders (often several tens of orders). In modern spectrographs a Cross-Disperser is introduced as illustrated here
The total design and the resolution and efficiency obtained then depends on the choice of many parameters. The equations are again found in the lecture notes, but the parameters are illustrated on this figure.
Recently, a high resolution spectrograph was brought into operation at the Nordic 2.5m telescope at the La Palma island. There is a homepage for FIES.
The spectrograph is a cross dispersed Echelle spectrograph connected with a fiber to the telescope. Acquisition of targets is done using the standby CCD camera on the telescope. Light is reflected off the fiber head and imaged on the CCD, where you can then control the position of the target star on the fiber entrance.
A high resolution (up to R = 100.000) spectrograph has been built for the VLT. The UVES instrument is a very powerful tool, which has been extremely popular and very much used for many projects, stellar as well as extragalactic. This is presently the most powerful instrument of its type with newly added options for taking spectra of many stars simultaneously.
A modest resolution of a few thousands can be achieved with less sophistication. The ALFOSC is a combined camera and spectrograph using grisms as the dispersive elements. A large set of filters and grisms are available in several wheels. The instrument has a very high efficiency and is well suited for extragalactic work. Similar instruments are attached to the VLT and other large telescopes.