Astronomical Spectroscopy - Physics - University of Cincinnati

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2.1.2. Choosing a grating
What drives the choice of one grating over another There usually needs to be some
minimal spectral resolution, and some minimal wavelength coverage. For a given detector
these two may be in conflict; i.e., if there are only 2000 pixels and a minimum (3-pixel)
resolution of 2Å is needed, then no more than about 1300Å can be covered in a single
exposure. The larger the number of lines per mm, the higher the dispersion (and hence
resolution) for a given order. Usually the observer also has in mind a specific wavelength
region, e.g., 4000Å to 5000Å. There may still be various choices to be made. For instance, a
1200 line/mm grating blazed at 4000Å and a 600 line/mm grating blazed at 8000Å may be
(almost) equally good for such a project, as the 600 line/mm could be used in second order
and will then have the same dispersion and effective blaze as the 1200 line grating. The
primary difference is that the efficiency will fall off much faster for the 600 line/mm grating
used in second order. As stated above (Equations 5 and 6), gratings fall off to 50% of their
peak efficiency at roughly λ b /m −λ b /3 2 and λ b /m+λ b /2m 2 where λ b is the first-order blaze
wavelength and m is the order. So, the 4000Å blazed 1200 line/mm grating used in first
order will fall to 50% by roughly 6000Å. However, the 8000Å blazed 600 line/mm grating
used in second order will fall to 50% by 5000Å. Thus most likely the first order grating
would be a better choice, although one should check the efficiency curves for the specific
gratings (if such are available) to make sure one is making the right choice. Furthermore, it
would be easy to block unwanted light if one were operating in second order in this example,
but generally it is a lot easier to perform blocking when one is operating in first order, as
described above.
2.2. Conventional Long-Slit Spectrographs
Most of what has been discussed so far corresponds to a conventional long-slit spectrograph,
the simplest type of astronomical spectrograph, and in some ways the most versatile.
The spectrograph can be used to take spectra of a bright star or a faint quasar, and the
long-slit offers the capability of excellent sky subtraction. Alternatively the long-slit can
be used to obtain spatially resolved spectra of extended sources, such as galaxies (enabling
kinematic, abundance, and population studies) or HII regions. They are usually easy to use,
with straightforward acquisition using a TV imaging the slit, although in some cases (e.g.,
IMACS on Magellan, discussed below in § 2.4.1) the situation is more complicated.
Table 1 provides characteristics for a number of commonly used long-slit spectrographs.
Note that the resolutions are given for a 1-arcsec wide slit.