Spectrographs

Spectrographs:

1. Take light from a source (into the telescope)
2. Pass it through a slit
3. Collimate it (force all beams to pass in a single direction)
4. Push it through a dispersing element (like a prism to separate it)
5. Pass it through a camera (to reform it)
6. Send to the observer

Diffraction gratings are often used as the dispersing element (in place of a prism) because prisms can get really expensive.

Sometimes, folks will sandwich prisms, gratings and another prism, to form a GRISM - this is the case at APO's KOSMOS.

Taking spectra

We don't need great seeing for spectra - so they can be useful to take on nights with low seeing. However, spectra require long exposure times for reasonable S/N ratio in every pixel.

Guiding can be done by a "slit view" camera (i.e. spectrograph in center, normal otherwise) or via an off-axis guider (like at MRO).

Resolving power

For spectra, resolution is defined as

$$R\equiv \frac{\lambda }{\Delta \lambda }$$ which can be $\ge 40,000$ for high-resolution spectra. $R$ is generally limited by slit-width.

Resolution can also be related to Doppler shift via

$$R\equiv \frac{\lambda }{\Delta \lambda }\approx \frac{c}{v}$$

Wavelength calibration

1. Find a lamp spectrum (i.e. using a dome light)
2. Extract an ID spectrum using the same aperture and trace
3. Using known lines and their pixel values, determine a dispersion solution
4. Apply it to the 1-D spectrum