# Information and notes for use with PrismSpect modeling and data display

## Constants and conversion factors

X-ray astronomers and high-energy-density plasma physicists often refer to temperatures by their equivalent energies, expressed in electron volts (eV) or kilo-electron volts (keV). The conversion factor is 11606, in the sense that 1 eV corresponds to 11606 K and 1 keV corresponds to 11.6 million degrees Kelvin. Energy and temperature are related by Boltzmann's constant: E = kT.

1 Angstrom (A) corresponds to 12398 eV (or 12.398 keV), and the relationship is inverse, according to E_{photon} = hν = hc/λ. So, E(eV) = 12398/λ(A) or λ(A) = 12398/E(eV) = 12.398/E(keV).

Note that you can combine the above to facts to relate wavelengths to temperatures. You will find a relationship similar to, but not exactly the same as, the Wien displacement law (λ_{max}T = 0.29), for the peak of a blackbody spectrum.

## A note about resolution

Spectral resolution is often expressed as a unitless ratio of the wavelenth observed to the width (in wavelength units) of the narrowest feature resolvable at that wavelength (technically, of the instrument response function). The width is usually, but not always, characterized as a full width at half maximum (FWHM). In the Wargelin et al. paper, they quote a resolution for the Tokamak data of R = λ/Δλ = 1270 at 8 Angstroms. Note that for gratings, the resolution is constant in Δλ and so R, as defined above, will change with the wavelength observed. The PrismSpect spectrum viewer tool allows you to set a value of R, but not of Δλ.