UVCS Tutorial: UV Spectrometer Slits

Coronal light reflected by the two telescope mirrors (each with UV coatings) is focused onto the spectrometer slits. We have now "officially" entered the spectrometer portion of UVCS. For detailed tutorials on how spectrometers work, see here or here.

One UV channel is optimized for observations of the neutral hydrogen Lyman alpha line (and is thus typically called the LYA channel) at 1216 Angstroms, and the other channel is optimized for observations of the pair of O VI lines at 1032 and 1037 Angstroms (and is thus typically called the OVI channel). The spectroscopic designation "O VI" denotes the sixth ionization stage of oxygen, starting with neutral oxygen as I, singly ionized oxygen as II, etc. Thus, O VI is an oxygen ion with 5 of its 8 electrons stripped off.

NOTE: The LYA channel has been turned off since late 1998. The analysis of UV observations taken more recently is thus done with the OVI channel alone.

Each spectrometer slit defines the "patch" of the sky which is imaged on the detectors. The long dimension of the slit (up to 2.5 solar radii in length) is perpendicular to the radial direction from Sun-center. The short dimension of the slit ("slit width") is in the radial direction, which is the same direction for scanning the mirror. UVCS can rotate around the Sun to position the slit at a chosen position angle (measured counterclockwise from the north solar pole).

The widths of the spectrometer slits are variable. When designing observations, a trade-off has to be performed between making the slits too wide or too narrow:

If the slits were made too narrow, not enough light would pass through, and we would not get enough of a count rate to be able to measure the detailed properties of the spectral lines.
If the slits were made too wide, the spectral lines would be artificially broadened because of the out-of-focus rays that are allowed to pass through. It would then be difficult, or impossible depending on the degree of broadening, to determine the actual coronal contribution to the line width.

Go back to the Top-level tutorial page.