Second-generation 4-position reflective slit for Star’Ex spectrographs
This "second generation" slit consists of quartz glass (synthetic quartz) measuring 16 millimetres square and 2 millimetres thick. One side is coated with an anti-reflective coating (to reduce ghost images), while the other side is covered with an opaque chrome layer, except for the narrow openings corresponding to the four slits, which have been engraved using a high-precision photolithographic technique.
Four independent slits are arranged in a square, with widths of 14, 20, 26 and 32 µm. In the centre is a hole with a diameter of 25 µm. The slit widths are indicated in the corners of the substrate. The 14-micrometre slit is ideal for spectrography with a small telescope, for example. In contrast, the 32-micrometre slit is better suited for low-resolution spectrography with a telescope with a diameter of 300 mm. The end of each slit is widened by a factor of 10 over a length of 0.5 mm. This extension has two functions: (1) to provide more information about the operating conditions of the instrument (degree of atmospheric turbulence, performance of the instrument, quality of the settings, evaluation of the instrument response); (2) to enable the acquisition of spectrophotometric data to facilitate the calibration of spectra in absolute flux values. Each slit is 4 mm long, allowing them to capture the spectrum of objects with large angular surfaces, such as nebulae or comets.
The 25 µm central hole is inconspicuous but represents a significant innovation. This hole allows the spectrograph to be set up safely and easily on a table without the need for a telescope or a starry sky: the hole acts as an artificial star.
