Campaign details

The spectra used to evaluate the orbit of a double spectroscopic star must be very carefully calibrated in wavelength. Indeed, the primary information sought is the precise radial velocity of the star at the time of observation. This is done using the wavelength position of one or more spectral lines.

We choose for this study the Halpha line of hydrogen located at the wavelength 6563 A. This line must therefore be in the region of the spectrum captured. Unless we observe with an echelle spectrograph this region will be a small portion of the spectrum because it is necessary for this study to work with a high spectral resolution.

The presence of the telluric H2O lines, at a fixed wavelength and in the immediate vicinity of the Halpha line, can help to refine and control the spectral calibration.

Preparation et telescope pointing

The first step to acquire a quality spectrum is to prepare your equipment well and to make a precise pointing on the celestial object in question. Make sure your telescope is properly aligned and in optimal focus. Use a guiding camera to maintain accurate pointing throughout the exposure.

Choice of the spectroscope configuration

This campaign is aimed at observers with a spectrograph with a resolution equal or higher than R=10000 (Lhires III 2400 t/mm, Star'Ex HR, eShel...).

Exposure time and image accumulation

The accuracy of measurement of the radial velocity increases with the signal to noise ratio. We aim here an accuracy of 1 km/s, or better. A signal to noise ratio in the continuum near the Halpha line should be if possible at least 150, however spectra affected by a higher noise level are also accepted, one of the results of the study being indeed to correlate the value of the signal to noise ratio and the quality of the radial velocity measurement obtained.

Image processing and spectrum extraction

The spectra sent must be calibrated, but you do not have to correct the radial velocity of the object (the proper radial velocity of the object and the topocentric velocity of the observer, i.e. the component related to the orbital velocity of the Earth around the Sun essentially).

You can use a specialized software to process the data, correct the artifacts and extract the spectrum (SpecINTI, ISIS, Demetra,...). Image processing should include background subtraction, correction of hot and dead pixels. Spectrum extraction must be done with care to minimize noise and maximize noise and maximize the quality of the final spectrum.