Campaign details

Information about the sigma Geminorum spectroscopic campaign.

How get target data ?

To ensure usable results and allow for precise comparative analyses, we recommend following the steps below:

  • Use the side illumination mode with four fibers placed in front of your telescope, or a neon bulb with a diffuser (tracing paper).
  • Minimum spectral coverage: from 6530 Å to 6685 Å, with a spectral resolution ≥ 13000..
  • Signal-to-noise ratio: SNR ≥ 150 between 6650 Å and 6654 Å.
  • If possible, use the side illumination mode for spectral calibration, with one or more fibers in front of the telescope or a neon lamp and diffuser (tracing paper).
  • Before or after observing Sigma Gem, acquire a spectrum of a nearby hot star. We recommend Theta Gem (HD 50019), of spectral type A2IV. This star is available in the Melchiors database. You must calibrate it in the same way as Sigma Gem.
  • Compute the instrumental response using a spectrum of Sigma Gem extracted from the Melchiors database.
  • Process the spectrum of the hot star (e.g., HD 50019) to measure the mean position of the telluric lines. Then apply the average measured shift to the spectrum of Sigma Gem.
  • In all cases, whether you observed the reference star or not, verify the quality of the wavelength calibration by performing a “check atm / verify telluric” in SpecINTI, and check the shift of the atmospheric line located at 6543.912 Å. If the shift is greater than 0.05 Å, apply the correction accordingly.

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=13000 (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.