The measurement of precise stellar radial velocities (RV) often requires a well-designed spectrograph that is highly stabilized. It is usually placed in a vacuum chamber and is stabilized thermally and mechanically, all at great costs. It is very difficult to get precise RV measurements with existing spectrographs if they were not originally designed for this radial velocity stability.
The iodine gas absorption cell method offers the astronomical community an inexpensive wavelength calibration technique that can be used on almost any spectrograph. A spectrograph with modest stability can achieve an RV precision of 3 m/s with a cell that only costs a few hundred Euro to construct. However, the effort and costs avoided at stabilizing the spectrograph is transferred to the effort required for the data reduction. This is mathematically complex, often requiring modeling of the so-called instrumental (IP) of the spectrograph for the best precision. Most research teams using an iodine cell essentially have to "re-invent the wheel" at considerable investment of human resources. This is often an obstacle for using these cells. More problematical, different codes can produce slightly different results. One would benefit from a universal computer program that the entire community can use.
High precision measurements with an iodine cell
For these reasons, we developed viper, a software for the determination of high precision radial velocities using an absorption cell. This new, flexible and general purpose software allows one to evaluate spectra taken with different instruments from various telescopes. This software is open source and available to the entire astronomical community. Scientists using absorption cells at other facilities can now invest more time on research rather than software development.
Some important features of viper make it highly versatile:
- It is written Python, a programming language in common use by the astronomical community, particularly young scientists.
- It is open source so that users can easily modify it for their own purposes.
- It can calculate radial velocities from different types of absorption cells in use at optical and infrared wavelengths.
- It can easily be applied to different instruments.
- It is easy to use with graphical interfaces, manuals, tutorials and a dedicated website to help first time users.
The development of viper began with Sireesha Chamarthi, a former intern at TLS and Mathias Zechmeister, a former Masters student at TLS currently at the Institute for Astrophysics at the University of Göttingen. Current development is done by Jana Köhler at TLS and Zechmeister. TLS received funding from the DFG to develop viper as an open source, general purpose program for the measurement of precise stellar radial velocities. A major goal of the project is to make viper the "industry standard" for the calculation of precise stellar radial velocities using a gas absorption cell.
Easily usable with data from many instruments
One of the design goals of viper was to make it easily usable with data from many instruments. So far viper has been adapted for the following instruments:
- CES: Coudé Echelle Spectrometer at La Silla, Chile (de-commissioned)
- CRIRES+: CRyogenic high-resolution InfraRed Echelle Spectrograph an ESOs 8.2-m-Very Large Teleskop (VLT) in Paranal, Chile
- GIANO-B: Near-Infrared High-Resolution Spectrograph am 3.6-m-Galileo National Teleskop (TNG) auf La Palma, Canary Islands
- HIRES: High Resolution Echelle Spectrometer am 10-m-KECK-Teleskop in Mauna Kea, Hawai'i
- OES: Ondrejov Echelle Spectrograph am 2-m-Teleskop in Ondrejov, Tschechische Republik
- TCES: Tautenburg Coudé Echelle Spectrometer am 2-Meter-Teleskop in Tautenburg
- Tull: Tull Coudé Spectrograph am 2.7-Meter-Teleskop am McDonald Observatory, USA
- UVES: Ultraviolet and Visual Echelle Spectrograph am VLT (Very Large Telescope) der ESO
This list of instruments is a testament to the versatily of viper.
viper is the first program developed that can calculate stellar radial velocities using gas absorption cells in the infrared. The CRIRES+ consortium developed a gas absorption cell for precise radial velocity measurements with the instrument. The European Southern Observatory contracted TLS and Jana Köhler to produce the RV pipeline of CRIRES+ based on viper.