6.1 HARPS @ ESO-3.6mThe very successful HARPS instrument can be considered as a precursor for ESPRESSO as far as radial velocity precision and stability are concerned. The further improvements foreseen for ESPRESSO will be based on the lessons learned in building and using HARPS. However, ESPRESSO will clearly be a new-generation instrument compared to HARPS, with major improvements regarding both its stability and photon collecting power.
6.2 High-resolution spectrographs on 10m-class telescopesThe existing instruments UVES@VLT and HIRES@Keck have been among the most productive astronomical facilities in the past decade. Their high resolving power coupled to the large collecting area of 10m-class telescopes have led to many breakthroughs in several fields of astrophysics. However, their lack of intrinsic stability represents a limitation for several major science cases, such as those described in this document. ESPRESSO will couple the high efficiency of these instruments with the highest intrinsic stability ever achieved.
6.3 ESPRESSO as a precursor for CODEXCODEX is also a concept for ultra-stable, high-resolution spectrographs but for the E-ELT. The prime science driver of CODEX is the exploration of the universal expansion history by detecting and measuring the cosmological redshift drift using QSO absorption lines. In order to achieve its science goals CODEX will have to deliver an exceptional radial velocity accuracy and stability, i.e. 2 cm s-1 over a timescale of about 20 yr. CODEX will represent a major development in high-resolution optical spectrographs compared to existing instruments such as UVES and HARPS. Thus, ESPRESSO will be a stepping stone towards CODEX both in a scientific and in a technical sense. Although the basic design concepts are already in place, several of the sub-systems needed to achieve the CODEX requirements do not currently exist and they will be implemented in ESPRESSO for the first time. Hence, in many respects ESPRESSO will be a CODEX precursor instrument that will allow us to test and gain experience with the novel aspects of these instruments.
Recently, Liske et al. (2008) found that a 42-m telescope would indeed have the photon collecting power to detect the redshift drift by monitoring the redshifts of QSO absorption lines over a timescale of about 20 years, providing a strong motivation for a CODEX-like instrument at the E-ELT. CODEX will implement a light scrambler and laser comb wavelength calibration. These two systems would be developed for ESPRESSO. Being able to test them would provide valuable experience and input for further improvements.
The scientific goals of CODEX are very challenging and require validation and demonstration of feasibility of many aspects of data handling and analysis. This includes data acquisition strategies, the tracking of CCD distortions, and the accuracy of flat-fielding, sky subtraction and scattered light corrections. The extraction of the cosmological signal from the data will also require testing. Issues include how to deal with QSO variability and the accuracy of the conversion to the cosmological reference frame. In addition ESPRESSO will allow us to determine the currently unknown intrinsic widths of the narrowest metal absorption lines in order to reassess their usefulness for the drift experiment, and to look for any sources of astrophysical noise so far overlooked. Data on QSOs collected with ESPRESSO for other scientific purposes would allow us to address all of the above issues. Observations of the brightest known QSOs would provide an end-to-end system verification, from data acquisition to signal extraction, at the level of about 30-40 cm s-1.
Liske et al (2007) examined also whether ESPRESSO, which has characteristics similar to those of CODEX, can be used to make a start on the redshift drift experiment. The idea is that, since ESPRESSO would be available several years before CODEX, data appropriately collected with ESPRESSO could serve as a “zeroth” epoch measurement, effectively extending the time baseline of the experiment for a few years, thereby improving the final result without delaying it. However, the improvement of the constraints is only quite modest, with the lower limit on ΩΛ increasing by about 20 per cent.