David W. Coutts

The MARVEL dedicated radial velocity facility will soon be commissioned at the Mercator Observatory at La Palma, Spain. MARVEL comprises four 0.8m telescopes, each feeding starlight via optical fibres into a single high-resolution spectrograph tuned for precision radial velocity measurements. The four telescopes and their domes have been successfully installed at the observatory site, and are regularly being used for initial photometric observations. In this proceeding we provide a general status update of the facility, covering the procurement, assembly, and laboratory testing of spectrometer optics, mechanics, and detector; developments in the calibration system; the PLC-based control software; the dedicated solar telescope; and a photometry programme that is already underway, making use of the MARVEL telescopes before the installation of the spectrograph.

Consumer-level digital single-lens reflex (DSLR) cameras are typically not used in professional astronomy because of the systematic errors present in the data as a result of the strong intra- and interpixel variations associated with each of the three different colors (RGB) of the Bayer color filter array. Nevertheless, because the cost of DSLRs compared with traditional astronomical CCDs is so much lower, they represent a potentially underexplored area of scientific quality astronomical imaging, especially in the area of wide-field transient surveys. We demonstrate an algorithm that can achieve ≈ 1 % level photometry in each of the RGB color channels from a stellar source and discuss the application of this algorithm to a ground-based transiting exoplanet survey. The algorithm primarily takes advantage of the large number of stellar sources available for statistical averaging within a single image, using a “lucky point-spread function” approach to identify sources in the image that exhibit systematic errors consistent with a chosen target from the same image. The selection of the appropriate“lucky” reference stars is accomplished through a comparison of the stellar image morphology as it appears on the Bayer array and the reference stars. These references are linearly combined to form a synthetic comparison star that can be used for differential photometry with the target. One key to the algorithm is that all data are retained at the individual pixel-level until the final differential comparison, which helps to alleviate systematic effects that might otherwise cancel each other out during the flux-summing process. We demonstrate the algorithm on HD 339461, a m_V = 8.93 G0-type star on which we achieve single-percent level photometry that approaches the fundamental noise floor possible from a single camera.

Fiber modal noise is a performance limiting factor in high-precision radial velocity measurements with multi-mode fiber fed high-resolution spectrographs. Traditionally, modal noise is mitigated by agitating the fiber, this way redistributing the light that propagates in the fiber over many different modes. However, in case of fibers with only a limited number of modes, e.g. at near-infrared wavelengths or in adaptive-optics assisted systems, this method becomes very inefficient. The strong agitation that would be needed stresses the fiber and can lead to focal ratio degradation. As an alternative approach, we propose to use a classic optical double scrambler and to rotate the scrambler’s first fiber end during each exposure. Because of the rotating illumination pattern of the scrambler’s second fiber, the modes that are excited vary continuously. This leads to very efficient averaging of the modal pattern at the fiber exit and to a strong reduction of modal noise. In this contribution, we present a prototype design and first laboratory results of the rotating double scrambler.

Precise wavelength calibration is a critical issue for high-resolution spectroscopic observations. The ideal calibration source should be able to provide a very stable and dense grid of evenly distributed spectral lines of constant intensity. A new method which satisfies all mentioned conditions has been developed by our group. The approach is to actively measure the exact position of a single spectral line of a Fabry-Perot etalon with very high precision with a wavelength-tuneable laser and compare it to an extremely stable wavelength standard. The ideal choice of standard is the D2 absorption line of Rubidium, which has been used as an optical frequency standard for decades. With this technique, the problem of stable wavelength calibration of spectrographs becomes a problem of how reliably we can measure and anchor one etalon line to the Rb transition. In this work we present our self-built module for Rb saturated absorption spectroscopy and discuss its stability.

Microbends in multimode optical fibers are shown to lead to focal ratio degradation which compromises fiber fed spectrograph design and performance. By propagating specific vortex mode patterns through multimode fibers containing a single controlled microbend the mechanisms for FRD can be understood. For example, we see both experimentally and through ray tracing analysis that a microbend can produce spiral patterns in the far-field. These patterns are potentially useful tools for diagnosing termination problems in fiber assemblies. For example, by analyzing the spiral patterns produced by a single microbend it is possible to determine the location of a microbend hidden in the fiber termination.

Ever more precise radial velocity instruments are needed to observe potential earth-like exoplanet targets that are beyond the range of current generation high resolution echelle spectrographs. Meanwhile, extreme adaptive optics systems at 8 meter class facilities have made ground based observations possible at the diffraction limit. In the field of Doppler spectroscopy, one way to take advantage of these AO capabilities is by the development of ultra-stable single mode fiber fed spectrographs.1 Coupling the light efficiently into SMFs with an extreme adaptive optics system offers significant advantage in removing modal noise, reducing instrument size, enabling superior environmental control and curbing cost. We report the design and challenges in building an ultra-stable spectrograph for the near infrared range. The design wavelength range is 650 to 1500 nm.

RHEA is a compact high-resolution single-mode spectrograph that can easily be produced in larger quantities as budgets allow. The instrument will be housed in a temperature-stabilized vacuum chamber which is surrounded by several layers of thermal shielding. The optical design employs cost-effective commercially available compo- nents, a cooled CMOS detector, and a double-fiber input which permits simultaneous wavelength calibration.

Precise wavelength calibration is a persistent problem for highest precision Doppler spectroscopy. The ideal calibrator provides an extremely stable spectrum of equidistant, narrow lines over a wide bandwidth, is reliable over timescales of years, and is simple to operate. Unlike traditional hollow cathode lamps, etalons provide an engineered spectrum with adjustable line distance and width and can cover a very broad spectral bandwidth. We have shown that laser locked etalons provide the necessary stability with an ideal spectral format for calibrating precision Echelle spectrographs, in a cost-effective and robust package. Anchoring the etalon spectrum to a very precisely known hyperfine transition of rubidium delivers cm/s-level stability over timescales of years. We have engineered a fieldable system which is currently being constructed as calibrator for the MAROON-X, HERMES, KPF, FIES and iLocater spectrographs.

High precision Doppler observations of bright stars can be made efficiently with small aperture telescopes. We are constructing a high resolution echelle spectrograph for the new 0.6 m telescope at Central Washington University. The spectrograph is fed by a multimode fiber and operates in the visible wavelength range of 380-670 nm. The spectrograph uses a white pupil design with 100 mm beam diameter and a monolithic R4 echelle grating.