LARS - An Absolute Reference Spectrograph for solar observations, Upgrade from a prototype to a turn-key system
J. Loehner-Boettcher, W. Schmidt, H.-P. Doerr, T. Kentischer, T. Steinmetz, R. A. Probst, R. Holzwarth
AAstronomy & Astrophysics manuscript no. LARS c (cid:13)
ESO 2018June 21, 2018
LARS – An Absolute Reference Spectrograph for solarobservations
Upgrade from a prototype to a turn-key system
J. Löhner-Böttcher , W. Schmidt , H.-P. Doerr , , T. Kentischer , T. Steinmetz , , R. A. Probst , , and R. Holzwarth , Kiepenheuer-Institut für Sonnenphysik, Schöneckstr. 6, 79104 Freiburg, Germanye-mail: [email protected] Max-Planck-Institut für Sonnensystemforschung, Justus-von-Liebig-Weg 3, 37077 Göttingen, Germany Max-Planck-Institut für Quantenoptik, Hans-Kopfermann-Strasse 1, 85748 Garching, Germany Menlo Systems GmbH, Am Klopferspitz 19, 82152 Martinsried, GermanyReceived 12 May 2017 / Accepted 04 July 2017
ABSTRACT
Context.
LARS is an Absolute Reference Spectrograph designed for ultra-precise solar observations. The high-resolution echellespectrograph of the Vacuum Tower Telescope is supported by a state-of-the-art laser frequency comb to calibrate the solar spectrumon an absolute wavelength scale. In this article, we describe the scientific instrument and focus on the upgrades in the last two yearsto turn the prototype into a turn-key system.
Aims.
The pursued goal was to improve the short-term and long-term stability of the systems, and enable a user-friendly and moreversatile operation of the instrument.
Methods.
The first upgrade involved the modernization of the frequency comb. The laser system generating the comb spectrum wasrenewed. The Fabry-Pérot cavities were adjusted to filter to a repetition frequency of 8 GHz. A technologically matured photoniccrystal fiber was implemented for spectral broadening which simplified and stabilized the setup. The new control software facilitatesan automated operation of the frequency comb. The second, quite recent upgrade was performed on the optics feeding the sunlightinto a single-mode fiber connected to the spectrograph. A motorized translation stage was deployed to allow the automated selectionof three di ff erent fields-of-view with diameters of 1 (cid:48)(cid:48) , 3 (cid:48)(cid:48) , and 10 (cid:48)(cid:48) for the analysis of the solar spectrum. Results.
The successful upgrades allow for long-term observations of up to several hours per day with a stable spectral accuracy of1 m s − limited by the spectrograph. The instrument covers a wavelength range between 480 nm and 700 nm in the visible. Stable,user-friendly operation of the instrument is supported. The selection of the pre-aligned fiber to change the field of view can now bedone within seconds. Conclusions.
LARS o ff ers the possibility to observe absolute wavelength positions of spectral lines and Doppler velocities in thesolar atmosphere. First results demonstrate the capabilities of the instrument for solar science. The accurate measurement of the solarconvection, p-modes, and atmospheric waves will enhance our knowledge of the solar atmosphere and its physical conditions toimprove current atmospheric models. Key words.
Instrumentation: spectrographs – Instrumentation: miscellaneous – Sun: photosphere – Line: profiles
1. Introduction
Observational solar physics relies strongly on precise and accu-rate spectroscopy. The optical solar spectrum with its thousandsof spectral lines provides a wealth of information about mate-rial motions in the light-emitting layers of the Sun, the presenceof magnetic fields, but also gas temperature, ionization state,etc. The Sun allows measuring spectral line profiles with highspectral and spatial resolution, to investigate the conditions inthe solar atmosphere in three dimensions. Asymmetric line pro-files provide information about gradients of physical quantitieswith height in the solar atmosphere, e.g. near-surface convection,acoustic waves, or magnetic reconnection. Moreover, due to thesolar activity, the conditions can vary at a temporal scale of sec-onds to hours and also depend on the heliocentric angle on thesolar disk.The investigation of motions and other physical conditionsin the solar atmosphere thus requires the precise measurementof the spectral line profiles and positions. A high temporal ca- dence, a well-defined averaging area, and a distinct repeatabilityfor the measurements are imperative besides the high spectralresolution. With existing grating spectrographs and filter spec-trometers, the typical wavelength accuracy amounts to around2 mÅ ( ∼
100 m s − ) in the visible range. Former techniques tocalibrate the wavelengths of the solar spectrum are the usageof iodine cells (Beckers 1977), spectral lamps (Lovis & Pepe2007), Fabry-Pérot interferometer (Reiners et al. 2014), or tel-luric lines as reference. However, the application of all givenmethods is limited, either by the low intensity level of the refer-ence lines, their irregular distribution across the spectral range,or their known accuracy. The Laser Absolute Reference Spec-trograph (LARS) overcomes all of these issues by using a laserfrequency comb (LFC) to calibrate the solar spectrum recordedwith a high-resolution echelle spectrograph, thus accomplishinga consistent accuracy better by two orders of magnitude. For so-lar physics, this makes LARS a unique instrument. Article number, page 1 of 10 a r X i v : . [ a s t r o - ph . I M ] J u l & A proofs: manuscript no. LARS
In astrophysical spectroscopy, a high spectral accuracy is im-perative to reliably measure small-scale velocities of the gas atthe solar surface. All systematic e ff ects (like orbital motions, ro-tations, and gravitational shifts) on the solar spectrum need tobe known or measured with su ffi cient accuracy, to guarantee thebest quality for a careful and consistent data calibration. In addi-tion, for many astrophysical investigations a high short-term andlong-term stability is essential. To investigate large-scale flows,or global properties, like the convective blueshift, long averagingtimes are needed to eliminate small-scale convective and oscil-latory motions that act as unwanted „solar noise“. With a high-resolution telescope like the German Vacuum Tower Telescope(VTT, Schroeter et al. 1985) observing only a small field-of-view, the significant systematic measurement of, e.g., the center-to-limb variation of the convective blue-shift may take weeksto month. They therefore require sensitive and very stable in-struments to avoid the detrimental influence of unknown drifts,caused, e.g. by changes in the index of refraction of the ambientair. At the VTT, these issues are solved with the Laser Abso-lute Reference Spectrograph (LARS, see logo in Fig. 1), a sys-tem based on an LFC as an absolute calibration source. LARSprovides an accurate wavelength calibration of each measure-ment over a large continuous wavelength range, and it guaran-tees long-term consistency of spectroscopic observations overmonths and years.In a pioneering work, Steinmetz et al. (2008) demonstratedthe feasibility of calibrating an astronomical spectrograph with alaser frequency comb. The authors used a comb operated in theinfrared range to successfully calibrate the echelle spectrographof the VTT. Between 2010 and 2013, an LFC-based wavelengthcalibration system was developed for the VTT spectrograph in acooperation between the Kiepenheuer Institute for Solar Physics,Freiburg, the Max Planck Institute of Quantum Optics, Garch-ing, and Menlo Systems GmbH, Martinsried. Initially, the sys-tem was planned to cover the visible spectral range from 500 nmto 600 nm with a mode separation of about 5 pm. Due to the highspectral resolution of the VTT spectrograph, the chosen modeseparation was considered to be narrow enough to have a su ffi -cient number of comb modes as calibration lines everywhere inthe spectrum. At the same time the separation was large enoughto observe clearly distinguishable comb lines with the shape ofthe spectrograph’s points-spread-function (Doerr et al. 2012b;Doerr 2015; Probst et al. 2015).This paper describes the LARS instrument in its currentscience-ready configuration that was reached after substantialupgrades and modifications in 2016 and 2017, compared tothe prototype version described in earlier works (Doerr et al.2012a,b; Doerr 2015; Probst et al. 2015). In Section 2, we brieflytouch the properties of the telescope and spectrograph, followedby a description of the final setup of the LFC that now allowsfor turn-key operation of the instrument. The final part (Section2.4) deals with the new opto-mechanical interface between thetelescope and the single-mode fiber feed with fields-of-view be-tween 1 (cid:48)(cid:48) and 10 (cid:48)(cid:48) in diameter. In Section 3, we demonstrate thescientific performance of the instrument. Section 4 provides anoutlook to the observing programs planned with LARS and thescientific opportunities with the instrument.Throughout the paper we stick to common practice in astro-physics and use air wavelengths when referring to spectral linesor observed wavelengths. The di ff erence to vacuum wavelength(which is linked to frequency ν and speed of light c through thewell-known relation λ · ν = c ) is about 0.14 nm in the rangearound 500 nm. An equation derived by Edlén (1953, 1966) isused to convert vacuum to air wavelengths, or vice versa. L A R S ! Fig. 1.
Logo of the LARS instrument. The emission lines of the laserresemble the shape of a comb. The colors from blue to red indicate theoperating range of the instrument in the visible. The symbols advert theassembly of laser and solar spectroscopy.
2. Instrumental setup
In this section, we describe the instrument in its final science-ready configuration which was reached after a substantial up-grade of the frequency comb in 2016, and of the optical setup ofthe solar light channel in 2017.
LARS combines a number of optical subsystems to one power-ful wavelength-calibrated solar spectrograph. A short overviewof the full instrumental setup and the involved subsystems isgiven in this section. The schematic overview of the setup isshown in Fig. 2. The telescope and spectrograph are describedin Sect. 2.2. Within the scope of the instrumental upgrades, thelaser frequency comb and the new fiber-coupling of the solarlight will be explained in more detail in Sects. 2.3 and 2.4.
Fig. 2.
LARS system. The femtosecond ytterbium laser operates at awavelength of 1060 nm with a repetition rate of 250 MHz and is lockedto a GPS-disciplined oscillator. The generated emission spectrum (up-per left) passes a pair of identical Fabry-Pérot cavities which filter themodes to an output repetition rate of 8 GHz (upper right). The frequencycomb spectrum is amplified, and then broadened by a photonic crys-tal fiber (PCF). With single-mode fibers (yellow lines), the signal fromeach light source (LFC, sunlight, flatfield lamp, HeNe laser, hollow-cathode lamp, ChroTel) is guided to its own entrance port at the fiberswitch. One output port is connected to the spectrograph (lower right).The light from the VTT falls onto a beamsplitter (lower left) and entersa fiber-coupling unit (1 (cid:48)(cid:48) , 3 (cid:48)(cid:48) , or 10 (cid:48)(cid:48) ) for spectral measurements and acamera for context imaging of the surrounding solar region. Acronymsare explained in Sects. 2.1 to 2.3. Figure adapted from Doerr (2015).
LARS was developed to perform solar observations sup-ported by an LFC as a source of frequency (or wavelength) cali-
Article number, page 2 of 10. Löhner-Böttcher et al.: LARS – An Absolute Reference Spectrograph for solar observations bration. The conjunction of both elements is illustrated in Fig. 2.The upper part sketches the generation of the frequency comb,the lower part shows the fiber-coupling of the solar light to thespectrograph. The sunlight collected by the VTT falls onto a cu-bic beam splitter which reflects 10% of the incoming light to acamera imaging the solar region as it appears in a selected nar-row wavelength range. The passing 90% of the sunlight is guidedto a fiber-coupling unit by which the light of a selected field aper-ture (1 (cid:48)(cid:48) , 3 (cid:48)(cid:48) , or 10 (cid:48)(cid:48) on the solar disk) is fed into a single-modefiber. The integrated signal is then guided to a fiber switch whichcan rapidly change between eight entrance ports. The single out-put port is connected to the spectrograph for spectral observa-tions.As depicted in Fig. 2, there are five di ff erent other lightsources connected with fibers to the switch: i) A tungsten lampwhich is used as a fiber-coupled flatfield lamp producing a con-tinuous white-light spectrum for the calibration of the spectro-graph camera; ii) a stabilized HeNe laser producing a sharpemission line to optimize the spectrograph alignment; iii) sev-eral hollow-cathode lamps which can be employed to measurethe spectral emission of their atomic transitions; iv) the inte-grated full-disk light from the Chromospheric Telescope (Chro-Tel, Kentischer et al. 2008); v) the laser frequency comb to ab-solutely calibrate the spectrograph and wavelength scale of allother spectra. For the sake of completeness, we briefly summarize the mainproperties of the VTT and its echelle spectrograph. Parts of thesetup are sketched in the bottom of the setup overview in Fig 2.More information can be found on the VTT webpage .The 70 cm telescope has a focal length of 45 m and imagesa circular fraction of the solar disk (270 (cid:48)(cid:48) Ø) at the main focalplane, with a plate scale of 4 . (cid:48)(cid:48) mm − . The entrance slit of thespectrograph is located at the focal plane in the direct, verticalbeam. A 45 ◦ fold mirror allows to redirect the beam to variousoptical laboratories. For observations with LARS, the entranceslit is replaced by a fiber-coupling unit. A single-mode fiber(SMF) with a mode-field diameter of a few µ m feeds the light(solar or artificial) to the spectrograph. This guarantees an iden-tical illumination of the spectrograph for all light sources. Thespectrograph design matches the telescope’s ratio between focallength and aperture of 64, whereas the numerical aperture of theSMF corresponds to an f-ratio of 3.8. To adapt the SMF to thespectrograph, an asphere with a short focal length is mounted be-tween the fiber and the entrance focal plane, to generate a beamwith an f-ratio of about 64 that illuminates the large 79-grooves-per-inch echelle grating. Note that the spectrograph is not a clas-sical, cross-dispersed echelle instrument. Instead, a pre-disperserproduces a spectrum with low dispersion (about 1 . − ) atthe entrance focal plane of the main disperser. The main part ofthe spectrograph is an asymmetric all-reflective Czerny-Turnerinstrument with a 15 m collimator and a 7.5 m imaging mirror.This leads to a demagnification of the spatial scale by a factor of2 compared to the telescope and to a dispersion of 19 pm mm − ( λ =
550 nm). The spectral resolution R =∆ λ/λ amounts to about800,000 at a wavelength of 550 nm. To record the spectra, weuse an ANDOR NEWTON CCD camera with a pixel size of13.5 µ m and a chip size of 2048 ×
512 pixels. This results in aspectral field of view of 0.53 nm at a wavelength of 550 nm (Do-err 2015). http: // / en / observatories / vtt / vtt-instrumentation / The major step from the expert-user prototype version of LARSto a turn-key systems was taken in May 2016. The upgrade ofthe LFC now enables a user-friendly long-term stable operationof the instrument. The setup and its key upgrade are described inthe following.
Fig. 3.
Broadened spectrum of LARS. For illustration, the light leavingthe optical fiber was dispersed with a small di ff raction grating and pro-jected to a nearby wall. Toward the red, the wavelength range is limitedby the transmission range of the single-mode fiber (and by the sensitiv-ity of the camera used for the photograph). The blue end depends onthe optical power used for spectral broadening of the frequency comb.In this example we reach a wavelength coverage of about 480 nm to700 nm. A femtosecond laser operating at a center wavelength of1060 nm generates a comb of equally spaced modes with a modeseparation of 250 MHz. The native comb spectrum is sketched inthe upper left of Fig. 2. The laser itself is referenced to a commer-cial 10 MHz oven-controlled quartz crystal oscillator (OQXO),that is GPS-disciplined. To convert a frequency comb to an in-strument suitable for astrophysical applications („astro-comb“),the repetition rate and the spectral band of the laser have to beadapted to the capabilities of the spectrograph under considera-tion. This is achieved by sending the laser light through a pair ofstabilized Fabry-Pérot cavities, which transmit only every 32ndcomb mode. The cavities are locked to the repetition rate of theLFC by the transmission signal of a continuous wave (CW) laser.The resulting output signal is sketched in the top right cornerof Fig. 2. The output repetition rate (mode separation) amountsto 8.0 GHz. The signal is amplified in a cladding-pumped Yb-doped fiber amplifier, and thereafter broadened in a tapered pho-tonic crystal fiber (PCF). The final, spectrally broadened LFCsignal is shown in Fig. 3 with low dispersion. The light exitingthe PCF is guided through a single-mode fiber to an additionalfree-space unit with a knife-edge (not depicted in Fig. 2). Byblocking parts of the light beam, the intensity level of the comblight is adjusted to the continuum level of the solar spectrum.The regulated comb signal is coupled to another single-modefiber and guided to the fiber switch. The general design of anastro-comb was already described by Steinmetz et al. (2008) andWilken et al. (2012). Doerr (2015) and Probst et al. (2015) dis-cussed the prototype version of the present instrument in greatdetail.In the prototype version (Doerr 2015), the infrared laser sig-nal was translated into the visible range by using a second har-monic generator, which doubled the frequency. An earlier ver-sion of the tapered PCF then broadened the laser signal to a rangefrom 460 nm to 700 nm. In the meantime, the PCF techniquematured by optimizing the taper geometry and PCF structure,now broadening the fundamental comb spectrum to a substan-tially wider range, making the use of a second-harmonic gen-erator obsolete. The frequency comb spectrum now spans from480 nm to 1300 nm. This was a major step forward simplifying The tapered elliptical core of the PCF spans only few micrometers.Article number, page 3 of 10 & A proofs: manuscript no. LARS
PCF
PCF box
SMF
Fig. 4.
The photonic crystal fiber (PCF) is connectorized at both ends,with sealed end facets, and is mounted in a segregated housing (boxin the right part of the picture) for quick and easy replacement. ThePCF output is coupled to a single-mode fiber (yellow) and guided to theknife-edge unit and fiber switch. the optical setup and, at the same time, substantially increas-ing the operational stability. The new PCFs do not only providemuch larger spectral broadening, they also have a significantlylonger life time of several thousands of operating hours. Theyare readily available as plug-and-replace units which can be ex-changed quickly. The PCF and its box are shown in Fig. 4. Asa by-product of this development, all optics are now fully con-tained in optical enclosures, which again improved the stabilityand decreased the sensitivity to changes in the ambient condi-tions. The new software package controls all instruments para-meters and regulates them automatically to the optimized setting.Eight weeks of observation time in 2016 have verified an unin-terrupted system stability of up to several days.In the prototype version, the native laser mode separation of247.5 MHz had been filtered to 5.445 GHz. This number trans-lated into a mode separation of 4.5 pm (at λ =
500 nm), well re-solved by the spectrograph. On the other hand, the mode spacingwas large enough to clearly identify the wavelength of an indi-vidual laser mode due to its proximity to a solar spectral line.Once the wavelength (and thus the frequency) of a single lasermode is identified, all other modes are exactly known, due to thegiven o ff set frequency and the constant spacing of the frequencymodes. For the final setup we decided to change the mode sepa-ration from 5.445 GHz to 8.0 GHz. Thus, only every 32nd modeis transmitted by the Fabry-Pérot cavities. The mode separationof 8.0 GHz (6.67 pm at λ =
500 nm) is still convenient for thevisible wavelength range and for the VTT spectrograph with itsextremely high spectral resolution. In addition, the larger repeti-tion rate would enable the use of LARS also for the near infraredGRIS instrument (Collados et al. 2012) at the GREGOR solartelescope (Schmidt et al. 2012).Intrinsically, the LFC provides a wavelength accuracy atlevel of a few cm s − . The stability of the GPS-disciplined ref-erence oscillator ultimately limits the frequency accuracy of thecomb to an absolute value of only 3 mm s − . It is the spectralfiltering of the native comb with the Fabry-Pérot cavities whichintroduces a minimal mismatch between the FPC transmissionpeaks and the equidistant comb peaks. As a result, the side-modes are suppressed with a slightly asymmetric weight. Then,fitting the transmitted modes with a Gaussian leads to a worst-case shift of 2 . − . In addition, during the power amplifica-tion for spectral broadening, the side-modes are unavoidable re-amplified to an intensity of around − ff ects (Wilken et al. 2012), but enters an additional de- fective shift of the LFC line centroids of 5 kHz, or 3 mm s − at630 nm (Doerr 2015). The second major upgrade of LARS was performed in April2017. The optical setup of the solar channel was restructured tofacilitate a quick change between three available field aperturesfor solar observations. On this occasion, the majority of the op-tical components was replaced.
Fig. 5.
New optical setup for solar observations with LARS. The fiber-coupling units (FCU), lenses (L), mirrors (M), translation stage, andContext Imager are sketched.
As illustrated in Fig. 2, LARS observes the solar light simul-taneously with two cameras. The Context Imager records a two-dimensional image of the solar region captured by the telescope.At the same time, a small circular field in the center of that regionis fed into a single-mode fiber and its field-integrated spectrumis observed by the spectrograph camera. Three fiber-couplingunits are available to measure the sunlight with an aperture cor-responding to either 1 (cid:48)(cid:48) , 3 (cid:48)(cid:48) , or 10 (cid:48)(cid:48) on the sky. For solar obser-vations, such a distinction of the integrated region is necessaryto perform exclusive and qualitative measurements of the atmo-spheric conditions for di ff erent spatially-resolved solar features,e.g., solar granules with a size of 1 (cid:48)(cid:48) or umbrae with a diameterof 10 (cid:48)(cid:48) . A pinhole in the final focus limits the field, a collimatinglens with short focal length feeds the light to the fiber facet . Forobservations, a change of the fiber-coupled aperture therefore in-volved a manual exchange of the pinhole and lens in front of thefiber. Since this change included a sensitive optical alignmentto maximize the coupling e ffi ciency, this process was laboriousand time-consuming. Therefore, the basic idea of the upgradewas to install three individual pre-aligned fiber-coupling units,one for each field aperture. A motorized translation stage withtwo mounted silver-coated mirrors guides the light to one of thefiber-coupling units. This automatization now allows a selectionof the di ff erent apertures within seconds.The old setup (Doerr 2015) consisted of a X95 rail and car-rier system to align the optical components with the light path.As part of the upgrade, the rail system was replaced by two60 mm optical breadboards for the new optics. The inner honey- Since it is not possible to image the observed aperture on the tinyfibre core, we project the telescope pupil on the fibre facet. This ensuresthat information from the whole aperture is coupled to the fibre. Butowing to the Gaussian acceptance profile of the fibre, the inner regionsof the aperture are covered with higher e ffi ciency than the outer regions.Nevertheless, the optics were designed such that the coupling e ffi ciencyin the outer regions of the aperture does not drop below 60%.Article number, page 4 of 10. Löhner-Böttcher et al.: LARS – An Absolute Reference Spectrograph for solar observations comb structure optimizes the damping of vibrations and acousticdisturbances below 100 Hz. The new optical design is displayedin Fig. 5. The transfer optics consisting of two lenses (L1 andL2, not shown here) with focal lengths of 1500 mm relay thescience focus of the VTT toward a stop wheel with a set of re-movable blends. Besides the free field, three pinholes with dif-ferent field apertures (1 (cid:48)(cid:48) , 3 (cid:48)(cid:48) , 10 (cid:48)(cid:48) on the sky) are mounted at thewheel to align the light path to the fiber and context camera. Athird 50 mm diameter achromatic lens (L3) with a focal lengthof 300 mm collimates the light to the silver-coated mirror (M1).The high-precision two-axes adjusters of the mirror mount facil-itate the optimized alignment of the light beam. After the mirror,a cubic beamsplitter reflects 10% of the incoming light to theContext Imager. A combination of a 50 mm lens and an inter-ference filter focus the wavelength-filtered light onto the CCDchip of the Context Imager camera. Information about the dataacquisition is given in Sect. 3.1. Fig. 6.
Fiber-coupling units. The solar light of the respective field-of-view (1 (cid:48)(cid:48) , 3 (cid:48)(cid:48) , or 10 (cid:48)(cid:48) ) is fed to the fibers. The integrated signal is guidedto the spectrograph. Compare Fig. 5.
The remaining 90% of the light passes the beamsplitter to-ward the fiber-coupling units (see Fig. 6). A 50 mm achromaticlens (L4) with a focal length of 300 mm focuses the light towardthe pinhole of the fiber-coupling unit. The heart of the system up-grade is the increase from one to three individual fiber-couplingunits. A motorized translation stage (TS) manufactured at theKiepenheuer-Institut handles the beam guiding. Two silver mir-rors (M2) with diameters of 25.4 mm are mounted on the move-able stage with an angle of incidence of ± ◦ to the incominglight beam. Three pre-adjusted positions (first, second, or no mir-ror) can be selected with micrometer repeatability. In combina-tion with an additional folding mirror (M3), an identical lengthof the light path to all fiber-coupling units is guaranteed. All mir-rors are mounted in two-axis adjusters to enable a precise andstable alignment.The fiber-coupling unit feeds the sunlight into the opti-cal single-mode fiber. The optical components are displayed inFig. 7. A circular pinhole is drilled into a brass plate to limit thefield aperture to 1 (cid:48)(cid:48) , 3 (cid:48)(cid:48) , or 10 (cid:48)(cid:48) on the solar disk. The lens withshort focal length collimates the residual light beam toward the Fig. 7.
Fiber-coupling unit with pinhole, lens and single-mode fiber(SMF) of the 1 (cid:48)(cid:48) field. fiber facet. To maximize the coupling e ffi ciency into the fiber,the fiber-coupling unit in total features nine degrees of freedomfor orientation and alignment. The whole unit is placed on topof a dovetail translation stage which can shift the unit by 25 mmin the direction of the light path. The pinhole and lens can bemoved in two axes, vertically and horizontally with respect tothe fiber. The fiber mount itself can be adjusted along six axes.To bring the fiber into the optimal position along the direction ofthe light beam, the unit is placed on top of a one-axis translator.The fiber head itself has a five-axes mount for optical alignmentin X, Y, and Z direction, as well as tip and tilt. The light coupledto the fiber (yellow cable in Fig. 7) is guided internally to thefiber switch. When the VTT channel is selected by the controlsoftware, the light enters the output fiber which is connected tothe spectrograph.All optical fibers of LARS are single-mode fibers. The fibersare specified for a peak transmission in the range from 450 nmup to 700 nm. Due to the operating range of the frequency combstarting at 480 nm, this limits LARS observations to the visiblepart of the spectrum from 480 nm to 700 nm. Toward the red, thedamping due to the fiber necessitates exposure times of a fewseconds to perform spectral observations. Unlike multi-modefibers, single-mode fibers guide only one propagation mode.This brings the advantage of well-defined beam properties whichare completely independent of the input coupling parameters. Astable and uniform illumination of the spectrograph grating isachieved, which in turn enables the high calibration accuracy ofthe spectra. On the other hand, the coupling e ffi ciency of the in-coming light is very low. But since the Sun (compared to otherstars) is an extended source with high intensity, the light leveltransmitted through the single-mode fiber is always adequate toallow for high-cadence ( ∼ For spectroscopic observations with LARS, a number of inde-pendent control units are involved. A schematic overview isgiven in Fig. 8. It contains: (i) the telescope operation systemand pointing, as well as the adaptive optics (AO) system, (ii)the spectrograph with its echelle grating and predisperser, (iii)the di ff erent units operated by the LARS control software, theseinclude the spectrograph CCD camera, the translation stage se-lecting the active fiber-coupling unit, the fiber switch to selectthe input source for the spectrograph, the Context Imager, and aknife-edge unit to regulate the light level of the frequency comb, Article number, page 5 of 10 & A proofs: manuscript no. LARS and (iv) the frequency comb itself with its operation control andmonitoring. The four main units are operated independently bythe observer. The following sections describe the last two units.
Observer
Telescope
Spectrograph
LARS
Frequency comb
Pointing
Adaptive optics
Echelle grating
Predisperser
Spectro. camera
Fiber coupling
Fiber switch
Context Imager
Knife-edge unit
Comb control
Comb watch
Fig. 8.
Schematic overview of the instrument control. The observer op-erates four individual control units and their sub-units.
The LARS control software commands the five sub-units listedin Fig. 8 (third column from the left). It sets the observational pa-rameters for the spectrograph camera, like the integration time ofthe CCD, the read-out area and binning mode, the temporal ca-dence and number of repetitions within the sequence, and thecamera cooling. It also controls the fiber switch to alternate be-tween the selected input channels of the di ff erent light sources.The Context Imager can be operated individually or triggeredby the spectrograph camera. The knife-edge unit is driven toregulate the light level of the frequency comb. The control soft-ware package was supplemented by the automated translation ofthe fiber-coupling units for the field-of-view. Before the opticalupgrade described in Sect. 2.4, the change of the fiber-couplingunit had to be performed by hand, including optical alignment tomaximize the coupling e ffi ciency. This was laborious and time-consuming. The new motorized translation stage and pre-alignedfiber-coupling units allow an automated selection of the di ff erentfields-of-view within a few seconds with a high repeatability. Itsoperation is now included in the LARS control software as partof the upgrade. Although being one of several light sources connected to thefiber switch, the LFC and its control system are operated inde-pendently from the rest of the instrument (see right column inFig. 8). The new version of the comb control system that camewith the upgrade in 2016 (see Sect. 2.3) now allows for an au-tomated operation of the comb. This was the decisive step toconvert the whole system from a complex prototype into a (stillcomplex, but easily manageable) turn-key instrument. The LFCcontrol system can be operated remotely, e.g., from the VTT con-trol room, or from any authorized computer outside the observa-tory. This remote capability allows for remote support by expertsin case of a malfunction. To check or reinspect the correct op-eration of the LFC system, an internal log file with all contextinformation is written.
To facilitate solar observations with LARS, all required controlscreens can be bundled as remote sessions at the remote con-trol station (see Fig. 9) in the control room of the VTT. Experi-enced users are thus able to perform LARS observations single-
Fig. 9.
Remote control station at the VTT. All instruments and devicesneeded for LARS observations are accessible via remote desktop toolsand are displayed on an array of screens. handedly. All remote sessions can also be controlled from anyother authorized computer.
3. Observations and data processing
In this section, we discuss the observations which are performedwith the two cameras of LARS – the Context Imager (Sect. 3.1)and the spectrograph camera (Sect. 3.2). We thereby focus on theacquired data and give a summary of the data calibration for bothchannels.
Observation:
The Context Imager records a two-dimensionalimage of the Sun. The sunlight is filtered to one narrow wave-length range of typically 1 nm or less. Various spectral pre-filterscan be inserted to yield context information, e.g. about photo-spheric, chromospheric, or magnetic dynamics in the solar at-mosphere. An example of a recorded context image is displayedin Fig. 10. It shows a fully-developed sunspot recorded on May20th 2016 at 08:46 UT close to the solar disk center at a wave-length of 430.8 nm (G-band). The reasons to select this spectralband were the simplified distinction between magnetic and non-magnetic regions, and the naturally higher spatial resolution inthe blue. With the appropriate pre-filter, the Context Imager canbe operated at any wavelength in the visible. The CCD cam-era has a detector size of 1360 × (cid:48)(cid:48) × (cid:48)(cid:48) on the Sun. The Context Im-ager can be synchronized to the spectrograph camera. Then, forevery acquired spectrum, a context image is recorded simultane-ously. The Context Imager can also be operated independently.Sequences with up to 20 frames per second and exposure timesof a few milliseconds can be achieved. Adaptive optics can beenabled to correct for image distortions caused by atmosphericturbulences. Image reconstruction techniques are then applied toreach a spatial resolution close to the di ff raction limit of the tele-scope. The two-dimensional image serves as context informationfor the spectroscopic observation. The three di ff erent fields-of-view and their alignment with respect to the context image areindicated in Fig. 10 as red (1 (cid:48)(cid:48) ), yellow (3 (cid:48)(cid:48) ), and blue (10 (cid:48)(cid:48) ) cir-cles. Article number, page 6 of 10. Löhner-Böttcher et al.: LARS – An Absolute Reference Spectrograph for solar observations
LARS Context Imager, Gband, 2016-05-20T 08:46:24UT ”
10” 3” 1”
Fig. 10.
Sunspot observed with the Context Imager of LARS. The im-age sequence was recorded on May 20th 2016 at 08:46 UT with an in-terference filter centered on the spectral G-band around 430 nm. TheSpeckle-code KISIP was used to reconstruct the displayed image. Thefull field-of-view covers a size of 100 (cid:48)(cid:48) × (cid:48)(cid:48) . The light from a circularregion with a diameter of either 1 (cid:48)(cid:48) (red), 3 (cid:48)(cid:48) (yellow), or 10 (cid:48)(cid:48) (blue) isintegrated to a fiber and guided to the spectrograph. Data calibration:
The raw data recorded with the Context Im-ager contain several defects like dissimilar pixel sensitivities,dust or dirt on the detector, and atmospheric distortions of thesolar image which are corrected by the data calibration. Theset of calibration data includes a sequence of „dark“ and „flat-field“ images to evaluate the background signal and inten-sity defects, and to calculate the average gaintable image forthe flatfield-correction of the image sequence. The calibration LARS data calibration
Spectrograph camera
Context Imager
Dark seq.
Flatfield seq.
Spectral sequence (e.g., Comb/Sun)
Dark avg.
Gaintable avg.
Flatfielded spectral sequence (Comb/Sun)
Dark seq.
Flatfield seq.
Image sequence
Dark avg.
Gaintable avg.
Flatfielded image sequence
Speckle-reconstructed solar image
Calibrated sequence of the solar spectrum
Ephemeris context
Fit of comb spectra
Reference mode
Wavelength calibration for the spectral seq.
Continuum rectification
KISIP code R a w d a t a P r o ce ss i ng d a t a F i n a l d a t a D a t a c h a nn e l Fig. 11.
Calibration scheme for LARS data. The data reduction fromthe raw to the final state is displayed for both LARS channels – thespectrograph camera (red colors) and the Context Imager (blue colors).Black arrows indicate the direction of processing. Raw, processing, andfinal state products are drawn with solid borders around the text box.Intermediate calibration steps and tools are indicated by dashed borders. scheme for the Context Imager data is shown on the right sideof Fig. 11. As a matter of routine, a set of context images isrecorded which are not named in the processing scheme. The The telescope performs a fast movement over a Quiet Sun regionclose to the observed solar target while the camera takes an image se-quence long enough to smear out all solar structures. An AO-stabilized image of an US Air Force test target is taken togain information on the spatial resolution. A pinhole image is obtained flatfielded image sequences can already be used for scientificinvestigations but still su ff ers from residual atmospheric distor-tions which can not be coped by the adaptive optics system. Se-quences recorded with a rate of 10–20 frames per second and ashort exposure time ( <
50 ms) are post-facto reconstructed em-ploying the Kiepenheuer-Institute Speckle Interferometry Pack-age (KISIP, Wöger & von der Lühe 2008) to approach the adi ff raction-limited spatial resolution. Observation:
The spectrograph camera records a one-dimensional spectrum of the selected input source (see Sect. 2.1).We use an ANDOR NEWTON CCD camera with a pixel size of13.5 µ m and a chip size of 2048 ×
512 pixels. Owing to the singlefiber feed of the spectrograph, the signal is concentrated in onlyfew (typically two to three) adjacent pixel rows of the CCD chip.However, due to e ff ects of instrument internal seeing, or changesof the ambient air pressure and temperature, the spectrum su ff ersfrom potential drifts. Detailed characterizations of the spectro-graph stability (Doerr 2015) revealed that long-term variations ofthe environmental conditions can lead to shifts of the LFC spec-trum of a few hundred meters per second within several hours.In addition, the short-term variation shows an instrumental jitterat the scale of 10 s with an amplitude up to a few m s − . With theLFC spectra recorded at a cadence of a second or below, we canaccount for these drifts. To consider spatial shifts of the spec-trum across the camera sensor, a region of around twenty rowsaround the illuminated pixels is read out and binned perpendic-ular to the dispersion axis. With a dispersion of 19 pm mm − at awavelength of λ =
550 nm, the 2048 pixel wide spectrum coversa spectral range of 0.53 nm. The spectral resolution of the spec-trograph amounts to about 800,000 at that wavelength. A typicalobservation sequence with LARS consists of the repetition of atwo-channel cycle. For solar observations, one cycle constitutesof the successive measurement of the frequency comb spectrumand the solar spectrum. An exemplary cycle is shown in Fig. 12.Typical integration times are around 1 s, depending on the solartarget. In case of a Quiet Sun region at disk center and a sin-gle exposure for 1 s, a signal-to-noise of 200 is reached for thespectral continuum around 550 nm. At this signal level, dark cur-rent and read noise do not play a significant role. As reported inDoerr (2015), the signal level decreases only to a minor fractionwhen changing between the 10 (cid:48)(cid:48) , 3 (cid:48)(cid:48) , and 1 (cid:48)(cid:48) fiber-coupling units.This has the fundamental reason that the coupling e ffi ciency ofthe multi-mode light source to a single-mode fiber increases withdecreasing field-of-view, in our case by one magnitude from 10 (cid:48)(cid:48) to 3 (cid:48)(cid:48) , and 3 (cid:48)(cid:48) to 1 (cid:48)(cid:48) each.Given the temporal scales of seconds, very fast camera read-out is not important. During the cycle, the fiber switch canchange the input channel within 2 ms. This sets a total cycle timeof a few seconds. By repeating the cycle, the frequency comband solar spectra are observed alternately. So the solar spectrumcan be calibrated by the interpolation between the preceding andsucceeding comb spectrum. The user defines the number of rep-etitions and, by this, the observation time of the sequence. Thefully-automated data capture software writes the data sequenceas one file in FITS format. The first dimension contains the2048 pixels, the second dimension successive cycles (or time). to verify the position of the AO lock point. Finally, context informationon the location of the fiber and the size of the integrated region is takenfor the spectral observation. The respective field stop at the filter wheelis inserted and a field-of-view image of the fiber is recorded.Article number, page 7 of 10 & A proofs: manuscript no. LARS N o r m a li z ed i n t en s i t y Spectral region with solar spectrum and Frequency CombFrequency CombSolar spectrum Si I O I Sc II Fe I Fe I Fe I Ti I GHz 475.838
THz M H z Fig. 12.
Spectral region with a typical cycle of solar spectrum and fre-quency comb signal. The spectrograph camera records a spectral regionwith a wavelength width of 5.6 Å, here centered around 6301.5 Å. Thesolar spectrum in red was normalized to the continuum intensity andconsists of several photospheric lines (elements in gray). The three un-named narrow lines are telluric O lines. The frequency comb spectrumis overlaid in blue and consists of 52 equally spaced (8 GHz) emissionmodes with a full width at half maximum of 700 MHz. One frequencymode (here 475.838 THz) has to be unambiguously identified throughits proximity to a spectral line core. Note that the solar and the comblines are indeed observed sequentially, to avoid any deterioration of thesolar line profile through the superposition of the comb lines. The metadata required by the data pipeline is written into theFITS header.
Data calibration:
The observed raw data of the spectrographcamera still contains systematic defects like dissimilar pixel sen-sitivities and dust on the detector, the background readout, andlarge-scale gradients in the spectrograph transmission. A set of„dark“ and „flatfield“ sequences is recorded to reduce these er-rors from the spectral sequences. The scheme of the spectral datacalibration is shown in Fig. 11 on the left side. Two short „dark“sequences taken with the covered camera define the readout andbackground signal of the camera. Since the camera can be cooledto − ◦ C, the dark signal is very low. The exposure times ofthe darks have to be the same as for the spectral sequence (e.g.,„LFC / Sun“) and „flatfield“ sequence, respectively. A tungstenlamp emits the continuous flatfield spectrum which contains theinstrumental errors when recorded by the camera. The exposuretime for the flatfield is adjusted to match the continuum inten-sity level of the solar spectrum. The raw data (yellow boxes inFig. 11) is entered to the semi-automated data pipeline. Aftereach dark and flatfield sequence is averaged in time, the gaintablespectrum is calculated as the normalized di ff erence between flat-field and dark. The flatfielded spectral sequence is obtained bysubtracting the corresponding dark spectrum from each solar orcomb spectrum and dividing by the gaintable spectrum.In the next processing steps, the solar spectrum has to be cali-brated to an absolute wavelength grid (using the comb spectrum)and corrected from inherent systematic wavelength shifts (calcu-lated by an ephemeris code). Given the strong e ff ect of system-atic relative motions between the telescope and the light source,the latter substantially depends on a high accuracy of the appliedmodels. The applied ephemeris code developed by Doerr (2015),which in turn is based on NASA’s Navigation and Ancillary In- formation Facility Spacecraft Planet Instrument C-matrix Events(SPICE) toolkit (Acton 1996), computes the relative motion be-tween the observing telescope and the Sun with an accuracy offractions of mm s − . For a ground-based telescope, it includesthe orbital motion of the Earth around the Sun, as well as theterrestrial rotation at the location of the observatory, which canadd up to a rapidly changing line-of-sight velocity of the order of ± − . The next systematic component is the gravitationalshift caused by the Sun and Earth according to the General The-ory of Relativity which, taken together, amounts to a redshift of633 . − , everywhere on the solar disk. If desired, the di ff er-ential rotation of the Sun reaching a line-of-sight velocity of upto ± − at the solar limb can be modeled and reduced foreach heliographic position on the solar disk. The ephemeridescode calculates the wavelength shifts for each time step of theobserved data sequence. The generated file is later applied to thecalibrated solar spectrum.To get the absolute wavelength grid for the solar spectrum,the comb spectrum has to be unambiguously calibrated. The sep-aration of the comb mode is fixed and amounts to 8.0 GHz. Withthis knowledge, each pixel of the detector can be assigned to afraction of a mode number. To determine the positions of thewhole emission modes, each individual comb line is fitted witha Gaussian model with four degrees of freedom (centre, width,amplitude, o ff set). Due to the very good side-mode suppressionby the two high-finesse FPCs, the Gaussians fit the comb linecenter extremely well. The typical statistical error for the mea-sured mode positions is estimated to 5 cm s − for a single cali-bration exposure. Only potential asymmetries of the instrumen-tal profile can become a severe systematic issue. In this case, thecomb lines shift toward the centroid of the instrumental profile.However, the deviation of the final mode profile from the per-fectly symmetrical Gaussian fit is minor, so that this e ff ect is lim-ited to a shift well below 1 m s − in average. In the next calibra-tion step, the overall pixel-to-mode-number solution is fitted bya polynomial, thus yielding an intermediate frequency solution.For a single measurement, its absolute accuracy was determinedto about 60 cm s − . The limiting unmodelled sub-structure in thefrequency solution is considered to stem from small ( ∼
50 nm)pixel spacing variations of the CCD detector. Since the repetitionrate and o ff set frequency ( −
100 MHz) of the comb spectrum areknown, it is su ffi cient to unambiguously identify only one modewith its mode number. We select a well-known solar spectral lineand enter its air wavelength. The closest comb mode serves asthe reference mode to assign all other modes with a well-definedfrequency (e.g., in Fig. 12 the reference mode close to the O i lineat 6300.3 Å has a frequency of 475.838 THz). With this informa-tion, all intermediate calibration steps are combined to computethe final frequency (or wavelength) solution. Each individual so-lar spectrum is then calibrated by the temporal interpolation ofthe two adjacent LFC calibration functions. To achieve a highspectral accuracy for our measurements, a temporal cadence ofonly few seconds or less is necessary. In fact, the Spectrographinstability introduces the largest error, compared to the line shiftsof a few cm s − from the LFC and CCD detector. Instrument in-ternal seeing, i.e. a spectrograph jitter at the scale of 10 s, in-fluences the repeatability of the observation. In periods of goodinstrumental seeing, the error typically is of the order of a fewcm s − but can increase up to a few m s − . For measurementswith 1 s cadence, this limits the total accuracy to around 1 m s − ,or better.Since the transmission plateau of the spectrograph is quitenarrow and the observed spectral band is not always well cen-tered within the di ff raction order selected with the predisperser), Article number, page 8 of 10. Löhner-Böttcher et al.: LARS – An Absolute Reference Spectrograph for solar observations gradients in the spectral continuum may occur. To correct for thise ff ect, the Fourier Transform Spectrometer (FTS, Neckel 1999)atlas spectrum is overplotted to the LARS observations. The FTSatlas features a spectral resolution of 400,000 and an accuracy ofaround ±
50 m s − in the visible which is su ffi cient for a compari-son of both spectra. By selecting individual continuum positions,a polynomial correction function is fitted and stored. In the laststep of the data calibration, this correction function and the com-puted wavelength calibration are applied to the sequence of solarspectra. A final, calibrated LARS spectrum is plotted in Fig. 12as red curve.
4. First results and outlook
In 2016, three observation campaigns were carried out with theLARS instrument. In total 58 days of telescope time at the VTTwere at our disposal to test the instrument with the upgradedLFC and to perform solar observations. All data were calibratedwith the processing techniques described in Sect. 3. The maingoal was to measure spectral line shifts caused by the solar con-vection and acoustic waves with an unprecedented spectral ac-curacy. In a series of forthcoming papers, we will present the re-sults for the center-to-limb variation of the convective blueshiftobserved with several frequently used spectral lines (e.g., the Fe i lines at 6301.5 Å, 6302.5 Å, 6173 Å, 5250 Å, and Na i i i N o r m a li z ed i n t en s i t y −400 −300 −200 −100Doppler velocity [m s −1 ]0.30.40.50.60.70.80.9 Fe I Å, profile and bisector at solar disk center
Fig. 13.
Spectral line profile and bisectors of Fe i − . With a spectral resolution ( ∆ λ/λ ) of 700,000 – 800,000 and aspatial sampling of a 1 (cid:48)(cid:48) – 10 (cid:48)(cid:48) wide region, LARS enables state-of-the-art spectroscopic investigations of line profiles. In the leftpanel of Fig. 13, the intensity profile of the Fe i (cid:48)(cid:48) fiber-coupling unit in a Quiet Sun regionat the solar disk center. The time series of 20 min was wavelengthcalibrated and temporally averaged. The figure is a magnifica-tion of the spectral range also shown in Fig. 12. The line has adepth of more than 70% and a full width at half maximum of around 0.14 Å. To attain detailed information on the line shapeand Doppler velocities along the sampled atmospheric layers, abisector analysis was performed on the profile. The center wave-lengths are computed for 30 equidistant height levels startingfrom the line minimum up to 96% of the continuum intensity.The single positions are marked as black asterisks in Fig. 13.The interpolated curve was added as solid red line and high-lights the asymmetric shape of the line profile. To convert theshifts from air wavelength to Doppler velocities, the exact ref-erence position is required. We typically take the observed airwavelength from the atomic spectra database of the National In-stitute of Standards and Technology (NIST, Kramida et al. 2015).The reference wavelength is subtracted from the observed wave-length and the remaining di ff erence is translated ( ∆ λ/λ = ∆ v/ c )into a Doppler shift. The subtraction of the gravitational redshiftof 633 m s − (uniform for the solar disk) yields the correct line-of-sight velocities in the solar atmosphere. Thanks to the abso-lute wavelength calibration, there is no need to refer to some (ar-bitrarily chosen) area of quiet Sun, or the like. At the solar diskcenter, a redshift is really a downflow, and a blueshift is an up-flow in our case. The right panel in Fig. 13 reflects the bisectoranalysis from the left panel, plotted against Doppler velocities inm s − .The di ff erential shifts leading to the „C“-shaped bisectorcurve originate from the physical conditions in the solar atmo-sphere (Balthasar 1984). The upflowing material in the centersof granules is hotter than the downward moving gas in inter-granular lanes, hence the upflows contribute more to the spatiallyaveraged line profiles, causing a net blueshift of the line. Sincethe convective flow speed decreases with atmospheric height, thenegative (upward) velocities decrease toward the line core. Incase of the Fe i −
440 m s − at a normalized intensity of 0 . −
50 m s − at the line minimum.Beyond this example, the high spectral resolution, wave-length accuracy, and fast temporal cadence of LARS observa-tions enable the detection of subtle and fast changes of the phys-ical properties of the solar atmosphere.The investigation of acoustic waves in the solar atmospherewith LARS can become invaluable for the research field of lo-cal Helioseismology. Magneto-acoustic waves in sunspots andpores can be observed with highest spectral accuracy. Precisemeasurements of absolute p-mode oscillations can be performedin a confined region of the Sun. Two examples of p-mode obser-vations in Quiet Sun regions are presented in Fig. 14. The panelsshow the temporal evolution of the line center of Fe i ± (cid:48)(cid:48) fiber-coupling unit. The left time series (green aster-isks) was recorded on the southern axis of the solar meridianat an heliocentric angle of α = ◦ , or associated heliocentricparameter µ = cos α = .
9. The right time series (blue aster-isks) was observed at an heliocentric angle α = ◦ , or µ = . For some spectral lines the reference wavelength positions can beenmeasured with the hollow-cathode lamps available with LARS. This is limited by the accuracy of the reference wavelength, which isbetter than ± Angle of incidence α between the line of sight and the local normalto the solar surface Article number, page 9 of 10 & A proofs: manuscript no. LARS A i r W a v e l eng t h [ Å ] µ = I Å line core position D opp l e r v e l o c i t y [ m s − ] µ = Fig. 14.
Temporal variation of the observed Fe i µ = . µ = . − . the velocities by 150 m s − to negative values. P-mode oscilla-tions with a period of 5 min and amplitude of up to 300 m s − areclearly recognizable in both examples. In addition, a destructiveinterference of p-mode waves is apparent in the first half of theright series.3D HD and MHD simulations of the quiet solar atmospherehave been claimed to be realistic (Pereira et al. 2013) or highlyrealistic (Scott et al. 2014), and have been used to calibrate thecenter-to-limb variation of the convective blueshift (de la CruzRodríguez et al. 2011). LARS provides measurements of un-precedented quality that will allow for a detailed comparison ofa number of spectral lines. We will measure the center-to-limbvariation of the convective blueshift, and of the correspondingline asymmetries (C-shapes). Since these line properties are in-timately coupled with the temperature and velocity gradients inthe solar atmosphere, the center-to-limb variations of these quan-tities will give tight constraints for realistic numerical models ofthe solar atmosphere. As models of the solar atmosphere are fun-damental for the interpretation of stellar spectra, such measure-ments can also have great impact to our understanding of stellaratmospheres. Convective blueshifts on stars other than the Sunare a source of significant systematic errors for the determina-tion of correct radial velocities. Realistic models of stellar at-mospheres must faithfully reproduce the observed spectral linesand their center-to-limb variation in the solar atmosphere, beforethey can be applied to stellar atmospheres.The observed temperature of sunspot umbrae suggests thepresence of convection, which would also produce some (weak)convective blueshift. To investigate this topic, we intend to deter-mine the absolute wavelength of suitable spectral lines in sunspotumbrae to derive the combined e ff ect of gravitational redshiftand convective blueshift. The di ff erent center-to-limb behaviorof both e ff ects will allow distinguishing between the two con-tributions. The main limitation for this investigation may comefrom the rapidly approaching minimum of the current solar ac-tivity cycle.Using the improved possibilities for spectral calibration withlaser frequency combs and Fabry-Pérot interferometers, new so-lar flux atlases are produced. The atlases of Molaro et al. (2013)using HARPS and of Reiners et al. (2016) using the Fouriertransform spectrograph at the Institute for Astrophysics Göttin- gen provide a wavelength accuracy on the order of ±
10 m s − ,thus exceeding the accuracy of the renowned FTS atlases of Ku-rucz et al. (1984), Neckel (1999), or Wallace et al. (2011) byone order of magnitude. With LARS, we could construct a disk-center spectral atlas as well. But since the narrow spectral win-dow of the LARS spectrograph does not permit a large wave-length coverage, the fragmented measurement would be verytime-consuming and laborious. For mutual benefit, we will pro-vide accurate wavelengths and asymmetries for selected lines ofhigh astrophysical interest to which future studies and atlasescan refer to. Acknowledgements.
We thank all colleagues at the Kiepenheuer-Institut, atMenlo Systems, and at the Max Planck Institute of Quantum Optics who workedon the development of the instrument, and the upgrade of the laser frequencycomb in 2016. We especially thank Thomas Sonner, as well as Roberto Simoes,Frank Heidecke, Andreas Fischer, and Oliver Wiloth for their help in realiz-ing the upgrade of the solar fiber feed optics in 2017. The development of theLARS instrument and the operation of the Vacuum Tower Telescope at the Ob-servatorio del Teide on Tenerife were performed by the Kiepenheuer-Institut fürSonnenphysik Freiburg, which is a public law foundation of the State of Baden-Württemberg. This work is part of a Post-Doc project funded by the DeutscheForschungsgemeinschaft (DFG, Ref.-No. Schm-1168 / References
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