Atmospheric Dispersion Corrector Research Articles

Analyzing the atmospheric dispersion correction of the Gemini Planet Imager: residual dispersion above design requirements

Analyzing the atmospheric dispersion correction of the Gemini Planet Imager: residual dispersion above design requirements

Optical system for extremely large spectroscopic survey telescope

This article presents research work on a spectroscopic survey telescope. Our idea is as follows: for such a telescope, a pure reflecting optical system is designed, which should have an aperture and a field of view (FOV) both as large as possible and excellent image quality, and then a strip lensm (lens-prism) atmospheric dispersion corrector (S-ADC) is added, only for correcting the atmospheric dispersion. Given the fund limitation and the simplicity of scaling up, some 12-m telescopes are designed as examples. Su, Korsch, and Meinel put forward the four-mirror Nasmyth systems I and II, which are used in this paper. FOVs of 1.5°, 2°, and 2.5° are selected. For all systems, the image qualities are excellent. Because the S-ADC relaxes the optical glass size restriction, this 12-m telescope with a FOV of 2.5° can be magnified in proportion to a 16-m telescope. Its etendue (from French étendue) and focal surface will now be the largest in the world. In such a telescope, a pure reflecting optical system can also be obtained. A subsequent coudé system is designed with excellent image quality.

The Measurement and Correction of Atmospheric Dispersion in 4-Meter Telescope With Adaptive Optical System

The Measurement and Correction of Atmospheric Dispersion in 4-Meter Telescope With Adaptive Optical System

Assembly and alignment of the 4-metre multi-object spectroscopic telescope wide field corrector

The 4-metre multi-object spectroscopic telescope (4MOST) is a fiber-fed multi-object spectrograph for the VISTA telescope at the European Southern Observatory (ESO) Paranal Observatory in Chile. The goal of the 4MOST project is to create a general purpose and highly efficient spectroscopic survey facility for astronomers in the 4MOST consortium and the ESO community. The instrument itself will record 2436 simultaneous spectra over a ∼4.2 square deg field of view and consists of an optical wide-field corrector (WFC), a fiber positioner system based on a tilting spine design, and three spectrographs giving both high and low spectral dispersion. The WFC comprises of six lenses grouped into four elements, two of which are cemented doublets that act as an atmospheric dispersion corrector. The first lens element is 0.9 m in diameter while the diameter of the other elements is 0.65 m. For the instrument to meet its science goals, each lens was aligned to be well within ∼100 μm—a major challenge. This was achieved using contact metrology methods supplemented by pencil beam laser probes. In particular, an off-axis laser beam system has been implemented to test the optics’ alignment before and after shipment. This work details the alignment and assembly methods and presents the latest results on the achieved lens positioning and projected performance of the WFC.

Design and optimization of a support system for large aperture wedge prisms based on an integrated opto-mechanical analysis.

A stable and reliable support system for large aperture wedge prisms is the priority of the atmospheric dispersion corrector (ADC). The prism is not a rotationally symmetric component, and the stress distribution on large aperture wedge prisms caused by the support system is different compared with the rotationally symmetric mirror. A scheme of support forces passing through the prism center of gravity (COG) is proposed in this paper. Comparing with the scheme of support force passing through the prism geometry center of rotation (COR) under the same conditions, the root-mean-square (RMS) value of the optical surface shape error shows that the proposed scheme obtains better optical surface quality when the prism rotates from 0° to 360° under the conditions of gravity coupling at 2°C and 42°C. In addition, based on the proposed scheme, a multi-island genetic algorithm (MIGA) is used to optimize the position parameters of the supports. The results show that the RMS value of the optical surface deformation of the wedge prism decreases effectively. Under the conditions of gravity coupling at temperatures of 2°C and 42°C, the RMS value decreases from 260.7 nm to 107.8 nm with 58.6% and from 108.6 nm to 69.5 nm with 36.0%, respectively.

Demonstration of an imaging technique for the measurement of PSF elongation caused by Atmospheric Dispersion

ABSTRACT Elongation of the point spread function (PSF) due to atmospheric dispersion becomes a severe problem for high-resolution imaging instruments, if an atmospheric dispersion corrector is not present. In this work, we report on a novel technique to measure this elongation, corrected or uncorrected, from imaging data. By employing a simple diffraction mask, it is possible to magnify the chromatic elongation caused by the atmosphere and thus make it easier to measure. We discuss the theory and design of such a mask and report on two proof of concept observations using the 40-cm Gratama telescope at the University of Groningen. We evaluate the acquired images using a geometric approach, a forward modelling approach and from a direct measurement of the length of the PSF. For the first two methods, we report measurements consistent with atmospheric dispersion models to within 0.5 arcsec. Direct measurements of the elongation do not prove suitable for the characterization of atmospheric dispersion. We conclude that the addition of this type of diffraction mask can be valuable for measurements of PSF elongation. This can enable high-precision correction of atmospheric dispersion on future instruments.

Simple concept for atmospheric dispersion corrector on-sky commissioning tests

Differential atmospheric dispersion, due to the wavelength-dependent index of refraction of the atmosphere, affects ground-based observations. To correct this effect, the usage of an atmospheric dispersion corrector (ADC) is fundamental. Insufficient or wrong correction of the atmospheric dispersion produces a spectrally elongated shape instead of a circular white one for the observed target. The commissioning tests of ADCs with on-sky observations are not an easy task. In fact, the residual dispersion is expected to be of a few tens of milliarcsec, with the object for a seeing limited telescope being almost 1 arc sec. A procedure was developed, based on ellipse fitting of several cuts from the guiding camera images, to determine the levels of oblongness in an object image caused by atmospheric dispersion. The characterization of the data allows for the validation of the ADC alignment by determining the dispersion direction and minimizing the ellipticity. The ellipse fit method was tested on ESPRESSO using the guiding camera images. The procedure was tested and demonstrated using simulated data that mimics the expected images using real sky dispersion models and real sensor characteristics. The accuracy of the method is highly dependent on the observational conditions and on the ratio between expected elongation (dispersion) and image size, but it is expected that the method can be more sensitive than traditional ADC on-sky alignment methods.

A New Type of Atmospheric Dispersion Corrector Suitable for Wide-Field High-Resolution Imaging and Spectroscopy on Large Telescopes

Atmospheric dispersion produces spectral elongation in images formed by land-based astronomical telescopes, and this elongation increases as the telescope points away from the zenith. Atmospheric Dispersion Correctors (ADCs) produce compensating dispersion that can be adjusted to best cancel out the atmospheric effect. These correctors are generally of two basic types: Rotating Atmospheric Dispersion Correctors (R-ADCs), and Linear Atmospheric Dispersion Correctors (L-ADCs). Lately, a third type, the “Compensating Lateral ADC” (CL-ADC) has been proposed. None of these design approaches allow for large corrector systems (with elements greater than 1 m in diameter), in which the secondary spectrum is corrected to small residuals, of the order of tens’ of milliarcseconds. This paper describes a new type of large corrector (>1 m diameter elements), which can achieve the correction of the secondary spectrum to the order of 10 milliarcseconds. This correction is achieved by combining the R-ADC and CL-ADC approaches to dispersion correction. Only glass types readily available in metre diameters are required.

On-sky measurements of atmospheric dispersion – II. Atmospheric models characterization

ABSTRACT Differential atmospheric dispersion is a wavelength-dependent effect introduced by Earth’s atmosphere that affects astronomical observations performed using ground-based telescopes. It is important, when observing at a zenithal angle different from zero, to use an Atmospheric Dispersion Corrector (ADC) to compensate this atmospheric dispersion. The design of an ADC is based on atmospheric models that, to the best of our knowledge, were never tested against on-sky measurements. We present an extensive models analysis in the wavelength range of 315–665 nm. The method we used was previously described in the paper I of this series. It is based on the use of cross-dispersion spectrographs to determine the position of the centroid of the spatial profile at each wavelength of each spectral order. The accuracy of the method is 18 mas. At this level, we are able to compare and characterize the different atmospheric dispersion models of interest. For better future ADC designs, we recommend to avoid the Zemax model, and in particular in the blue range of the spectra, when expecting residuals at the level of few tens of milli-arcseconds.

On-sky measurements of atmospheric dispersion – I. Method validation

ABSTRACT Observations with ground-based telescopes are affected by differential atmospheric dispersion due to the wavelength-dependent index of refraction of the atmosphere. The usage of an atmospheric dispersion corrector (ADC) is fundamental to compensate this effect. Atmospheric dispersion correction residuals above the level of ∼100 milliarcseconds (mas) will affect astronomical observations, in particular radial velocity and flux losses. The design of an ADC is based on atmospheric models. To the best of our knowledge, those models have never been tested on-sky. In this paper, we present a new method to measure the atmospheric dispersion on-sky in the optical range. We require an accuracy better than 50 mas that is equal to the difference between atmospheric models. The method is based on the use of cross-dispersion spectrographs to determine the position of the centroid of the spatial profile at each wavelength of each spectral order. The method is validated using cross-dispersed spectroscopic data acquired with the slit spectrograph UVES. We measure an instrumental dispersion of $\rm 47 ~ mas$ in the blue arm, and 15 and 23 mas in the two ranges of the red arm. We also measure a 4 per cent deviation in the pixel scale from the value cited in UVES manual. The accuracy of the method is ∼17 mas in the range of 315–665 nm. At this level, we can compare and characterize different atmospheric dispersion models for better future ADC designs.

Quantification of the expected residual dispersion of the MICADO Near-IR imaging instrument

ABSTRACT MICADO, a near-infrared imager for the Extremely Large Telescope, is being designed to deliver diffraction limited imaging and 50 microarcsecond (μas) astrometric accuracy. MICADO employs an atmospheric dispersion corrector (ADC) to keep the chromatic elongation of the point spread function (PSF) under control. We must understand the dispersion and residuals after correction to reach the optimum performance. Therefore, we identified several sources of chromatic dispersion that need to be considered for the MICADO ADC. First, we compared common models of atmospheric dispersion to investigate whether these models remain suitable for MICADO. We showed that the differential dispersion between common atmospheric models and integration over the full atmosphere is less than 10 μas for most observations in H band. We then performed an error propagation analysis to understand the uncertainty in the atmospheric dispersion as a function of atmospheric conditions. In addition, we investigated the impact of photometric colour on the astrometric performance. While the differential refraction between stars within the same field of view can be significant, the inclusion of an ADC rendered this effect negligible. For MICADO specifically, we found that the current optomechanical design dominates the residual dispersion budget of 0.4 milliarcseconds (mas), with a contribution of 0.31 mas due to the positioning accuracy of the prisms and up to 0.15 mas due to a mismatch between the dispersive properties of the glass and the atmosphere. We found no showstoppers in the design of the MICADO ADC for achieving 50 μas relative astrometric accuracy.

The impact of atmospheric dispersion in the performance of high-resolution spectrographs

ABSTRACT Differential atmospheric dispersion is a wavelength-dependent effect introduced by the atmosphere. It is one of the instrumental errors that can affect the position of the target as perceived on the sky and its flux distribution. This effect will affect the results of astronomical observations if not corrected by an atmospheric dispersion corrector (ADC). In high-resolution spectrographs, in order to reach a radial velocity (RV) precision of 10 cm s−1, an ADC is expected to return residuals at only a few tens of milliarcseconds (mas). In fact, current state-of-the-art spectrograph conservatively require this level of residuals, although no work has been done to quantify the impact of atmospheric dispersion. In this work, we test the effect of atmospheric dispersion on astronomical observations in general, and in particular on RV precision degradation and flux losses. Our scientific objective was to quantify the amount of residuals needed to fulfil the requirements set on an ADC during the design phase. We found that up to a dispersion of 100 mas, the effect on the RV is negligible. However, on the flux losses, such a dispersion can create a loss of ∼2 per cent at 380 nm, a significant value when efficiency is critical. The requirements set on ADC residuals should take into consideration the atmospheric conditions where the ADC will function, and also all the aspects related with not only the RV precision requirements but also the guiding camera used, the tolerances on the flux loss, and the different melt data of the chosen glasses.

Design and development of an adaptive optics system in visible and near-infrared for Inter-University Centre for Astronomy and Astrophysics 2-meter telescope

Robo-AO is the first robotic autonomous laser-guided adaptive optics (AO) system operating in the sky. It is a very economical AO system especially suitable for observations with 1- to 3-m class telescopes. A second Robo-AO system, which works both in the visible and near-infrared wavelengths, has been developed to improve the image quality of the 2-m diameter telescope at Inter-university Centre for Astronomy and Astrophysics Girawali Observatory in India. We present the optomechanical design and development of the Laser Guide Star Facility (LGSF) and the Cassegrain AO facility with various test results. Effects of different projection geometries of the LGSF have been discussed with modeling results. Comprehensive study of an atmospheric dispersion corrector with dispersion model and development of a generic software are elaborated with experimental results. Toward the end, AO loop test results in the presence of artificial turbulence generated in the laboratory are presented.

On-sky Closed-loop Correction of Atmospheric Dispersion for High-contrast Coronagraphy and Astrometry

Adaptive optic (AO) systems delivering high levels of wavefront correction are now common at observatories. One of the main limitations to image quality after wavefront correction comes from atmospheric refraction. An atmospheric dispersion compensator (ADC) is employed to correct for atmospheric refraction. The correction is applied based on a look-up table consisting of dispersion values as a function of telescope elevation angle. The look-up table-based correction of atmospheric dispersion results in imperfect compensation leading to the presence of residual dispersion in the point spread function (PSF) and is insufficient when sub-milliarcsecond precision is required. The presence of residual dispersion can limit the achievable contrast while employing high-performance coronagraphs or can compromise high-precision astrometric measurements. In this paper, we present the first on-sky closed-loop correction of atmospheric dispersion by directly using science path images. The concept behind the measurement of dispersion utilizes the chromatic scaling of focal plane speckles. An adaptive speckle grid generated with a deformable mirror (DM) that has a sufficiently large number of actuators is used to accurately measure the residual dispersion and subsequently correct it by driving the ADC. We have demonstrated with the Subaru Coronagraphic Extreme AO (SCExAO) system on-sky closed-loop correction of residual dispersion to <1 mas across H-band. This work will aid in the direct detection of habitable exoplanets with upcoming extremely large telescopes (ELTs) and also provide a diagnostic tool to test the performance of instruments which require sub-milliarcsecond correction.

Gemini/GMOS Transmission Spectral Survey: Complete Optical Transmission Spectrum of the Hot Jupiter WASP-4b

Abstract We present the complete optical transmission spectrum of the hot Jupiter WASP-4b from 440 to 940 nm at R ∼ 400–1500 obtained with the Gemini Multi-Object Spectrometers (GMOS); this is the first result from a comparative exoplanetology survey program of close-in gas giants conducted with GMOS. WASP-4b has an equilibrium temperature of 1700 K and is favorable to study in transmission due to its large scale height (370 km). We derive the transmission spectrum of WASP-4b using four transits observed with the MOS technique. We demonstrate repeatable results across multiple epochs with GMOS, and derive a combined transmission spectrum at a precision about twice above photon noise, which is roughly equal to one atmospheric scale height. The transmission spectrum is well fitted with a uniform opacity as a function of wavelength. The uniform opacity and absence of a Rayleigh slope from molecular hydrogen suggest that the atmosphere is dominated by clouds with condensate grain sizes of ∼1 μm. This result is consistent with previous observations of hot Jupiters since clouds have been seen in planets with similar equilibrium temperatures to WASP-4b. We describe a custom pipeline that we have written to reduce GMOS time-series data of exoplanet transits, and present a thorough analysis of the dominant noise sources in GMOS, which primarily consist of wavelength- and time-dependent displacements of the spectra on the detector, mainly due to a lack of atmospheric dispersion correction.

A High-precision Technique to Correct for Residual Atmospheric Dispersion in High-contrast Imaging Systems

Direct detection and spectroscopy of exoplanets requires high-contrast imaging. For habitable exoplanets in particular, located at a small angular separation from the host star, it is crucial to employ small inner working angle (IWA) coronagraphs that efficiently suppress starlight. These coronagraphs, in turn, require careful control of the wavefront that directly impacts their performance. For ground-based telescopes, atmospheric refraction is also an important factor, since it results in a smearing of the point-spread function (PSF), that can no longer be efficiently suppressed by the coronagraph. Traditionally, atmospheric refraction is compensated for by an atmospheric dispersion compensator (ADC). ADC control relies on an a priori model of the atmosphere whose parameters are solely based on the pointing of the telescope, which can result in imperfect compensation. For a high-contrast instrument like the Subaru Coronagraphic Extreme Adaptive Optics (SCExAO) system, which employs very small IWA coronagraphs, refraction-induced smearing of the PSF has to be less than 1 mas in the science band for optimum performance. In this paper, we present the first on-sky measurement and correction of residual atmospheric dispersion. Atmospheric dispersion is measured from the science image directly, using an adaptive grid of artificially introduced speckles as a diagnostic to feedback to the telescope’s ADC. With our current setup, we were able to reduce the initial residual atmospheric dispersion from 18.8 mas to 4.2 in broadband light (y- to H-band) and to 1.4 mas in the H-band only. This work is particularly relevant to the upcoming extremely large telescopes (ELTs) that will require fine control of their ADC to reach their full high-contrast imaging potential.

FORS2 observes a multi-epoch transmission spectrum of the hot Saturn-mass exoplanet WASP-49b

Context: Transmission spectroscopy has proven to be a useful tool for the study of exoplanet atmospheres, and has lead to the detection of a small number of elements and molecules (Na, K, H$_2$O), but also revealed that many planets show flat transmission spectra consistent with the presence of opaque high-altitude hazes or clouds. Aims: We apply this technique to the $M_P=0.38 M_{jup}$, $R_p=1.12 R_{jup}$, $P=2.78d$ planet WASP-49b, aiming to characterize its transmission spectrum between 0.73 and 1 $\mathrm{\mu}$m and search for the features of K and H$_2$O. Methods: Three transits of WASP-49b have been observed with the FORS2 instrument installed at the VLT/UT1 telescope at the ESO Paranal site. We used FORS2 in MXU mode with grism GRIS_600z, producing simultaneous multiwavelength transit lightcurves throughout the i' and z' bands. We combined these data with independent broadband photometry from the Euler and TRAPPIST telescopes to obtain a good measurement of the transit shape. Strong correlated noise structures are present in the FORS2 lightcurves, which are due to rotating flat-field structures that are introduced by inhomogeneities of the linear atmospheric dispersion corrector's transparency. We accounted for these structures by constructing common noise models from the residuals of lightcurves bearing the same noise structures, and used them together with simple parametric models to infer the transmission spectrum. Results: We present three independent transmission spectra of WASP-49b between 0.73 and 1.02 $\mu m$, as well as a transmission spectrum between 0.65 and 1.02 $\mu m$ from the combined analysis of FORS2 and broadband data. The results obtained from the three individual epochs agree well. The transmission spectrum of WASP-49b is best fit by atmospheric models containing a cloud deck at pressure levels of 1 mbar or lower.

大气色散对4 m望远镜成像分辨力的影响与校正

This paper counts and analyzes the effect of atmospheric dispersion on image resolution of 4 m optical telescope system. It is introduced that atmospheric dispersion will influence image resolution when the zenith angle is larger than 15°. With the zenith angle increasing,especially up to 45°,the image quality would be degraded greatly,so it is quite necessary that the atmospheric dispersion corrector( ADC) is applied to compensate atmospheric dispersion. By comparing the advantages and drawbacks of three modes for ADC: tilt mode,move mode and rotate mode,the glue prism pairs is used to eliminate the influence of atmospheric dispersion for 4 m telescope system,which would rotate with each other when the zenith angle changes. In 4 mtelescope optical system,the ADC is positioned in front of imaging elements of collimated optical layout. Then we analyze the relation between zenith angle and rotating angle of ADC. The largest wedge angle of the ADC is9. 65°,and the rotating accuracy is ± 0. 1°,having an effect of 1 /1 000 on resolution. So it is obviously concluded that the results satisfied the request of image resolution,and it is an effective way for the analysis of atmospheric dispersion and the application of ADC.

Design, Implementation, and On-Sky Performance of an Advanced Apochromatic Triplet Atmospheric Dispersion Corrector for the Magellan Adaptive Optics System and VisAO Camera

ABSTRACTWe present the novel design, laboratory verification, and on-sky performance of our advanced triplet atmospheric dispersion corrector (ADC), an important component of the Magellan Adaptive Optics system (MagAO), which recently achieved first light in December 2012. High-precision broadband (0.5–1.0 μm) atmospheric dispersion correction at visible wavelengths is essential both for wavefront sensing (WFS) on fainter guide stars, and for performing visible AO science using our VisAO science camera. At 2 airmasses (60° from zenith) and over the waveband 500–1000 nm, our triplet design produces a 57% improvement in geometric rms spot size, a 33% improvement in encircled energy at 20″ radius, and a 62% improvement in Strehl ratio when compared to a conventional doublet design. This triplet design has been fabricated, tested in the lab, and integrated into the MagAO WFS and the VisAO science camera. We present on-sky results of the ADC in operation with the MagAO system. We also present a zero-beam-deviation triplet ADC design, which will be important to future AO systems that require precise alignment of the optical axis over a large range of airmasses in addition to diffraction-limited broadband dispersion correction.

On-Sky Demonstration of a Fluid Atmospheric Dispersion Corrector

ABSTRACTThe first on-sky demonstration of a fluid atmospheric dispersion corrector (FADC) is presented using the Anglo-Australian Telescope at Siding Spring Observatory. The atmospheric dispersion correction was observed with a three-colour CCD camera at the telescope’s Cassegrain focus. The FADC contains a pair of immiscible fluids in a small glass container placed very close to the telescope focal plane. A pair of fluid prisms is formed and the apex of the two prisms varies with telescope zenith angle because of gravity. Three chemicals were identified and tested for this purpose. We experimentally measured the FADC dispersion properties versus zenith angle and it is shown that its dispersion follows the tan(Z) law. We have been able to observe 6 stars at different zenith angles and show that the FADC can correct atmospheric dispersion up to 1′′ at a zenith angle of 52° across the visible spectral range of 400–700 nm. It is demonstrated that an FADC can function as a passive atmospheric dispersion corrector without any moving parts. Our on-sky measurement results show excellent agreement with the optical ray-tracing model.