Study of the performance of the Atmospheric DispersionCorrectors from VLT/SPHERE.

Pascale Hibona, Arthur Viganb, Kjetil Dohlenb, Julien Millia, Julien Girarda, Jean-LucBeuzitc, and David Mouilletc

aESO, Alonso de Cordova 3107, Vitacura, Santiago, Chileb LAM, UMR6110, CNRS/Universit de Provence, B.P. 8, F-13376 Marseille Cedex 12, France

cUniversite Grenoble Alpes, IPAG, F-38000 Grenoble, France

ABSTRACT

SPHERE was installed at the UT3 Nasmyth focus of the Very Large Telescope in April 2014 and first lightoccurred on May 4th that year. Its common path and infrastructure (CPI) features two Atmospheric Disper-sion Correctors (ADCs) : a Near Infrared one, and a Visible one. In this paper, we give an overview of thecharacterization and performance of these ADC units obtained on sky.

Keywords: optics, SPHERE, ADC, Atmospheric Dispersion Corrector, Instrumentation

*P. Hibon, phibon@eso.org

1. INTRODUCTION

The atmospheric refraction is the bending of light due to the Earths atmosphere. It is a consequence of the

wavelength-dependent index of refraction of the atmosphere. The light from celestial objects is continuously

refracted on its way through the Earths atmosphere because it moves through a media with continuously varying

refraction index (caused by the variations of pressure, temperature and water vapor concentration with height).

As the refraction depends on the wavelength the effect is a deviation of the light beam from its original direc-

tion, and also a spectral widening of the beam, with shorter wavelengths being more refracted than longer ones.

The effect depends on the tangent of the zenith angle, being 0 at the zenith, and on altitude, humidity, and

wavelength. At large zenith angles, the differential refraction between red and blue can be several arcsecs. The

device that compensates for this effect is called an Atmospheric Dispersion Corrector (ADC). The image of a

star with a ground-based telescope without an ADC is a vertical spectrum, especially noticeable at large zenith

angles. With an ideal ADC this same image shows no dispersion. There are two basic requirements for an ADC:

the variable dispersion to compensate that of the atmosphere at a given zenith angle and the zero-deviation at

some mean wavelength Since this dispersion varies with the zenith angle, the correction is usually implemented

by installing two counter-rotating prisms to adjust the total refraction angle. Only two prisms cannot satisfy the

Figure 1. Layout of the SPHERE Common Path and Infrastructure.

zero-deviation condition. Thus, to satisfy the given requirements, an ADC is a set of four prisms in two cemented

doublet pairs. If the two refracting prisms are made of two different glasses with the same refractivity at the

central wavelength, the light passing through this pair of prisms at this central wavelength will have no deflection.

The Spectro-Polarimetric High-contrast Exoplanet Research (SPHERE) instrument (1) available on one of

the Unit Telescopes of the Very Large Telescope (VLT) saw its first light on May 4th 2014 and aims to unveil the

mechanisms of formation and evolution of planetary systems. It is equipped with a visible and an infrared arms

to send the light to different modules : the Integral Field Spectrograph (IFS), the infra-red dual band imager

and spectrograph (IRDIS) and an imaging polarimeter (ZIMPOL). The following papers give details on each

SPHERE modules : (2; 3; 4; 5). Figure 1 shows the position of both ADCs, one for the visible and one for the

infrared, in the SPHERE optical path. Table 1 enumerates their adopted requirements.

Table 1. Adopted ADC requirements

Humidity 0 to 95%Operational air temperature range +5C - +18CWavelength range - VADC 500 - 900 nmWavelength range - IADC 950 - 2320 nmZenith Distance 1≥ ZD ≥ 60degAirmass < 2Field of view - ZIMPOL 3 x 3 sq.arcsecField of view - IFS 1.77 x 1.77 sq.arcsecResidual dispersion in IR atmospheric bands < 1.2masResidual dispersion in IR dual imaging filter pairs < 0.3masResidual dispersion across the visible range < 10mas

2. OPTICAL AND MECHANICAL OF THE SPHERE ADCS

SPHERE has two ADCs, on visible (VADC) and one infrared (IADC). Each ADC consists of four prisms,

mounted in pairs in two independently rotating ADC units. The two prisms forming a pair are made of different

materials and mounted in opposite directions so that the beam deviation is compensated while creating the

required dispersion, simulating that of the atmosphere. In the IADC, the two units are identical. In the VADC

a slight difference is introduced for ghost control purposes.

Each ADC is located in a pupil plane and cannot be extracted from the optical path. One particular feature of

the ADC control law used in SPHERE is its capacity to compensate for the fixed instrumental dispersion due

to the wedge in the dichroic separator between the ZIMPOL path and the WFS path.

The coating designs for each ADC was chosen to ensure that it will minimize the stress on surface and minimize

the polarization for nominal incidence angle. Figure 2 presents the visible ADC (VADC) design and Figure 3

the design for the infrared ADC (IADC). SPHERE also requires modification of the standard ADC rotation law

due to two effects:

• Manufacturing errors. Slight manufacturing and misalignment errors of the ADC prisms cause a difference

between the theoretical and actual dispersion of each ADC unit. Their effects are non-negligible in the

context of SPHERE and will need to be compensated by the adjustment of the control law.

• Instrumental dispersion. The dichroic plate separating between WFS and ZIMPOL is prismatic in order

to avoid ghosts. This introduces a small dispersion which must be compensated by the ADC control law.

Residual dispersion is also present due to other compoenents in the system. The effect of these features

are of the order of 1-10mas. SPHERE specifies alignment and stability of the star on its coronagraphs to

0.5mas (goal 0.2mas), making correction of these effects mandatory.

3. ON-SKY CHARACTERIZATION AND SCIENTIFIC PERFORMANCE

3.1 Infrared ADC

We studied two different kinds of datasets to understand the performance of the IADC.

First we looked at IFS commissioning data. Stars were observed without coronagraph, in direct imaging with

Figure 2. Visible ADC design. The glasses and dimensions are indicated on the figure.

Figure 3. Infrared ADC design. The glasses and dimensions are indicated on the figure.

the IFS extended mode using the YJH prism to disperse the light from each lenslet. This covers the wavelength

range : 0.9 to 1.7 microns. These datasets used to study the IADC performance when used with the IFS are

presented in the Table 2.

Table 2. Summary of the datasets for the IFS IADC performance.

Target Obs. Date Alt. angle SPHERE Config. Obs. Conditions(deg) [Seeing, Tau0, Winds]

HD1160 2014-08-13 60 IFS-YJH 0.6” - 3ms - 6.9m/sHD140573 2014-08-08 55 IFS-YJH 1.2” - 1.4ms - 4.6m/sHD140573 2014-08-08 45 IFS-YJH 0.8” - 2ms - 3.9m/sHR591 2014-10-10 50 IFS-YJH NA - 3ms - 11.3m/sHR7581 2014-07-18 72 IFS-YJH 1.1” - 3ms - 5.7m/sHR8799 2014-08-12 41 IFS-YJH 0.86” - 2.8ms - 13.4m/sgammaSgr 2014-08-09 84 IFS-YJH 0.76” - 2.7ms - 6.08m/sPZTel 2014-08-08 64 IFS-YJH 0.98” - 1.7ms - 1.85m/sTauCeti 2014-08-16 70 IFS-YJH 0.72” - 4.4ms - 4.9m/s

Figure 4. Variation of the centroid for stars observed in IFS YJH direct image mode at different elevations.

These datasets were reduced using the SPHERE/IFS reduction pipeline described in (6). The centroids

were determined on the direct image final datacubes using SExtractor (7). We were then able to determine the

residual of the atmospheric dispersion which corresponds to the variation of the centroid position as a function

of wavelength. Figure 4 shows the comparison of the centroid dispersion for several stars observed at different

elevations. These datasets need to be carefully compared as the weather conditions could be different during

these observations.

Between 1350nm and 1500nm, the expected effect due to atmospheric absorption can be seen for each target.

The SPHERE specifications require a residual dispersion inferior to 1.2mas until an airmass of 2.

In Figure 5, we compare the on-sky performance at ZD=51.6 deg and ZD=60.6deg to a model. In both cases,

the performance obtained are mostly within the nominal operating range for a zenith distance inferior to 60deg.

The right panel of Figure 5 shows the extreme case where the ADC was used outside of its operating range and

therefore shows the maximum dispersion.

To understand and complete the characterization of the IADC, IRDIS Long-Slit Spectroscopic data observed

with the very wide filter covering the YJHK wavelength range should be taken and analyzed. We could also

design a test for observing the same star at different altitude first and then repeat the test with different wind

Figure 5. Comparison between the model and the data for two star observed at ZD=51.6deg and ZD=60.6deg in IFSmode. The obtained datapoints are in agreement with the model. The IADC has been designed to work until a zenithdistance of 60deg. The right figure, showing the model for a zenith distance of 60.6deg, present then the maximumdispersion which can be produced by the IADC.

speed and other weather conditions to understand and constrain better the results we would obtain.

3.2 Visible ADC

The Visible ADC is only used when observing with ZIMPOL, the imaging polarimeter. The datasets used

to understand the VADC performance were obtained during the last SPHERE commissioning. The following

strategy was applied :

• Observe the image positions in different ZIMPOL filters

• Compare the image positions with flat-field coronagraph positions to eliminate the filter prism effect

• Use a constant reference filter in the second ZIMPOL channel to eliminate temporal evolution error

The data were reduced using the SPHERE DRH pipeline described in (8).

It is important to note that the data used for the VADC characterization were affected byt the known low-

wind effect which deteriorates the PSF of the images, especially in visible bands. Figure 6 shows the centroid

error obtained in both axis in function of wavelength. Different position angle were tested. As mentioned in

Table 1, the residual dispersion in this observing mode should be smaller than 10mas. From the Figure 6, we

can conclude that this specification is respected. However, to be able to obtain characterize better the VADC,

Elevation Sky - 0deg

Sky - 90deg Sky - 180deg

Figure 6. Variation of the centroid for stars observed in IFS YJH direct image mode at different elevations and positionangles.

it would be very useful to obtain data not affected by the low-wind effect and therefore measure with better

precision the residual dispersion on ZIMPOL data.

4. CONCLUSION

The data we have currently in our hands for characterizing the visible and infrared ADCs allows us to confirm

that both ADCs are adjusted properly and are not affecting the data. Both give results within the required

specifications up to a zenith angle of 55deg approximately. However, more data could be useful to constrain

better the ADC performance and the wind effect, amongst others, on them.

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List of Figures

1 Layout of the SPHERE Common Path and Infrastructure.

2 Visible ADC design. The glasses and dimensions are indicated on the figure.

3 Infrared ADC design. The glasses and dimensions are indicated on the figure.

4 Variation of the centroid for stars observed in IFS YJH direct image mode at different elevations.

5 Comparison between the model and the data for two star observed at ZD=51.6deg and ZD=60.6deg

in IFS mode. The obtained datapoints are in agreement with the model. The IADC has been

designed to work until a zenith distance of 60deg. The right figure, showing the model for a zenith

distance of 60.6deg, present then the maximum dispersion which can be produced by the IADC.

6 Variation of the centroid for stars observed in IFS YJH direct image mode at different elevations

and position angles.

List of Tables

1 Adopted ADC requirements

2 Summary of the datasets for the IFS IADC performance.

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