OSCA — The Coronographic Mask Device for NAOMI

S. G. Els¹, S. Thompson², P. Doel², P. Jolley¹, C. Benn¹, M. Blanken¹, T. Gregory¹, R. Østensen¹, F. Prada¹, I. Soechting¹

1: Isaac Newton Group; 2: University College London


In May 2002 the adaptive optics system NAOMI on the WHT received an instrument upgrade: the coronographic mask device OSCA (Optimized Stellar Coronograph for Adaptive optics).

Coronography in astronomy intends to overcome huge brightness differences on small spatial scales —meaning high contrast imaging. Thus the main science driver for coronography is the investigation of the close enviroment of bright stellar objects, e.g. looking for faint companions or dusty material.

Seeing limited imaging techniques can only provide poor contrast ratios between the peak of the point spread function (PSF) and its wings. Employing adaptive optics (AO) techniques allows for diffraction limited images to be obtained in which the detectivity of faint objects/structures close to bright objects is greatly improved. It is important to note that even using AO the form of the PSF still contains a halo the size of the seeing disk on which the diffraction limited core is located. Adaptive optics is best suited for coronography as the amount of suppression achievable with a coronograph depends directly on the image quality (e.g. FWHM and Strehl ratio).

The coronograph OSCA has been designed and built at University College London by a team led by P. Doel. Here we give only a brief overview over the optical system of OSCA. For further details, please refer to the paper by Thompson et al. (2003). OSCA is permantly mounted on the optical bench of NAOMI. To deploy it into the light path a pneumatic system lifts OSCA from its parking position into the light beam (see Figure 1). Once in the lightpath, a mirror (a) picks up the converging beam coming from NAOMI and directs it onto the focal plane masks (b) and then onto the first off axis paraboloid (c). Currently six hard edged masks with sizes between 0.25" and 2.0" are installed. These masks are not fully opaque thus enabling good centering of the target behind them and also allowing good astrometry to be obtained as the light centre of the target can be well measured. In addition to these masks, two gaussian shaped masks with FWHM=0.5" and 0.6" for optical wavelengths, have been available since early 2003. All masks are deposited onto wedged substrates giving a circular field of view of approximatly 20" in diameter. After passing a Lyotstop (c) the beam leaves OSCA via an optical system (d) which conserves the focal point and f-ratio of the NAOMI beam. Therefore OSCA can be used with any instrument fed by the AO system.

Figure 1. Photograph of OSCA from above the NAOMI bench. The light path is indicated by the arrows. The dashed red line shows the lightpath without OSCA. The pink letters indicate the OSCA optical components and are described in the text. [ JPEG | TIFF ]

First on-sky tests show that OSCA allows contrast ratios to be overcome of about ΔH~8 mag compared to the peak intensity over a distance of 2" or a suppression of about 0.5 – 1 mag compared to the non-coronographic image. This is comparable to other coronographic systems on 4m class telescopes using AO systems. As OSCA is not a cooled coronograph it will not be used at wavelengths longer than H-band which is also the current limitation of INGRID used with NAOMI. Also, as OSCA is part of the NAOMI system the same restrictions apply for OSCA as for NAOMI observations. For details see the NAOMI webpage: http://www.ing.iac.es/Astronomy/instruments/naomi/index.html.

With OSCA, ING now offers to its users a coronographic device in conjunction with its adaptive optics system NAOMI. At present it can be only used for near infrared imaging using ING’s infrared camera INGRID. Observers interested in using OSCA can apply for time in the same way as for other instruments at ING.

With its move to the new temperature controlled Nasmyth station – GRACE – in early 2003 ING’s adaptive optics facility NAOMI is expected to perform better and with greater stability in the near future, positively influencing coronographic work.

ING is going to also offer a unique facility to combine AO-fed integral field spectroscopy with coronography. A new instrument for the adaptive optics system will be the integral-field-spectrograph OASIS, installed and commissioned during summer 2003. OASIS will receive an AO corrected input beam from NAOMI. OSCA can then be operated as a NIR imaging coronograph but as well as in the following instrument combination: NAOMI+OSCA+OASIS.

Updated OSCA informations are provided on its webpage: http://www.ing.iac.es/Astronomy//instruments/osca/index.html.¤

Figure 2 (left). Example of a star behind the 2² coronographic mask. As the mask is not fully opaque the stars light peak can still be seen and indicates in this case that the star is not perfectly centered behind the mask [ JPEG | TIFF ]. Figure 3 (right). Suppression test of OSCA. The upper line indicates a radial cut of a star without OSCA in the NAOMI beam. The relatively low signal to noise of that cut is due to the short integration time of that image. The lower line shows a radial cut through a coronographic image taken with OSCA. [ JPEG | TIFF ]

References:

• Thompson, S., Doel, P., Bingham, R., et al., 2003, SPIE Proc, 4839, 1085. [ First citation in text | ADS ]

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Email contact: Sebastian Els (sels@ing.iac.es)