The ACS Coronagraph

Ardila, D.R., Krist, J., Hartig, G., Clampin, M., Golimowski, D.A., Ford, H.C., and Illingworth, G.D., Burrows, C., Bartko, F., Benitez, N., Blakeslee, J.P., Bouwens, R., Broadhurst, T.J., Brown, R.A., Cheng, E., Cross, N., Feldman, P.D., Franx, M., Gronwall, C., Infante, L., Kimble, R.A., Lesser, M., Martel, A.R., Menanteau, F., Meurer, G.R., Miley, G., Postman, M., Rosati, P., Sirianni, M., Sparks, W.B., Tran, H., Tsvetanov, Z.I., White, R.L., and Zheng, W.

The Advanced Camera for Surveys provides a selectable coronagraphic mode that allows high contrast imaging of faint objects near bright stellar-like sources. The coronagraph operates in the aberrated beam of HST before it is corrected by the ACS optics. This requires the use of relatively large (D=1.8" and 3") occulting spots. On-orbit measurements show that the coronagraph suppresses the diffraction pattern of the central object to a level below that of the scattered light caused by imperfections in the HST optical surfaces, resulting in a 10x reduction in PSF wing brightness. Using images of another star or the same object at a different orientation of the telescope, it is possible to subtract most of the remaining light, resulting in a total PSF wing supression of 500x-5000x. The high resolution, multibandpass capabilities of the ACS coronagraph will allow more detailed observations of circumstellar and circumnuclear material than was possible with previous HST instruments.

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