Suppressing speckle noise for simultaneous differential extrasolar planet imaging (SDI) at the VLT and MMT

We discuss the instrumental and data reduction techniques used to suppress speckle noise with the Simultaneous Differential Imager (SDI) implemented at the VLT and the MMT. SDI uses a quad filter to take images simultaneously at 3 wavelengths surrounding the 1.62 μm methane bandhead found in the spectrum of cool brown dwarfs and gas giants. By performing a difference of images in these filters, speckle noise from the primary can be significantly attenuated, resulting in photon noise limited data. Non-trivial data reduction tools are necessary to pipeline the simultaneous differential imaging. Here we discuss a custom algorithm implemented in IDL to perform this reduction. The script performs basic data reduction tasks but also precisely aligns images taken in each of the filters using a custom shift and subtract routine. In our survey of nearby young stars at the VLT and MMT (see Biller et al, this conference), we achieved H band contrasts >25000 (5σ ΔF1(1.575 μm) >10.0 mag, ΔH > 11.5 mag for a T6 spectral type object) at a separation of 0.5″ from the primary star. We believe that our SDI images are among the highest contrast astronomical images ever made from ground or space for methane rich companions.