An Analysis Of Optical Pick–up In SCUBA Data

The Sub-millimetre Common User Bolometer Array (SCUBA) at the JCMT employs a chopping and nodding observation technique to remove variations in the atmospheric signal and improve the long term stability of the instrument. In order to understand systematic effects in SCUBA data, we have analysed single–nod time streams from across SCUBA’s lifetime, and present an analysis of a ubiquitous optical pick–up signal connected to the pointing of the secondary mirror. This pick–up is usually removed to a high level by subtracting data from two nod positions and is therefore not obviously present in most reduced SCUBA data. However, if the nod cancellation is not perfect this pick–up can swamp astronomical signals. We discuss various methods which have been suggested to account for this type of imperfect cancellation, and also examine the impact of this pick–up on observations with future bolometric cameras at the JCMT.