Background generated by moving beams on mirrors with temperature gradients and dust contamination

摘要

The aim of this report is to illustrate under what conditions a telescope system incorporating active optics or beam steering mirrors can induce an unacceptable IR (infrared) background noise into a SOFIA (Stratospheric Observatory for Infrared Astronomy) science instrument. This report is based on two previous reports, one by Wright, and another by Davidson and Erickson. As explained in these reports, infrared telescopes are 'choppers' to cancel background sky and telescope emission from the emission of an astronomical source. The result is an ac-signal at the frequency of the chop with thermal 'shot' noise contributions from the astronomical source, sky and telescope. At IR wavelengths the Noise Equivalent Power (NEP) due to the thermal shot noise from the telescope and sky is approximately one-millionth of their thermal emission. Many interesting astronomical sources have signals on the order of this NEP. Thus it is important to (1) substract the background emission and (2) not introduce any further noise while doing so. The report by Davidson and Erickson illustrated that a 'nodding plus chopping' routine can be successfully employed using a chopping secondary in a Cassegrain telescope configuration to cancel the sky and telescope background emission down to the NEP level of the sky and telescope. This idealized substraction was achieved without introducing noise, since the chop and nod mechanisms were assumed perfect and so each placement of the beam in the chopping/nodding sequence was reproduced exactly. However, in practice such accuracy is not possible; low level random motion of the beam on the primary (or any other mirror surface) occurs, introducing noise if the mirror surface has temperature gradients, defects of dust contamination. Active optics will also introduce noise to the signal in a similar way. We will qualitatively and quantitatively discuss the noise introduced by beam motion on a telescope mirror surface in the following sections. Sections 2 and 3 will estimate the noisse generated by mirrors with temperature gradients and dust contamination, respectively, and section IV will give examples illustrating how to use the information derived in sections 2 and 3.