Optimized in-flight absolute calibration for extended CMB surveys

¹ Università degli Studi di Milano, via Celoria 16, 20133 Milano, Italy
² INAF-Osservatorio Astronomico, via G. B. Tiepolo 11, 34131 Trieste, Italy
³ SISSA/ISAS, Astrophysics Sector, via Beirut 4, 34014 Trieste, Italy
⁴ IASF, CNR, via Bassini 15, 20133 Milano, Italy

Corresponding author: B. Cappellini, Benedetta.Cappellini@mi.infn.it

Received: 8 October 2002
Accepted: 4 June 2003

Abstract

Accurate measurements of the Cosmic Microwave Background (CMB) anisotropy call for high precision and reliability of the in-flight calibration. For extended surveys the CMB dipole provides an excellent calibration source at frequencies lower than ~200 GHz; however poorly known foreground emissions, such as diffuse galactic components, complicate the signal and introduce a systematic error in the calibration. We show that introducing a weight function that takes into account the uncertainty in the a priori knowledge of the sky, allows us to substantially improve the calibration accuracy with respect to methods involving galactic latitude cuts. This new method is tested for Planck-lfi radiometers at 30 and 100 GHz. On short time scales (less than 1 day) the absolute calibration of each channel can be recovered with an overall 1–2% accuracy. We also consider the effect of CMB anisotropy itself on the calibration, and find that knowledge of the CMB pattern on large scales is needed to keep the short-time scale calibration accuracy within 1%.