Cable Reflections in Autocorrelation Delay Spectra

Abstract

For the Cosmic Dawn key science goal of detecting the redshifted 21cm signal around redshift z=17 with OVRO-LWA-352, we wish to minimize spectral structure in the uncalibrated bandpass of the instrument. An effective method to quantify bandpass spectral structure is to use the delay spectrum of the bandpass. The delay spectrum is the Fourier transform along frequency of the bandpass. In this memo, we briefly review the delay spectrum as it pertains to redshifted 21cm Cosmic Dawn measurements and then analyze existing OVRO-LWA autocorrelation spectra as a proxy for bandpass information of the current system. Any uncorrected bandpass structure in the telescope, whether from the chromaticity in the antenna beams or from the signal chain, will convolve with the intrinsic foreground delay spectrum, spreading the foregrounds to higher delay modes and causing more contamination with the 21cm signal. Thus, one of the primary goals for 21cm instrument design is to minimize the instrument's bandpass structure. While sky-based calibration can correct some bandpass structure, minimizing the instrument's intrinsic structure alleviates burden on the calibration. We expect the 21cm fluctuations to be ~50 dB below the total foreground power during Cosmic Dawn, although this is dependent on the frequencies and spatial scales that will be probed. Some explanations for the EDGES absorption profile predict larger 21cm fluctuations. Nevertheless, we would ideally set a requirement that instrumental bandpass structure be suppressed be at the -60 dB level on the spectral scales that correspond to 21cm fluctuations.

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