Estimated observing efficiency for past and current telescopes

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Colin Bischoff, 2018-09-24


In this posting, I try to estimate the relative observing efficiency for telescopes at South Pole vs Chile. It is hard to get a clean answer to this question because every experiment has its own unique circumstances and there are a limited number of data points to examine.

The method I will use here is to compare a survey weight (units of μK-2) calculated from published BB bandpowers to a survey weight calculated from instantaneous sensitivity and observing time. Note that survey weight is the quantity that should scale linearly with efforts, so the survey weight at 150 GHz for the BK14 paper is equal to the BICEP2 2010--2012 survey weight plus Keck Array 150 GHz survey weight for 2012--2014.

  • The "bandpower weight" is easier to define unambiguously -- in a previous posting I calculated the N and effective fsky for many different experiments that have published BB results. From these results, I calculate the bandpower weight as fsky / N.
  • The "tod weight" is calculated from instantaneous sensitivity (NEQ) of the full experiment and observing time (τ) as τ / NEQ2. While this definition is quite simple, there are many possible choices for how to select τ and it can be difficult to do this in a consistent way across experiments.

The idea behind these statistics is that tod weight describes the experiment on paper, i.e. "I will put together an array of detectors with NEQ = 15 μK s1/2 and then observe for three years". The bandpower weight describes the results that were actually obtained, including data cuts, instrument downtime, filtering, inefficiencies in sky coverage, etc, etc. Note however that I am using actual array NEQ as reported in published results to calculate tod weight, so detector yield, noisy detectors, and increased NEQ from marginal weather all get baked into the tod weight to some extent and should not lead to a discrepancy between the two statistics.

Figure 1 is a plot of the ratio of tod weight to bandpower weight for BICEP/Keck, ACTpol, ABS, and QUIET. I didn't include SPTpol or POLARBEAR because I couldn't array NEQ numbers for those instruments. Points are color-coded according to observing band (red for 95 GHz, green for 150 GHz, and blue for 220 GHz). A larger value of the weight ratio (y-axis) means that the statistical power of the bandpower result fell short of what we might expect from the instrument sensitivity and time on sky. For most experiments I include two points (connected by a line) that make different choices for how to define observing time. See below the figure for details.

Figure 1

...posting still in progress...