Analysis of normalized point source sensitivity as a performance metric for large telescopes

Abstract

We investigate a new metric, the normalized point source sensitivity (PSSN), for characterizing the seeing-limited performance of large telescopes. As the PSSN metric is directly related to the photometric error of background limited observations, it represents the efficiency loss in telescope observing time. The PSSN metric properly accounts for the optical consequences of wave front spatial frequency distributions due to different error sources, which differentiates from traditional metrics such as the 80% encircled energy diameter and the central intensity ratio. We analytically show that multiplication of individual PSSN values due to individual errors is a good approximation for the total PSSN when various errors are considered simultaneously. We also numerically confirm this feature for Zernike aberrations as well as for the numerous error sources considered in the error budget of the Thirty Meter Telescope (TMT) using a ray optics simulator. Additionally, we discuss other pertinent features of the PSSN, including its relations to Zernike aberration, RMS wave front error, and central intensity ratio.

Adaptive Optics for Astronomy

“…The missing budgeting capability of the encircled energy led Seo et al (2009) to propose the normalized point source sensitivity (PSSN) metric. The PSSN for AO characterization is defined as the ratio of the sum of an observed point source image intensity I PSF squared over that of a perfect imaging system working at the diffraction limit:…”

Estimation of normalized point-source sensitivity of segment surface specifications for extremely large telescopes

“…Therefore, the overall PSSN estimation is based on the multiplicative feature of PSSN [6]. A similar technique can be used for other science metrics.…”

“…We numerically install this full set of TMT M1 segments in our ray-tracing modeling tool, modeling and analysis for controlled optical systems (MACOS), from which we obtain the simulated wavefront OPD. Then, we compute PSSN using the OPD [6]. We repeat the above processes multiple times (Monte Carlo) to get averaged values of PSSN, PSSN b .…”

“…Therefore, we use the α SR value of 2∕rad 2 or 2 × 4π∕500 2 ∕nm 2 , which is the maximum possible value for the α value [6].…”

“…The PSSN is a recently developed performance metric that has been adopted for the Thirty Meter Telescope (TMT) and European Extremely Large Telescope for their error budgeting and performance evaluation [1][2][3][4]. This metric is directly related to the photometric error of background-limited observations so that it accurately represents the efficiency loss of science due to increased telescope observing time [5,6].…”