Seeing-limited imaging sky surveys -- small vs. large telescopes

TL;DR

This paper analyzes the efficiency of small versus large telescopes for sky surveys, introducing metrics like grasp and information-content grasp, and argues that technological advances favor multiple small telescopes over single large ones for seeing-limited observations.

Contribution

It provides an analytic framework for comparing telescope survey capabilities and advocates for small telescope arrays enabled by recent technological advancements.

Findings

Multiple small telescopes can be more cost-effective than a single large telescope for seeing-limited surveys.

Optimal exposure time in background-dominated noise is three times the dead time.

Technological advances like large-format CMOS detectors support the use of small telescopes for wide-field surveys.

Abstract

(Abridged) Typically large telescope construction and operation costs scale up faster than their collecting area. This slows scientific progress, making it expensive and complicated to increase telescope size. A metric that represents the capability of an imaging survey telescopes, and that captures a wide range of science objectives, is the telescope grasp -- the amount of volume of space in which a standard candle is detectable per unit time. We provide an analytic expression for the grasp, and also show that in the background-dominated noise limit, the optimal exposure time is three times the dead time. We introduce a related metric we call the information-content grasp, which summarizes the variance of all sources observed by the telescope per unit time. For seeing-dominated sky surveys, in terms of grasp, etendue, or collecting-area optimization, recent technological advancements make it more cost effective to construct multiple small telescopes rather than a single large telescope with a similar grasp or etendue. Among these key advancements are the availability of large-format back-side illuminated CMOS detectors with <4 micron pixels, well suited to sample standard seeing conditions given typical focal lengths of small fast telescopes. We also discuss the possible use of multiple small telescopes for spectroscopy. We argue that if all the obstacles to implementing cost-effective wide-field imaging and multi-object spectrographs using multiple small telescopes are removed, then the motivation to build new single large-aperture (>1m) visible-light telescopes which are seeing-dominated, will be weakened. These ideas have led to the concept of the, currently under construction, Large-Array Survey Telescope (LAST).