The precise determination of the instrumental response function versus wavelength is a central ingredient in contemporary photometric calibration strategies. This typically entails propagating narrowband illumination through the system pupil, and comparing the detected photon rate across the focal plane to the amount of incident light as measured by a calibrated photodiode. However, stray light effects and reflections/ghosting (especially on the edges of filter passbands) in the optical train constitute a major source of systematic uncertainty when using a at-field screen as the illumination source. A collimated beam projector that projects a mask onto the focal plane of the instrument can distinguish focusing light paths from stray and scattered light, allowing for a precise determination of instrumental throughput. This paper describes the conceptual design of such a system, outlines its merits, and presents results from a prototype system used with the Dark Energy Camera wide field imager on the 4-meter Blanco telescope. A calibration scheme that blends results from at-field images with collimated beam projector data to obtain the equivalent of an illumination correction at high spectral and angular resolution is also presented. In addition to providing a precise system throughput calibration, by monitoring the evolution of the intensity and behaviour of the ghosts in the optical system, the collimated beam projector can be used to track the evolution of the filter transmission properties and various anti-reflective coatings in the optical system.
|Original language||English (US)|
|Title of host publication||Observatory Operations|
|Subtitle of host publication||Strategies, Processes, and Systems VI|
|Editors||Alison B. Peck, Robert L. Seaman, Chris R. Benn|
|State||Published - 2016|
|Event||Observatory Operations: Strategies, Processes, and Systems VI - Edinburgh, United Kingdom|
Duration: Jun 27 2016 → Jul 1 2016
|Name||Proceedings of SPIE - The International Society for Optical Engineering|
|Conference||Observatory Operations: Strategies, Processes, and Systems VI|
|Period||6/27/16 → 7/1/16|
Bibliographical noteFunding Information:
MWC was supported by the National Science Foundation Graduate Research Fellowship Program, under NSF grant number DGE 1144152. CWS is grateful to the DOE Oce of Science for their support under award DE-SC0007881.