A new method of CCD dark current correction via extracting the dark information from scientific images

TL;DR

This paper introduces a novel method to derive and apply dark current correction frames directly from scientific CCD images, effectively reducing noise and improving photometric accuracy when traditional dark frames are unavailable.

Contribution

The authors present a simple, effective technique to extract dark current information from scientific images, enabling accurate correction without dedicated dark frames, especially useful at high CCD temperatures.

Findings

Dark current correction reduces noise to sky photon noise levels

Method improves photometric precision significantly

Applicable to other projects with similar constraints

Abstract

We have developed a new method to correct dark current at relatively high temperatures for Charge-Coupled Device (CCD) images when dark frames cannot be obtained on the telescope. For images taken with the Antarctic Survey Telescopes (AST3) in 2012, due to the low cooling efficiency, the median CCD temperature was -46$^\circ$C, resulting in a high dark current level of about 3$e^-$/pix/sec, even comparable to the sky brightness (10$e^-$/pix/sec). If not corrected, the nonuniformity of the dark current could even overweight the photon noise of the sky background. However, dark frames could not be obtained during the observing season because the camera was operated in frame-transfer mode without a shutter, and the telescope was unattended in winter. Here we present an alternative, but simple and effective method to derive the dark current frame from the scientific images. Then we can scale this dark frame to the temperature at which the scientific images were taken, and apply the dark frame corrections to the scientific images. We have applied this method to the AST3 data, and demonstrated that it can reduce the noise to a level roughly as low as the photon noise of the sky brightness, solving the high noise problem and improving the photometric precision. This method will also be helpful for other projects that suffer from similar issues.