Photometer Calibration via Simultaneous Night Sky Measurements
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Solution Overview
Problem
Calibrating large groups of photometers simultaneously and within a reasonable time is challenging due to differences in spectral response and dispersion among various photometers, complicating the calibration process, especially when used in different telescopes or the same telescope.
Innovation Solution
A method using simultaneous night sky measurements to calibrate photometers by comparing dark measurements between a reference and a photometer to be calibrated, applying constraints on Sun, Moon, and galactic plane elevation, and using a least squares algorithm to correct systematic errors, thereby reducing calibration errors.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If laboratory calibration method with integrating sphere is used, then calibration can be performed under controlled conditions, but calibration time is excessive and cannot be completed within reasonable time frame
Solution Approach 1:
The calibration method uses the night sky itself as the calibration source, eliminating the need for laboratory equipment and procedures. Each photometer calibrates itself by measuring the night sky brightness, allowing simultaneous calibration of multiple photometers without requiring time-consuming laboratory procedures.
Solution Approach 2:
The night sky serves as a universal calibration source that can simultaneously calibrate multiple different types of photometers with different spectral responses. This single calibration source replaces the need for separate laboratory calibration procedures for each photometer.
2Productivity
If multiple types of photometers are calibrated simultaneously, then productivity increases, but dispersion in measurements increases due to different spectral responses
Solution Approach 1:
The method changes the calibration parameter from laboratory-controlled monochromatic light to natural night sky spectrum. By accepting and adapting to the natural spectral distribution of the night sky, the method accommodates different photometer spectral responses without requiring precise matching, thus reducing dispersion while maintaining high productivity.
Solution Approach 2:
The calibration method accounts for the specific spectral response characteristics of each photometer type by using the night sky spectrum that naturally interacts with each detector's unique spectral sensitivity, allowing each photometer to be calibrated according to its local spectral characteristics.
3Measurement precision
If constraints on Sun, Moon, and galactic plane elevation are applied, then calibration accuracy improves by reducing spectral variability, but the period available for calibration is reduced
Solution Approach 1:
The method optimizes the calibration parameters (elevation constraints) to achieve the best balance between accuracy and availability. By setting specific elevation thresholds for the Sun, Moon, and galactic plane, the method identifies the optimal calibration windows when spectral variability is minimized while maximizing the number of available calibration nights.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This method achieves lower calibration errors, allowing for mass calibration of photometers in less than seven nights, with a calibration error of the order of a hundredth, and decouples the effect of original calibration errors from dispersion in simultaneous measurements.
Implementation Method 1
Two photometers, a reference (can be replaced by a calibrated photodiode for absolute calibration) and the one to be calibrated are attached to these ports, in which a uniform angular radiance that completely fills the effective field-of-view of the photometers is expected
Data Source
Figure 1
Figure 2A~2B
Figure 3A~3B
AI summary
The calibration method consists in the comparison of simultaneous dark measurements between the photometer to be calibrated and a reference one, to obtain the Zero Point of the former. Constrains on the elevation of the Sun, Moon, clouds and distance to the galactic plane are defined to reduce variability due to spectral differences between devices and the RANSAC method is applied to eliminate extreme values. Specifically, the method comprises the steps of: defining a first reference photometer; taking two simultaneous dark measurements (SDM) at the same place by a photometer and the reference photometer; filtering the SDMs; repeating the previous steps to obtain N filtered SDMs; fitting a line to the filtered SDMs with a least squares algorithm to extract inliers; and obtaining the mean and standard deviation of the inliers.