7. Magnitude fitting

Command: wisp-fit-magnitudes

Important Parameters

• single-photref-dr-fname

• correction-parametrization

• magfit-catalog-max-magnitude

Description

In ground based applications, the night sky is imaged through variable amount of atmosphere, which itself is subject to changes (i.e. clouds, humidity, etc.). In addition various instrumental effects are generally present. The purpose of the magnitude fitting step is to eliminate as much as possible effects that modify the measured source brightness within an image in a manner that depends smoothly on the properties of the source.

In short, a reference frame is selected (and later generated). Then for each individual frame (target frame from now on) a smooth multiplicative correction is derived that when applied to the brightness measurements in the target frame matches the brightness measurements in the reference frame as closely as possible.

In the pipeline this is actually done multiple times. The first time, a single frame which appears to be of very high quality (sharp PSF, high atmospheric transparency, dark sky etc.) is used as the reference frame. The corrected brightness measurements of the individua frames are then stacked to produce a much highe signal to noise “master reference frame”, which is then used in a second iteration of the magnitude fitting process and so on.