Constant background value

[louisquilley]

Hello, when using --background-value, the background map has the constant input level, as expected, but it seems that the modelling does not take it into account. Indeed, we should have image_data - background = model + residual, for the flux in each pixel. It works when using --background-cell-size but not with --background-value. The offset can be as large as 70% of the background-value, which is also 1/6 of the peak model in my example.

We noticed this issue because varying the background-value had little impact on the residuals, whereas a similar variation in the constant background with SExtractor led to some satisfying residuals for some background value.

I can provide you with the files in which I saw this error if needed.

Thank you.

[marcschefer]

Yes, that is by design.

The --background-value parameter affects the background on the detection image. A fixed background for the measurement images can be set for each individual image by using the constant_background parameter in the python configuration file when loading them.

I think this needs to be added to the documentation.

Thank you for your feedback

[louisquilley]

Hello, thank you for your answer.

We managed to force the constant background and find the expected result by adding the constant_background parameter in load_fits_images().

I would like to confirm one point : when specifying --background-cell-size, it is used both for the detection image and the measurement images, is that right ?

Thank you in advance.

[marcschefer]

Hi,

Yes, I checked the background analyzer is always configured the same for the detection and measurement images, including cell-size.