Output magnitudes are, on average, fainter than the input ones

[x12red]

Hi All,

I've been testing the pipeline on simulated exposures with MIRAGE and I noticed that no matter what I do (e.g., also adopting a different approach to subtract the background compared to the standard pipeline), the final output magnitudes are, on average, fainter (~0.15/0.2 mag) than the input ones.

So, I started to think that maybe I am missing something in how MIRAGE simulates sources starting from the source catalogue I give as input.

Did you notice this trend too?

Thanks in advance!

[bhilbert4]

Our tests in the past comparing input source magnitudes to output magnitudes have shown good agreement. What types of sources are these (point sources, galaxies, extended)? And what instrument/mode are you working in? Are the sources systematically fainter, or is there a spread of differences?

Sorry about the delay. JWST commissioning is taking up all of my time these days, so I haven't had a lot of time to think about Mirage.

[x12red]

Hi Bryan,

No worries at all. I can only imagine what it means to work in the JWST commissioning!

So, I am testing NIRCam adopting the imaging mode. That trend I see is true for module A and module B (all the detectors). Other people I work with have found the same trend. Yes, basically I find that my sources (which are point sources, galaxies, and extended - I am simulating HUDF + simulated sources) are systematically fainter than the input ones. To estimate the output magnitudes, I use the following keyword: PIXAR_SR. Then, I simply estimate mag_output = -2.5*log10(flux * 1E6 * PIXAR_SR) + 8.90.

So, I wonder if my mistake is in how I estimate output magnitudes...

[bhilbert4]

Hmm. That seems like a reasonable way to calculate output magnitudes. I guess one way to check would be to convert from countrate to magnitudes and see if you get the same answer. If you use the rate.fits file, the conversion that Mirage uses is here: https://github.com/spacetelescope/mirage/blob/master/mirage/utils/utils.py#L314 and the photfnu value can be pulled from the table here: https://github.com/spacetelescope/mirage/blob/master/mirage/config/NIRCam_zeropoints.list