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Problems applying SE++ to a VIS coadd image #469
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Top left: residual From the residual it looks like the model fitting is not completely breaking down, the negative pixel value in the centre is around a quarter of the flux of the original image - so the model is 25% too high in the centre - not too unusual I guess. |
I have done some tests last Friday, the model fitting is clearly happening so it's not like something is completely wrong. Overall the model check image looks mostly correct by eye, just a bit off in flux. Could it be a PSF related problem? Maybe the variable PSF is not being correctly applied? |
As discussed in the telecon, I ran the dataset with 3 different PSF models (Gaussian, psfex, coadded PSF). The offsets are similar in all three cases. In another test I used a constant RMS image with the background RMS value. Did not help. |
I did a fairly complete parameter study and changed, from a baseline solution, all conceivable parameters that could have an effect on the fitting (can provide a protocol if desired). The only parameter that changed the photometry offset significantly was changing |
It's really persistent isn't it.... Depending on how the coadd was constructed, you might want to set the gain to Because the counts are scaled to 1s of exposure in many coadd images. |
Happy to check that out if I know exactly what and how. The image indeed has an effective exptime of 1s. There are 4 exposure with 565s each, that would mean: Would that be correct (that's quite a high value, similar to inf.=0.0)? |
This seems like the answer. |
Emmanuel should probably check this, but I think what you want is, poisson_noise = numpy.sqrt(numpy.fabs(img_data)*exp_time*gain)/(exp_time*gain) total noise = numpy.sqrt(poisson_noise**2 + background_rms**2) |
If |
With this formula:
Shouldn't there be the exposure time in that equation? |
Emmanuel's formula is a simplification of the one I wrote, where he's using the effective gain, (effective) gain = gain * exposure time So it is in there implicitly. Does that clear it up, or where you thinking it should appear somewhere else? |
No, exposure time is out of the equation because the |
When fitting a bulge+disk model to VIS coadded data the comparison between auto_mag (== TU mags) and the fitted mags looks like this:
The PSF is a bit naive (2.0pix Gaussian). I tried a lot but could not significantly improve the offset(s) and I wonder whether the known deficiencies in the data and the setup can explain the large offsets.
The data is here:
https://deepdip.iap.fr/#folder/624e9eeb28cafb12e8553f0c
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