SN2025mg

Hello @all

I should receive soon a set of RAPAS filter (thanks @thierrymidavaine), I still take some images of the new transients and SN :

SN2025mg :

Mag (R) measured using tycho tracker 16.11 +/- 0.08 (using ATLAS (R) catalog)

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DSS image :

SN 2025mg is a type II supernova detected by ZTF on 2025-01-15 07:48. More details are available here: Astro-COLIBRI

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Latest lightcurve from ATLAS / ZTF :

How to you measure acuratly SN magnitude when source background is not a plane ?

For stars, using tycho and this aperture configuration :

When doing photometry on reference stars from ATLAS in the right filter, I get results here the error distribution is coherent with the standard deviation of the measure (it has to be, it is estimated using these stars…)

But when measuring a SN like SN 2025mg using the same aperture diameter :

I get results that are wayyyy of what ATLAS and ZTF are giving in the lightcurves :

MAG (R) 15.412 +/- 0.009

@marc.serrau2 @roger.hellot.astro @Fabian.Schussler do you have any advices ?

Hi !
I’m not very familiar with using Tycho-tracker’s photometry module. I mostly use AMJ (AstroimageJ). But I am surprised to see in your first image a star in the sky anulus because this anulus is used to measure the background of the sky which is cut off from the brightness of the main aperture, the star in the circle. Beside the ratio of the two gives the signal-to-noise ratio. It is true that some software removes the value of this star from the ring, but I will remain very cautious.

In the example of SN2025mg, I will shrink the star’s aperture ring to 1.5 times the FWMH, or even a little less, at the closer, and I will significantly increase the diameter of the dead zone to have a very clean ring of sky, with no stars or galaxies inside. It seems feasible to me. The aim is to get the value of the brightness of the sky background of the image. In addition, in this specific case, it is difficult to subtract from the star light the relative part of the galaxy’s background.
But the diameter of the circles have no reason to be in equal and identical proportions in all cases. We have to adapt their diameter to all particular situations.
Roger

Thank you Roger,


MAG(R) : 16.031+/- 0.049. → still strangely high

It seems odd to me to proceed this way, won’t the galactic disc will participate a lot in the flux ? but maybe I don’t get every detail of your technique

Hi,
It looks much better for me. Perhaps, you can try to reduce the diameter of the external disk of the anulus, in a way that you will have less stars inside. Depending of the number of pixels you have for the middle star, a number of pixels in the anulus 10-100 times more could be enough to get a good S/N ration, what is the aim. Do not forget that the aim is not to get an absolut value of the magnitude, what is quite impossible, but a relative value which could be compared with that of tomorrow. Keep in mind to use tomorrow the same diameters. What could be very efficient is to measure 15 images of 15 days observations at the same time. It can give you a very significant curve.
Let me know you results, because you case is interesting and recurrent.
Roger.

Perhaps you can send me an original calibrated image with that small galaxie and I will analyse it with AMJ. It will give you a chart of what we are talking about.
Roger

Thanks, PM sent with a link to transfert the calibrated images

Crazy idea : but let’s try

Photometry on the Star output of StarNet (https://starnetastro.com/)

How bad could it be ?

So on stars, using the StarNet AI to remove all but the stars is making an increase off 10% of the photometry error, but the photometry measure of the SN in the image was significantly changed to a lower magnitude !!!