YZ Ret - MGAB-V202 - Strange behaviour or wrong photometry?

Sun, 10/09/2022 - 21:44

Hello everybody,

I'm quite new on reporting observations in AAVSO.

Some weeks ago, I performed a CCD observation of YZ Ret (also known as MGAB-V202) with V, B, G and I filters. The data matches the data from other observers pretty well.

I perfomed another observation the 3rd of October, and now I've analyzed it. The obtained results are hard to believe. Not only the magnitudes have increased noticeably, but also the "colour", as the difference between magnitudes on each filter are completely different than they used to be.

I'm pretty sure that my lack of experience has made me mess it up somehow, but I can't find the mistake. I've tried changing the reference variable star, but the result is the same, and also I've double checked that I'm analyzing the desired target, which I am pretty sure it is.

Could you please check if it's a real result and there's something going on with this star? Or if I had the bad luck to get a different object (an asteroid?) just passing in front of my star? Or if I made a wrong photometry analysis somehow?

If anyone is interested I can submit the .fits files.


American Association of Variable Star Observers (AAVSO)
YZ Ret obs

Hi David,

Welcome to joining the AAVSO data contributors!

Looking at the data in the LCG, the last obs do stick out. That always spooks me. Good that you checked.
https://www.aavso.org/LCGv2/static.htm?DateFormat=Julian&RequestedBands=&Grid=true&view=api.delim&ident=yz ret&fromjd=2459132&tojd=2459862.472&delimiter=@@@

The airmass for all the observations is pretty high (2.8) so maybe conditions sensitive?

Are the reported CMAG and KMAG values the instrumental values? Most software will generate instrumental values that are negative.

One test is to compare your measured standardized K magnitude to the reference value from the chart. Reasonable match?

Strange result?: Collect more data!

If you can load the images into VPhot and share them to SGEO and/or MZK, we can take a look.





Hi George,Thanks for your…

Hi George,

Thanks for your answer.

They definitely stick out, and that's why I'm also reluctant about if the results are correct.

I didn't include the link to the chart as I was unsure it was a correct procedure in this forum.

This is how I report my data, let me know if I'm doing anything wrong:

- Reported Comparison Magnitude is the one shown in the chart.

- I calculate the difference between the Comparison Magnitude and the magnitude provided by the software. Apparently there are no stars overlapping that may affect the result.

- I calculate Check Magnitude by adding the obtained magnitude difference (taking into account the sign of this difference) to the magnitude calculated by the software. The difference between this Check Magnitude and the one reported in the chart is around 0.1 - 0.2 magnitudes. I understand that it's a bit higher than what it should (given the small magnitude error of less than half this value), but nothing too spooky.

- Finally, I calculate the target star magnitude (KMAG?) using the same procedure as the Check Magnitude. But the offset compared with the previous observations is huge (0,5 - 2 mag depending on the filter). I triple-checked that I'm adding the magnitude difference with the correct sign.

I'll try to load the images into VPhot this evening (European time).


Edit 10/10 21:15 (UTC+1): FITS images are in queue to be processed in VPhot with the title "MGAB-V207_8294091". I noticed that the target star is just above a row of dead pixels in all the images, so I guess that's the issue here. I thought that this dead row would reduce the aparent brightness (increasing the magnitude), not the opposite... If you confirm that this is the source of error, I'll delete the reported and suspicious observations.

Wrong photometry - observations deleted


I've just analyzed another observation of a different star, and I'm having exactly the same issue, so I have the strong feeling that the line of dead pixels just in the middle of the file (where the target star lies) is affecting the photometry.

I've deleted the suspicious observations and I'm not reporting the one that I've just analyzed.

As I'm using an array of telescopes, I'll just deselect the telescope with this issue for next observations.

Thanks for your help!