# Light Curve or Reference Star question photometry

Analyzing TCrB star sequence I notice a discrepancy in the light curve (perhaps a misunderstanding on my part). I believe the light curve provides the differential magnitude of the selected variable (the first star selected) from the reference star in the light curve Y axis (which is the last star manually selected and in my case seqsetmag 10.55) . The desktop plot (attached) shows a different differential magnitude than the light curve (attached). I am thinking perhaps my understanding of the reference star selection/setting is incorrect?

In the â€śPlotâ€ť tab, you only see the curve of each star, independently of the others.
Therefore, you cannot compare manually with the light curve.

Thanks for the response, but can you help me understand why this is so. Again, shouldnâ€™t there be a rough correspondence between the ( reference star - variable star magnitudes) and the averages of the scatter distribution shown in the light curve plot? The desktop plot shows about a -0.4 to -0.8 difference in magnitude over the sample periods, yet the light curve shows approximately -1.5 to -2 difference between the two. The difference range is similar, approximately 0.5, but the absolute difference values are not. can you provide a bit more insight regarding this?

Hi Gary
There is a small mistake in your workflow. To understand, let me ask you two questions:
-why the ref star should be the last selected one?
-whatâ€™s the use of the other selected stars?

The photometry process in Siril is designed to allow the use of multiple comparison stars, which is necessary for the study of exoplanets.

For variable stars, only one ref star can be used.

And finally, yes, you will get one relative light curve wrt a reference. So this â€śdifferentialâ€ť curve will not account with the absolute magnitude you set. Note that It would not make sense if you use several comp stars (like in an exoplanets process).

Fred

Hi Fred,
Thankyou for this response. Your statement does help me see that I am not understanding Siril photometry correctly. As to your questions:

• I selected the reference star as the last in the sequence because that is the star Seqsetmag references when it re-adjusts/calibrates the other selected stars magnitudes? I use an AAVSO star table to select a known reference star, in this case the one closest to the variable TCrB whoâ€™s magnitude I wish to measure, and then seqsetmag to that star magnitude I read in the AAVSO table.

• The other stars are my check stars, whoâ€™s magnitudes are readjusted (as is the variable star). In this way I can inspect their magnitudes to see if they match up the AAVSO star table mags as a further check on my data collected.

Your note suggests (Iâ€™m guessing) that I am applying an incorrect process by selecting the additional check/comparison stars which Siril uses to calibrate the light curve as part of an exoplanet measurement process?) If so, I might have misread the exoplanet tutorial as I didnâ€™t get this understanding from it.

Gary

Hello. Next version will have a AAVSO export module: Light curves â€” Siril 1.3.0 documentation

You also have the formula used for the light curve.

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Thanks, this looks very helpful. Good to see the file export capability too.
Gary

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Hi Gary
One comment on the curves you send.
The black plot shows the mag of each star, corrected with the 10.55 mag offset.
According to this curves, you identified a difference between the ref (rose curve) and the variable (purple) curve.
The command seqset mag applied the offset you manually got to all the sequence. Perfect.

On the second plot (whith one), you performed a light curve process. The goal is to compare the variable star to a â€śsyntheticâ€ť reference star (variable minus synthetic). This star is computed to be the mean of the flux of all the comp stars you selected. Thatâ€™s why this is not only the last one. The light curve only shows the variation in magnitude (V-C).
As all your comp+check stars show a flat (mag vs time) behaviour, this leads to a constant offset.
And thatâ€™s why you found the -1.5 to 2.0 difference in this plot.

In short, the use of the command seqsetmag is not necessary if you perform a light curve process. This command does not set the ref star that is used in the LC process.

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Thank you both very much for your responses. The 1.3.0 docs and Fredâ€™s comments provided the clarification I needed!
Gary