Light curves

In astronomy, a light curve is a graph of light intensity of a celestial object as a function of time, typically with the magnitude of light received on the y axis and with time on the x axis. Siril is able to generate such curves when analyzing stars.

There are now two ways of selecting the variable and references (also called comparison) stars: manually, or using a list of stars obtained by the N.I.N.A. exoplanet plugin.

Manual star selection

Start by selecting stars and running photometry analysis on the sequence for each, as explained here.

One star is the variable (purple with a V) and the 5 others are used as references.

Warning

Make sure to not select variable stars for references. If the astrometry is done on your image, do not hesitate to use the SIMBAD request to know more about the stars.

Tip

It is preferable to choose references whose magnitude is close to that of the variable.

Once done, Siril automatically loads the Plot tab as shown in the figure below. This shows FWHM curves expressed as a function of frame number.

The plot tab as showed right after the quick photometry on sequence.

What interests us in this part is to display the magnitude curves. Simply go to the drop-down menu and change FHWM to Magnitude. The magnitude curves of each analyzed star are then displayed. This also results in the button Light Curve being sensitive. It is also recommended to check the Julian Date button in order to plot magnitude as a function of a date.

Switching to magnitude view make the Light Curve button sensitive.

Once the analysis is completed with a number of reference stars of at least 4 or 5 (the higher the number, the more accurate the result), you can click on the Light Curve button. Siril will ask for a file name to save the data in csv format, then the light curve will be displayed in a new window. The csv file can of course be used in any other software or website to reduce the data.

Warning

As already mentionned, the software gnuplot must be installed to be able to see light curves.

Light curve of an exoplanet transit.

NINA exoplanet button

In order to automate the process of transit analysis of exoplanets, lists of reference stars, also called comparison stars, could be obtained from star catalogues, with the appropriate criteria: similar magnitude, similar color (to not change their relative magnitude with atmospheric extinction at different elevations), proximity.

The capture software N.I.N.A has an exoplanet plugin that will show such stars and allow the list to be saved in a CSV file, such as csv file:

Type,Name,HFR,xPos,yPos,AvgBright,MaxBright,Background,Ra,Dec
Target,HD 189733 b,2.6035068712769851,1992,1446,1640.3703703703704,39440,1917.0601851851852,300.18333333333328,22.709722222222222
Var,SW Vul,2.8626145609282911,2972,276,26.14,2012,1905.445,300.02171,22.93517
Var,DQ Vul,2.372369130017419,3006,1040,28.180555555555557,2048,1906.9027777777778,300.01254,22.78103
Var,HQ Vul,3.8351043206620834,157,1690,49.393939393939391,2104,1905.7454545454545,300.55808,22.64067
...
Comp1,ATO J300.3222+22.7056,2.4268101078425852,1367,1465,352,4496,1913.9504132231405,300.32229415181337,22.705681453738887
Comp1,HD 189657,2.5343988482845927,2527,2808,23.814814814814813,2012,1906.5061728395062,300.08714683055996,22.4400393728
...
Comp2,000-BJP-946,2.2738807043120195,1832,750,29.962962962962962,2024,1910.0648148148148,300.23741666666666,22.846999999999998
Comp2,000-BJP-942,2.0977710589704297,2760,1572,31.083333333333332,2096,1908.6527777777778,300.025875,22.704777777777778
...

In the Plot tab, Siril can load this file using the NINA exoplanet button. To use this, a few prerequisites must be met:

• the sequence of calibrated images must be already loaded

• the reference image of the sequence must be plate solved, to make sure we identify the correct stars from their equatorial J2000 coordinates

• gnuplot is installed to create or show the light curve, otherwise only the data file will be created.

From there, everything is automatic, showing the light curve for the selected comparison stars at the end of the process.

The following video shows an automated processing of light curve with comparison star list from NINA:

Commands and automatic operation

It is also possible to automate or create the light curve remotely using the light_curve command. As blind operation needs as much automation as possible, the configuration of the background annulus radii can be automated with the -autoring argument: it runs a star detection in the reference image and multiplies the mean FWHM with a configurabe factor to obtain the inner and outer radii that should work with the sequence.

Siril command line

light_curve sequencename channel [-autoring] { -at=x,y | -wcs=ra,dec } { -refat=x,y | -refwcs=ra,dec } ...
light_curve sequencename channel [-autoring] -ninastars=file

Analyses several stars with aperture photometry in a sequence of images and produces a light curve for one, calibrated by the others. The first coordinates, in pixels if -at= is used or in degrees if -wcs= is used, are for the star whose light will be plotted, the others for the comparison stars.

Alternatively, a list of target and reference stars can be passed in the format of the NINA exolpanet plugin star list, with the -ninastars= option. Siril will verify that all reference stars can be used before actually using them. A data file is created in the current directory named light_curve.dat, gnuplot plots the result to a PNG image if available

The ring radii for aperture photometry can either be configured in the settings or set to a factor of the reference image's FWHM if -autoring is passed.

See also SEQPSF for operations on single star

Links: seqpsf