In this entry I would like to quickly show how to try to find variables in some area of sky. As input data you will need images of the same sky area made in some range of time – so this is actually the data you have, when you collect subframes for making pretty pictures. Time span of these subframes will determine what variable types you can seek for. If all subframes were made during one night session, then only short period variables can be found. But if you have subframes from several sessions spread over week or month, then also long period variables can be looked for.
To start work you need to import images to Muniwin software and process them in standard way – this was described in the first part Photometry with Muniwin. You can proceed with this tutorial till the moment when you are ready to plot the light curve. Instead of this, you need to call Tools -> Find variables option.
This is the process that compares matched star brightness, finds a comparison star and then plots magnitude standard deviation versus magnitude difference. In the plot in the top left part of the window we have at the vertical axis difference in magnitude between comparison star and all stars – the fainter the star is, the more to the right it is placed. And vertical axis shows standard deviation of the examined star over the whole time serie. The plot is more spread in the right part, because fainter stars have their magnitude calculated with less precision (because signal to noise ratio is lower). And in this plot we need to look for the stars, that are placed above the star cloud in the plot. If the star is placed there, then it means its magnitude varies more, than calculation error would indicate, and it can be possible variable. We can click at this star and in the lower part there will be light curve for this star displayed, and the star will be marked in the small chart at the top right window part. If light curve plot looks like variable star changes, we can save this selection with Save as… button at the right for further examination with the name for example “possible variable nr 1”. When we save all candidates, we can close Find variables window and open regular Light curve window, and there we can select any of our saved selections, zoom and pan and try to identify our candidates. We can try to find them with our planetary software or using VSX index at https://www.aavso.org/vsx/ page (when you click More you can do coordinate based search). At the example I pasted above the possible variable star was identified as https://www.aavso.org/vsx/index.php?view=detail.top&oid=245723 , and you need to be prepared, that most probably the variable you discovered has been already found 🙂
One important thing here is the shape of Find variables plot you examine – it should have the shape similar to the one I pasted, so it should become more spread to the right part. When the shape is not like that, for example if it is more spread in the middle or left part, then most probably you found and matched too many stars, and some noise pixels also have been matched. Then you need to get back to the photometry and match stars process and narrow matching criteria, so less stars will be identified and matched.