Free course

Astronomy with an online telescope

The standard view of this forum does not always work well with assistive technology. We also provide a simpler view, which still contains all features. Switch to simple view.

You need to be enrolled on this OpenLearn unit before you can participate in this forum. Enrol now.

Picture of Paul Mckay

Paul Mckay Post 1

20 Jan 2021, 21:10

Flux Values in Light Curve

Hi Alan and Jo

I finished the course on 23 December 2020 and thoroughly enjoyed it, particularly the practical side of requesting, editing and viewing images taken through a telescope on the top of a volcano thousands of miles away.  Plotting the results on a real light curve constructed from images requested from many other students was the icing on the cake/snow on the summit! What a fantastic way of engaging and giving a sense the satisfaction to students.

This is my 7th Open Learn course on astronomical subjects and is light years ahead of the others. The use of this blog is a great addition and you really do feel part of a student community. Your answers to our questions are always prompt, informative and full of encouragment.

My technical question concerns the value of 'Flux' used on the light curve. There was a question in the Badge Quiz at the end of Week 8 and although I gave the correct answer, I could not find where in Weeks 7 or 8 this was covered. I have tried calculating the flux ratio: (target/reference) and tried inverting it, but I cannot get it to match the flux figure for the data points I created on the light curve. Have I missed something?

Finally, I posted an explanation of the mathematical relationship between brightness and magnitude just minutes after yours appeared. I struggled to get the wording right and your repy was not there when I started to write mine. I hope it is correct and not confusing because it gives more information than the course requires. My understanding of this topic is a legacy of doing GCSE Astronomy online in 2017-18, shortly after I retired. Another wonderful course vastly under-rated by schools.


Picture of Alan Cayless

Alan Cayless Post 2 (unread) in reply to 1

21 Jan 2021, 16:42 Edited by the author on 21 Jan 2021, 16:43

Hi Paul;

Thanks for asking about this, and that's great to hear that you've completed the course and got so much out of it.  And thanks for the explanation on the magnitude thread - it seems fine.

Yes, the flux plotted on the light curve is the relative brightness described in Section 2.2 of week 8 - so it should indeed be the target divided by the reference.  You're right that we could perhaps make this a bit clearer.

How different are your values from the ones on the light curve ?


Picture of Paul Mckay

Paul Mckay Post 3 (unread) in reply to 2

22 Jan 2021, 13:08

Hi Alan

Data from my 3 observations, photometry measurements and the data points (in red) plotted on the light curve are given in a screen shot in the attached Word document.

By hovering over my data points on the light curve I can verfy which data set they came from and the Averaged Flux figure. Comparing the averaged flux figures with the Target/Reference flux ratio calculated using the listed figures shows a clear mis-match. The data for one of the jobs (date completed 17.12.20) is listed below as an example but all 3 data sets show a mismatch. Please explain what I am I doing wrong? Is the ratio calculated from the flux figures averaged in some way. Note that my data points do lie on the Folded light curve.

Job J368839.  Target flux 164858.  Reference flux 215282.  Ratio 0.7658.  Averaged Flux from chart 1.0676.


Picture of Alan Cayless

Alan Cayless Post 4 (unread) in reply to 3

23 Jan 2021, 16:25

Hi Paul;

Thank you for this detailed info.   It really helps to know which image. We'll check it out and get back to you.



Take your learning further371

Making the decision to study can be a big step, which is why you'll want a trusted University. The Open University has 50 years’ experience delivering flexible learning and 170,000 students are studying with us right now. Take a look at all Open University courses372.

If you are new to university level study, we offer two introductory routes to our qualifications. Find out Where to take your learning next?373 You could either choose to start with an Access courses374or an open box module, which allows you to count your previous learning towards an Open University qualification.

Not ready for University study then browse over 1000 free courses on OpenLearn375 and sign up to our newsletter376 to hear about new free courses as they are released.

Every year, thousands of students decide to study with The Open University. With over 120 qualifications, we’ve got the right course for you.

Request an Open University prospectus371