Good afternoon everyone
How do I follow that? I have a short talk, about ten minutes, based on a paper I delivered about a month ago to the Pro-Am collaboration stream at the 2014 European Planetary Science Congress in Lisbon. How I came to be there is one of those weird tales, but let me just say “Be careful what you wish for, you never know where your hobby might take you!”
Let’s reframe the title of the talk to make it a bit less academic.
I’ve followed a trail that has led me to what looks like a good idea for a collaborative project, and Nick has kindly lent me ten minutes of his time to present it here.
Let me Introduce myself and do the “thanks” first.
I am the boy who forgot he was interested in astronomy, and remembered in his mid-fifties. I only remembered because I met to Stuart Atkinson who runs the Eddington Astronomical Society in Kendal, where I live. Thanks Stuart, for reminding me that I am interested in astronomy! Stuart is here today, if you haven’t chatted to him yet then do yourself a big favour and go and introduce yourself. Ask him about Mars. Don’t ask him about OSIRIS.
Thanks to Nick who moved behind the scenes to get my paper to the EPSC And of course here too..
Huge thanks to Andrew and Sue for taking a blind leap of faith to let a random amateur loose on stage at your astronomy festival.
I am here not just as an amateur, but still feeling pretty much a beginner. Although I am starting to study for an astronomy degree this week, my astronomy activities so far have been dominated by my interest in DSLR photography. In effect, taking pretty pictures of the night sky and then presenting them to my astronomy club.
With a bit of luck, this project might also fit in with my degree studies.
Presenting comet photos to astronomy club, the most frequent question is “how bright will it become?” When you try to look up the answer to this, you find that comet magnitude is a tricky subject. Visual magnitude for a point object, like a star, is easy to understand, but magnitude for a diffuse object is not so straightforward. So I also tried to find out “how” we answer the question. How do we take readings? How de we calculate the light curve?
I opened the textbooks and by coincidence, stumbled across this out of print journal article.
There he is. Bobrovnikoff. I love the idea that the historical archive of comet magnitude data is full of readings taken using methods which today seem curious or downright crazy. The “Bobrovnikov” method is based on looking at the comet through a telescope and memorising its appearance. You then move the telescope to one of the designated reference stars, put it out of focus so that the star appears diffuse like a comet and then ask yourself how it compares with your recollection of the comet that you memorised. This seems about as unscientific a way to take measurements as it is possible to conceive.
But that’s how they did it.
And this presents the amateur with an ideal opportunity for experimentation and study of method, and I invite other amateurs to join me in putting this project together.
I have tried a variation of the Bobrovnikov method, using a DLSR. Let’s work it through.
Here’s my capture of C/2012 K1 (PANSTARRS) earlier this year. Calibrated, stacked and filed with the BAA.
Here’s a single frame, unprocessed. The stars are points, the comet is diffuse.
Here’s a single frame out of focus so that the stars look like the comet nucleus. I’ve wound up the brightness to make it easier to see.
We can now investigate the onscreen brightness of the stars…
…and of the comet.
Our display tells us the on-screen brightness of the pixels which we can read with just about any image processing software. I put the mouse pointer over the pixels in the circle and read the value from the bottom of the screen.
So I can make a list of the on-screen brightness, what about the magnitudes?
Here’s the same field on SkySafari (other Apps are available!)…
…where I can click on each star…
…to be told its magnitude.
Now I have a list of screen brightnesses and a list of magnitudes. Time for a graph.
Let’s break this down.
Here are those stars and their known magnitudes.
Here are the onscreen brightnesses of the stars and of the comet.
By plotting the magnitudes against the brightnesses, we can calculate the magnitude corresponding to the onscreen brightness of the comet.
When I had finished this simple task, the thrill I got (even aged 56) from realising that I had just made my first scientific measurement of an object in the night sky, that thrill was real and palpable., If we can encourage that feeling in – what – 11 year olds?, we will be making the scientists of the future.
Given sufficient readings, we get the light curve – the progressive line of best fit from the data over a period of time. When I compare my reading from late April it’s a pretty good fit.
So what’s my problem? Why don’t I just choose a comet and get on with it?
Well, I’d love to, but although I live in one of the most beautiful parts of the UK, it’s cloudy, on average, about four weeks per month. So it would probably work best as a collaborative project.
How to structure the project is open to discussion. I could simply ask people to send out-of-focus images, and do all the analysis myself. Or, contributors could analyse their own images and send me their results. Or we could have teams taking images and teams doing analysis.
Yes, there are many technical questions and objections. Should we calibrate the frames to eliminate the sky background? Should we use total luminance or just the green pixels as being closest to visual magnitude? Does it matter that different cameras have different colour filter arrays? Should we standardise our reference star catalogue? And so on.
My standard response to these questions has become “NO” or “IT DOESN”T MATTER”. Because by gathering a large quantity of data it should be possible to eliminate the effect of these objections. If the volume of data is big enough, eventually, statistically, the answer always lies in the middle.
While the professionals seek to take “more accurate” measurements, I believe we can arrive at the same answer by simply taking “more measurements”.
Of course, it’s already been done. You can download light curves of most comets from online data sets. You can automate the reading of the on screen brightnesses. But the purpose here is to be a learning project, so I would advocate doing as much as possible without these resources.
I finish with this thought. This is the light curve for C/2012 S1 (ISON), Comet “Icarus” that flew too close to the Sun last year. Readings submitted to the Minor Planet Centre are in red and International Comet Quarterly are in blue. Look at the vertical spread of the readings. Not only are the two data sets not in agreement with one another, but they vary by as much as 5 magnitudes in a given measurement interval. That’s a one-hundred-fold variation. Surely we can do at least as well as this.
So, please join me in this project if you think it might be of interest to you, your club, society or school.
Here are my contact details, or come and grab a card or leave me your contact details if you prefer.
Thank you to all who have stayed to listen. I’m happy to take questions, or do come and see me afterwards.