Real-time photometry software

[Dawson]

I want to make a model to demonstrate a dip in light as an exoplanet travels in front of a star. I’ve got a star (light source) and an orbiting planet; it will be run in a darkened room.

Is there a way to take a feed from a DSLR (via USB) to the and plot the light intensity against time in real-time? Else a feed from a ZWO camera.

Any help, gratefully received.

James

[Dr Andrew Smith]

Jame kook at post 10 in the attached link https://www.aavso.org/content/best-software-very-accurate-differential-photometry-work 

Maybe what you need.

Regards Andrew

[Dawson]

My DIY/electronics mate thinks he can do something with an Arduino / Pi to detect and display the data real time, like this:

https://www.instructables.com/id/Plotting-real-time-data-from-Arduino-using-Python-/

I’ll report back.

James

[Robin Leadbeater]

If it is just a simple demonstration of a dip you are after, perhaps you could use the guiding program PHD which can produce a graph of the “mass” of the selected star in the field in real time.  (using a zwo camera with its all sky wide field lens?)Not sure if you can scale the graph to show a small dip though

Cheers

Robin

[Dawson]

That is another good idea. Thanks.

On an SGL thread I started on this topic, someone suggested a bit of software which uses a webcam into a PC and does real time light curves, which is brilliant and I think is the solution I’d go for. It needs Adobe Flash and I can only get it to work in FireFox, but otherwise it is very good:

http://www.planetarium-activities.org/shows/sp/lightgrapher

The other thing the SGL thread has raised, which I had thought about and won’t factor into my model, is limb darkening of the distant star. All interesting stuff:

https://stargazerslounge.com/topic/342034-exoplanet-transit-model/

James

[Grant Privett]

Has a humble webcam have the sensitivity and well depth required?

[Dawson]

I think a webcam will have sufficient sensitivity for my model; but may struggle if I put it on a scope and point at a real star.

[Dominic Ford]

James: I had been wondering about doing something similar on 11 November. Set a DSLR to take frames of fixed exposure, pointed either at the sky, or in fact at the ground. Averaging the brightness across the frame, I wonder how easy it would be to detect a brightness step at Mercury’s ingress and egress from transit…

[Robin Leadbeater]

It was (just) possible with a modified webcam 🙂

http://www.threehillsobservatory.co.uk/astro/TrES_1.htm

[Robin Leadbeater]

That sounds tough. A back of envelope calculation suggests ~1/32000 change (10 arcsec/30 arcmin)^2

Robin

[Grant Privett]

Ah, I see. A 20 min running average certainly helps.

Impressive. Noise level at the 8-10mmags level perhaps.

[Robin Leadbeater]

Hi Grant,

Mostly beginners luck really. Back then I was just scratching around for interesting science based projects to do with my modified webcam and knew very little about measuring variable stars, otherwise I probably would not have even tried!

[Dominic Ford]

That’s about right, I think.

I agree it’s tough, and almost certainly impossible under the UK’s changeable skies. But with a 6D at a good site, averaging over 20 megapixels, it seems tantalisingly within reach? Depends how well-behaved the noise is, and I don’t have a feel for how “good” a good site would need to be.

[Dominic Ford]

But in this case you’ve got no shortage of photons. You are quite right that it would be madness to propose detecting a transit of a Mercury-like exoplanet.

[Dr Paul Leyland]

1/32K difference in flux corresponds to a roughly 1/32 millimag dip in brightness.  Either I’m missing something important, which I do quite often, or I’d change “tough” into “a chance somewhere between nil and negligible”.  That said, I’m a great fan of understatement.

In the ARPS meeting today we were advised, correctly in my opinion, to concentrate on objects with a transit depth of at least 10 millimags.  Three hundred times deeper, in other words.

My experience is that good observations of a transit depth of, say, 5 millimags is achievable but not entirely trivial.  I couldn’t manage one millimag.

[Dr Paul Leyland]

Good point.  I knew I was missing something.

[Robin Leadbeater]

Yep plenty of signal to knock down the stochastic errors and a large area to average out the seeing.  The question is how stable is the camera electronics (gain and dark current)  and the atmospheric transparency at the sort of timescale we a talking about?  

[Dawson]

A noisy child’s orrery I have (currently very noisy as running at 12v and not 4.5v) and the light grabber software work well. I need an external webcam ideally and not use the built in one on the laptop.

thanks for the discussion.

james

Link to a video of the set up on my dropbox: https://www.dropbox.com/s/inv87rgfm4ht08u/IMG_1529.MOV?dl=0

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