28 September 2019 at 1:00 pm #574410
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.
James28 September 2019 at 2:05 pm #581404Andrew SmithParticipant
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 Andrew28 September 2019 at 3:34 pm #581405
My DIY/electronics mate thinks he can do something with an Arduino / Pi to detect and display the data real time, like this:
I’ll report back.
James28 September 2019 at 6:47 pm #581406
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
Robin28 September 2019 at 7:25 pm #581407
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:
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:
James28 September 2019 at 9:22 pm #581408Grant PrivettParticipant
Has a humble webcam have the sensitivity and well depth required?28 September 2019 at 10:12 pm #581409
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.28 September 2019 at 11:01 pm #581410
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…29 September 2019 at 12:06 pm #581412
It was (just) possible with a modified webcam 🙂29 September 2019 at 12:32 pm #581413
That sounds tough. A back of envelope calculation suggests ~1/32000 change (10 arcsec/30 arcmin)^2
Robin29 September 2019 at 1:29 pm #581414Grant PrivettParticipant
Ah, I see. A 20 min running average certainly helps.
Impressive. Noise level at the 8-10mmags level perhaps.29 September 2019 at 4:14 pm #581415
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!29 September 2019 at 4:20 pm #581416
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.29 September 2019 at 9:16 pm #581418
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.29 September 2019 at 9:21 pm #581417Paul LeylandParticipant
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.29 September 2019 at 9:22 pm #581419Paul LeylandParticipant
Good point. I knew I was missing something.29 September 2019 at 9:46 pm #581420
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?1 October 2019 at 9:07 pm #581425
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.
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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