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We are running a session at my local society on transits and occultations. One station will focus on exoplanet transits, and we'd like to build a very simple model to demonstrate this. We have a star (light source) and an orbiting "planet" but I need to work out how to detect the changes in light intensity and display this on a laptop, like a classical transit photometry trace below (taken from https://heasarc.gsfc.nasa.gov/docs/tess/primary-science.html).

Is there a way to take a feed from a DSLR through the USB output to do this, else I could get an adapter for my ZWO and put an EOS lens on the front of that. I really do want a light intensity vs time trace in real time on the laptop. This model will be run in a darkened room.

Thanks for any comments.

James

 

transit_white.png

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Interesting idea.

Depending on distance and size of light source and occulter, I think that you will benefit from longer focal length lens. Think your best bet is to use ZWO camera for this, and I'm afraid you'll need a bit of custom software to do it.

Easiest way is to do "calibration" of image upon setup - quite easy thing to do - you just need to "mark" circle that represents diameter of light source. After that you take sequence of images and calculate sum of pixel values in marked region, and output real time graph based on this value. Make sure you don't saturate light source, so use fairly short exposure for your subs (this helps with "feed" being real time).

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Thanks. I was thinking of using a telephoto lens anyway, 70-300. 
 

The other option I thought was to just run canon utilities on the laptop with live view, and then make a suction cup to stick onto the screen with a light dependant resister inside and attach that with some simple circuitary with the help of a friend, to a small oscilloscope...

 

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2 minutes ago, jambouk said:

Thanks. I was thinking of using a telephoto lens anyway, 70-300. 
 

The other option I thought was to just run canon utilities on the laptop with live view, and then make a suction cup to stick onto the screen with a light dependant resister inside and attach that with some simple circuitary with the help of a friend, to a small oscilloscope...

 

Well yes, that is also interesting option and I believe it should work as well.

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James there is a very simple piece of software (freeware) called LightGrapher which is made exactly for this purpose - no need to muck about with arduino or anything else.  You can even use it with a simple web cam; here's the link for the download and suggested activities. I've used it in school and it produces excellent transit graphs. Depending on your model set up you can experiment with effect of distance to star, size of planet, transit time etc. For the star I used a globe style lamp and for planets I had a snooker ball,  a computer mouse ball and a polystyrene ball rotating round the globe on a simple turntable.  The planets were placed on an arm which allowed the orbital distance to be varied. If it would be helpful I can photograph the setup but it won't be until Monday. 

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

Globe Lamp - IKEA

 

Jim 

Edited by saac
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2 hours ago, saac said:

For the star I used a globe style lamp

I was just thinking about this, and this is excellent way to do simulation to get proper results. Have not thought about it previously, but this thread gave me food for thought.

I was under impression that star is providing uniform illumination (or rather I always thought of stars in two terms - either single point of light or uniformly lit disc), but above transit graph shows it is not the case. If one wants to properly simulate this and get exact curve - this needs to be taken into account.

I'm talking about this region of graph:

image.png.4a1e6068b3f5a6ddbd384e9d226bcbdd.png

It shows section of the transit where planet is fully "inside" stellar disc. Why should there be dip in this section of the graph? If you observe purely geometrical aspect - you have two surfaces - that of the star and that of the planet - both circles, and "total light emitting" surface is equal to their difference. However this line of reasoning does not include the fact that amount of emitted light from the star depends on angle on its surface!

I'm sure I've seen this effect before, but have not payed attention to it, and here is an example of it:

image.png.d7273861664671017b92748762c91e65.png

If you look at this Ha image of our sun - you can see that it is not uniformly lit (disregard features and look at average brightness of surface). It is brighter at the center than towards the edges. Planet disc near the edges will remove less light than that close to center of the disk - that is why there is "dip" in curve.

You should either use image of the sun on a black background evenly illuminated with some source, or as suggested - globe type lap as it should provide similar effect.

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Thanks both. The LightGrapher thing is good; it just took time to work out how to get it to run on my laptop, I can only get it to run with FireFox for the moment and only with the inbuilt camera on the laptop, not another one via USB, but will try again tomorrow.

The limb darkening thing is interesting and I'd not thought about it before, though actually I think I recall someone talking about this at a meeting when exoplanet detection was all the rage. For the simple model I need, I don't think it is necessary to factor this in, but maybe worth mentioning for the more advanced members of the audience.

I also recall from a meeting where an academic was showing transit traces, that the dips in light intensity during a transit were not symmetrical; the onset of the dip had a different morphology to the offset of it. I went up and asked him afterwards and he explained why, but I've totally forgotten what he said...!

James

 

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3 minutes ago, jambouk said:

I can only get it to run with FireFox for the moment and only with the inbuilt camera on the laptop, not another one via USB, but will try again tomorrow.

Try installing WDM drivers for USB camera (if it is ZWO camera) - software probably uses WDM type driver to access the camera.

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32 minutes ago, jambouk said:

Thanks both. The LightGrapher thing is good; it just took time to work out how to get it to run on my laptop, I can only get it to run with FireFox for the moment and only with the inbuilt camera on the laptop, not another one via USB, but will try again tomorrow.

 

James I run it on MS Edge and it woks ok.  I also just use a bog standard web cam so I can have greater freedom to position the camera but the laptop cam works fine as well. You may just need to update some drivers (uses Adobe Flash on MS Edge)

Jim 

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  • 3 weeks later...

The LightGrapher software worked a treat and several people asked about it.

A very sucessful evening on transitsand occultations where we all learnt a great deal, and had fun. Amateur indoor astronomy at its best.

James

 

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