# What practical considerations are there for amateur observations of transiting exoplanets?

Obviously, I am not referring to actual viewing of the exoplanets themselves, but detecting their effects on the brightness of the light emitted from the parent star (as in the diagram below from The Institute of Astronomy, University of Hawaii).

I would imagine that a good quality telescope, would be about to detect and record the effects of the exoplanet transiting the parent star.

What practical considerations in terms of equipment, software etc. would be required for 'backyard' amateur observations of the effects of transiting exoplanets?

If anyone has actually tried this, what has been your experience?

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This is actually quite straightforward with digital CCD's (it used to be quite tricky with film cameras as you'd have to carefully develop film that moved past the lens and assess the width of the trail)

Get yourself a good telescope - a 12" Dobsonian or above if you want to give yourself a good chance of picking out the fluctuations against the noise background. Then select a decent CCD. Five hundered pounds gets a reasonable one, but expect to pay a couple of thousand pounds for a cooled CCD, which will also help to reduce noise.

(Buying in US dollars? A reasonable CCD runs about $1000. A cooled CCD will cost you at least$1500.)

You'll want a good quality equatorial mount, with computer controlled servos to track the target smoothly over long periods of time.

Ideally you will also slave a second telescope and CCD, pointed along the same path but slightly off - this will help you cancel out cloud and other fluctuations from our own atmosphere.

Oh, and get as far away from a city as possible - up into the mountains can be a good plan :-)

Then arrange your viewing for a series of full nights. The more data points you can get, the better the noise reduction. Imagine the exoplanet is orbiting every 100 days, in order to get any useful data, you will need to track it over some multiple of 100 days. So assume you set up to track your target star for 2 years, plan for 3 or 4 star shots per night to give you a range of data points.

These 600+ days of 4 data points per night gives you a reasonable stack of data - the challenge now is to work out whether there are any cyclic variations. Various data analysis tools can do this for you. As a first step, if you find a cycle around 365 days, it probably isn't the target, so try and normalise for this (of course this will make it very difficult to discover exoplanets with a period of exactly 1 year)

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"quite straightforward"... and then you tell us it takes a couple of years, a mountain location and a few thousand quid of equipment. +1 for the wry British humour.. – Owen Boyle Dec 10 '14 at 8:50
I don't understand this answer. Are you suggesting trying to discover transits in this way?! 4 data points per night is completely insufficient. My reading of the question is the desire was to observe known transiting planets, which can be done in a few hours. – Rob Jeffries Sep 14 at 22:10

IEEE Spectrum recently ran a piece on detecting exoplanets: DIY Exoplanet Detector - You don’t need a high-powered telescope to spot the signature of an alien world

Canon EOS Rebel XS DSLR. With old manual-focus lenses now useless to most photographers, I was able to acquire a 300-millimeter Nikon telephoto lens... An Arduino-controlled star tracker...

More info here: Detect known exoplanet with DSLR/telephote lens

The star in question, HD 189733A, has a visual magnitude of 7.6. Dimmer targets would naturally require more light gathering power than an old telephoto lens can supply.

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I'd question the statistics of that detection, but that's some feat. I had no idea you'd be able to get anything at all with consumer-level hardware! – Warrick Dec 19 '14 at 9:59
@Warrick Yes, a bigger objective lens'd give you less noise, but cameras have gotten unspeakably sensitive this past decade. – Wayfaring Stranger Dec 19 '14 at 12:38

If you stick to observing "hot Jupiters", then this is very much in reach of "amateur" technology.

I would agree that a 10-inch+ telescope is probably needed, along with a cooled CCD.

Hot Jupiters (giant planets orbiting close to their parent stars) produce transit signals of about 0.01-0.02 mag amplitude. The transits last of order a couple of hours, occur every 1-10 days and the times of transit are well predicted. In principle you could collect all the data you need in about 6 hours of observing. But, the amplitude of the transit dip is small, so you need to get very precise differential photometry. Your best bet is to observe targets which have plenty of other stars in the same CCD field that can act as comparisons - this may mean you need a CCD with a wide field of view. On the other hand, you must make sure that the seeing disk of the star is well-sampled by the CCD pixels (e.g. in your typical seeing is 2 arcseconds, each CCD pixel must not image more than 1 arcsec on the sky, and preferably less).

Other strategies for success involve observing at low airmasses, which should improve the quality of the differential photometry, and don't bother with anything observed even through the thinnest cirrus. Observing multiple transits will allow you to improve your data by "phase-folding" on the known planet orbital period.

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I'm still working on my internet skills, but google "Kepler: Amateur Astronomy" and click on the first thing that comes up. I think you will find it interesting.

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