Report by ASSNE Member Mark Munkacsy
I’m starting the process of trying to observe an exoplanet transit. My plan is to look for a known exoplanet and to monitor the brightness of the star as seen from the earth while the planet crosses the star’s disk. (See the Wikipedia article for more on exoplanet transit photometry). The star’s brightness dips while the exoplanet crosses in front of the star’s disk. This image shows a typical exoplanet transit light curve obtained by NASA from the Kepler space telescope:
The American Association of Variable Star Observers (AAVSO) publishes a guide to exoplanet transit photometry (the measurement of star brightness). The guide advises making a series of measurements of the transit event, starting 30 minutes before the scheduled start, covering the entire time of the transit, and ending 30 minutes after the scheduled end. Since exoplanet transits can last anywhere from one hour to 6+ hours, this turns into a long sequence of images of the same patch of sky.
A major technical challenge is detecting the relatively small change in brightness of the star’s light during the transit. To a good first approximation, the amount of the star’s light blocked by the exoplanet during the transit is equal to the ratio of the apparent area of the planet to the apparent area of the star, which equals the square of the ratio of the diameters of the two. If we take our solar system as an example, Jupiter’s diameter is almost exactly 1/10 the sun’s diameter. So, an alien watching our solar system would see a transit of Jupiter as reducing the sun’s brightness by (1/10)^2 = 1%. In reviewing data on known exoplanets, a brightness change of 1% is typical for a “hot Jupiter” exoplanet.
However, I usually figure that my own photometric accuracy is only around 2-3%. To reduce those errors, I’ll need to change my typical practices; this is explained in some detail in the Exoplanet Observing Guide I mentioned earlier. In particular, <url=https://starcircleacademy.com/2013/07/flat-frames>normal CCD flatfielding is not good enough for exoplanet transit measurements. The Guide recommends making fresh flat-frames for every transit observing session, and also recommends strong enough camera guiding that each star in the image remains on the same pixel throughout the entire 6+ hour observing session.
This is a challenge for me, because I don’t normally do auto-guiding, relying instead on the tracking of my mount. However, even though there is no measurable periodic error with this mount, there is slow image drift due to imprecise polar alignment and imperfect mount error modeling. I typically see a drift of about 0.3 pixels/minute, and over a period of 6 hours, this would add up to a lot of pixels. However, I could measure this drift and could issue guiding commands based on each image I take (something which I’m starting to call “drift guiding”).
And so, this seems to be where I am starting from:
- Exoplanet transit depth goal: about 1% brightness change
- Exoplanet choice: will select a few days ahead of time from the NASA exoplanet site
- Guiding: slow “drift guiding” using drift estimates updated after every image (new software that I haven’t starting working on yet)
- Brightness & exposure time: anything between mag 9 and 13 should be good? Exposures of 30-60 seconds each?
- Filters: not sure this matters, but there seems to be a preference for images made through a standard photometric filter. I can do this, as long as blue isn’t important, since my camera’s sensitivity to blue is pretty terrible. For now, I’ll plan on using green (Johnson V)
- Photometric measurement from images: using the software package called “AstroImageJ” which is specifically written to handle some of the idiosyncrasies of exoplanet transit observation. Which I have exactly zero experience with!
I’ll keep posting updates as I learn more and start gathering data.
Read more about this project, including Q&A with other amateur astronomers, on the ASSNE Forum.