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If you thought it was impossible to observe explanets from your backyard, so did we. But since Michel Mayor and Didier Quelloz's discovery in 1995, amateur astronomers' equipment has evolved considerably. Here's a brief description of the adaptations to our system that may help you further research and even contribute to scientific publications, for the most dedicated of you.
The ExoClock project
The ExoClock project
I first heard about the ExoClock project in an article of the excellent French-language astronomy magazin Ciel&Espace. An international team develops all necessary to tools to make explanet observation fun and interactive. By "observation" I mean: measuring the temporary loss of brightness of a star during the transit of one of its planets.
Basically, almost each astrophotography setup equipped with a CCD camera can do the job, and suprisingly well. So why not trying?
Because our observatory specializes in narrowband photography, the March-April period (relatively poor in nebulae and rich in galaxies) isn't the most suitable. We therefore decided to dedicate a few hours of observation to the ExoClock project and enjoy the pleasure of participating in a research program alongside professionals.
To our great surprise, the very first attempt was a success, even if the data collected wasn't particularly good. It seems that a decade of experience in astrophotography is a significant asset!
Which star?
Which star?
We had no experience with exoplanets or how to plan an observing session. Fortunately, after a quick setup of the artificial horizon coordinates and our observatory settings, ExoClock suggests an observing plan for the same day, or for the next few days. Easy!
KELT-3b seems to be visible tonight! Lots of observations have been done already, and therefore the priority is low. Minor matter: each observations helps to increase the accuracy.
Enter the star coordinates in your favorite automation, use the astrometry to precisely slew the mount and abracabra, the start is in the very center of the image.
For our observatory, 10mmag is the limit we can achieve. This means that the star will lose approximately 10‰ (ie. 1%) of its luminosity.
The transits will start at 21:13 and will end up at 00:22, but the measurement shall start 1h before the ingress and end up 1h after the egress.
20 arcsec of astrometry accuracy. Good enough... :-)
Camera settings and focus
Camera settings and focus
Filter
Filter
Surprisingly, some transits should be photographed through a red filter, for rather complex reasons, which are also very well explained by the tutorials available on the Exoclock website. However, we obtained much better results in Luminance.
Exposure time
Exposure time
The main parameter remains the exposure time. Too short an exposure time results in large standard deviations in brightness measurements. Consequently, this does not necessarily increase the temporal resolution, which is the purpose of this type of observation.
An excessively long exposure time would result in a loss of temporal resolution, but also the risk of saturating the sensor, which would distort the measurement.
As an order of magnitude, we use exposure times ranging from 30s (magnitude 10) to 80s (magnitude 15)
Take bad photos!
Take bad photos!
What?
Yes...
Be out of focus
Be out of focus
To spread the light flux over a wider area, and thus make the most of the sensor's linear range without ever saturating it, it is strongly recommended to defocus the camera.
Generally, we add between 70 and 100 focuser steps, corresponding to approximately the same number of micrometers.
No dithering
No dithering
Unless all the photos are realigned, dithering will cause the star to move within its reference annulus, without significantly improving measurement accuracy through improved calibration. Furthermore, each dithering operation wastes valuable observation time. In short, we do not recommend it.
Reduce the sensor size
Reduce the sensor size
Why shall we save 50MB for each image if only the center is useful?
Since we don't use any reference stars at the edge of the image during post-processing, we prefer to save only the central portion of the photo, which is 1/9th of its original size, and therefore with 1/9th of its original file size. Your hard disk will love you for that.
Deactivate the automatic focus
Deactivate the automatic focus
You are already out of focus... :-) Do not forget to deactivate this feature!
Example of an exoplanet sequence for Gaia 2b
Did this article make you curious? Sign up on the ExoClock website and take part in a wonderful citizen science project!
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