Detection of HD189733_b  Exoplanet

I want to report the second successful detection of an exoplanet from my observatory in Central Ohio .

 On the night of September 28, 2007 I was able to record the transit of HD189733b thanks to unusual good evening weather with stable atmosphere.

Plan of the Imaging Session

Applying the lessons learned from my previous experience with TrES-2 I laid out a plan the previous evening.

Listed below are the elements of the plan and their sources:

As part of my imaging equipment I use a Pyxis camera rotator so with the help of the Field of View (FOV) indicator in the Sky 6 I can look for and designate the comparison stars before I start the imaging run and in addition find the optimal guide star for my guider.  To find the comparison stars I center the host star in the middle of the FOV indicator and rotate it until the camera’s field of view includes a suitable set of stars.  I then look at the FOV indicator for the guider and adjust the position so there is a suitable guide star in the guider’s field of view.  I then record the camera angle relative to North.

 At this point I have a good idea of where in the sky the host star would be, the exact start and end time that I needed to be able to record the exoplanet transit and the camera angle I needed to ensure that I capture not only the host star but suitable comparison stars.

At the Octadome

 With plan in hand I arrived two hours prior to the required start time, enough time to open up the Octadome, begin the cool down of the telescope, setup the imaging and other electronic equipment and ensure that everything was in proper working order.

 I set the camera at -15°C, homed the Paramount ME, rotated the camera to the planned camera angle and waited until I could see a few stars so I could begin to focus the telescope.

 Once it got dark enough I focused the telescope and then slewed the telescope to the host star HD189733 and proceeded to take a test image.  Here is where the pointing accuracy of a good mount really pays off; the target star was no more that a few arc seconds away from the center of the CCD detector.  I took a few more images to confirm that the stars that were going to be used for comparison were present in the field of view as well as ensuring that HD189733 was not saturated at the selected shutter speed (0.7 seconds).  I reconfirmed that I had a good guide star in the guider, calibrated the guider and started guiding.

 Finally I set up the sequence in the image acquisition program (Maxim) to take a 0.7 second image every minute and began the planned 5 ½ hour acquisition run.

By the end of the acquisition run HD189733 was very low in the western sky and I saw the deterioration of both signal and focus during the last hour or so of imaging.

Data Reduction

I got up late the next day and I was very curious about the results so I decided to try reducing the data as soon as I could.

 I had taken a total of 275 images so the first step was to use the Batch Save and Convert function in Maxim to calibrate (dark subtract) all the images.  Once the images were calibrated I proceeded to align them all.  Once the images were aligned I used the “Add Reference Star” plug in for Maxim to add a perfectly Gaussian reference star to the upper left corner of each of the 270 images. 

 As I found out during my last session, when using the Photometry tool in Maxim I can only load about 150 images in memory before it gets into trouble so I divided the 270 images into two groups.   I selected the synthetic star as the reference with a magnitude of 0, selected HD189733 as the Object to be measured and selected 4 more “Check” stars for comparison.  The photometry tool generated the curves and the Comma Separated Value table and after following this process twice I had all the data I needed to perform the data analysis with the spreadsheets.

Results

The light curve shown above illustrates the results of the analysis on the data for exoplanet HD189733b on the night of September 28, 2007 .

As before, there are several components that are part of the light curve generated.  The small red dots represent each actual 0.7-second images that passed the acceptance criteria for both, extra losses and outlier rejection. The large red dots are 9-point non-overlapping, median combines of the accepted data.  The two vertical lines at the top indicate the predicted ingress and egress times.  At the right of the legend, in the bottom, there are a few more notes; at the upper left it shows that an aperture radius of 9 was used for measuring star fluxes and this resulted in an RMS for 1-minute data of 0.44 mmag, which corresponds to an RMS of 0.98 mmag for 5-minute averages.  

The thick grey line is a model for this exoplanet transit.  The model illustrates the “U” shape expected with the detection of this exoplanet.

The reduced data has been accepted by and is now available at the Amateur Exoplanet Archive (AXA) and Transitsearch.org, a site sponsored by the University of California at Santa Cruz and managed by Professor Gregory Laughlin.

The detection of the second exoplanet from my observatory validates the viability of performing this kind of scientific endeavor from places with less than weather patterns such as Central Ohio .   

 Isaac Cruz

The Octadome

10/12/07