Photographing Exoplanet Transits using Amateur Astrophotography Equipment
Updated: Nov 15, 2020
You'd think that photometry was reserved for scientists and professional astronomers, well think again! You can get very good results with very modest amateur equipment. Here's what my plot of the exoplanet Qatar-1b came out as:
For this you will need:
A GoTo computerised mount
A medium/long focal length Telescope or Lens (bigger the better)
Astronomy application capable of plate solving to target
Optional - Some form of autoguiding
To get started, head over to http://var2.astro.cz/ETD/predictions.php and enter your east longitude and your latitude. Look for a star that's nice and high in the sky. The larger the depth, typically the better as this will have more of a magnitude dip and is easier to detect on smaller setups. You will also want to start imaging around 20-30 minutes before and after the predicted start and end of the transit to have a flat line either side of the dip.
Once you're out in the field and all setup, calibrate your guiding and plate solve over to your intended target. I would recommend an application that allows you to see the ADU of a specific point in the image as this will come in very handy. Take a base 60 second exposure and measure the target star for its ADU. You won't want this value to be too low as to underexpose the image, nor do you want it to be overexposed and saturated as this will render your data useless. Typically most applications will measure ADU in 16 bit so aim for around 20,000-30,000 ADU. For me, this was 60 seconds.
Using a mono camera will of course help, but colour cameras and DSLR's should be fine too. To reduce the effects of the atmosphere, I chose a red filter on my mono CMOS.
Be sure to take dark, flat and bias frames.
Once you've taken all your data, now we can process it and produce a light curve.
I'm going to assume that you know how to create master dark, bias and flat frames already so I will skip that step.
Open up AIJ and go click on the blue and red DP (Data Processing) button along the main bar.
Note: If you're using a OSC or DSLR, I would recommend extracting the green from your data and using that.
It'll bring up this menu:
Make sure the settings are as follows:
Filename Pattern Matching - Specify the directory for your lights and make sure it has the wildcard (*.fits) as that selects all .fits files in that folder.
Bias Subtraction - Check Enable and specify the directory and file.
Do the same for dark and flat subtraction
Image correction - make sure both linearity correction and remove outliers are unchecked
FITS Header Updates - Check General and make sure target coordinate source and observatory location are both manual entry
Leave post processing alone and enable Save Calibrated images, saving them as either 16 or 32 bit files
Now we can update the FITS header by clicking on the compass in FITS Header Updates
A menu will pop up that looks like this. Simply enter the name of the host star in SIMBAD Object ID, make sure the Observatory is set to custom and enter your Longitude, Latitude and altitude.
You can now press start and wait for it to calibrate all of the images.
Once completed, close DP and navigate to File > Import > Image sequence. Select on one of your calibrated light files. Make sure virtual stack is checked as this will make it work from the hard drive and save on RAM, especially when dealing with larger files.
You should now get something that looks like this.
Find your target star and ALT-click on it
A graph should pop up like this. Take note of the Radius, Back> and <Back values
Go back to where the view where your image is. Along the top there should be a drop down called Analyze. Nagivate to Analyze > Multi Aperture.
This menu will pop up. Make sure that all the tick boxes are the same as mine. Adjust the Radius of object aperture, Inner radius and Outer radius to match the values from the graph earlier. In my case this was 8, 14 and 21.
Click on Aperture Settings
Again, make sure the check boxes are the same. What you will want to adjust is your CCD gain, readout noise and dark current for your camera. You can find these values online depending on your model. Now click OK and click on place apertures.
Place the first one around your target star then place around 4-8 comparison stars by clicking on others. Now right click and the photometry should begin.
You should now get several windows pop up and a plot of measurements like this. Find the Measurements window and go to File > Save. Save the file as a CSV. Now close everything but the main toolbar down. Along the top there should be a button that looks like a graph called Multi Plot Main, click on that.
The menu should look like this
Download this config file:
Now go to File > Open plot configuration template and select the config.
Also go to File > Open table from file, and select your measurements CSV.
Again, multiple windows will pop up.
On Multi-Plot Main make sure all the settings are the same except V. Marker 1 and V. Marker 2. On Exoplanet Transit Database, locate when your transit begins and ends in UTC.
Go to this calculator - https://www.aavso.org/jd-calculator
Enter the values and convert it into JD. Take the value for the beginning of the transit for UTC after the decimal point (eg. 0.458) and input that into V.Marker 1 and do the same for the end of the transit and V.Marker 2. Now click the copy button with the two diverging arrows in Fit and Normalize Region Selection. You should see two lines appear. These are predictions of when the transit begun and ended. You'll also a fitted region where there should be a dip where the transit occurs.
Navigate to one the data set window and it should look like this.
Go to - http://exoplanet.eu/catalog/ and search for your planet.
Look for the values Orbital Period and Rsun. Input the period in orbital parameters at the top of the window and the Rsun value in Host Star Parameters. Take note of the Radius
Now go to - http://astroutils.astronomy.ohio-state.edu/exofast/limbdark.shtml and select the planet with the corresponding filter used. V if using a OSC camera. Click submit.
Enter the first value in Quad LD u1 under Transit Parameters and the second value for u2. Tick the lock box for both of them.
Now on Detrend Parameters, click use on any of the boxes. In Fit Statistics, note the value of BIC. When you apply one of the detrend parameters, if the value goes down by more than 2, keep it. If not, then don't apply it. Experiment with applying different combinations and whether the value goes down or not.
If all went well, you should now have a nice fitted transit graph that looks something like this.
We can now save, model and upload our data to ETD.
On Multi-Plot Main - File > Save data subset to file
Make sure yours looks the same as this and save this file either as a .txt or .CSV file.
Head to - http://var2.astro.cz/ETD/protocol.php and select your file and planet. Change from magnitude to flux and click Compute.
Should pop up like this. Take note of the depth uncertainty. The lower this is the better along with the duration. If something's not quite right, these uncertainties will become higher.
It might also be beneficial to correct by square power. This could reduce the uncertainty.
You can now click Send protocol to TRESCA database and fill in all the details. You should hopefully see your light curve on the website.
Hopefully that helps anyone out there.
This is my first guide and post on my website so I hope you enjoyed reading!
If there's any issues, just contact me and I'll get back to you ASAP.