As well as trying to take nice images of night sky objects, why not use your imaging skills to capture real science data. Perhaps not so pretty but quite mind-blowing if you think about the stellar processes going on. Here’s an example of something you could try – and it’s not as tricky as you might think. Create a light curve for an eclipsing binary star system.
You can also use the same technique to get an asteroid’s light curve as it tumble through space – with one additional step – see below.
The basic process I went through is this:
- Choose your object – probably best to choose something relatively bright and with a short period to start with.
- Find you object – using you telescope’s goto capabilities with plate solving certainly helps here.
- Calculate the length of exposure to use to give a good dynamic range without saturating the star.
- Take a large number of exposures (lights) throughout the transit – ideally with a V filter but full colour or mono are also fine.
- Take darks, flats and bias as you would normally for imaging.
- Pre-process your lights.
- Identify your target star and one or more comparison stars.
- Perform photometry on all your lights.
- Graph your results.
You should download and install AstroImageJ as this is the software we will use to automate a lot of the steps.
Here’s a video showing just what’s happening in a binary system and why the light curve looks like it does:
Credit: ESO/L. Calçada
In more detail, here’s a worked example for each of the steps. And here’s some test data for V523 Cas if you want to try out processing.
1. Choose your object
There is a very useful website with details of eclipsing binary stars: the Mt. Suhora Astronomical Observatory TIDAK
Pick a constellation from the list, one that will be well placed for you for some hours. Then choose a star from that page – one that is sufficiently bright for your setup and one with a short period. The star I chose is in Cassiopeia and the star is V0523 Cas. It is a W Ursae Majoris type of variable, also known as a low mass contact binary. The two stars are actually in contact with each other and orbit very fast!
It has a V magnitude of 10.91 and a period of 0.2336933 days. So you should get a full curve approximately every 5.5 hours – 1 primary and 1 secondary minima.
2. Find your object
Here you’ll need to plug in the coordinates into your goto system / software / plate solving – whatever you use. If you click on the V0523 link you will be taken to a page which contains a link to Simbad, an online astronomical database containing a lot of information about the star plus an image that will help you to identify the star.
You can also see a list of primary and secondary transit times for your star.
Another useful resource that we will use later is the AAVSO Variable Star Plotter. With this you can create a finder chart plus a list of useful comparison stars for your object. These we will use in step 7.
Create your chart by entering the name of the star (V523 Cas without the extra “0”) plus other parameters as shown:
3. Calculate the length of exposure
This is a bit of hit-and-miss to start with unless you know your setup well. I know that for a 10th magnitude star, a 25s exposure will not saturate the pixels but I tried 10s and 40s; each pixel will have a maximum value of 65535. You want to aim for the target star being less than three quarters of that, where your chip has a linear response. Take a series of exposures of different length, examine the target star and work out it’s value. You may already have something that does this. For example NINA, Nebulosity and Siril all have this feature and others will too.
In AstroImageJ, load the FITS file into the program and hover over the target star. The pixel value will be displayed top right. Two values are shown: the value under the cursor (13,972) and the maximum value within the apertures (17,071). So this is an OK exposure for this star – possibly aiming for a little higher would have been better. Aiming to get around 50% is ideal, but it must be in the region where your camera sensor’s response is linear and you have to take into account the brightnesses of comparison stars but ideally they should be similar anyway.
4. Take a large number of lights
The number depends on the exposure, the length of time you want to observe, when your minima occur and which part of the curve you want to catch. Using a V filter (from UVBRI) is standard for this type of photometry but we are just after a simple curve here, so anything should do fine.
Try and keep the star as central as possible during the session. Autoguiding is ideal. Also, defocusing the star a little can give better results as the star is spread across more pixels which averages out the varied response of each pixel and also stops the star from saturating.
5. Take darks, flats and bias frames
This is what you would normally do for imaging. 10 – 20 of each will be sufficient.
6. Pre-process your lights
Separate your lights, darks, flats and bias frames into separate subfolders for convenience. Open up AstroImageJ and select the DP button – CCD Data Processor.
This will bring up CCD Data Processor window:
Set the folders and filename patterns for each of your lights, bias, darks and flats as shown. If you have them set correctly, the number of each type of file will be shown on the right. Click START and your lights will be pre-processed using your flats, darks and bias frames. Masters will be created and the processed lights will be placed in the specified folder (pipelineout).
7. Identify your target star and comparison stars
Now we need to load up all the pre-processed images into a virtual stack. From the main toolbar select File – Import – Image Sequence…
Select the first image in the pipelineout directory.
Make sure you select Use virtual stack. Click OK.
8. Perform photometry on all your lights
The first image in the stack will be displayed. Use the chart above to identify your target and comparison stars. You can use View – Invert X / Invert Y to get the image to look the same as the finder chart above.
First you need to identify the size of apertures to be used for the photometry. You can zoom in and out using your mouse scroll wheel.
Alt-left-click on your target star. This will display the Seeing profile window.
Click Save Aperture and close the window. Now we need to place the apertures around the Target and Comparison stars. Click on the perform multi-aperture photometry button.
Here you will see the apertures have been saved. If you select Prompt to enter ref star apparent magnitude then you will be able to get an actual magnitude rather than just a relative curve. In this case it doesn’t matter but it can be useful.
NB For asteroids – since the target changes position between frames, you’ll need to also select the Use single step mode option and click on the asteroid in each frame to identify it in the next stage after hitting the ENTER button.
Now we need to place the apertures. Click Place Apertures.
Left-click once on your target star. This will place green apertures around it and mark it T1.
Left-click once on each of the comparison stars chosen, C2, C3… If you chose to enter magnitudes you will be prompted to enter them each time. Here you can see the magnitude displayed for each star after the = sign. The magnitude of the target star is also calculated and displayed.
Comparison stars should be close in angular distance, brightness and colour to the target star for accurate photometry.
To remove a star, click on it again.
9. Graph your results
Once you are happy with the selections, hit the ENTER button on your keyboard. This will perform the photometry, create a Measurments.txt file containing the data and start to display the graphing functions. Select options similar to these to create a graph of the curve. (For an asteroid you’ll need to click on the asteroid in each frame.)
You can play around with the controls to display the curve as you wish. Choosing different Bin Sizes can smooth out the curve.
If you want to play around with the graph functions at a future stage, you can simply load the Measurements.txt file again rather than repeating all the other processing steps.
If you monitor an eclipse for long enough you can get both primary and secondary dips and peaks.
Article: Mark Phillips
Banner image credit: ESO/L. Calçada
Other images: Mark Phillips, AstroImageJ