4 Plan your own variable star observations using COAST

In this exercise you will use the knowledge and experience that you have gained from your observations of Messier objects in earlier weeks to help you to plan your observations of the eclipsing binary star.

1. Select target and plan observations

The table below gives a list of potential eclipsing binary stars identified by SuperWASP. Just as with the Messier objects that you observed in Week 4, these are located in different parts of the sky, and so different objects in this list will be best placed for observing at different times of the year.

The celestial coordinates (RA and Dec) are listed alongside each object. Using your knowledge of how the RA determines when an object is well placed for observation, select a target from this list that will be observable over the next week. You may wish to refer back to Section 2.1 of Week 4 to help you with this. You can also use Stellarium to check – it will be difficult to identify the individual stars from this list and they may not be shown at all, but you can still check that the coordinates are visible at night during the week that you will be observing.

2. Request images from COAST

Log in to the telescope.org website and select Use telescope from the menu on the left hand side. Previously, you selected Messier objects from a list, but this time you will enter the coordinates of your chosen target directly. Select or click here to enter coordinates under the Enter the name or catalogue ID box. This brings you to a screen where you can enter the RA and Dec coordinates from the table. Enter the coordinates carefully as shown in this example:

Figure 10 Entering RA and Dec coordinates in telescope.org. The example shown here is for target EA1. Check the visibility pane on this screen to confirm that your target is visible at this time of year.

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Image request form from the telescope.org website. In the telescope.org colours of orange on black, the form shows the Step 1 – Select Target screen. There are white fields for entering RA and DEC coordinates, and below these another white field labelled Name and containing the name 'EA1'. To the right is a box labelled RA DEC information with a horizontal coloured bar labelled Visibility – By Month. This bar is divided into twelve segments labelled with the months of the year and coloured red, orange or green to indicate visibility of the object. November and December are coloured red to indicate that the object is not visible in those months. The months from March to August are coloured green to indicate good visibility and the remaining months either side of the green zone are coloured orange indicating poor visibility. The month of July is highlighted to indicate the date of observation with the legend 'Current: Good' confirming good visibility at this time.

Figure 10 Entering RA and Dec coordinates in telescope.org. The example shown here is for target EA1. Check the visibility pane on this ...

Enter the name of your target in the Name box that appears below the coordinates and click Next to take you to the Step 3 – Customize screen.

3. Filter selection

In order to compare results, it is important that all measurements are made using the same filters. For this investigation we will use the green (V) filter. This is a standard filter for this type of work as it covers the middle part of the visible spectrum. On the Step 3 – Customize screen, select the (V) filter option under pick a filter (advanced) on the right-hand side.

4. Exposure bracketing

As part of your imaging request you will need to select an exposure time. In order to determine the correct exposure, you should request several images with a wide spread of different exposure times: for instance 15, 30, 60 and 120 seconds. This technique of doubling the exposure each time is known as exposure bracketing. When you get this first set of images back you can choose the image with the correct exposure and use this exposure time for any further images you want to request. This is another example of using information from one set of observations to help you plan subsequent observations.

For each image, confirm your request by pressing Submit on the Confirm request screen.

You are now ready for the final phase of the project. When your images of your chosen target come back you will be able to measure the brightness of the variable star and add your results to the light curve by following the instructions given next week.