Task 1 of 8: Taking the data

Checklist for Task 1:

• Load the 12 images for one of the three Cepheids into SalsaJ.
• Adjust the brightness settings so you can actually see the stars in the field.
• Set up a spreadsheet to enter the data or use Table 1.
• Locate the Cepheid using the guide given below.
• Measure the photon counts in each of the 12 images and enter the data in a spreadsheet (e.g. Excel) or in Table 1.
• Repeat for the remaining two Cepheids.

Loading images into SalsaJ

1) Open SalsaJ - either pre-installed or from:
http://www.euhou.cicrp.jussieu.fr/euhou/Resources/ Soft/index.html

2) Load the 12 images for one Cepheid (e.g. C31) either from your local disk or from by opening the folder containing the fi les for the Cepheid you are working on.

3) Import the fi les using the File menu and the sequence File -> Import -> Image Sequence. This loads all twelve images into one window. You can fl ick from one image to the next using the horizontal slider. As you change images SalsaJ keeps the same location in each image.

Adjust the brightness

1) Adjust the brightness of the images using the Image menu and the sequence Image->Adjust->Brightness/Contrast.

2) A pop up appears, click the Set button at the lower left and type in the following values: Minimum Displayed Value: 0. Maximum Displayed Value: 40.

Setting up a spreadsheet - getting ready to collect data

Before you begin to work with the data it is helpful to have a clear way of recording your results. Table 1 below suggests a way of organising the data.

1) Use a spreadsheet if possible. Spreadsheets make performing routine repeated calculations easy, and there are lots of these in this exercise.

2) Make three copies of the table - one for each Cepheid - you will have to fi ll out the fi ve empty columns in each case - explained fully in the next section - as you begin to collect the data.

3) When you are working with the fi les, work with one Cepheid at a time and take all the data (location and photometric for that Cepheid for each epoch (date)) before going on to the next epoch.

Cepheid name

Table 1: Cepheid measurements.

Locating the Cepheid

It is not trivial at all to locate the Cepheids. In the following three images (one for each Cepheid) each Cepheid is marked by an arrow. This should help you to locate the Cepheids in the remaining 11 epochs / images. If you have the images loaded as a stack the position of the Cepheid changes very little between each image, so once you have found the Cepheid in the top image, you can fl ick through and see how the Cepheid changes in each successive image.

1) Use the brighter stars in the fi eld to navigate your way to the Cepheid. It can help to pick a couple of bright stars in an image where you are sure you know which the Cepheid is, so that you have the relative positions of the Cepheid and a couple of references. Then in an image where the Cepheid is really faint you can navigate from the stars you have chosen as references to the location of the Cepheid.

2) Make use of the Zoom-tool in SalsaJ to home in on the star. If you use the Zoom-tool make sure you switch it off before you try to take any photometric measurements.

3) Locate the Cepheid in the image and enter the x and y co-ordinates (shown at the lower left of the main SalsaJ toolbar) into the fi rst two empty columns of your spreadsheet (or Table 1). The Cepheids are at roughly the same position on the different dates, but not exactly so, so expect a few pixels shift between each image.

4) Calculate the Photometric Setting y-coordinate using the equation:

Photometric Setting y-coordinate = 201 – Cepheid y-coordinate

and enter the results in the third empty column. We need this number because sometimes SalsaJ counts its y-coordinate from the bottom of the image upwards (in the main toolbar and the Photometric Results window) and sometimes from the top of the image downwards (in the Photometric Settings window), so we have to be careful we are always talking about the same location in the different windows. The x coordinate is the same throughout.

Figure 1: C22 is here marked with a purple arrow

Figure 2: C25 marked with a purple arrow

Figure 3: C31 marked with a purple arrow

Making photometric measurements

Once the Cepheid has been located we can take the photometric measurements that tell us how much light is coming from each Cepheid on a given date.

1) Start up the photometry by clicking in the Analyse menu using Analyse -> Photometry menu. A new blank window should pop up. This stays blank until you take some data.

2) Now set up the photometry in the Analyse menu using Analyse-> Photometry Settings. A second new window pops up and it looks like this:

Figure 4: Screenshot of the photometric settings window in SalsaJ

a. Since the Cepheid moves a little between images in the stack, use the Forced Co-ordinates option. Select the grey box and enter the x coordinate and the SECOND y co-ordinate (the one you calculated and entered in the Photometric Settings y-co-ordinate column).

b. The Sky settings should always be "Auto radius (FWHM)". The other two sky settings should be left blank.

c. The star radius should be set manually using the “Forced Radius” option while locating the exact centre for the Cepheid (set the Forced value of the radius to e.g. 3). Once the Cepheid is located exactly, use the “Auto Radius (FWHM)” setting to obtain the best estimate of the intensity of the Cepheid.

d. Take a measurement by clicking the image. SalsaJ puts a circle round the star it is measuring so you should be able to see that you have the right co-ordinates.

e. To check that you really have found the centre of the Cepheid try shifting the x and y coordinates by one pixel each way to fi nd the selection with the best fi t to the star. It should be the selection with the smallest FWHM circle around it.

f. The "Photometry" window is updated with the x and y co-ordinates, the radius of the measured circle and the photon count (intensity) for the selected area. The x- and y- co-ordinates in this window should match the values in the main SalsaJ application.

3) When you are sure you have the best fi t for the centre of the star enter the photon count number (from the Intensity column in the main photometry window) in the "Intensity (SalsaJ)" column in the spreadsheet.