2.3 Using finder charts to identify target and reference stars

The first thing you will have noticed about your image is that it contains a large number of stars. Before you can make the aperture photometry measurements it is important to make sure that you are measuring the correct stars.

In each image you will need to identify which star is the target (your chosen variable star) and which star is the reference star. This is especially important when combining results, as you will do in order to make the light curve; for the results to be consistent, everyone needs to use the same reference star!

If the telescope has moved exactly to the coordinates that you specified, the variable star you have chosen should be in the very centre of the image. Sometimes there are minor variations in the telescope drive and the target may not be exactly in the centre, so it is important to be sure that you are measuring the correct star. The reference will be another star in the same image, not necessarily near the centre.

To help you to identify these stars in your images we have produced finder charts for each of the variable star targets listed at the end of Week 7.

From the list, select the chart that corresponds to the variable star that you are investigating and save a copy to your computer. You may want to print this out and work from a paper copy.

One thing that you will notice about the finder charts is that they are negative images, with black stars on a white background. This makes them easier to use when printed out.

First, take a look at the finder chart and image in Figure 2 and see if you can identify the target and reference stars:

This short video goes into more detail on how to match up the stars in your images with those on the finder charts by looking for patterns in the stars near the target and the reference.

Download this video clip.Video player: finderchartrefstar.mp4

Show transcript|Hide transcript

Transcript

ALAN:

Let's take a closer look at how to match up stars on the finder chart with stars on the images that you're getting back from COAST. Here I've got a nice finder chart for variable star EA3. It's a negative image with black stars on a white background. And I've got a target and a reference star marked here. So the target is the variable star that we're interested in, and the reference is the one we're going to compare with. And my task is to identify the same two stars on the image from COAST which is the other way around, it's white stars on a black background.

But the first thing to note is that we can see we're looking at the same field of view. I've got bright stars in the same places on both images. And I've got a nice galaxy down here in both fields of view. So that means we are looking at the same part of the sky. And I'm reasonably confident that this target star here in the centre is this one in the centre of the image.

But to confirm that, what we can do is to look for distinctive patterns in both images in and around the stars that we're interested in. So the main thing to note here is that the target star has a brighter star slightly above and to the left of it here. And that star is at the end of a line of three stars here. There's a fainter one in the middle.

And then there's another line of stars below making a sort of narrow V shape here with the bright star at the apex and the target star, the variable star, at the end of this upper line of three stars here. And if I go to the COAST image, we can see we've got exactly the same pattern here. We've got the line of three stars, the narrow V shape. So that confirms that this is the target, the variable star, that we're interested in.

Now coming back to the finder chart, we can do the same thing for the reference, looking for a distinctive pattern in and around the reference star. So I've got a faint star just to the left of it. And then I've got a little group, a curved line of three fainter stars above the reference star here, very distinctive. And so we can look for that same distinctive pattern on the COAST image.

And we can see here I've got the faint star to the left, same distinctive pattern of three stars above it, confirming that this is my reference star here. And so by comparing patterns on both images we've confirmed that this is the target star here, the variable star EA3, and this is the reference star that we're going to compare with. And so we've now identified the stars on this image, ready to go to the aperture photometry software.

End transcript

Download

 

Now do the same with your own image and the corresponding finder chart from the list. Having identified the correct stars in your image, you are now ready for the next step – the aperture photometry.

Because of the way that the telescope moves, sometimes the images from COAST are rotated by 180 degrees compared to the finder chart. If this has happened to one of your images, use the Flip X and Flip Y options in the telescope.org Edit window and in the photometry tool (on the Image tab) as needed to make the pattern of stars in your image match the pattern in the finder chart.