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.