"Program and Standard Stars for the Catalog"

Trisha Borgman - March 15, 1996

    Good morning! I apologize for not being able to write anything last week-- I had 2 midterms and 3 papers due, and I was barely able to come into work at all! But now things have calmed down; Spring Break is next week, and I'm finished with all of my classes.

Our observing run is coming up quickly! Since next week is Spring Break, I will actually leave for Arizona on Wednesday (my parents live in Arizona, so I'll spend a few days with them before going down to Kitt Peak) The run begins on the night of March 25, and ends on the morning of March 29. I sure hope we'll have good weather!!

Yesterday, my boss sent me a list of all the objects which we should try to observe--I believe we're going to have a meeting on Monday to plan everything out. I have never done anything like this, so I'll be learning as much as you will!!

Since we're building a star catalog, it is very important for us to be certain that the data we collect is completely accurate. I mentioned before that part of my job is to remove the instrumental "signatures" left by the telescope itself. Another very important part of building the catalog is having a separate set of stars whose photometric information is already known to compare our stars to. We therefore call the stars we're observing for our catalog "Program stars"; the stars for which the information is already known are called "Standard stars." The standard stars come from a catalog which was compiled by an astronomer named Arno Landolt. He spent several years observing around the celestial equator (the celestial equator is just like the earth's equator-- stars near the celestial equator would be almost directly overhead to a person standing on earth's equator). Stars near the celestial equator are visible to people both in the northern hemisphere and the southern hemisphere. His catalog is filled with literally hundreds of stars from this region, and are often called the "Landolt standards."

Have you ever looked at what happens to sunlight when it passes through a prism? The light separates into the colors of the rainbow, right? Well, this visible light is just a small part of what scientists call the "electromagnetic spectrum." The electromagnetic spectrum includes the entire range of waves, waves can teach us about the amount of energy a star is producing. A picture of our sun in radio waves would look very different from a picture in x-rays, and both would look very different from what the sun looks like in the visible wavelengths -- wavelengths our eyes can see.

When collecting photometric information about the stars, astronomers typically use five of what we call 'passbands': U (ultraviolet), B (blue), V (visible), R (red), and I (near- infrared). A passband, as you might have guessed, is a narrow region of the electromagnetic spectrum. In order to only study within a certain passband, astronomers have to block out the rest of the spectrum with very sensitive filters. Of course, many astronomers choose other filters, but these five are probably the most common. The first GSPC catalog included data only from the B and V filters; the second GSPC (which I'm working on) will contain B, V, and R data and will also include much fainter stars.

So what does all this have to do with our observing run? Well, when we go out to the telescope, it's important not only for us to know which "program" stars to observe for our catalog, but also which "standard" stars to use. The standard stars help us to account for atmospheric distortions. When a star is directly overhead, its light will pass through much less atmosphere than when the star is near the horizon. We therefore observe these standard stars at many positions in the sky (overhead, at 30 degrees that we can take into account the effects of the atmosphere when reducing the data from our program stars. Does this make sense? We already know what the data from the standard stars SHOULD be, so when we measure them at many positions in the sky, we can compare the differences between what the standard star data SHOULD be and what it really is. We can then take that information and apply it to our program stars.

Pretty neat, huh?

Well, I'll be sure to write more next week before I leave, and I promise to write from Kitt Peak!!

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