Notes from the Gettysburg CLEA Workshop

Michael Richmond
June 8, 2003

Contents:

• Basic CCD processing: cleaning a raw image
• Some notes on parallax measurements of asteroids
• Some notes on photometry
• Beyond the "Large Scale Structure" exercise

It may be useful to look at the syllabus to a course on Observational Astronomy I taught at RIT.

Basic CCD processing: cleaning a raw image

You can find an overview and some links to more detailed information by going to

• Basic First-Aid for raw CCD images

Some notes on parallax measurements of asteroids

• Lee Smallwood and Debora Katz (of the US Naval Academy) and I have written a paper which describes how one can measure the parallax of asteroids which pass close to the Earth. You can download a PDF copy of the paper.

• I wrote up a brief account of our experience observing the asteroid 2002 NY40.

• We also attempted to detect the parallax of asteroid 2000 QW7.

• Other people have detected parallax, too. For example, you can read about the
  • Joint Observation of Asteroid 2000 GD2 from Munich and Verona
  • Measurements of 1999BJ8 from Florida and Kansas (beware the pop-up ad, and go down to the bottom of the page)

• If you want to work out all the geometry yourself, it might help to glance at some pictures showing the quantities involved in the experiment.

• The CLEA Project includes an exercise in which students measure the parallax of an asteroid. It also includes a very handy set of tools for doing these measurements yourself.

• For a different aspect of parallax, look at these pictures of asteroid trails taken by the Hubble Space Telescope:
  • Mapping the Motion of Asteroids
  • The Hickson Compact Group 87 Asteroid Story by Matt McMaster

Some notes on photometry

• The basic idea behind aperture photometry: measure light from a star within a circular region, and don't forget to subtract the background!

• Practice some aperture photometry

• What's so great about differential photometry? It can save you when you forget to move the dome, among other things.

• The signal-to-noise ratio
  • What is it?
  • Some illustrations
  • A tool to help you estimate S/N in CCD images

• Converting instrumental magnitudes to a standard system
  • What are "standard magnitudes?"
  • How to detect the color terms of your camera, and fix its measurements. In many situations, you don't have to go to all this trouble. It's only when you want to share your measurements with other astronomers and care about effects at the level of around five percent or less that this is really important.

• Complications: crowding, non-uniform background, moving targets
  • When the stellar density is so high that images of stars overlap (as in globular clusters, for example), simple aperture photometry doesn't work very well. You need to use a more sophisticated technique called PSF-fitting photometry. My favorite reference for this method is a paper written by Peter Stetson in 1987. It includes some examples of PSF-fitting in action.
  • The case of SN 1994D: a star on top of a varying background
  • If you are trying to measure the rotation of an asteroid over several nights, you can run into trouble because the comparison stars on the first night won't be visible in images taken the second night.

• Some good projects for small telescopes
  • There are thousands of variable stars bright enough to be measured precisely with small telescopes. Some take months to vary, some change in just a few hours. One class of binary stars, cataclysmic variables, (or CVs for short), occasionally flare up in an "outburst" and become very interesting for a few days or weeks. I spent a lot of time observing one in particular: WZ Sge.
    • Measurements of WZ Sge at RIT Observatory on July 26, 2001. While reducing the data, I noticed a previously unknown variable star. Bonus!
    • Astronomers in the VSNET collaboration tend to get a little excited when they talk about WZ Sge.
    • The Center for Backyard Astrophysics is a confederation of amateur and professional astronomers around the world who work together to study variable stars. Joe Patterson wrote a paper on WZ Sge which included some data I collected.

  • Every now and then, a supernova will explode close enough to the Milky Way that it is bright enough to study with small telescopes.

  • Larry Marschall will tell you about observing asteroids with small telescopes. One can focus on astrometry to help improve the orbits of known asteroids, or photometry to measure their rotation periods. The Collaborative Asteroid Lightcurve Link (CALL) page can help you find other astronomers who are interested in working together with you to determine light curves.
    • Good targets for the summer of 2003, from the CALL website.
    • Remember how many asteroids have been discovered? Look at how few have known periods of rotation.

Beyond the "Large Scale Structure" exercise

In the CLEA Large Scale Structure exercise, you and your students map a small portion of the local universe with several hundred galaxies. You end up with a picture like this:

It seems clear that galaxies are clumped together, rather than filling space uniformly. But there isn't a lot to see. Note that the most distant galaxy has a velocity of about 15,000 km/sec, which corresponds to a redshift of about 0.05.

In recent years, many astronomers have spent a great deal of time and effort to measure the spectra (and so relative distances) of LOTS of galaxies. Two of the main surveys are

• 2dF (2-Degree Field)
• SDSS (Sloan Digital Sky Survey)

Recent maps of the universe show many more galaxies over a much larger volume: this map from the 2dF goes out to a redshift of about 0.30, about six times deeper than the CfA survey. That means it includes a volume about 6^3 = 216 times larger.