2. Your Observing Challenge: White Dwarfs in Open Star Clusters

3. Star clusters are collections of stars born nearly-simultaneously. Once we know their ages, we can determine how different stars change with time. Messier 35 and NGC 2158 (CFHT)47 Tucanae (HST)

4. Models tell us how the appearance of stars should change with time.

5. The ages of star clusters are usually determined by the “main sequence turn-off” in a color-magnitude diagram.

6. The models that give white dwarf ages use different physics from the models that give us main sequence ages, giving us a sanity check! von Hippel (2005)

7. Your Observing Challenge: Take pictures of a star cluster with our 0.8- meter telescope and find candidate white dwarfs that might be useful for studying ages of the cluster. First, let’s examine your primary tools: the CCD camera and ImageJ

8. Light is the only probe we have for objects outside the solar system.

9. Telescopes are funnels that collect large amounts of light from stars, but must send light somewhere.

10. Several devices are capable of collecting light and storing information about it.

11. The first modern astronomers used their eyes, pencil and papers and brains. William & Caroline Herschel Heber Curtis Maria Mitchell

12. Eyes are very inefficient and cannot integrate.

13. Eyes can only do comparisons between objects, and have trouble making quantitative measurements.

14. The brain is not the most reliable image processing software.

15. Around the turn of the century, photographic plates became popular. John Draper 1839 Henry Draper 1880David Malin 1979

16. Plates can integrate and are modestly efficient.

17. Plates store images accurately for long periods of time.

18. Plates cannot be added together, making analysis hard.

19. CCDs are the modern astronomer’s weapon of choice for observing.

20. CCDs can integrate for long times at nearly 100% efficiency.

21. The images are read into a computer and stored on disk. Dr. Jana Pittichova

22. CCDs pictures can be added together, letting you see fainter. SingleStack of 5

23. CCDs convert light to electrical signals.

24. CCDs are based on Einstein’s photoelectric effect.

25. CCDs are made of individual pixels, each of which works independently.

26. The electrons knocked out of silicon are held in a well until the exposure is finished.

27. After the exposure, circuitry counts the number of electrons and reports it to the computer.

28. If the signal is too high, the circuitry can’t count the electrons – the pixel is “saturated.”

29. Colors are measured by taking images through colored glass filters.

30. CCDs only detect the number of photons, not their color.

31. Filters allow only one color of light through.

32. Measuring brightness through different filters gives us color. From Hubblesite.org

33. There are dozens of flavors of filters; the most common are the Johnson system U = ultraviolet B = blue V = green R = orange/red I = very red (not infrared) Wavelength blue red Throughput (%)

34. CCDs are not perfect, so astronomers must take calibration data every night.

35. Bias frames measure noise from the electrical circuitry.

36. Dark frames are needed if a CCD produces spurious electrons.

37. Professional astronomers rarely use these for optical light.

38. “Flat fields” correct for different efficiency in different parts of the chip.

39. “Vignetting,” or shadowing, reduces the light at chip edges.

40. These calibrations take care of most defects.

41. Images of “standard stars” allow images to be put on an absolute scale.

42. Different telescopes and instruments have different throughput.

43. Dust and haze change the amount of light reaching the ground.

44. Calibration using standard stars lets us compare data from different telescopes and nights.

45. With cameras using one or more CCDs, astronomers have been study very large areas of sky. Kitt Peak Mosaic Keck Observatory LRIS Steward Observatory 90Prime

46. Astronomy requires the collecting of light for later study.

47. Astronomers need cameras that take long exposures and are efficient.

48. CCDs fit the bill -- provided the right calibrations are taken.