Amateur Science - Getting Started in Photometry

1. Amateur Science - Getting Started in Photometry Tom Krajci CBA New Mexico (formerly CBA Tashkent, Uzbekistan) PO Box 1351 Cloudcroft, NM 88317 tom_krajci@tularosa.net http://overton2.tamu.edu/aset/krajci/

2. Introduction – variable stars are all around us • http://antwrp.gsfc.nasa.gov/apod/ap041012.html • Globular cluster M3 • Single image….

3. Introduction – variable stars are all around us • Time series!

4. Overview • Why photometry? • Photometry basics • Scope and mount – up to the task? • CCD image calibration – and nothing else • What targets to select? • Data – what do you do with it? • Me…write a scientific paper?!

5. Why photometry? • Appeal of discovery and exploration • Opportunity to collaborate with other interested amateurs and professionals • Desire to contribute to scientific knowledge • You love a challenge

6. But I’m competing with the pro’s • Don’t compete, complement their observing • 10 meter scopes look deep, but sky coverage is far short of 100% • Time is on your side as an amateur • It’s your equipment – no science board tells you what to observe • Easy to collaborate with amateurs and pro’s

7. Do I put visual observers out of business? • No • “The Future of Visual Observations in Variable Star Research” • http://www.aavso.org/publications/ejaavso/v33n1/65.shtml • Shows how visual and CCD observations can complement one another

8. Don’t I need a sophisticated/expensive rig? • Use what you have • (This is backwards from the way professionals approach photometry, but that’s OK) • Example: http://www.aavso.org/publications/ejaavso/ej37.pdf • Cookbook CCD, 50mm camera lens, stationary mount, urban light pollution • 30 – 39 degrees declination were surveyed • 75 new and previously suspected variables were detected

9. What is photometry? • Measuring the brightness of stars • Types of photometry • Absolute/All-Sky • Differential (covered in this presentation) • Filtered • Unfiltered (covered in this presentation) • Aperture (covered in this presentation) • PSF (Point Spread Function) Fitting • Differential Image Analysis (DIA)

10. Problems in photometry • The atmosphere: • Makes absolute/all-sky photometry difficult • The sky is not completely dark • Adds light to the star in all our images • How do we address these problems?

11. The Answer - Differential Aperture Photometry (unfiltered) • Compare brightness of one star to another • Differential photometry (through an aperture) • Solves the atmosphere problem (mostly) • Measure sky brightness around the stars • The annulus (donut) • Solves the sky glow problem (mostly)

12. Differential aperture photometry • Aperture Annulus

13. Photometry Basics • Imaging basics come first • Find, focus, track, proper sampling, proper exposure • Stay within linear limits of your CCD • Answer some important questions: • Are my CCD images properly sampled? • Is my CCD response linear?

14. Why is proper sampling important? • Undersampled (FWHM < 2) – star brightness depends on where it falls on the CCD array

15. Why is proper sampling important? • Oversampled (FWHM > 4) – Too much read noise because star’s light is spread over many pixels

16. Why is proper sampling important? • Proper sampling (2 < FWHM < 4) - good compromise between problems in under and oversampling

17. Are my CCD images properly sampled? • Full Width Half-Maximum (FWHM) • Less than 2: Defocus • More than ~4: Add focal reducer

18. Are my CCD images properly sampled? • Focus versus photometric precision • http://www.socastrosci.org/Files/SAS_Proceedings_2005.pdf(page 101)

19. Is my CCD response linear? • Linear CCD response is essential for proper photometric analysis • Most CCD’s are linear at lower pixel values: 40%, 50%, 60%? • Use a cloudy evening to measure and confirm your CCD’s behavior indoors • If your CCD has anti-blooming, turn it OFF

20. Is my CCD response linear? • Need a stable light source • LED and regulated voltage supply (good to approx 1%) • http://www.edn.com/archives/1997/071797/15di_02.htm (ultra stable light source) • More info: • AIP Handbook, chapter 8 • http://overton2.tamu.edu/aset/krajci/st-7/st-7.htm • http://overton2.tamu.edu/aset/krajci/st-7-new/st-7-new.htm

21. Is my CCD response linear?Bench test results: • Plot of avg ADU rate vs. avg pixel value • This CCD is linear (+-1%) up to 62K • (Except for the lowest light levels) • Typical CCD’s show a loss of linearity at higher pixel values

22. Is my CCD response linear? • Other bench tests and analysis will provide you with CCD: • Gain • Read noise • Dark Current • These values are important to determine the uncertainty in your photometry • Most software today makes this an easy task

23. Field testing! • Start with unfiltered photometry (KISS principle) • Take a time-series of any star field as an overall system test (use non-variable stars) • Nominal 60sec. Exposure • Use autoguider to keep field centered • Keep star pixel values at/below 50% of max ADU • Later, try Landolt standard fields (especially if you get photometric filters)

24. Field testing! • Second set of tests: turn off the autoguider! • Series of images with star field moved about CCD frame • Tests flat field calibration/vignetting issues • Can be more problematic if you use focal reducer • If you have a German mount, take test images in both hemispheres

25. Field testing of CCD linearity • This is what you should see in a time series: • A constant star minus a constant star = a constant (differential) brightness value

26. Field testing of CCD linearity • Another kind of sampling error – aperture too small! (Aperture = 2.5 pixel radius)

27. Field testing of CCD linearity • Find your minimum aperture and never use a smaller value!

28. What size aperture to use? • Rules of thumb: • Aperture 5 x sigma 2 x FWHM • Inner Annulus 7.5 x sigma 3 x FWHM • Outer Annulus 12.5 x sigma 5 x FWHM

29. What signal to noise ratio should I expect? • http://www.tass-survey.org/richmond/signal.shtml • Compare actual standard deviation of brightness (of constant stars) to predictions • Should be close, but can never be better • Bright stars will have higher SNR • A plot of magnitude versus standard deviation will have a characteristic curve….

30. What signal to noise ratio should I expect? • Magnitude versus standard deviation • Variable star will not fall on the characteristic curve • Variable? • Beware false alarms!

31. What signal to noise ratio should I try to achieve? • Depends on your observing goals • Eclipsing binary: SNR 50:1 • Exoplanet transit: SNR 200:1 • Faint cataclysmic variable: SNR 10:1 • Today: magnitude 17.5 • In a few years: magnitude 20 - 21

32. Comparison stars – some recommendations • Comp stars 0.5 to 1.5 magnitudes brighter than target variable • As close as reasonably possible in color • Minimizes differential atmospheric extinction effects • Often expressed as a B-V magnitude • Large B-V: object is red • Near-zero B-V: object is ‘white’ • Negative B-V: object is blue • NOTE: The redder the star, the more likely it’s variable

33. Comparison stars – how to get color data? • Planetarium software • Various star catalogs • Beware, many star catalogs are astrometric, not photometric…precision may suffer • If all else fails: http://www.nofs.navy.mil/data/FchPix/ • Examine blue and red magnitudes in the star list

34. Mount and focuser up to the task? • Polar alignment accurate • German mount ‘meridian flip’ • Mirror flop / differential flexure • Focus shift when temperature changes • DSC’s good enough, or use a flip mirror • How automated is your system?

35. Controlling “local seeing” and light pollution • Telescope fans • Observatory fans (or hose down) • Long dew/light shield • Moveable light screens?

36. Baffle your CCD Baffle size must accommodate your telescope’s f/ratio (steepness of light cone)

37. SCT baffling – may need improvement • No direct sky light falls on CCD, but…. • Only one oblique bounce/scatter allows light to reach CCD • At oblique angles flat black is not very black

38. SCT baffling – “2nd order baffling” • Larger central obstruction and long dew shield • No direct rays can even reach sky tube mouth • Now two oblique scatters are required to reach CCD…and at less glancing angles

39. Calibration frames (and nothing else) • Master bias • Master dark • Master flat (sometimes master flat-dark) • With repeatable temperature regulation, your master bias and dark frames are good for months • Only need to shoot flats every night • Use a light box, image in twilight • No dark time wasted on calibration frames

40. What target(s) to start with? • Eclipsing binaries of fairly short period • Compare your results to known ephemeris data • Heliocentric Julian Day? • http://www.rollinghillsobs.org/ • http://www.aavso.org/observing/programs/eclipser/omc/nelson_omc.shtml • http://var.astro.cz/ocgate/index.php?lang=en

41. What target(s) to start with? • One source of eclipse predictions:

42. What target(s) to start with? Compare your timing results to established values This is an O-C (observed minus calculated) plot of AB And

43. Make a finder chart • Saves hassle when your IQ is low at 3AM • Keep notes of important settings • Tailor your chart to your equipment needs and capabilities

44. More advanced projects • Cataclysmic variables (global collaboration) • Asteroid light curves (moving target, determine their shape/spin axis) • Exoplanet transits (low amplitude variability) • Gravitational lensing events (collaborate with pro’s doing large surveys) • Gamma Ray Bursts (optical transients) • And many others! • Often requires collaboration with other astronomers, pro and amateur • Compare you data against others’ work • A tremendous opportunity to learn!

45. Why global collaboration is good • Superhump feature has different period than the binary’s orbital period • Superhump • Eclipse

46. Why global collaboration is good

47. Why global collaboration is good

48. Why global collaboration is good • Observations from a single location would not cover this object’s behavior sufficiently

49. Check you data against others • If someone else was working the same target at the same time….

50. Find a mentor! • Mentors can be extremely helpful • Photometry skills • Source of software tools (light curve analysis) • Ideas for valuable and interesting targets • Establish collaborative links • Prevents isolation and loss of morale • http://www.aavso.org