Asteroid Light Curves

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Gordon Garradd, 2001, 9P/Tempel 1
Pedro Re, Linear N?, 2001
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Comet and Asteroid Photometry

• Introduction to Photometry with CCDs
• Broadband Photometry of Comets
• Asteroid Photometry
• Other Interesting Photometry Stuff

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Introduction to Photometry with CCDs

• Photometry Measuring the brightness of an
  astronomical object
• Photometry with Small Telescopes CCDs
• Recall
• Energy of photon -- electron -- digital counts
  (DNs)
• of accumulated electrons is proportional to
  brightness of object
• Photometry is basically counting the accumulated
  electrons
• Can make very accurate measurements of brightness
• With some work, can achieve to tenths of a
  magnitude
• With more work and excellent skies, can achieve
  to hundredths of a magnitude
• Work includes carefully reduced, unsaturated
  images
• Bias and dark subtraction
• Flat fielding
• Standard star calibrations (more work)

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Photometry

• Aperture Photometry ?
• Term derived from the days when photoelectric
  photometers used fixed apertures in the focal
  plane of the telescope to measure stars.
• Everything within the aperture was measured into
  one bucket!
• A comparison field, free of stars, was also
  measured and subtracted to obtain the brightness
  of the object being measured.
• Todays CCDs simplify the process!
• Many buckets (pixels) -- Get a picture of the
  object AND the sky
• Doing Photometry, in a nutshell...
• Observe an object and standard stars.
• Avoid saturating the object and the standard
  stars.
• For a list of sources for stellar magnitudes for
  comet and asteroid photometry, visit
  http//cfa-www.harvard.edu/icq/ICERec.html.
• Reduce the CCD frames by correcting for bias,
  dark current, and flat fielding.

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Photometry

• Measure the instrumental count rates of the
  object using photometry software
• Draw an aperture around the object.
• Calculate the size of the aperture in pixels,
  Aaper.
• Total the counts of all pixels in the objects
  aperture.
• Outline an area of the background sky that is
  free of stars.
• Calculate the size of the sky area in pixels.
• Total the counts of all pixels in the skys
  aperture and divide by the area to get the sky
  signal per pixel, Ssky.
• Convert the image exposure time to seconds, Texp.
• Then, the instrumental magnitude is defined as
• Minst -2.5 log( ( Naper - Aaper Ssky) /
  Texp )
• An instrumental magnitude is specific to your
  telescope, CCD, and filter setup.

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Photometry

• Correct instrumental magnitudes to zero airmass
  (i.e., the observed magnitude outside the
  atmosphere.
• At this point, you can stop and begin recording
  magnitude changes of over hours, days, weeks,
  months, and years. Just keep in mind that your
  measurements are specific to your setup and will
  be hard to compare to other observers data.
  But, this is a great starting point to get a feel
  for photometry!
• Derive transformation equations that relate the
  instrumental magnitudes of your setup to a
  photometric standard system. For this talk, we
  will refer the broadband UBVRI photometric
  system.
• Fortunately, this is usually a one-time effort,
  until you change something in your setup.
• To do this, you measurements standard stars and
  refer to published magnitudes for the filter
  system you use.
• Derive uncertainties in your measurements to get
  more accurate results.
• For the gory details, refer to the photometry
  references at the end.

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Broadband Photometry of Comets

• Suggested instrumentation
• Short focal length telescope with an 8 inch
  aperture.
• Good, commercial CCD.
• FOV of at least 7 arcminutes because comets are
  extended sources.
• Research grade, broadband UBVRI filters.
• V and R are most useful for comet and asteroid
  work
• V isolates C2 emissions in cometary comas
• R (and I) isolates light reflected off dust in
  cometary comas
• Wide passbands -- More light is received than
  with narrowband filters that isolate molecular
  emissions such as CN -- You dont have to
  work as hard to get some good results
• Can take unfiltered images but these are harder
  to reconcile with other observers data and
  contain light from gas emissions and dust.

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Broadband Photometry of Comets

• Broadband filters isolate certain light emitted
  by certain gases, dust, and the nucleus for
  studying
• Rotation rates and pole orientation
• Gas and dust production
• A few observing hints
• Keep an observing log with accurate UTCs
• Generate an ephemeris for that includes distances
  of target to the sun (r) and to the earth
  (del). Use this information to convert
  instrumental magnitudes to
• Heliocentric magnitudes (i.e., normalized to a
  constant distance of 1AU from the earth)
• Absolute magnitudes (i.e., normalized to a
  constant distance of 1AU from the earth and 1AU
  from the sun)
• Track at the rate of motion of the comet.
• Do not saturate the coma. Usually not a problem
  for 2-5 minute exposures of faint comets.

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Representative Comet Spectra
CN, C2
C2
Dust
Dust
CN OH
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Comet Photometry - Sample Lightcurves
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Comet Photometry - Sample Lightcurves
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Comet Photometry - Sample Lightcurves
Mikuz and Dintinjana, 1994, ICQ
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Photometry of Asteroids

• Suggested instrumentation
• Short focal length telescope with an 8 inch
  aperture.
• Good, commercial CCD.
• FOV of at least 7 arcminutes, but not a critical
  because are asteroids are point-sources similar
  to stars.
• Light-pollution-blocking filter.
• Some observing hints
• Using visible light (unfiltered), so youre work
  is a little easier when compared to comets.
• Track at the rate of motion of the asteroid.
• Observe known asteroids.
• Image asteroids over a night, a week, a month,
  and even one or more years.
• Many, short-term observations -- rotation rate.
• Need many observations over an orbital period --
  pole orientation.

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Asteroid Photometry - Sample Lightcurves
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Other Interesting Photometry Stuff

• Discovered August 6, 2000 by Keith Rivich and his
  fellow observers, Bill Dillon and Cynthia
  Gustava, while imaging comet 9P/Tempel 1 for The
  Small Telescope Science Program for NASA's Deep
  Impact Mission.
• The team used the George Observatory's 0.46 meter
  telescope with a CCD camera.

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Photometry References and Guides

• Henden, A. and Kaitchuck, R. 1982 Astronomical
  Photometry, A Text and Handbook for the Advanced
  Amateur and Professional Astronomer
• Budding, E. 1993 Introduction to Astronomical
  Photometry
• Howell, S. 2000 Handbook of CCD Astronomy
• CCD Astronomy (back issues from Sky and
  Telescope)
• International Comet Quarterly (ICQ)
• http//cfa-www.harvard.edu/icq/cometphot.html
• Center for Asteroid Physical Studies (CAPS)
• http//www.minorplanetobserver.com/CAPS/Photometr
  yGuide.html
• Mikuz, H. and Dintinjana, B. 1994 CCD
  Photometry of Comets in the ICQ
• http//www.fiz.uni-lj.si/astro/comets/CCDPhot/iw
  ca5.html
• Romanishin, W. An Introduction to Astronomical
  Photometry Using CCDs
• http//observatory.ou.edu/book4512.html

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Obtaining UBVRI Filters

• Omega Optical
• http//www.omegafilters.com
• Bessell Photometric Filters (UBRVI)
• Custom Filters from UV to mid-IR
• SBIG
• http//www.sbig.com
• Bessell Photometric Filters (UBRVI)
• Special Application Filters (H-alpha, O-III,
  S-II, etc.)
• Off-the-shelf filters cost about 200 each.
  Youll want a filter wheel, too!

The Bessell filter prescription,of the
Johnson-Cousins filter set, is specifically
designed for photometry with CCD cameras.
Off-the-shelf filters cost about 200 each.
Youll probably want a filter wheel, too!
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Some Photometry Software Packages

• Many of these packages let you perform photometry
  as well as reduce your data
• MIRA
• DAOPHOT II
• FitsPro
• IRAS