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Information about photometry

Published on January 16, 2008

Author: Maurizio


Slide1:  Gordon Garradd, 2001, 9P/Tempel 1 Pedro Re, Linear N?, 2001 Comet and Asteroid Photometry:  Comet and Asteroid Photometry Introduction to Photometry with CCDs Broadband Photometry of Comets Asteroid Photometry Other Interesting Photometry Stuff Introduction to Photometry with CCDs:  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”) Photometry:  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. Today’s 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 Reduce the CCD frames by correcting for bias, dark current, and flat fielding. Photometry:  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 object’s 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 sky’s 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. Photometry:  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. Broadband Photometry of Comets:  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 don’t 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. Broadband Photometry of Comets:  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. Representative Comet Spectra:  Representative Comet Spectra CN, C2 Dust Dust C2 CN OH Comet Photometry - Sample Lightcurves:  Comet Photometry - Sample Lightcurves Comet Photometry - Sample Lightcurves:  Comet Photometry - Sample Lightcurves Comet Photometry - Sample Lightcurves:  Comet Photometry - Sample Lightcurves Mikuz and Dintinjana, 1994, ICQ Photometry of Asteroids:  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 you’re 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. Asteroid Photometry - Sample Lightcurves :  Asteroid Photometry - Sample Lightcurves Other Interesting Photometry Stuff:  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. Photometry References and Guides:  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): Center for Asteroid Physical Studies (CAPS): Mikuz, H. and Dintinjana, B. 1994 CCD Photometry of Comets in the ICQ: Romanishin, W. An Introduction to Astronomical Photometry Using CCDs: Obtaining UBVRI Filters:  Obtaining UBVRI Filters Omega Optical Bessell Photometric Filters (UBRVI)* Custom Filters from UV to mid-IR SBIG Bessell Photometric Filters (UBRVI)* Special Application Filters (H-alpha, O-III, S-II, etc.) Off-the-shelf filters cost about $200 each. You’ll 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. You’ll probably want a filter wheel, too! Some Photometry Software Packages:  Some Photometry Software Packages Many of these packages let you perform photometry as well as reduce your data: MIRA DAOPHOT II FitsPro IRAS

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