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Astronomy A305 - Modern Observational Techniques Lab Assignment 7

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Astronomy A305 - Modern Observational Techniques Lab Assignment 7 Astronomy A305 - Modern Observational Techniques Lab Assignment 7 Timing an Eclipse in an Algol Binary Goal: The goal of this lab assignment is to learn how to acquire and reduce time-resolved differential CCD photometry on an Algol-type eclipsing binary star. You will obtain an eclipse light curve for an Algol-type binary and determine the time of the eclipse. What to hand in: Submit a complete lab report including: • An introduction describing what an Algol binary is and what is known about the star you observed. • A description of your observing procedure and observations, including a log. • A description of your analysis procedures and plots, including error analysis. • A discussion of your observations, comparing the depth, width, timing, and character of your eclipse with the literature. An appropriate length is 3-5 typed pages, single spaced, 12-point font, with 1" margins, plus figures and tables. You may work with partners to obtain the data for the lab, but your analysis and writeup should be entirely your own. Be sure to review the general lab instructions on the "lab options" sheet. Observing Preparation Select a few stars in the table of Algol eclipsing binaries in the table below, and determine on which nights eclipses will occur. Arrange for observing on Sunrise or Sunset the night(s) you need. If you are not checked out for unassisted observing, confirm times with one of the Lab Instructors so that someone is available to help you. Because of possible problems with clouds and weather on the nights you are observing, be sure to prepare 2-3 possible targets for a given night (a primary target and two backups). You can find the predicted times of eclipse for the stars on the list from the website of the American Association of Variable Star Observers (www.aavso.org, see http://www.aavso.org/observing/programs/eb/eb2005.pdf). A copy of the variable star ephemeris for Nov. and December is available in from the Lab Instructors. It gives the approximate time of the eclipses for program stars. When does the eclipse occur? Be sure the star us up and that you will be able to observe it for 3-5 hours centered on the time of the predicted eclipse. Use "The Sky" to determine • The position of the object in the sky at the predicted time of the eclipse • The effect of the moon (phase and distance from the star) • Whether two suitable comparison stars for differential photometry are available in the field of view of the ST-8 CCD. • If the star is bright enough to observe under the moon conditions, especially during the eclipse? • How long will the star be available for viewing? • Make finding charts! It is critical that an accurate time be recorded for each CCD exposure. Check beforehand that the image header contains the correct time. The AAVSO website is useful for finding out what an Algol variable is, and for general information on observing variable stars. They also provide finding charts on their website, and you can download lightcurves from previous eclipses. At the Telescope Make observations in the V filter. Obtain twilight flats before it gets dark. The optimal exposure time for your variable star observations will depend on the system, but it will typically be 3-4 minutes. The drop to minimum typically takes a couple of hours. Depending on the brightness of the system, exposure times of 1.5 to 10 minutes might be appropriate. You need to get well-exposed images (neither saturated nor too faint) for both the variable and one or two comparison stars in the field. Use autotrack if a suitable guide star is available. If you can't use autotrack, 3 minute exposures are near the maximum that will provide untrailed images. You need a 3-6 hour sequence of images on your star. You will need to see portions of both the decline to minimum and the rise to maximum. The star brightness may or may not pause at minimum for 0.5-1.5 hours, depending on whether the eclipse is partial or total. Because this is differential photometry, you may observe through clouds or haze as long as you are getting enough signal for the variable and the comparison stars. Record all exposures on your log sheet. Reducing the data Correct all images with IRAF using the sky flat frames. Use aperture photometry (the "r" key in imexamine is sufficient) to determine the instrumental magnitudes of the variable and comparison stars in each image. Calculate the Julian date for the midpoint of each exposure. The AAVSO website has a UT to JD converter (http://www.aavso.org/observing/aids/jdcalendar.shtml). Analysis • Plot the magnitude of the variable vs. time • Plot the magnitudes of all comparison stars vs. time • Plot the difference in magnitude between two comparison stars vs. time • Plot the difference in magnitude between the variable and each comparison star vs. time. • Use the method of bisectors and/or the IRAF curve fitting routine to determine the time of the eclipse and an uncertainty. Approx transit Star RA Dec Max Min Depth Period(d) time in mid- Nov W Del 21 18 9.4 12.7 3.1 4.81 7 pm TT Del 21 08 10.6 12.5 1.9 2.87 7 pm CM Lac 22 44 8.5 9.5 1.0 1.60 8 pm AQ Peg 22 14 10.3 13.0 2.7 5.55 8 pm VX Lac 23 38 10.9 12.3 1.4 1.07 9 pm Y Psc 00 7 9.0 12.0 3.0 3.77 10 pm U Cep 01 82 6.7 9.8 3.1 2.49 11 pm WZ And 01 38 11.6 12.6 1.0 3.00 11 pm XZ And 02 42 10.0 13.0 3.0 1.36 Mid X Tri 02 28 8.9 12.0 3.1 0.97 Mid RZCas 03 69 6.4 7.8 1.4 1.20 1 am AB Cas 03 71 10.2 12.2 2.0 1.37 1 am Z Per 03 42 9.9 12.4 2.5 3.06 1 am RT Per 03 46 10.6 12.0 1.4 0.85 1 am ST Per 03 39 9.7 13.2 3.5 2.65 1 am .
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