Photoelectric Photometry of the Pleiades

A Manual to Accompany Software for the Inductory Astronomy Lab Exercise
Contemporary Laboratory Experiences in Astronomy
Photoelectric Photometry of the Pleiades
Document SM 2: Version 1
Department of Physics
Gettysburg College
Gettysburg, PA 17325
Telephone: (717) 337-6028
email: clea@gettysburg.edu

Document modified for online use by Mariet Hofstee on 4/1/03.
Download the PhotoLab.EXE file (754 kb) from CLEA software by right clicking the file and selecting (save as ...). This file is a self-extracting file (.exe) and will install itself when you run it on your computer. It is probably best to save it to a separate directory on your harddrive or desktop (not in 'My Documents') and then run it.

Goals

You should be able to use photometry to determine the relative apparent and absolute magnitude of stars in a cluster in order to calculate the distance to the cluster.

Objectives:

If you learn to ....... You should be able to ......

Introduction

The computer program you will use is a realistic simulation of a UBV photometer attached to a moderate sized research telescope. The telescope is controlled by a computer that allows you to move from star to star and make measurements. Different filters can be selected for each observation, and the integration time (the length of time the photometer samples the starlight) is adjustable. The computer also does much of the busy work needed to convert photon counts into apparent magnitude and provides an estimate of the quality of the collected data.

You will use this instrument to collect data on 24 stars in the region of the Pleiades star cluster. The apparent magnitudes will be measured for each star, in the Blue (B) and Visible (V) colors. We will assume all of these stars are approximately the same distance away. This is a necessary assumption, and reasonable because all of the stars are members of the same cluster. If we did not make this general assumption, the apparent magnitudes of the stars would also depend on their individual distances, an effect we cannot easily take into account in this lab.

From this information you will plot a Hertzsprung-Russell (H-R) diagram which will display the apparent magnitude of the cluster of stars as a function of their color index. The color index, B-V, is the apparent blue magnitude (B) minus the apparent visual magnitude (V). All the data will be recorded in the Photoelectric Photometry Data Sheet. This excel file has a graph included that is all set up to display your data. The first datapoint is already recorded and the resulting graph is shown below in figure 1.


Figure 1: HR diagram for the Main Sequence and the Pleiades data
The y axis on the left side corresponds to the calibration data from the Main Sequence. The y-axis on the right side is for your data. Once you have recorded all your data points, you will need to adjust the right hand axis in order for your data to fall on top of the callibration data. To adjust the right hand scale, right click the right hand y-axis of the plot and select 'Format Axis'. You will then get a pop up window like the one on the left. Select the 'Scale' tab and adjust the minimum and maximum values by the same amount to shift your data points up or down. Once the two data sets are alligned, you will be able to relate the apparent magnitude (right hand scale) of a cluster star to an absolute magnitude (M, left hand scale) from the main sequence plot. For example in the case of figure 1, M=0 and m=5.

Knowing the apparent and absolute magnitude of a star, you can determine its distance (in parsecs) from the equation:

d= 10 x 10(m-M)/5                             (1)
where     m = the apparent magnitude (V as measured)
               M = the absolute magnitude (V in figure 1)
                d = the distance in parsecs

Finally, a few stars will be measured, and their distance compared to data in the Hipparcos catalogue.

Operating the Computer Program

First , some definitions:
PRESS Push the left mouse button down (unless another button is specified).
RELEASE Let the mouse button up.
CLICK Quickly press and release the mouse button.
DOUBLE CLICK Quickly press and release the mouse button twice.
CLICK AND DRAG Press and hold the mouse button. Select a new location using the mouse, then release.

You will use this program in the following order:

If the program is running properly the CLEA logo screen should appear in a short time. Position the cursor over the LOG IN... on the menu bar and click the mouse button ONCE to continue to the Student Accounting screen.

Entering Student Accounting Information

Enter your name (first and last). When all the information has been entered to your satisfaction, click OK to continue, and click YES when you are asked if you are finished logging in. The opening screen of the photometry lab will appear.

Accessing the HELP Files

Click HELP on the menu bar, and then click on GETTING STARTED. A text window will appear on top of the opening screen. Position the cursor over the lower right hand border of the text window. When the cursor is properly positioned, the cursor changes from a pointer to a diagonal double arrow. Click and drag the corner of the window until you can view the entire width of the text area. Then, click on the word GETTING STARTED at the top of the text window and drag the window off to one side so you can see the opening screen again. Use the scroll bar on the right side of the text window to read all information in the help window. You can interactively move back and forth between the text window and the photometry program by clicking anywhere in the desired window.
When you have finished with the GETTING STARTED help screen, close it by double clicking on the gray button in the upper left hand corner of the window.
In the same fashion, select HELP from the menu bar again open and read the TAKING DATA help screen in its entirety.

Open the Observatory

Click the START option on the menu bar on the opening screen. The center of the screen is the view of the night sky. Controls and readouts are to the left and right of the view. Take a moment to study the various controls available to you using the following explanation:
 

Telescope and Controls and Readouts (left hand side of photometer)

 
DOME  Opens and closes the observatory dome. Open the dome to activate the controls 
TRACKING  Turns on/turns off the telescope drive. Turning tracking ON causes the telescope to counteract the effect of the Earth’s rotation and is necessary in order to take measurements. Turning tracking OFF allows the star field to drift through the field of view as the Earth turns. 
SLEW RATE Controls the rate of telescope movement when the N, E, S, W direction controls are pressed. The slew rate can be set to 1, 2, 4, 8, or 16. The larger the number, the faster the movement rate. Slow speeds are useful for centering a star image. Faster rates are useful for quickly moving from star to star. 
N, S, E, W  Directional controls. Click one of these buttons to cause the telescope to move north, south, east, or west. When the telescope is moving, a red light next to the direction button glows. Movement in the selected direction continues until the same button is clicked again, or a different direction button is similarly selected. 
RIGHT ASCENSION Displays the celestial coordinates of the center of the field of view. Right Ascension is displayed in hours, minutes and seconds. 
DECLINATION Declination is displayed in degrees, minutes and seconds.
MONITOR  Click to select the FINDER mode or the INSTRUMENT mode. Select FINDER to see a wide angle view of the stars. The red square identifies the field of view when in the PHOTOMETER mode. The PHOTOMETER mode shows a close-up of the star field. It is necessary to select the PHOTOMETER mode to use the photometer. The numbers directly beneath the MONITOR button display the field of view of the screen, that is , how much of the sky is being viewed. 
SET COORDINATES  When the dome is open, you can click on this button and enter a desired Right Ascension and Declination to move to. After you have entered the coordinates, clicking on OK moves the telescope to the new location in the sky. 

Photometer Controls and Readouts (right hand side of the photometer)

 
FILTER Clicking this button cycles the color filters through U (ultraviolet) , B (blue) and V (visible). 
SECONDS Selects the duration of any given integration, or how long light is collected for each reading. The time can be set from 0.1 to 10 seconds. The dimmer a star, the longer the integration time will have to be in order to get an accurate measure of the light. 
INTEGRATIONS  Adjusts the number of times a measurement is repeated. Multiple readings are averaged. 
TAKE READING  When the dome is open and the MONITOR is set to the PHOTOMETER mode, clicking this button begins a set of measurements, (how many is determined by the setting of INTEGRATIONS), of a particular time each (how long for each measurement is determined by the setting SECONDS), through the filter selected by the FILTER button. 

NOTE: Before you can start taking readings of stars you must first take initial “sky” readings through all three colored filters. If you forget to take a sky reading, an error message is displayed and a real reading is not taken.

Some additional information is displayed once a reading of the sky or a star is taken:
 
OBS UT  The UTC, or Universal Coordinated Time, at the prime meridian at Greenwich, England. 
JD The Julian Date of the OBS UT. The Julian Date is a continuous count of the number of days and fractions of a day since the year -4712. The new Julian Day starts at noon UTC. 
ELAPSED  The number of SECONDS that have elapsed since any particular reading began. 
COMPLETED  The number of INTEGRATIONS done so far in the sequence of readings being taken. 
RAW COUNTS The photometer simulates a photon-counting photometer. The count of the number of photons captured during the individual reading. 
SN RATIO We assume that the only error in the counting if the photons is their randomness defined by quantum mechanics. The square root of the sum of the RAW COUNTS is the fractional error in the reading. A SN RATIO of 100 is needed for a fractional error of 1% (which is 0.01 magnitude). The higher the SN RATIO the lower the fractional error. The SN RATIO, if not high enough, can be raised by increasing the SECONDS or the number of INTEGRATIONS. 
MEAN SKY COUNT/SEC  The number of MEAN counts contributed by the sky through a particular filter (determined when the sky reading is taken) divided by the SECONDS, giving a normalized photon rate in counts per second. 
MAGNITUDE  The apparent magnitude of the star, through a particular color filter, based on the MEAN of the counts adjusted by the appropriate MEAN SKY COUNTS/SEC.

Results


Name Student: 
e-mail address: 

Move to a Star

Once the observatory is opened, the directional controls can be clicked to move the telescope around in the sky. The red square in the center of view is the field of view seen by the photometer when you engage the PHOTOMETER mode (by clicking on MONITOR). When you are in the PHOTOMETER mode, the small red circle in the center of the field of view is called the photoelectric aperture and is the portion of the sky being examined by the photometer. The desired star must be carefully centered within the aperture. Otherwise, some of the light from the star spills out of the aperture and is missed during measurement.

The rotation of the earth will cause the stars to drift through the view. Observe this phenomenon by carefully watching the stars with TRACKING turned off. Does the Earth rotate to the east or to the west?
Earth rotates to the 
Do the stars seem to drift east or west?
The stars drift 

Telescopes are equipped with a motor drive which moves the telescope in a direction opposite to the drift and at the same rate. The motor (often called the clock drive) cancels the effect of the earth’s rotation and the star seems to stand still permitting extended study. Click on TRACKING and note how the stars cease to drift. You must have the telescope tracking before you can operate the photometer.

The directional controls, N, E, S, W, move the telescope with respect to the sky. Moving the telescope to the west appears to make the stars move to the east. Try it. The SLEW RATE control adjusts in steps and changes how much the telescope is moved when the directional controls have been activated. Try various settings of the SLEW RATE and move the telescope around in all directions.
 

Set up and Take SKY Readings

Since the aperture is much larger than the star under study, and the sky is not perfectly dark, the sky within the aperture contributes a certain number of photons. These unwanted photons are counted by taking a sky reading. The star to be measured is then centered in the aperture and a star reading is taken. It is important that both the sky and the star reading be taken through the same color filter. Then the true photon count for the star by itself is approximated by the star reading minus the sky reading.

Make sure the TRACKING is on, and click on the MONITOR button to turn on the PHOTOMETER mode. Move the telescope until the aperture (red circle) is free of any star. Select a FILTER. Set SECONDS to 10 seconds, and INTEGRATIONS to 5. Then click on TAKE READINGS and wait for the reading. Repeat the procedure for each color filter and record the results in your logbook.
 

Take a Reading and Record the Results

The right ascension and declination of the aperture center are displayed in the upper right of the screen. Select a filter (U, B, or V - they cycle when clicked). Select an integration time. Use short integration times for bright stars to save time, and long integrations for faint stars. Integration times are in seconds. Bright stars generate many photons, and cause high counts.

Carefully center a star in the aperture and take a reading. The computer will take a series of integrations depending upon the setting of INTEGRATIONS, and display the individual and average photon counts in the raw count box on the right. After the integrations are completed, the computer considers the appropriate sky reading for the filter you used and the apparent magnitude of the star is displayed in the lower right corner of the photometer.

Also displayed is the signal-to-noise ratio or SN RATIO of the reading. A high SN RATIO means you have a lot of desired photons, and only a little noise. You should strive for SN RATIOs of 100 or more. You can increase the SN RATIO by increasing the integration time because the SN RATIO is directly proportional to the square root of the total collected raw counts.

Note the identification number of the star you are measuring and use the Hipparcos catalog to verify your
measurements.  Note the value of the parallax (field H11). Record your measurements in your logbook and in the PPDS excel file (se link below).

1. For each star on the Photoelectric Photometry Data Sheet, use the right ascension and declination to locate the star. Carefully center the star in the aperture and measure the B and V apparent magnitudes. Record all magnitudes to the nearest 0.001 magnitude on the data sheet. Turn off the tracking and close the observatory when you are done. Calculate the color index B-V for each star to the nearest 0.01 magnitude on the worksheet. Hot blue stars have low, and even negative B-V. Cooler red stars have B-V values somewhat over 1. Include your data in the excel file.

2. Identify, by RA and Dec, three possible red giant stars.
Star Name  RA  DEC 

3. Consider the star near RA 3h 44m and Dec 24d 35". It seems curiously out of place with respect to the main sequence. What type of star might this be? Upon what did you base your decision?

 

Distance to the Cluster

The data from Table 3 below (see worksheet 2 in the Excel file) gives the color index versus the absolute magnitude for main sequence stars. Use the procedure outlined in the introduction to allign the experimental data with the absolute magnitude callibration data. The cool red stars in the lower right of your paper graph are quite scattered and may not fit very well. Notice that once the two main sequences are aligned, a fixed relationship is established between the apparent and absolute magnitude scales, given by the value of your offset.


 

Use the equation (1) in the Introduction to calculate the distance to the cluster in parsecs. Then convert your answer to light years.
Measured distance modulus: 
Distance to cluster:  parsecs
Distance to cluster: light years
The Pleiades (M45) form an Open Cluster at a distance of 380 ly. How does your calculated value compare?

The startfield contains many other stars that might be part of the Pleiades. Measure at least three bright stars and compare the values to the Hipparcos or Tycho data. You might need to include the letters HD in the search parameters for the Hipparcos catalog.  Include these measurements in worksheet 3 of the Excel file and fill in the table below. Estimate the distance modulus (m-M) for these three stars from your graphs and calculate the distance from the distance modulus. Use excel to calculate the distance from the parallax in the Hipparcos catalog. Do the different methods for determining the distance agree? Why (not)? (Hint: Look at the spectral class):

- Compare the parallax and proper motion (RA and DEC components)  and determine if these stars are part of the Pleiades? Why (not)?
HD Star Number  RA  DEC  V (H5) B-V (H37)  Parallax (H11) distance from m-M distance from parallax part of Pleiades?

When you have completed you data collection and made the graphs, send the complete excel file WITH YOUR SLATE USERNAME AS THE FILENAME to mhofstee@mines.edu.



Last updated 4/6/03 15:21 MST by MAH