CLEA Photometry
Photoelectric Photometry of the Pleiades This is a realistic simulation of using a UBV photometer attached to a telescope at a nearby observatory. It closely approximates what you would be doing using the real telescope. (In fact, we use this as a "trainer" to make sure students get the process down before spending a night at an observatory.) A brief discussion of what Photometry is all about has been described by the Royal Observatory in Greenwich, England.
Equipment Needed:
What To Do:
- Scientific Calculator
- Graph Paper
- Excel Spreadsheet Program
The Observatory:
- Access Clea Pleiades software program - This is an icon on your main menu.
- Login with your lab group or by yourself if you are doing this alone.
- Open the Observatory Dome
- For each star and with each filter you will take "sky" counts and then "star" readings. This is important. You see, when you look at a star you are seeing more than just the star. You are seeing the light from the star and light from the sky. Yes, the light from the sky is a small amount, but nevertheless, it is a contributing factor. So to get the true value of the light from a star, you must take the total light and subtract from that value, the light from the sky. The remainder is the light from the star. Fortunately for us, the computer does that nitty gritty work of subtracting and calculating. But we must actually point the telescope and take a reading of the sky from a location right near the particular star. You need to do this for every star. In practice, since we have a cluster of stars and they are pretty close in the sky, you might not do this every time when you go to an observatory. But you will need to do this here.
Observatory Dome This opens the dome. You ought to see the Pleiades star cluster. Note it appears to move to the west. Tracking
This turns the telescope drive on and off. As the earth rotates to the East the stars appear to move to the West. Thus the telescope must drive to follow the stars. Turning the Tracking ON allows the telescope to stay with the star selected. SLEW Rate There are several rates to Slew, or move the telescope ratrher quickly from object to object. Rates are 1, 2, 4, 8, or 16. N, S, E, W This moves the telescope one way or the other. Play with the controls and familiarize yourself. Right Ascension Celestial Coordinate of where the telescope is pointing Declination Celestial Coordinate of where the telescope is pointing Field Instrument
This allows you to select the wide field where you see the entire cluster, as you might in the finder scope of a telescope system. It also allows you to select the Instrument i.e., the Photometer. Set Coordinates You can enter a specific right ascension and declination but that is not necessary here. Photometer Controls and Readouts:
Filter This allows you to select the U, B, or V filter. For this lab you will only need to use the B and V filters. Seconds This selects the time of integration. Typically a ten second integration time is adequate for these objects. This means the photometer will add up counts for ten seconds. Basically you want to get at least a 10 to 1 signal to noise ratio so you know the counts are really essentially coming from the object and not background scattering. Integrations This is the number of times you will integrate and average the counts for each integration. Three is good for this project. The computer takes the average for you. Take Reading This starts acquiring data. It takes as many sets (number of integrations) and averages them for you, and computes the apparent magnitude at that filter wavelength. These are mV and mB . Don't forget that before you measure the star you must measure the sky background. The computer then, can subtract out the counts due to the sky and not coming from the star. You must do this for every star and each filter. Obs UT This is the Universal Time JD Julian Date, a continuous count since year -4712. Elapsed Number of seconds into the integration. Completed How many integrations are already done. Raw Counts The number of photons counted during a particular integration. SN Ratio The higher the better. This gives a ratio of signal to moise. Mean Sky Counts This is counts per second. If you are counting 10 seconds then the total ius divided by 10. Magnitude This is the apparent magnitude based on filter, background, etc.
- You will acquire apparent magnitude data for B and V filters for twenty five stars. You are to collaborate so as not to duplicate any effort or certainly if you are doing this alone, there won't be any duplication. It will require at least twenty five stars to make a minimal plot of an H-R diagram for this cluster. Record the B and V magnitudes for these stars.
- Calcultae B-V for each star. Simply subtract the value of V from the value of B. This is called the Color Index. Knowing what we know about black body curves and magnitudes, hot blue stars have low, perhaps even negative B-V values. Cool stars have values somewhat over 1.
- For your information (adapted from Allen, Astrophysical Quantities) the following values give you an idea of what the B-V values tell us in terms of spectral class:
B-V Spectral Class -0.35 05 -0.31 B0 -0.16 B5 0.00 A0 0.13 A5 0.27 F0 0.42 F5 0.58 G0 0.70 G5 0.89 K0 1.18 K5 1.45 M0 1.63 M5 - Fill in the following worksheet:
Star RA hr min sec
Dec deg min sec
B ..........
V .........
B-V .........
1 3 41 05 24 05 11 . . . 2 3 42 15 24 19 57 . . . 3 3 42 33 24 18 55 . . . 4 3 42 41 24 28 22 . . . 5 3 43 08 24 42 47 . . . 6 3 43 08 25 00 46 . . . 7 3 43 39 23 28 58 . . . 8 3 43 42 23 20 34 . . . 9 3 43 56 23 25 46 . . . 10 3 44 03 24 25 54 . . . 11 3 44 11 24 07 23 . . . 12 3 44 19 24 14 16 . . . 13 3 44 27 23 57 57 . . . 14 3 44 39 23 27 17 . . . 15 3 44 39 24 34 47 . . . 16 3 444 45 23 24 52 . . . 17 3 45 09 24 50 59 . . . 18 3 45 27 23 17 57 . . . 19 3 45 28 23 53 41 . . . 20 3 45 33 24 12 59 . . . 21 3 46 26 23 41 11 . . . 22 3 46 26 23 49 58 . . . 23 3 46 57 24 04 51 . . . 24 3 47 29 24 20 34 . . . - Enter the values into Excel spreadsheet program. You are going to construct a graph (use an x-y scatter plot) with the visual magnitude on the left side (vertical axis) and the B-V on the horizontal axis. Remember that the smaller or more negative a magnitude the brighter a star is. So you have to plot the small value higher on the vertical axis. Using the chart wizard, plot an H-R diagram. Excel typically won't let you put the small values on top and the large values on the bottom (of the vertical axis.) So instead, plot 1 over (divided by) the actual V magnitude. This will give us a scaling that works. If you aren't comfortable doing it on Excel, you can do this by hand. It works out. Basically, you have actually constructed an H-R diagram all by yourself, for the Pleiades star cluster.
- Discussion:
Assuming you are successful in constructing your own H-R diagram, discuss what this all means to you. First of all, the Pleiades star cluster is an open cluster. The many stars are all the same distance and were developed from the same intial chemical composition. All the stars are basically the same age. The difference is the initial mass which caused individual stars to have different spectral classes, temperatures, etc. i.e., because of the different initial mass they evolved differently. Look at the main sequence, and discuss its shape. Sure, we don't have a ton of stars but there should be a pattern evolving. How about red giant or white dwarf stars? What can you say in general about the cluster? Any hint of how old it might be? There's a lot we can tell from the H-R diagram. Have fun and good "observing" to you.
By the way, this is basically a "real" kind of project being done by astronomers today. Here's a paper recently published that you may be interested in reading.
Credit: The original Clea (Computer Laboratory Exercises in Astronomy) were developed at Gettysburgh College under NSF funding. The computer exercise and much of the above information is attributed to NSF and Gettysburgh College.