the open cluster NGC 6939

1) Download and save the following 2 open cluster images from
http://hou.lbl.gov/cgi/listimages?St_Marys+open_cluster+ngc6939

    6939B1.fts (a B-filtered image)
    6939G1.fts (a V-filtered image)

Make sure that the files are saved with an .fts extension

2) Open the HOU image processor and then open both images in the HOU image processor

3) The two images needed to be shifted by a small amount in x and y so that any given star has exactly the same coordinates on the two images. 
Determine, perform, and record the shift that you made.

4) Use the Find command to measure the x and y positions and the Counts value of all stars higher than sky + 6x deviation on the G image (henceforth called the V image).  (Empty the Results box before doing this for the later convenience in saving the results to a text file that will load into Excel.)

WARNING: WE WILL HAVE TO MERGE EVERYONE'S SPREADSHEET INTO ONE GIANT SPREADSHEET,
SO YOU WILL NEED TO USE THE SAME SPREADSHEET FORMAT AS EVERYONE ELSE

5) Save the results file as a .txt file.

6) Open Excel and open your results file for the V image.

7) Repeat steps 4-6 for the B image and save the results in a separate .txt file.

8) Cut and paste the B .txt file data into the V .txt file. 
Note that all stars recorded in the V image's text file will not necessarily show up in the B image's text file (and vice versa). 
You will have to hand delete any stars that dont show up in both OR go back and
use aperture to measure the counts total of any stars that weren't found in one of the Find commands done in steps 4/7 above.

9) You are responsible

0 - 80 315 - 340
80 - 125 340 - 385
125 - 155 385 - 420
155 - 230 420 - 500
230 - 255 500 - 580
255 - 315 580 -

10) The star located at x = 386 and y = 181 in the original B image is a standard star and has the following standard magnitudes:  B = 11.874 and V = 11.247.
Use the same method as in the recent Photometry lab to determine the B and V magnitudes from your data. 
Use the power of the spreadsheet to do the magnitude calculations!

11) After you have calculated both the B and V magnitudes for each star,
create another spreadsheet column that calculates the B-V magnitude for each star. 
Note: for later convenience, it would useful to have the B-V column to the left of the V magnitude column;
Excel always chooses the leftmost column of the two columns selected to become the x-axis and the rightmost column to be the y-axis.

12) AT THIS POINT WE WILL HAVE TO MERGE THE INDIVIDUAL SPREAD FILES,
SO EVERYONE MUST SEND ME YOUR RESULTS IN EXACTLY THE SAME FORM:

a) create a new spreadsheet that contains 4 and only 4 columns,
and containing no column labels (i.e., only the numbers described below);
if you don't know how to do this easily, ask me or someone else

b) the format must be Excel with extension .xls
no other formats please!

c) the name of the file is yourlastname (no capitals!),
which means the file name with extension will be yourlastname.xls

d) the first column contains the x-location of the star in pixels (no more than decimal place in the number, please)
e) the second column contains the y-location of the star in pixels (ditto above)
f) the third column contains the B-V magnitude (exactly two decimal places please)
g) the fourth column contains the V magnitude (again, exactly two decimal places)

h) save your file in the directory.................

13) Once we have all the data collected into 1 file (with the 4 columns described above),
you will plot the color-magnitude diagram of the cluster.
In order to create an embedded graph in Excel:

a) select the column of numbers that will become the y axis
(select only the V numbers by clicking the mouse and dragging over only numbers; do not include any blank cells or cells with text; do not select the whole column by clicking on the top letter of the column)
b) hold CTRL down and with the mouse, select the column numbers that will become the x axis (these are the B-V magnitude numbers)
(at this point, both columns should be highlighted)
c) click on Chart Wizard
d) click on XY Scatter Graph
e) choose type of graph. etc.

(after the graph is finished, remember that to reverse the vertical axis, so that the V magnitude axis increases downward! 
in some versions of Excel, you can right click on the vertical axis and select "reverse axis scale")

14) By comparing the (apparent) V magnitudes (often denoted m by astronomers) of stars on your color-magnitude diagram
with the absolute magnitudes of the same B-V stars (see green book), find the distance to the cluster. 
Use the entire cluster, not just one (or a few) B-V values. 
Provide ample documentation for your results.

15) Search the ADS data base for a published H-R diagram of NGC 6939 in the astronomical literature.
How does your color-magnitude diagram (and cluster distance) compare with professionally published data?

You will notice that our H-R diagram has only the brighter stars in the cluster;
much of the lower portion of the published diagram's main sequence will be missing. 

16) Determine the age of the cluster using the main sequence turnoff point. 
Document your steps.  Again compare to accepted values.