Once you have reduced your CCD images, there are many ways to do photometry on them.
Photometry with imexamine | |
Photometry with daofind | Creating DS9 Region Files |
Instructions for using Phot | Running Phot in Batch Mode |
Running Phot in Interactive Mode |
Photometry with imexamine
Imexamine is the most useful task when getting started with
photometry. It can be accessed from the cl prompt and any
other package. Enter imexamine filename for the
image you want to look at. (If you don't enter a filename, IRAF will
prompt you for one.)
The cursor will jump to the image on display in DS9, and the arrow
will change to a blinking circle. This represents the annulus which
will be used to measure the counts on a star and the average
background around the star.
Imexamine will display information about the area under the cursor
when you hit certain keys. Be sure that each time you want to get
information, the DS9 window is active. (You may have to click on the
window to make it active. You can also change the computer settings
so that the window in which the cursor is located will become the
active one. Go to Programs Settings
GNOME Control Center and then click on Focus
Behavior and edit the parameter When does the mouse pointer
affect the input focus.)
Here is a list of the key commands that will probably be most
useful for you. You can get more details about the commands, as well
as a full list of the Cursor Key Commands by entering help
imexamine.
? | Print help. This also will give you a listing of the Cursor Key Commands and descriptions of the outputs for 'a' and 'r' |
a | Prints useful information about the star under the cursor. |
First, the parameters will be listed in two rows, and then the values will be listed underneath in the same fasion. | |
c | Displays a 2D graph for a column of data through the cursor position. |
d | loads a new image without having to quit imexamine |
The cursor will move to the IRAF command window, and you will be prompted for a new image to display. | |
e | Displays a contour plot for the area beneath the cursor. |
l | Displays a 2D graph for a horizontal line of data through the cursor position. |
m | Prints statistics about the area under the cursor (such as mean, median, mode, and standard deviation) |
q | Quits imexamine |
r | Displays a radial profile of the star under the cursor. IRAF will fit a Gaussian to the data points, which will also be plotted. |
s | Displays a surface plot for the area under the cursor. |
x | Prints the cursor's coordinates |
, | Prints more information about a star under the cursor, including the Maximum Full Width Half Max (FWHM) |
Photometry with daofind
Daofind is a useful task for examining star clusters, because IRAF
will automatically find stars using an algorithm and do photometry on
them. To access this task, open the digiphot and then
apphot packages.
Before running daofind, you will need to take some preliminary
measurements with imexamine. Enter imexamine, and place the
cursor over random spots in the sky. Press 'm' to print
statistics about the area under the cursor. Note the standard
deviation value (sigma) for the sky. Do this several times and decide
on an appropriate approximation for later.
Now place the cursor over a star. Press 'r' to see a
radial profile for the star. Estimate the Full Width Half Max (FWHM)
by choosing the radius which seems to intersect with the value half
the height of the Gaussian. The FWHM is twice the radius. Check your
estimate by viewing the radial profile for other stars. The FWHM
should be about the same for all of them, and a value between 2 and 4
is normally fine. Stars which have saturated the CCD are not good for
estimating the FWHM. Another good way to check is to press ','
while the cursor is over a star. This will print data about the star,
and the last column shows the Maximum FWHM. However, it is better to
view the radial profile and choose an integer value for the FWHM.
Quit imexamine by pressing 'q'.
Now enter daofind. IRAF will prompt you with the following:
input image: | enter the filename for the image you want to process |
FWHM of features in scale units: | enter the FWHM you estimated earlier |
standard deviation of background in counts: | enter the sigma value you chose earlier |
detection threshold in sigma: | enter a whole number |
When daofind is examining the image, it will use an annulus with a diameter equal to the FWHM to find the peak number of counts inside and then estimate the average background count behind the star. It subtracts the background count from the peak, and then divides that value by sigma. If the quotient is above the detection threshold, that spot will be counted as a star. | |
It is wise to choose values from 4 to 6, although the values can vary, depending on the data. The higher the threshold, the fewer false stars will be detected. However, daofind may not pick up other real stars if the threshold is too high. | |
minimum good data value: | |
maximum good data value: | set the min value very low, and the max value very high. 1000 and 60000 are good values, unless all your stars are near saturation. Then 1000 and 65000 will be more appropriate. (These numbers are for the Brackett telescope CCD chip, which saturates at 65000 counts.) |
Checking Stars Found with tvmark
Tvmark is an incredibly useful command that allows you to see and edit the stars found
by daofind. You can edit the parameters by entering epar tvmark to mark the stars
in a variety of ways.
mark | Determines what type of mark is used on top of the stars found in the coordinate file. |
cross -- displays an x on top of the star | |
point -- displays a dot on top of the star | |
circle -- displays concentric circles over the star | |
rectangle -- displays rectangles over the star | |
plus -- displays a + on top of the star | |
color | Determines the marking color |
202 -- black | |
203 -- white | |
204 -- red | |
205 -- green | |
206 -- blue | |
207 -- yellow | |
208 -- cyan | |
209 -- magenta | |
label | yes will cause the star coordinates to be printed with the mark |
no will turn this option off | |
number | yes will cause the star number to be printed with the mark |
no will turn this option off | |
The stars do not have to be numbered in the coordinate file in order for this to work. Tvmark will number them in order | |
pointsize | Determines how big the points will be if "point" is the chosen mark |
txtsize | Determines how big the text will be, and the marking size if "cross" or "plus" is the chosen mark |
interac | yes turns interactive mode on, and no turns interactive mode off |
An easy way to change this parameter without having to enter the parameter listing, is to type interac+ with the tvmark command to turn interactive mode on or interac- to turn interactive mode off | |
radii | Determines the sizing of the concentric circles if "circle" is the chosen mark. |
Separate the various radii sizes by commas when you want to use more than one circle | |
nxoffset | |
nyoffest | Determines the X and Y offset (in pixels) for the star numbers. |
If these options are set to zero, the numbers will be displayed with the lower left corner at the center of the mark. With some markings, the number will be nearly impossible to read unless you modify this parameter |
Running tvmark in Interactive Mode
When interactive mode is turned on, the cursor will change to a blinking annulus.
You can execute a number of tasks by using keystrokes.
You can put marks on the frame by pressing: | |
+ for a plus sign | |
x for a cross | |
. for a point | |
c for concentric circle | |
r for rectangles | |
l | marks all objects on the coordinate list |
a | adds the object nearest the cursor position to the coordinate list and marks it |
d | deletes the object nearest the cursot position from the coordinate list and marks it |
Remember, when you are deleting points from the coordinate list, the markings will not disappear. It is not until you reload the display and reload tvmark will you see that the coordinates have been deleted. | |
q | quits tvmark |
Creating DS9 Region Files
You can also create region files with the coordinates given by daofind
by using the task txdump. It will
work in the same package as daofind. Just enter digiphot and
then apphot.
Don't worry about setting parameters. All the parameters you will
need can be entered with the command as follows:
Instructions for using Phot
Phot is IRAF's basic photometry package. It can be loaded
by entering digiphot and then apphot starting from the
cl prompt. The phot task has five parameter lists with lots of
variables, but IRAF will prompt you only for the critical ones.
Entering epar phot will bring up the main parameter list.
The important variables that you will need to change are:
image | the name of the file you're doing photometry on | output = default | The default setting will attach .mag to the end of the filename, with a number. You can choose an alternate filename for the results if you like. |
coords | the name of the coordinate list you are using |
Any .coo or .reg type of file, such as the ones creates with daofind, should work. | |
interac = no | Interactive mode is on when "yes" is entered. Batch mode is on when "no" is entered. |
In interactive mode, the cursor will change to a blinking annulus before doing photometry. It will then wait for key commands before running. This will happen even if you have specified a coordinate file for phot to use. Type in help phot in the IRAF command window to see a full list of the key commands. | In batch mode, phot will not prompt you for any key commands. It will automatically run through the coordinate file you have specified. You will also be prompted for the vital parameters. You should run phot in batch mode unless you want to check your photometry by varying the parameters before saving the results to file. |
An easy way to change this parameter at the command prompt is to type interac+ to turn interactive mode on or interac- to turn interactive mode off. This parameter should be assigned after typing in the full command. | |
verify = yes | make sure that verify is on so that the critical parameters in using phot will be verified when you are in batch mode |
Running Phot in Batch Mode
There are various parameters the phot command uses in order
to do photometry. You will be prompted for the most critical
parameters. Just make sure interactive mode is off, and enter
Input image: | enter the image you want to do photometry on or press enter to accept the one in parenthesis |
Centering algorithm (centroid): | controls the centering algorithm, which finds the stars' centers |
If you are using the coordinate file from daofind, then the centers are probably good enough, and you can choose none. (It saves a tiny bit of time.) | |
If you created your own coordinate list or you have chosen a number of stars manually with tvmark, choose centroid. (This is the default, and if shown in parenthesis, just press enter.) | |
Centering box width in scale units: | controls the size of the box that IRAF uses to find the stars' centers |
The default is 5 pixels, and normally good enough. If this is the number shown in parenthesis, press enter or enter your own value. | |
Be sure that the cbox size is not too high, or else more than one star may fit in the box and cause confusion | |
Sky fitting algorithm (centroid): | This algorithm controls how the sky value is calculated from the annulus |
You can choose from mean, median, mode, and centroid. Massey and Davis, authors of A User's Guide to Stellar CCD Photometry have suggested using mode. | |
Centroid is the default, and will probably appear in parenthesis everytime you run phot. Just enter the algorithm you want, or press enter to accept the one in parenthesis. | |
Inner radius of sky annulus in scale units: | choose a radius or press enter to accept the one in parenthesis (10 pixels is a good choice) |
Width of the sky annulus in scale units: | choose a width for the sky annulus or press enter to accept the one in parenthesis |
File/list of aperture radii in scale units: | change the aperture radius or press enter to accept the one in parenthesis |
A number around twice the FWHM is good for fitting the entire star inside aperture radius. | |
Minimum good data value (INDEF): | press enter to accept "indefinite", unless you have an idea of what you want the datamin to be |
Maximum good data value (INDEF): | press enter to accept "indefinite", unless you have an idea of what you want the datamax to be |
Running Phot in Interactive Mode
If you leave the interac parameter set to "yes" in the
epar phot listing, the cursor will become a blinking annulus
(just like imexamine), even if you have specified a coordinate file.
However, interactive mode can be useful because you can edit the
critical parameters and view the photometric results before saving it
to an output file.
Enter phot filename at the ap prompt to
start. The cursor will jump to the image display and become a
blinking annulus. You can execute commands with the following keystrokes. (For a full list, enter help phot in the IRAF command window.)
v | the IRAF window will prompt you for the critical parameters |
w | saves the parameters to the essential parameter listings |
i | allows you to interactively set parameters with the current star |
A window will appear. Press enter to accept the extraction width. A radial profile plot will appear. Press ? to see a list of keystrokes you can use to interactively set certain parameters. Press q to quit | |
f | prints photometry for the current star in the IRAF command window, but does not save it to the output file |
space | does photometry on the current star and saves it to the output file |
m | moves to the next star in the coordinate list |
When you start phot, it will not start using the stars on the coordinate list until you press 'm'. The cursor will not jump, but if you keep pressing 'm' and then 'f', you will see that the data changes. | |
n | does photometry on the next star in the coordinate list and saves the data to the output file |
l | does photometry on the remaining stars in the coordinate list and saves the results to the output file |
r | (rewind); sends the coordinate list back to the beginning |
You will have to press 'm' again in order to go to the first star on the list. Otherwise you will stay on the star you were using before the list was rewound | |
q | exits phot |
c | fits the center for the current star |
t | fits the sky around current centered star |
p | does photometry on the current star using the current sky |
o | does photometry on the current star using the current sky, and saves the results to the output file |