Name:_____________________________________________ Due Date:__________Score ____ / 35
IRAF Exercise IV - Aperture Photometry
1. <1 pt> How can you tell from the image header whether CCDPROC has been run on a frame?
2. <1 pt> As a follow on to the ‘set-airmass’ step: List two reasons why the blue images might need a longer
exposure time to reach a similar flux level as the visual images.
3. <3 pts> "Setairmass" is one way to "patch" an image header. Find two other tasks that will allow you to add,
delete, or modify a keyword in an image header, and summarize briefly how these tasks work.
4. <3 pts> The way that QPHOT defines an aperture and sky region differs from what we saw when we used
RIMEXAMINE in a previous exercise. Review both and explain the differences briefly here. (NOTE: if you
unlearn imexamine you should also separately unlearn rimexamine.)
5. <2 pts> How can you find out what quantities are listed in the m920004.mag.1 file? Specifically, which
column in the file lists the magnitude error?
6. <5 pts> Work with TXDUMP and with just the m920004 and m920006 images. Extract the xcenter, ycenter,
magnitudes, and ifilter identification for the 9 stars, and put the information into a file. Making sure you have
matched the data for each star between files, list the V magnitudes, B magnitudes, and B – V magnitudes. Plot V
against B – V, making sure that the smallest V magnitude is at the top of the y-axis and the largest at the bottom.
Attach your plot to this worksheet. SAVE YOUR X-Y COORDINATES AND MAGNITUDES FOR BOTH
FILTERS FOR USE IN EXERCISE IRAF V.
2/12/16
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7. <2 pts> Show your graph to your instructor or TA and have him/her initial here: _____________
Using multiple apertures for comparison of photometry results
The qphot task can be used to determine aperture photometry values for a set of apertures, rather than a just one
single one. Load the digiphot and apphot packages, then set a list of multiple apertures:
<ecl> epar qphot
image = "m920004"
Input image
cbox = 5.
Centering box width in pixels
annulus = 15
Inner radius of sky annulus in pixels
dannulus = 10
Width of the sky annulus in pixels
apertures = "1,2,3,4,5,10"
List of photometry apertures
(coords = "")
Coordinate list
(output = "default")
Results file
(plotfile = "")
Plot metacode file
(zmag = 25.)
Zero point of magnitude scale
(exposure = "exptime")
Exposure time image header keyword
(airmass = "airmass")
Airmass image header keyword
(filter = "filters")
Filter image header keyword
(obstime = "ut")
Time of observation image header keyword
(epadu = 14.)
Instrument gain
(interactive = yes)
Interactive mode
(radplots = yes)
Plot radial profiles in interactive mode
(verbose = no)
Print messages
(graphics = "stdgraph")
Graphics device
(display = "stdimage")
Display device
(icommands = "")
Image cursor: [x y wcs] key [cmd]
(gcommands = "")
Graphics cursor: [x y wcs] key [cmd]
(mode = "ql")
8. <2 pt> Run qphot on m920004 again, this time with multiple apertures. (Try killing the graphics window
each time before restarting qphot). Concentrate on star #7 of the finding chart. Place the cursor on the star and
hit the space bar. Fill in Table 1, where Mag(i) refers to the magnitude obtained within a radius of i pixels.
Table 1. Aperture Photometry data for Object 7 in m920004.fits
X
Y
Sky
Mag(1)
Mag(2)
Mag(3)
Mag(4)
Mag(5)
Mag(10)
9. <2 pts> How much more flux (in ADU’s) fell within a 5 pixel radius, compared to the flux within a 2 pixel
radius? Recall that m1 – m2 = –2.5 log(f1/f2). Do not simply subtract the fluxes that the program returns; show all
of your quantitative logic.
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10. <2 pt> Now repeat the multi-aperture qphot measurement, but for star #9, and fill in Table 2 below. All you
may need to do is place the cursor over the object and hit the space bar.
Table 2. Aperture Photometry data for Object 9 in m920004.fits
X
Y
Sky
Mag(1)
Mag(2)
Mag(3)
Mag(4)
Mag(5)
Mag(10)
11. <1 pt> By now you have hopefully realized that it requires a large aperture to actually integrate all the light
from an object. In crowded regions, this is problematic due to other stars, and because it also folds in a lot of sky
photons that add to the noise in the measurement. As long as the Point Spread Function (PSF) is constant across
the image, you can make good relative measurements (i.e. differences in magnitudes between objects) with a
small aperture. Use the results from Tables 1 and 2 to complete table 3, with
Dmag = mag(object 7) – mag(object 9).
Table 3. Aperture Photometry comparison for Objects 7 and 9 in m920004.fits
DMag(1)
DMag(2)
DMag(3)
DMag(4)
DMag(5)
DMag(10)
12. <3 pts> Does the magnitude difference between objects 7 and 9 depend very strongly on the aperture
chosen? Be quantitative and also include how you have defined "very strongly." You should consider the error
in the magnitudes when answering this question. (For example, one could compare the range in magnitudes to
the average magnitude error for these 2 stars.) Be sure to state what effect using an aperture of 1 pixel had on the
first DMag value.
13. <3 pts> What does your answer to the previous question imply for differential photometry?
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14. <5 pts> This IRAF assignment is one of the hardest of the course as we take fairly sizeable leaps in using the
packages and tasks. In general and understandable terms, describe the steps needed in photometry to produce a
reliable color-magnitude diagram for a globular or an open cluster. You are addressing an intelligent audience,
but perhaps one where most of the participants have not gone through this process. Satisfy some or all of the
learning goals stated in the tutorial within the process of your writing. (It is perfectly fine to type your essay and
attach to this answer sheet. It is strongly encouraged if your handwriting is difficult to read.) We will be
checking spelling and your use of the English language.
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