IRAF Exercise II - Quick-look Photometry with Imexamine

Name:________________________________________ Due Date:____________ Score: _____ / 30
IRAF Exercise II - Quick-look Photometry with Imexamine
There is an image called "spicamfocus.fits.gz" in the directory:
/astro/classes/Astro_480/exercises/exercise2/
that you should copy to your working directory. It is a typical focus frame, taken with an instrument
called SPIcam on the Apache Point 3.5 meter telescope. Each pixel spans 0.28 arcseconds on the sky.
Thought Question: You notice that one of the brightest stars has a diameter on the chip of about 10
pixels. How many arc seconds is that?
The picture contains multiple images of each star each at a different focus setting. This is accomplished
by doing 5 iterations of 1) open the shutter for 10 sec, 2) close the shutter, 3) shift the charge on the
detector down by a few rows, 4) change the telescope focus, 5) repeat. The CCD is then completely
read out. To keep track of things, there is a double-skip in the charge transfer on the last iteration. For
this frame, assume the focus was set to (1100, 1200, 1300, 1400, 1500) microns with the 1500 micron
setting corresponding to the images that are "double-spaced."
Suggestion: Use tools within ds9 viewer to rotate the image so that your view looks like this one. Also,
there are overscan regions to the left, bottom, and right of the image that will play a role later in this
exercise. Be sure to adjust parameters in IRAF display task to view image logarithmically or open the
file directly from ds9. You will want to have the contrast such that you see the entire “stack” and the
overscan regions.
Fig. 1 It’s hard to replicate via sketches how the
image on the left was created, and the actual method
involved reading out just part of the CCD chip and
taking another image. However, imagine that you
have just 2 stars in the field. You take an image. You
read out a number of rows of the CCD, and take
another image. The images of any given star will be in
a straight line vertically. It’s like stacking transparent
sheets of paper that have the same pictures on them,
but the vertical alignment is offset a bit for each sheet.
Caution: Be sure to keep the focus settings straight between the image and the tables below. Also, the
stars you choose cannot be saturated in any of the focus frames, so check all 5 images of the star
before actually choosing it. In general, choose the faintest stars where you can still identify all 5
images of them.
4/17/12 iraf2_exercise 1

1. By how many rows is the image single-space shifted between focus settings? ______
2. When the telescope has its best focus, the stellar images should be axisymmetric (ellipticity
= 0), smallest (min FWHM), and have the maximum peak intensity. Check out the 5 images of the
faint star whose 3rd image is at about (693, 470). Notice that the 3rd image seems better than the rest.
This is a clue that the best focus setting will likely be around 1300 microns. We will now confirm this
quantitatively. Edit the "rimexam" parameter file so that the object radius=4, background
buffer width=2 and background width=1. Now choose two well-behaved (unsaturated and
isolated) stars and fill in the tables below.
Star 1: Double-skip centroid: x = ________ y = ________ (pixels)
Focus Setting Peak Flux Ellipticity
1100
1200
1300
1400
1500
Focus Setting
1100
1200
1300
1400
1500
FWHM
(pixels)
Star 2: Double-skip centroid: x = ________ y = ________ (pixels)
Peak
Flux
Ellipticity
FWHM
(pixels)
FWHM (arcsec)
FWHM (arcsec)
3. What would you choose as the best focus setting for the telescope? On what basis? Which
FWHM value did you use?
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In your answering of the next question, we need to make sure that you
understand completely how the spicamfocus image was made.
•What information do the overscan regions contain? _________________
•How many “layers of sky” are in the brightest part of the image? ______
•In which part of the image will you get values of the real sky? ________
•Since that part will also contain read noise, how will you get just the
values of the sky in ADUs? ____________________________________
4. Using just the ds9 pixel readout (accessed from the Analysis
menu at the top), what would you estimate as the background sky value for
this image, in A/D units? ______________

5. Investigate how the rimexam parameters affect the results you obtain for aperture
photometry. The settings of the object radius, buffer width, and background width will affect the
determination of the sky values and object flux. Edit the rimexam parameter file and set rplot=15
(NOTE: this is not the aperture radius), buffer=1, width=1. For the values of object radius given, fill in
the table below (see next page). As you fill in the table, notice how the sky value in the radial plot
varies. Use the isolated star at (523,497) that is in focus. [Don’t forget to set up a log file to save your
work. Also, make sure that the iterations parameter in rimexam is set to 1!]
(radius = 1.) Object radius #change this value as you work through this step
For Object at x = ___523___ y = ___497___
Object radius Magnitude Flux Sky Peak FWHM
1
2
3
4
8
12
16
6. From your data in the table above, explain how the sky, peak and FWHM values depend on
the object radius. There were probably unexpected results or trends. If so, list one and come up with
your guess as to why it occurred.
7.
a) Graph the FWHM and magnitude (smallest numbers at the top of the axis for both) versus
aperture radius values in the graph shown on the last page (use different colors or symbols and each
axis will have different values and units).
b) Do they tend to asymptotic values as the object radius gets larger? Explain.
8. If the "right" answer is obtained with a very large object radius, what disadvantages might
dissuade you from using an object radius of, say, 25 pixels?
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9. How does the task imexamine determine the astronomical flux from a star? We are not
looking for the steps you took (such as, epar’ing, typing ‘r,’ making a log file, etc.), but the actual
process going on within the programs. What is IRAF doing to get the peaks, FWHM, sky values, and
magnitudes? This means digging into phelp imexamine.
10. Learning objectives are given at the top of the tutorial page for this exercise. In narrative
form, using good writing style, meet the first 5 objectives. For the proficiency goal: What do you think
you will easily remember as we go on to the next IRAF exercises, and why?
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Name ____________________________________
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