Name:______________________________________ Due Date:____________ Score: _____ / 30

Astronomy 480 - IRAF Exercise II
Quick-look Photometry with Imexamine

There is an image called "spicamfocus.fits.gz" in the directory: /net/projects/Astro_480/exercises/exercise2/ that you should copy to your directory. It is a typical focus frame, taken with an instrument called SPIcam on the Apache Point 3.5 meter telescope. Each pixel in the image spans 0.28 arcseconds on the sky. 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."

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 shift the telescope slightly down a bit, and take another image. If the shift is perfect, then 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.

1. <1 pt> By how many rows is the image single-space shifted between focus settings? ______

2. <4 pts> 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 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 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)

Star 2: Double-skip centroid: x = ________ y = ________ (pixels)

3. <2 pts> What would you choose as the best focus setting for the telescope? On what basis? Which FWHM value did you use?

4. <1 pt> Just using 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. <5 pts> 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. 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!]

For Object at x = ___523___ y = ___497___

6. <2 pts> From your data in the table above, explain how the sky, peak and FWHM values depend on the object radius.

7. <4 pts> Graph the FWHM and magnitude (smallest number at the top of the axis) values vs aperture radius values in the table below (use different colors or symbols and each axis will have different values and units). Do they tend to asymptotic values as the object radius gets larger? Explain.

8. <2 pts> 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?

9. <4 pts> 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. <5 pts> 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?