UW ASTR 480 - Study Notes (6 pages)

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Study Notes



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Study Notes

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Pages:
6
School:
University of Washington
Course:
Astr 480 - Astro Data Analys
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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 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 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 FWHM pixels FWHM arcsec 1100 1200 1300 1400 1500 Star 2 Double skip centroid x y pixels Focus Setting Peak Flux Ellipticity FWHM pixels FWHM arcsec 1100 1200 1300 1400 1500 3 2 pts What would you choose as the best focus setting for the telescope On what basis Which FWHM value did you use 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 1 pt 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 2 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 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 2 pts 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 4 pts 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 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 3 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 4 Name 5 We have intentionally put only this sentence on this page 6


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