Name:________________________________________ Due Date:___________Score ________ / 30

Astronomy 480 - IRAF Exercise III
Image Processing: Bias Frames and Flatfielding

The questions on this sheet are intimately linked with the tutorial. If you do not understand a question, it could be that you’ve not gotten to that part of the tutorial yet.

1. <2 pts> One can use imsum or imarith to add images. Are these two tasks interchangeable as far as processing images? Why or why not.

2. <3 pts> In a similar vein, imsum and imcombine can both average images. List three differences between these two tasks. List one possible instance where you might prefer one over the other.

3. <1 pt> Use phelp imarith to see what operations are available under this task, and list them here.

4. <1 pt> Use imstat to determine the following quantities:

5. <1 pt> What is the useful imaging region ([x1:x2,y1:y2]) for this detector? _____________________

6. <1 pt> What is a sensible overscan region to use for bias level determination? ________________

7. <1 pt> Looking at m920001.fits, is the “bias” value constant over the entire imaging region? How big (in A/D units) is the variation across a row? (Implot might be helpful here.)

8. <1 pt> It is obvious that the bias level is higher over a small range of the left-hand columns. If one does not account for this, and if the object images do not have this trend, what effect on this region of the image will subtracting the bias images from the object images have? Choose the correct answer.

    1. the object images will not be affected
    2. these columns in the object images will have a lower-than-average count than the rest of the image
    3. these columns in the object images will have a higher-than-average count than the rest of the image

9. <1 pt> In your bias-subtracted object images, check to see if the noise level is flat across the image. Comment here on the results.

10. <1 pt> Why do you need to do distinct division operations for the sensitivity correction for the two passbands B and V?

11. <3 pts> Why do we want to normalize each flat field by its mode rather than, say, the average? (HINT: What will be the most frequently occurring number after you divide by the mode?) How does dividing by the normalized flat affect the pixel values of the object (science) image? Consequently, what’s the purpose as far as further processing of the object images is concerned?

[You’ve gone through the rest of the exercise, through CCDPROC, and survived.]

12. <2 pts> Show your results from CCDPROC to your instructor or TA and have one of them initial here: _____

Understanding more about these images:

13. <4 pts>

    1. Find the RA and dec of M92 in one of the science image headers (m920004 - m920007). RA = ___________ dec = ___________
    2. Could we observe this cluster this quarter?
    3. How did you figure out this answer?
    4. What was the local time (KPNO) when these images were taken?
    5. What was the approximate hour angle when these images were taken?
    6. What is the airmass for the image m920004?

14. <3 pts> Getting intimate with implot: ecl> implot m920006; How would you go about:

    1. getting the average of columns 100 to 200?
    2. expanding between 2 regions of the chip?
    3. setting a number of lines or columns to average?

15. <5 pts> Using good writing techniques, clearly explain what you learned in this tutorial and exercise assignment. The learning objectives listed at the top of the tutorial can be your guide, but you may also want to include comments on such topics as: Which steps or processes do you feel you need more practice on? What accomplishment do you feel most proud of? Did the multiple practice of IRAF commands help you remember what do do next? Feel free to add additional suggestions as well.