1. A Description of the Script
The script (wfpc2_script1) performs its work on HST observations of Omega-Cen (= NGC 5139) taken with WFPC2 on February 15, 1995. Two 16-second images using the F555W filter (u2g40o05t.c0h and u2g40o06t.c0h) and two 14-second exposures using the F814W filter (u2g40o09t.c0h and u2g40o0at.c0h) are used in the script. Observers are welcome to modify the script for other datasets and for their own purposes, but they should keep in mind that it is meant primarily as a demonstration tool. A more careful treatment may be required to get the most from your data (e.g., correction for CTE, color terms, geometric distortions, aperture corrections, etc.) This section contains a brief description of each step.
See Part 2 for instructions on getting the files and notes for running wfpc2_script1. After getting into IRAF and loading the packages required, follow the following steps.
Step 1: Remove Working Files
del ttt *
WARNING: The script uses files beginning with "ttt" as working files and deletes them at the start, so make sure this won't cause any problems. Remember to copy any files you want to save after running the script.
Step 2: Cosmic Ray Removal
gcombine u2g40o05t.c0h,u2g40o06t.c0h ttt1 reject=ccdcrrej combine=average hsigma=3 rdnoise=5 gain=14 gcombine u2g40o09t.c0h,u2g40o0at.c0h ttt1b reject=ccdcrrej combine=average hsigma=3 rdnoise=5 gain=14
There are several programs available for removing cosmic rays (e.g., gcombine, crreject, and a new program developed by Rick White currently being implemented into STSDAS). As is standard, two separate Omega-Cen exposures have been taken to help identify the cosmic rays. Wfpc2_script1 uses gcombine for this step. An examination of the resulting images (ttt1 and ttt1b) shows that this task is quite successful at removing cosmic rays for these short exposures which inherently have few cosmic rays, and so have very few cases where cosmic rays fall on the same pixels on the two separate images (using the display task described on page 7). This becomes a bigger problem for long exposures. The  after each image name means we will work only on group 1 data (i.e., the PC chip) in this script. Note that the calibration data are taken with gain=14, while most observations are taken at gain=7.
Step 3: Hot Pixel Removal
cosmicrays ttt1 ttt2 answer=no threshold=5 fluxrat=3 npasses=5 window=5 interactive=no cosmicrays ttt1b ttt2b answer=no threshold=5 fluxrat=3 npasses=5 window=5 interactive=noBefore we can identify the stars, we need to remove a few hot pixels that might be mistaken for stars. This is not critical for these very short exposures since the background is not high enough to allow any but the "hottest" pixels to be evident. The only hot pixels removed during this step are [135,189], [609,623], and [154,107]. However, hot pixels may be the primary difficulty when looking for very faint point sources on long exposures.
For some scientific programs, such as finding relatively bright stars on short exposures, using the IRAF task cosmicrays to remove hot pixels may be sufficient. In other cases you may want to retrieve the current hot pixel list from WWW, or recalibrate using updated dark images from the archives. Another technique for removing hot pixels is to dither the images (i.e., observe them with slight offsets; see articles in the January and February 1995 versions of the Space Telescope Analysis Newsletter available via WWW or several articles in the February 1995 ST-ECF Newsletter available via WWW. You might also wish to apply the distortion image available on WWW (f1k1552bu.r9*) at this point.
A word of warning is in order about using cosmicrays. If you set the criteria too low, you may find that you begin removing the centers of real stars. Carefully examine the images before and after using this task (e.g., by "blinking" the images), or use imarith to subtract the two images to make a mask that shows where changes have been made. For example, a close look at [609,623] and [154,107] suggests that these are probably real objects, hence a threshold of 5 was probably too low. A value of threshold = 7 would be a better choice.
Step 4: Trim Off the Region Occulted by the Pyramid
imcopy ttt2[44:799,52:799] ttt3 imcopy ttt2b[44:799,52:799] ttt3b
Before identifying the hot pixels we need to trim off the regions occulted by the pyramid, as well as the last column or two, or we will get lots of false identifications. Note that if you modify this script to work on the other chips, you will need to change the region that is trimmed since the occulted region is different for each chip. See Table 2.5 of the WFPC2 Instrument Handbook for information on the regions occulted on each chip.
Step 5: Identify the Object
daofind ttt3 output=ttt3.coo.1 sigma=0.5 nsigma=1.5 fwhmpsf=1.5 thresho=10 readnoi=7.1 epadu=28 sharplo=.2 verify=no interactive=no
The daofind task from DAOPHOT (Stetson, 1987, PASP, 99, 111) is used to identify the stars. The current setup searches for objects that are more than 10 sigma above the local background and writes them to the file ttt3.coo.1. Sigma must be set manually. For example, the command "imstat u2g40o05t.c0h[200:300,220:320] lower=-2 upper=2" has been used in this case to determine the background in a very blank region.
Some of the other parameters defined here are:
nsigma = 1.5 (width of convolution kernel in sigma) fwhmpsf = 1.5 (full-width-half-maximum of star) thresho = 10 (threshold in units of sigma) epadu = 28 (electrons per adu [counts]; used to estimate +/-) (this is often called "gain" by other programs) readnoise=7.1 (readnoise)
The coordinate list written to ttt3.coo.1 will look something like this:
#K IRAF = NOAO/IRAFV2.10.3BETA version %-23s #K USER = whitmore name %-23s #K DATE = 03-17-95 mm-dd-yr %-23s #K TIME = 15:00:56 hh:mm:ss %-23s #K PACKAGE = apphot name %-23s #K TASK = daofind name %-23s # #K SCALE = 1. units %-23.7g #K FWHMPSF = 1.5 scaleunit %-23.7g : : #N XCENTER YCENTER MAG SHARPNESS ROUNDNESS ID \ #U pixels pixels # # # # \ #F %-13.3f %-10.3f %-9.3f %-12.3f %-12.3f %-6d \ # 294.201 78.858 -2.345 0.911 -0.109 1 697.219 104.314 -0.158 0.815 0.118 2 : : 561.083 668.179 -1.480 0.920 0.060 34 654.141 739.081 -1.659 0.957 -0.066 35
See A User's Guide to Stellar CCD Photometry with IRAF by P. Massey and L. E. Davis (available from NOAO) for lots more details about DAOPHOT (e.g., running it interactively to tune the parameters for your own use).
Step 6: Aperture Photometry
phot ttt3 skyfile="" output=ttt3.mag.1 coords=ttt3.coo.1 readnoi=7.1 epadu=28 exposure="" aperture=3 salgori=median zmag=24.575 annulus=10 dannulus=5 calgori=none interactive=no verify=no phot ttt3b skyfile="" output=ttt3b.mag.1 coords=ttt3.coo.1 readnoi=7.1 epadu=28 exposure="" aperture=3 salgori=median zmag=23.428 annulus=10 dannulus=5 calgori=none interactive=no verify=no txdump ttt3.mag.1 xcenter,ycenter,mag,merr mode=h > ttt4 txdump ttt3b.mag.1 mag,merr mode=h > ttt4b tcalc ttt4 c1 "c1+43" tcalc ttt4 c2 "c2+51"
Aperture photometry is performed using an aperture with a radius of 3 pixels ("aperture=3"), the background determine by the median ("salgori=median"; the median is less sensitive to contaminating objects in crowded fields), within an annulus from 10 pixels ("annulus=10") to 15 pixels ("dannulus=5"). The coordinate list from the previous step is used with no recentering ("calgori = no"). The zero point is 24.575 for the F555W observations and 23.428 for the F814W observations. For example, an object with 100 counts in the F555W filter has a magnitude of -2.5 log (100) + 24.575 = 19.575 mag.
WARNING: These zero points are appropriate only for the particular choice of apertures and exposure times used in this script. See Holtzman et al. (The Photometric Performance and Calibration of WFPC2, Jon Holtzman et.al., 1995, in prep., and Photometry with the WFPC2, Brad Whitmore, 1995, Proceedings of the 1995 HST Calibration Workshop; both papers available at WWW), for further discussion on this subject.
The results are written to ttt3.mag.1 for the F555W results and to ttt3b.mag.1 for the F814W results, and will look something like this:
: : #K SALGORITHM = median algorithm %-23s #K ANNULUS = 10. scaleunit %-23.7g #K DANNULUS = 5. scaleunit %-23.7g : : # #K WEIGHTING = constant model %-23s #K APERTURES = 3 scaleunit %-23s #K ZMAG = 24.575 zeropoint %-23.7g # # #N IMAGE XINIT YINIT ID COORDS LID \ #U imagename pixels pixels ## filename ## \ #F %-23s %-10.3f %-10.3f %-5d %-23s %-5d # #N XCENTER YCENTER XSHIFT YSHIFT XERR YERR CIER CERROR \ #U pixels pixels pixels pixels pixels pixels ## cerrors \ #F %-14.3f %-11.3f %-8.3f %-8.3f %-8.3f %-8.3f %-5d %-13s # #N MSKY STDEV SSKEW NSKY NSREJ SIER SERROR \ #U counts counts counts npix npix ## serrors \ #F %-18.7g %-15.7g %-15.7g %-7d %-6d %-5d %-13s # #N ITIME XAIRMASS IFILTER OTIME \ #U timeunit number name timeunit \ #F %-18.7g %-15.7g %-23s %-23s # #N RAPERT SUM AREA MAG MERR PIER PERROR \ #U scale counts pixels mag mag ## perrors \ #F %-12.2f %-15.7f %-15.7f %-7.3f %-6.3f %-5d %-13s # ttt3 294.201 78.858 1 ttt3.coo.1 1 \ 294.201 78.858 0.000 0.000 INDEF INDEF 0 NoError \ -0.0150283 0.3968335 -0.2390791 389 3 0 NoError \ 1. INDEF INDEF INDEF \ 3.00 184.4735 28.5715 184.9029 18.908 0.020 0 NoError ttt3 697.219 104.314 2 ttt3.coo.1 2 \ 697.219 104.314 0.000 0.000 INDEF INDEF 0 NoError \ -0.0236741 0.3923617 -0.1982743 391 2 0 NoError \ 1. INDEF INDEF INDEF \ 3.00 29.98758 28.52468 30.66288 20.858 0.085 0 NoError : : ttt3 654.141 739.081 35 ttt3.coo.1 35 \ 654.141 739.081 0.000 0.000 INDEF INDEF 0 NoError \ -0.05124146 0.4315955 -0.1955512 302 2 0 NoError \ 1. INDEF INDEF INDEF \ 3.00 92.23106 28.44914 93.68884 19.646 0.035 0 NoError
These can be a little hard to read. To clarify, some of the main parameters from the first object are:
XINIT= 288.201 (X position) YINIT=70.858 (Y position) ID = 1 (ID number) MSKY = -0.0150283 (counts in the background annulus; NOTE: The background has been slightly over-subtracted in this image.) SUM = 184.4735 (total counts in the object aperture) AREA = 28.5715 (area in pixels2 in the object aperture) MAG = 18.908 (object magnitude) MERR = 0.025 (magnitude uncertainty)
As seen above, there are 184.47 counts in the main aperture, a background of -0.01503 times 28.5715 = -0.43 needs to be subtracted off, so the corrected flux is 184.47 + 0.43 = 184.90, hence the final result is -2.5 log(184.90) + 24.575 = 18.908 mag.
The relevant information is then written to ttt4 and ttt4b, and the offset used in step 4 is added back in.
Step 7: Merge the Tables into one Table and Calculate the Mean Value of (F555W - F814W)
tmerge ttt4,ttt4b ttt5 allcols=yes option=merge tcalc ttt5 new c3-c5 tstat ttt5 c7
This step merges the tables obtained for the F555W and F814W filters into a single table (ttt5), determines the value of (F555W - F814W) which it puts into a new column (c7), and then determines the mean value of (F555W - F814W) along with a few other statistics. The statistics are written to the screen, but are not saved in a file.
Step 8: Compare the Results with Harris et al.
copy ttt5 ttt6 tsort ttt6 c3 joinlines ttt6,omegacen_1 > ttt7a tselect ttt7a ttt7 "c15 == 0" tcalc ttt7 new c3-c11 tcalc ttt7 new c5-c13 tstat ttt7 c16 tstat ttt7 c17
Table 5 is copied and sorted by F555W magnitude, and then joined with the file omegacen_1 which contains the published ground based photometry from Harris, AJ, 105, 1196. The file omegacen_1 must be downloaded along with the script (see Appendix A). The differences between the WFPC2 and Harris et al. measurements are then computed and put into columns c13 and c14, and the statistics on the comparison are written to the screen but are not copied to a file.
The results indicate that the zero points are accurate and the scatter in the measurements is 0.022 mag for F555W and 0.019 for the F814W measurements.
The final results for the stars in common with Harris et al. are in ttt7, as shown below. The full listing for all the detected stars is in ttt5 (sorted in order of Y) and ttt6 (sorted in order of F555W magnitude).
# X Y V +/- I +/- V-I X Y ID gr V +/- gr I +/- f V - gr V I - gr I # ------- ------- ------ ----- ------ ----- ---- --- --- ---- ------ ----- ------ ----- - --------- --------- # 143.036 453.827 14.792 0.002 13.678 0.002 1.114 143 454 1629 14.799 0.004 13.656 0.004 0 -0.007 0.022 # 315.165 191.122 16.339 0.005 15.346 0.005 0.993 315 191 1461 16.359 0.008 15.348 0.006 0 -0.02 -0.002 # 313.878 662.084 16.629 0.005 15.601 0.006 1.028 314 663 1714 16.598 0.006 15.598 0.007 0 0.031 0.003 # 716.766 196.045 17.748 0.01 16.882 0.012 0.866 717 196 1425 17.751 0.008 16.906 0.013 0 -0.003 -0.024
Step 9: Display the Image and Mark the Objects
display ttt2 1 zrange=no z1=-5 z2=100 ztrans=linear tvmark 1 coords=ttt4 mark=circle radii=5 color=202 label=no number=yes nxoffse=10 nyoffset=0 pointsi=1 txsize=1 interac=no tvmark 1 coords=ttt7 mark=rectangle lengths=15 color=203 label=no number=yes nxoffse=10 nyoffset=-30 pointsi=3 txsize=3 interac=no
The image is displayed and the stars are identified (see Figure 1). The stars within the black circles are from the full object list, while the stars within the white squares are 2 of the 4 in common with the Harris et al. measurements.
Step 10: Use IGI to Plot the Color-Magnitude Diagram
Make a plot of the color-magnitude diagram.
NOTE: To make a hardcopy rather than plotting on your computer screen just change "stdgraph" to "stdplot" below and remove # in front of "gflush". To make a PostScript file, change stdgraph to "psi_land" (landscape) or "psi_port" (portrait), and remove the # in front of "gflush".
igi device=stdgraph data ttt5 xcol 7 ycol 3 limits .5 1.5 22 14 box xlabel (V-I) ylabel V ptype 12 3 points bye # gflushColor magnitude diagram of the stars in a PC image of Omega-Cen as generated by igi.
Step 11: Make a LaTeX File and Preview the Results
Make a LaTeX file of the results, convert it to PostScript, and preview it.
NOTE: table1header.cd is the header file which should be downloaded along with the script and the data files.
tcreate ttt5.tab table1header.cd ttt5 hist=no tprint ttt5.tab pwidth=80 plength=40 showrow=yes columns=* option=latex > ttt5.tex !latex ttt5 !dvips ttt5 !ghostview ttt5.ps &Positions and magnitudes of the stars in a PC image of Omega-Cen as generated by tcreate.