Документ взят из кэша поисковой машины. Адрес оригинального документа : http://www.stsci.edu/hst/wfpc2/documents/tir/TIR/tir_9710.pdf Дата изменения: Wed Dec 3 18:42:58 2008 Дата индексирования: Thu Jan 15 04:40:54 2009 Кодировка: Поисковые слова: coma

Technical Instrument Report WFPC2 - 97-10

WFPC2 Photometry from Subtraction of Observed PSFs
J. Surdej1,2, S. Baggett3, M. Remy1, M. Wiggs3 October 3, 1997

ABSTRACT Based on observed PSFs from the WFPC2 calibration programs, a series of PSF subtraction tests have been performed and the resulting photometry analyzed. We find that using a composite observed PSF, constructed from optimally selected PSFs based on location and breathing values, yields single photometric values affected by an RMS of ~0.01-0.02 mag. While resampling does not appear to have much of an effect on the photometric results, the color of the PSF employed is important.

1. Introduction
Subtraction of scaled PSFs from direct CCD images of quasars (or stars) not only offers the possibility of detecting the presence of host or foreground galaxies (or companions like brown dwarfs or planets) but also provides a means of deriving accurate photometry of the primary objects. The observations used for the tests described here were taken from the WFPC2 photometric monitoring programs; primarily F555W in the PC was used, although separate independent tests were also done with a subset of the F814W and F439W, PC and WF3, images. The target in all cases was the spectrophotometric standard GRW+70D5824, a DA3 white dwarf (V=12.77, B-V=-0.09). Details of the F555W images are provided in the table in Appendix A; tabulated are the image rootname, the PSF positions on the original chip, the row and column position of the PSF in the mosaic frame (see Figure in Appendix A), the date and MJD for the start of the observations, the exposure time (in seconds), the relative defocus of the secondary mirror (in microns) and finally, the X,Y components of coma (in microns) of wavefront error. The relative focus was determined using the phase retrieval code of Krist and Burrows (1995) to reproduce the detailed shape of the observed PSFs; the derived focus positions are illustrated in Figure 1 as a function of Modified

1. 2. 3.

Institut d'Astrophysique, UniversitИ de LiХge, Belgium Research Director (FNRS, Belgium) STScI 1


Julian Date (MJD). Figure 5 in Appendix A provides a greyscale mosaic of the 43 observed PSFs. Figure 1: Relative focus positions (in microns) as a function of Modified Julian Date.

In the MIDAS environment, Remy (1996, Ph.D. thesis) has developed a general, automatic procedure to derive optimal photometric measurements of (multiple) point sources. A composite PSF is determined by summation of the selected images of the spectrophotometric standard regularly observed with WFPC2, after recentering at the same position by bi-quadratic interpolations. Photometric measurements of single observations are then determined by fitting in flux and position the above composite PSF, using a chi-squared minimization method. A description of this automatic procedure may also be found in Remy et al. (1997).

2. The Photometry
Single Star - F555W, PC As a baseline for comparison, photometry using a single PSF star (characterized by various defocusing values) was performed on the 43 individual F555W images. Depending upon the precise focus value of the observed PSF used, the final average magnitudes ranged from 12.79 to 12.85, with scatter typically ~0.02 mag (somewhat higher, 0.04 mag, for the highest focus PSF, ~0.5 microns). These tests indicate that using a single PSF leads

2


to adequate results provided that it is close in location and focus to the target. However, the use of an observed PSF whose relative focus position differs by 6 microns or more may result in a systematic photometric error that exceeds 0.1 mag. Composite of 42 Observed PSFs - F555W, PC A composite PSF image constructed from 42 of the observed F555W PSFs is shown in Figure 2 (one of the 43 PSFs, namely the file with rootname u2a70105t, was omitted since it was so far out of focus compared to the others). Figure 2: Composite F555W PC PSF based on 42 direct images of GRW+50D5824.

This composite PSF was subtracted from each of the 43 original observations; a greyscale representation of the subtraction results is illustrated in Figure 5 in Appendix A. Because of the PC image undersampling, concentric rings are indicative that the centering may be slightly different from the center of the composite; fainter features around the periphery are probably due to focus differences (breathing, i.e. the PSF variations seen over the timescale of an HST orbit). The photometry results achievable when using this 42-image composite PSF are plotted in Figure 3 in the form of magnitudes obtained as a function of focus. The scatter affecting the photometric results of the 34 reliable (see below) observations of GRW+70D5824 is 0.014 mag. Note that no systematic dependence of the derived V magnitude as a function of the relative focus position is noticeable. These good photometric results are certainly due to the high S/N of the composite PSF constructed from the 42 single observations.

3


Figure 3: Magnitudes, obtained via PSF subtraction of the 42-image composite, as a function of focus (in microns).

Composite of 34 Observed PSFs - F555W, PC Immediately apparent in the photometric results of the previous test (Figure 3, using the composite of 42) are eight outliers; five of these appear to be due to PSFs with larger coma or PSFs which are in a substantially different location on the chip while three of the PSFs were taken under different conditions: clocks=ON and therefore, the exposure time was slightly shortened (two in Dec 94) and at a warmer operating temperature (one in Feb 94). For these reasons, a second observed composite was constructed, omitting these questionable PSFs as well as the one PSF dropped earlier (very far from the average focus). The photometry results using this composite of 34 images are also listed in Table 1. The photometric results derived from the 34 composite PSF are comparable (0.014 mag scatter) to those based on the 42 composite PSF.

4


Position and Spectral Type Dependence Figure 4 demonstrates the scatter in the resulting photometry when TinyTim (Krist, 1993) model PSFs at a range of angular distances away from the target are used. Also in the same figure (at distance = 0) are the results when using TinyTim PSFs of various spectral types. As can be seen, a mismatch in spectral type can result in nearly as much error as when using a PSF ~15-20" away from the target. Figure 4: Changes in photometry as a function of radial distance from center (in arcsec). Additional data points at dist = 0 are the results of using TinyTim PSFs of various spectral types (B-V= -0.297, -0.155, 0.126, 0.619, and 1.590; note: for dist > 0, magnitudes were computed using B-V=-0.155).

5


3. Conclusions
Table 1 below summarizes the average magnitude and scatter obtained from the PSF subtraction photometry of GRW+70D5824 when using the noted (composite or single) PSF type and focus range. These results indicate that the best PSF subtraction photometry is obtained when using a composite PSF close in relative focus to the target, originating from a location on the chip as close as possible to the target (typically less than 10") and having a spectral type as similar to the target's type as possible, in that order of importance. Owing to the better S/N, a composite PSF fared better than a single PSF, however, the specific number of PSFs used in the composite was found to be relatively unimportant. In addition, resampling did not improve the resulting photometry. Additional tests were also done with a subset of F814W and F439W PC and WF3 images, corroborating the F555W PC results. A WFPC2 PSF Library has been established to enable users to carry out more experiments (see WWW page, under WFPC2 Software Tools). An investigation of photometric measurements based upon subtraction of TinyTim model PSFs has been performed by Remy et al. (1997). Table 1. Summary of the F555W PSF subtraction photometry results. The average magnitudes and scatters were calculated from the 34 reliable observations (see text).

test case

relative focus of composite PSF (in microns)

no resampling

2x2 resampling

mag observed PSF, single F>0.492 0.492>F>=-0.820 -0.820>F>=-2.459 -2.459>F observed PSF, composite of 42 observed PSF, composite of 34 2.295>F>-11.311 2.295>F>-11.311 12.806 12.809 12.849 12.792 12.736 12.769

scatter 0.042 0.022 0.024 0.024 0.014 0.014

mag 12.826 12.750 12.826 12.746 12.770 12.767

scatter 0.087 0.065 0.050 0.049 0.016 0.021

6


4. References
Hasan, H., and Bely, P., 1994, in The Restoration of HST Images and Spectra II, R. Hanisch and R. White, eds., p. 157. Casertano, S., 1995, Instrument Science Report, OTA 18. Krist, J., 1993, in Astronomical Data Analysis Software and Systems II, ASP Conference Series 52, R. J. Hanisch, R. J. V. Brissenden, and J. Barnes, eds., 530. Krist, J., and Burrows, C., 1995, Applied Optics, 34, 4951. Remy, M., 1996 (thesis). Remy et al., 1997, in Calibration Workshop Proceedings. Wiggs, M., Baggett, S., Surdej, J., and Tullos, C., 1997, Instrument Science Report # Tiny Tim manual http://scivax.stsci.edu/~krist/tinytim.html

5. Appendix A
Table 2. Log of PC1 F555W images used for tests described in this paper.
rootname u2a70305t u2a70605t u2a70905t u2a70c05p u2a70i05t u2a70l05t u2a70o05t u2a70r05t u2a70u05t u2a70x05t u2a71005p u2a71305t u2a71605t u2a71905t u2a71c05t u2a71f05t x 472 417 402 431 355 345 331 329 357 369 381 367 395 412 452 458 y 458 428 444 477 496 486 420 432 385 372 363 340 361 356 325 328 ix 1 2 3 4 5 6 7 1 2 3 4 5 6 7 1 2 iy 1 1 1 1 1 1 1 2 2 2 2 2 2 2 3 3 obs date 8/03/94 20/03/94 25/03/94 1/04/94 21/04/94 1/05/94 8/06/94 14/06/94 4/07/94 16/07/94 25/07/94 4/08/94 23/08/94 2/09/94 21/09/94 26/09/94 MJD 49419.1328 49431.6602 49436.6875 49443.8555 49463.7852 49473.3750 49511.1367 49517.3555 49537.6523 49549.9766 49558.8164 49568.8477 49587.1289 49597.9805 49616.0547 49621.5391 expt 1.6 1.6 1.6 1.6 1.6 1.6 1.6 1.6 1.6 1.6 1.6 1.6 1.6 1.6 1.6 1.6 focus -1.4333 0.4610 0.3345 0.1025 -1.3445 -11.3925 -3.0977 -2.4649 2.1136 0.5301 0.5056 0.2755 -0.7914 0.7010 -0.6605 -0.3941 x-coma 0.0029 0.0054 0.0023 0.0043 0.0055 0.0017 0.0149 0.0088 0.0045 0.0078 0.0093 0.0090 0.0091 0.0099 0.0035 0.0056 y-coma -0.0029 0.0005 -0.0023 -0.0044 -0.0066 -0.0103 -0.0104 -0.0061 -0.0047 -0.0072 -0.0056 -0.0070 -0.0048 -0.0080 -0.0048 -0.0066

7


Table 2. Log of PC1 F555W images used for tests described in this paper.
rootname u2a71i05t u2a71l05t u2a71o05t u2a71r05t u2a71u05t u2a71x05t u2a72605t u2a72905t u2a72c05t u2a72f05t u2n10203p u2n10403t u2o00501t u2s61101t u2s61201t u2s61301t u2s61401t u2s61501t u2s61601t u2s61701t u2s61801t u2s62101t u2s62201t u2s62301t u2s62401t u2s62501t u2s62601t Column Column Column Column Column Column Column x 483 489 504 507 511 522 509 498 465 459 409 392 332 406 405 427 408 418 419 418 417 416 416 416 416 417 448 y 345 352 379 388 423 462 497 513 504 505 593 590 504 419 419 436 451 410 411 415 417 410 412 413 415 416 429 ix 3 4 5 6 7 1 2 3 4 5 6 7 1 2 3 4 5 6 7 1 2 3 4 5 6 7 1 iy 3 3 3 3 3 4 4 4 4 4 4 4 5 5 5 5 5 5 5 6 6 6 6 6 6 6 7 obs date 20/10/94 25/10/94 14/11/94 21/11/94 12/12/94 20/12/94 11/01/95 21/01/95 11/02/95 13/02/95 7/03/95 13/03/95 7/05/95 27/07/95 6/08/95 21/08/95 31/08/95 18/09/95 28/09/95 16/10/95 29/10/95 13/11/95 22/11/95 12/12/95 18/12/95 10/01/96 15/01/96 MJD 49645.2461 49650.8750 49670.7617 49677.9805 49698.8633 49706.0312 49728.6172 49738.8750 49759.3242 49761.8711 49783.5586 49789.7695 49844.9141 49925.2930 49935.1953 49950.0586 49960.9609 49978.4453 49988.5430 50006.0664 50019.1992 50034.7266 50043.9023 50063.0547 50069.2031 50092.9648 50097.0508 expt 1.6 1.6 1.6 1.6 1.0 1.0 1.6 1.6 1.6 1.6 3.5 3.5 1.2 3.5 3.5 3.5 3.5 3.5 3.5 3.5 3.5 3.5 3.5 3.5 3.5 3.5 3.5 focus -2.0015 -3.8875 -3.1423 -0.9612 -1.6872 -4.1893 -3.6870 0.1799 -0.8905 -1.9657 2.1728 -0.2908 -0.5168 -1.7113 -2.8893 -5.2985 -1.2288 2.0072 0.8492 1.0692 -0.2110 -0.5944 0.5878 -0.9875 1.2373 -0.7580 -4.2018 x-coma 0.0084 0.0087 0.0045 0.0077 0.0080 0.0168 0.0051 0.0119 0.0060 0.0086 0.0133 0.0122 0.0113 0.0098 0.0115 0.0132 0.0092 0.0126 0.0123 0.0094 0.0108 0.0133 0.0116 0.0126 0.0098 0.0132 0.0084 y-coma -0.0069 -0.0050 -0.0063 -0.0052 -0.0078 -0.0088 -0.0096 -0.0085 -0.0070 -0.0097 -0.0083 -0.0092 -0.0087 -0.0053 -0.0067 -0.0122 -0.0082 -0.0066 -0.0071 -0.0096 -0.0049 -0.0050 -0.0050 -0.0053 -0.0060 -0.0024 -0.0054

1: image file rootname. 2 & 3: coordinates of the center of the star image on the original frames. 4 & 5: row and column positions of the star image in the mosaic frame (Figure 5). 6 & 7: date and Modified Julian Date for the start of the observations. 8: nominal exposure time in seconds. 9: relative defocusing of the secondary mirror in microns. 10 & 11: X,Y components of coma, given in microns of wavefront error.

8


Figure 5: Greyscale representation of the 43 GRW+70D5824 observed PSFs (top) and residuals after subtraction of the composite observed PSF (bottom).

9