5.6.1 Aperture Corrections vs. Field Position

The amount of energy encircled by an aperture used for stellar photometry will depend on the aperture size, and on any variations in the PSF with field position, time, etc. In general, larger apertures will provide more stable results in the presence of PSF variations. However, large apertures will also exacerbate many problems: contamination from residual cosmic rays, scattered light from nearby stars, and the lower signal-to-noise (S/N) that typically results.

Gonzaga et al. (1999) have measured aperture corrections and characterized their change as a function of field position and filter. The differences in photometric magnitude between apertures with various radii (i.e. aperture corrections), and their mean and standard deviations for the F555W filter, are presented in Table 5.5. For example, the first row of the table indicates that stars measured with a 1 pixel radius aperture will be about 0.887 magnitude fainter than if a 5 pixel radius aperture were used (averaged over entire PC CCD), and this difference will vary by about 0.054 magnitudes RMS across the CCD.

Variations in the PSF with field position will, of course, cause a position dependence in the aperture corrections. Figure 5.7 illustrates how the aperture correction varies with distance from the CCD center, R, for different pairs of aperture sizes. The scatter in the plots is due to contamination from residual cosmic rays and nearby faint stars within the larger aperture. While the data are somewhat incomplete, a clear trend is present: the aperture correction generally increases linearly as a function of distance from the CCD center. For example, the aperture correction between 1 to 5 pixel radius is about 0.82 magnitudes at the PC center, and increases to about 0.94 magnitude at the far corners of the CCD. (The average correction is about 0.89 magnitude, as given in the first line of Table 5.5:.) The other WFPC2 CCD chips show results similar to the PC chip.

Table 5.5: Magnitude differences produced by different aperture sizes. Results given for PC, WF2, WF3, and WF4 in F555W.

Chip	Filter	Aperture Radii (pixels)	Number of Stars	Mean Magnitude Difference1	RMS of Magnitude Difference2
PC	F555W	1 vs. 5	116	0.887	0.054
PC	F555W	2 vs. 5	115	0.275	0.028
PC	F555W	2 vs. 10	115	0.401	0.075
PC	F555W	5 vs. 10	115	0.106	0.055
WF2	F555W	1 vs. 5	558	0.608	0.130
WF2	F555W	2 vs. 5	558	0.160	0.085
WF2	F555W	2 vs. 10	544	0.310	0.257
WF2	F555W	5 vs. 10	548	0.133	0.204
WF3	F555W	1 vs. 5	660	0.680	0.133
WF3	F555W	2 vs. 5	656	0.188	0.076
WF3	F555W	2 vs. 10	649	0.376	0.308
WF3	F555W	5 vs. 10	647	0.154	0.233
WF4	F555W	1 vs. 5	828	0.672	0.129
WF4	F555W	2 vs. 5	831	0.198	0.115
WF4	F555W	2 vs. 10	815	0.386	0.350
WF4	F555W	5 vs. 10	814	0.160	0.252

1Magnitude difference averaged around CCD. 2RMS magnitude difference around CCD.

In practice, the aperture correction also depends on defocus. The interplay between aperture correction and defocus may be complex, since the optimal focus changes with field position. A full correction has not been established, but the TinyTIM PSF model (see Section 5.7) can be used to estimate the extent of the variation in the aperture correction. In general, we recommend that a minimum aperture radius of 2 pixels be used whenever possible, in order to reduce the impact of variations of the aperture correction with focus and field position. If the field is too crowded and a smaller aperture is needed, we recommend that users verify the validity of the corrections on a few well-exposed stars.

The following section includes a discussion of aperture corrections as a function of OTA focus.

Figure 5.7: Aperture correction (delta) between two given apertures within the PC chip versus radial distance of the target from the center of the chip. Open symbols indicate spurious data.