Point Spread Function

5.7 PSF Variations with Time

The shape and width of observed PSFs varies slightly over time, due to the change in focus of the telescope. The focus variation consists of two terms: a secular change, due to the ongoing shrinkage of the Metering Truss Assembly, at a currently estimated rate of 0.85 mm month-1, and short-term variations, typically on an orbital time-scale (the so-called
"breathing" of the telescope). The breathing is probably due to changes in the thermal environment as the telescope moves through its orbit, and has a typical peak-to-peak amplitude of 4 mm; larger variations are occasionally seen.

These small focus shifts will impact photometry performed with small (few pixel radius) apertures. Typical +/- 2 micron focus shifts will result in photometric variations in the PC1 of 6.8%, 4.5%, 2.0%, and 0.2% for aperture radii of 1, 2, 3, and 5 pixels, respectively in F555W. This is based on the focus monitoring data taken over the period from July 1994 to January 1996 (Figure 5.7). Hence, "breathing" is often one of the major sources of errors for small-aperture photometry. However, relative photometry (i.e. the difference in magnitudes of stars in the same image) is not affected by this variation very much since all the stars in an image are impacted by the defocusing in a similar way. The variations are, of course, smaller for the WF chips because the pixels are roughly twice as large.

Figure 5.7: Measured OTA Focus Position (microns) as Function of Number of Days Since January 1, 1994.

Figure 5.8: Measured Aperture Correction V(r) - V(r=10 pix), in Magnitudes as Function of Shift from Optimal Focus.

Systematic errors due to the secular focus drift can be corrected using the aperture corrections phased with the focus change (see Figure 5.8):

ap_corr = ap_corr_nominal + a(r) x d

where ap_corr_nominal is the nominal aperture correction (mag) as derived from Table 2a in Holtzman et al. (1995a), a(r) is the flux variation per 1mm of focus drift (mag per micron) using an aperture with radius r (pixels), and d (mm) is the focus shift from the nominal position. The monitoring data mentioned above yield for PC1, F555W, the following values for a(r):

a(1 pix) = 0.0338 +/- 0.0038

a(2 pix) = 0.0226 +/- 0.0024

a(3 pix) = 0.0100 +/- 0.0018

a(5 pix) = 0.00105 +/- 0.0015

Values for other filters and other chips will be compiled in an instrument science report later in 1996.

Large focus changes, with amplitudes up to 10mm, are seen occasionally (See Hasan and Bely, Restoration of HST Images and Spectra II, p. 157). On May 1, 1994, and February 27, 1995, a short-lived defocusing of the telescope of up to 10mm was seen, probably due to extreme thermal conditions. Such a defocusing causes an increase of the PSF width by about 5-10% and a significant change in its shape, especially evident in the PC both because of its higher resolution and its astigmatism (which makes the out-of-focus image appear elongated). The change in the PSF appears to be modeled adequately by the TIM software. (See Hasan and Bely, Restoration of HST Images and Spectra II, p. 157). (See sample PSF subtraction in Figure 7.2 on page 148).