46.8 PSFs

• A sharp, 0.1 arcsecond radius core which contains about 15% of the energy.
• Broad, extended wings out to about 4".
• Strong position dependance due to camera vignetting.
• Wavelength dependance due to diffraction effects.
• Time variability due to dependance on the telescope's focus position.
• Shutter diffraction effects for exposures of less than 1/2 second.
• OTA focus variations and image motion on short time scale (< 1 hour).
• Spacecraft jitter. Ideally, PSFs should be observed in a variety of positions on each chip, in all filters used and close in time to the observation, a time-consuming process. In addition, the shorter exposures needed to avoid saturating the PSF cores generally provide insufficient S/N in the wings. For these reasons, model PSFs will generally be needed to supplement the available observed PSFs. The Tiny Tim software package, which can be used to generate model PSFs, is available at:

http://scivax.stsci.edu/~krist/tinytim.html

Header keywords from a typical PSF image are described in Table 46.3. The secondary mirror actuator position keywords were populated using the results presented in the OTA Instrument Science Report #7.

PSF Header -Keywords

Keyword	Sample Value	Definition
Keywords Listed in WWW Memo
filtnam1	F785LP	First STSCI filter name
camera	PC	WF or PC
detector	6	1-4 for WF, 5-8 for PC
rootname	W0MN0A02T	Rootname of observation with PSF image
data_fil	C3U1428IW.R7H	Name of the PSF image file
targname	SAO107200	Target name
spectral	unknown	Spectral type of source, if known
exptime	0.23	Exposure time in seconds
date_obs	23/06/91	UT date
xcorner	502	X pixel of (1,1) corner in PSF image
ycorner	38	Y pixel of (1,1) corner in PSF image
xcenter	630	X coordinate of PSF center on the chip
ycenter	166	Y coordinate of PSF center on the chip
Keywords in Headers, but not Listed in WWW Memo
filtnam2	F122M	Second STScI filter name
mode	FULL	Full or area
origin	HST	Data source, e.g., HST, TIM or other
mjd	48430.	Modified Julian date (JD-2400000.5)
calibrat	T	Was image calibrated (see Chapter 45)?
flatfile	c1916444w	Name of flatfield image (or INDEF)
psfscale	1.	Divisor used to normalize PSF (1 if none)
obsmode	PC,6,F,DN,F785LP	Observation mode (for synphot)
refspec	CRCALSPEC: AGK_81D266_002.TAB	Reference spectrum
actuat25	-1492	Position of secondary mirror actuator 25 (and actuator keywords 26 through 30)

PSF Limitations and Effect of Jitter

Except for very short exposures, both the observed and modeled PSFs will not perfectly match WF/PC-1 images due to the jitter in the HST pointing. The jitter is primarily caused by the motion of the telescope in response to the solar array oscillations, and possibly oscillations of the aperture door and the high gain antennas as well (SESD Report 92-15). The effect is greatest during earth terminator crossings, but is present at other times as well. On April 12, 1992, software called SAGA II was installed to correct for some of the jitter, effectively reducing it by about 50%. That is, the rms jitter dropped from 17 mas to 7 mas entering orbital day and fell from 12 mas to 6 mas entering orbital night.

The effects of the remaining 5-10 mas jitter, while not predictable, can be partially determined for observations taken in Fine Lock, via some of the associated engineering data. (Note that this is engineering data which is not part of the standard WF/PC-1 image set; users must submit a special request to help@stsci.edu to obtain more information on a particular observation from the Observation Monitoring System¹.) For images obtained on Coarse Track, the effects of jitter can only be estimated.