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Astronomical Data Analysis Software and Systems IV
ASP Conference Series, Vol. 77, 1995
R. A. Shaw, H. E. Payne, and J. J. E. Hayes, eds.
Simulation of HST PSFs using Tiny Tim
J. Krist
Space Telescope Science Institute, 3700 San Martin Dr., Baltimore, MD
21218
Abstract. Tiny Tim has been used for generating Hubble Space Tele­
scope (HST ) point­spread functions (PSFs) for deconvolution, algorithm
testing, proposal planning, and optical studies. The most recent ver­
sion, V4.0, includes new mirror zonal error maps and revised aberrations,
significantly improving the PSF models.
1. Introduction
Tiny Tim is a stand­alone program for simulating HST PSFs as viewed with the
on­board imaging cameras (WF/PC­1, WFPC2, FOC, COSTAR/FOC). PSFs
generated with Tiny Tim have been used extensively for deconvolution, algo­
rithm testing, proposal planning, and optical studies. The most recent version,
V4.0, is significantly better than earlier releases, producing more accurate PSFs
due to better maps of the HST mirror zonal errors and improved aberration
values. The software package is available via anonymous ftp at ftp.stsci.edu in
the software/tinytim directory.
2. What is Included in Tiny Tim
The HST focus changes slowly over time due to shrinkage of the telescope truss
from desorption. The secondary mirror has occasionally been moved away from
the primary to compensate. The desorption changes have been characterized by
measuring observed PSFs. Tiny Tim computes the focus for an aberrated cam­
era with adjustments for desorption and mirror moves, based on a user­specified
date of observation. Since desorption has nearly stopped, it is no longer included
in the focus values for the corrected cameras (WFPC2 and COSTAR/FOC).
Each camera has its own set of aberrations (astigmatism, coma, spherical,
etc.) which have been determined using phase retrieval methods. The measured
aberrations for FOC f/96, WF/PC­1 PC6, WFPC2, and COSTAR/FOC are
used by Tiny Tim. The PC6 values are used for the other WF/PC­1 channels,
which may result in some PSF mis­matches. The measured focus offsets among
the WFPC2 cameras are also included.
Circular zones in the HST mirrors resulting from the polishing process
have important effects on the PSFs. In the aberrated PSFs, they affected the
diffraction ring structures, and in the corrected ones, they cause scattering which
results in a low level halo extending out to about 3 00 . Maps of these errors were
initially obtained from pre­launch interferograms by Perkin­Elmer and were used
1

2
by Tiny Tim (up to V3.0). Recent maps obtained from phase retrieval of on­
orbit WFPC2 data are much better and significantly improve the PSF models.
These improvements were introduced in V3.0: they are illustrated in Figures 1
and 2.
The WF/PC­1 and WFPC2 instruments contain Cassegrain repeater optics
with their own secondary mirrors and spiders. Since these obscurations are in the
same plane as those from the telescope, they appear to shift with respect to the
telescope's depending on field position. This leads to position­dependent PSFs.
These effects were significant in WF/PC­1, but are less so in WFPC2. The shifts
are determined by Tiny Tim based on user­specified object positions. Multiple
positions can be simulated in a single run by providing a list of coordinates.
The HST PSF varies significantly with wavelength. In narrow­band filters,
where the PSF does not change much over the bandpass, the diffraction rings are
sharp. However, in wide­band filters the changes in the PSF result in blurring
of the diffraction structures. These effects are accounted for in the software by
adding together PSFs from different wavelengths with weights appropriate for a
given filter. By default, Tiny Tim will create a PSF integrated onto detector­
sized pixels. It can, however, creates PSFs at finer samplings. Such PSFs are
useful in deconvolution of undersampled data (like WF/PC­1 or WFPC2) and
in peak­centering analysis. Note that in WFPC2 there is a sub­pixel variation
which is not included (except as a general pixel­level scattering function applied
only to normally sampled PSFs).
3. What is Not Included in Tiny Tim
Despite including all of the above parameters, Tiny Tim does not account for
all important factors. This is typically because they are not well characterized,
or vary on short timescales.
The focus of the telescope is known to change slightly over the period of an
orbit due to thermal effects (``breathing''). Also, it is known to change sometimes
depending on where the telescope is pointed. While a model for breathing has
been developed, it relies on thermal data for a given observation which is not
readily available. The pointing­dependent effect, which is about three times
worse than breathing, has not been characterized. Note that these effects are
not limited to changes in focus but also in coma and astigmatism.
Tiny Tim does not include large angle scattering. The scattering at angles
larger than about 3 00 from the core was below expected levels in WF/PC­1
(the limited FOC dynamic range does not allow such scatter to be measured in
that camera). In WFPC2, however, it is about an order of magnitude greater
and includes streaks which radiate from the star. This is due to scattering
by the electrode structure of the front illuminated CCDs (WF/PC­1 was back
illuminated). It is not possible to model this effect (which is typically seen only
in highly saturated star images). This will lead to possibly significant errors at
large angles from the core in the models.
The WF/PC­1, WFPC2, and COSTAR/FOC instruments have as­designed
field dependent astigmatism. However, the changes over the field of view are
practically unmeasurable, and thus are not included in the models. Geometric
distortions in WF/PC­1 and WFPC2 have been measured but are not included.

3
New Map No Map
Observed Old Map
Figure 1. FOC f/96 observed and Tiny Tim model PSFs (pre­
COSTAR) at 253 nm. The PSF generated using the old mirror map
is typical of those produced by Tiny Tim previous to V3.0. The PSF
using the new mirror map was created using V4.0b.

4
New Map No Map
Observed Old Map
Figure 2. FOC f/96 observed and Tiny Tim model PSFs (pre­
COSTAR) at 486 nm.