Tiny Tim HST PSF Modeling (original) (raw)

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About the Mission

The Hubble Space Telescope (HST) is a cooperative program of the European Space Agency (ESA) and the National Aeronautics and Space Administration (NASA) to operate a space-based observatory for the benefit of the international astronomical community. HST is an observatory first envisioned in the 1940s, designed and built in the 1970s and 80s, and operational since the 1990. Since its preliminary inception, HST was designed to be a different type of mission for NASA — a long-term, space-based observatory. To accomplish this goal and protect the spacecraft against instrument and equipment failures, NASA planned on regular servicing missions. Hubble has special grapple fixtures, 76 handholds, and is stabilized in all three axes. HST is a 2.4-meter reflecting telescope, which was deployed in low-Earth orbit (600 kilometers) by the crew of the space shuttle Discovery (STS-31) on 25 April 1990.
Responsibility for conducting and coordinating the science operations of the Hubble Space Telescope rests with the Space Telescope Science Institute (STScI) on the Johns Hopkins University Homewood Campus in Baltimore, Maryland. STScI is operated for NASA by the Association of Universities for Research in Astronomy, Inc. (AURA).
HST's current complement of science instruments includes three cameras, two spectrographs, and fine guidance sensors (primarily used for accurate pointing, but also for astrometric observations). Because of HST's location above the Earth's atmosphere, these science instruments can produce high-resolution images of astronomical objects. Ground-based telescopes are limited in their resolution by the Earth’s atmosphere, which causes a variable distortion in the images. Hubble can observe ultraviolet radiation, which is blocked by the atmosphere and therefore unavailable to ground-based telescopes. In the infrared portion of the spectrum, the Earth’s atmosphere adds a great deal of background, which is absent in Hubble observations.
The Hubble Space Telescope being repaired by astronauts with the Earth in the background.
The first servicing mission to the Hubble Space Telescope saw astronauts install a set of specialized lenses to correct the flawed main mirror in the telescope.

Hubble Through the Years

When originally planned in the early 1970s, the Large Space Telescope program called for return to Earth, refurbishment, and re-launch every 5 years, with on-orbit servicing every 2.5 years. Hardware lifetime and reliability requirements were based on that 2.5-year interval between servicing missions. In the late 70s, contamination and structural loading concerns associated with return to Earth aboard the shuttle eliminated the concept of ground return from the program. NASA decided that on-orbit servicing might be adequate to maintain HST for its 15-year design life. A three-year cycle of on-orbit servicing was adopted. HST servicing missions in December 1993, February 1997, December 1999, March 2002 and May 2009 were enormous successes and validated the concept of on-orbit servicing of Hubble.
The years since the launch of HST in 1990 have been momentous, with the discovery of spherical aberration in its main mirror and the search for a practical solution. The STS-61 (Endeavour) mission of December 1993 corrected the effects of spherical aberration and fully restored the functionality of HST. Since then, servicing missions have regularly provided opportunities to repair aging and failed equipment as well as incorporate new technologies in the telescope, especially in the Science Instruments that are the heart of its operations.

Mission Operations and Observations
Although HST operates around the clock, not all of its time is spent observing. Each orbit lasts about 95 minutes, with time allocated for housekeeping functions and for observations. "Housekeeping" functions includes turning the telescope to acquire a new target, switching communications antennas and data transmission modes, receiving command loads and downlinking data, calibrating the instruments and similar activities. On average, the telescope spends about 50% of the time observing astronomical targets. About 50% of the time the view to celestial targets is blocked by the Earth, and that time is used to carry out these support functions.
Each year the STScI solicits ideas for scientific programs from the worldwide astronomical community. All astronomers are free to submit proposals for observations. Typically, 700-1200 proposals are submitted each year. A series of panels, involving roughly 100 astronomers from around the world, are convened to recommend which of the proposals to carry out over the next year. There is only sufficient time in a year to schedule about 1/5 of the proposals that are submitted, so the competition for Hubble observing time is tight.
After proposals are chosen, the observers submit detailed observation plans. The STScI uses these to develop a yearlong observing plan, spreading the observations evenly throughout the period and taking into account scientific reasons that may require some observations to be at a specific time. This long-range plan incorporates calibrations and engineering activities, as well as the scientific observations. This plan is then used as the basis for detailed scheduling of the telescope, which is done one week at a time. Each event is translated into a series of commands to be sent to the onboard computers. Computer loads are uplinked several times a day to keep the telescope operating efficiently.
When possible, two scientific instruments are used simultaneously to observe adjacent target regions of the sky. For example, while a spectrograph is focused on a chosen star or nebula, a camera can image a sky region offset slightly from the main viewing target. During observations the Fine Guidance Sensors (FGS) track their respective guide stars to keep the telescope pointed steadily at the right target.
Engineering and scientific data from HST, as well as uplinked operational commands, are transmitted through the Tracking Data Relay Satellite (TDRS) system and its companion ground station at White Sands, New Mexico. Up to 24 hours of commands can be stored in the onboard computers. Data can be broadcast from HST to the ground stations immediately or stored on a solid-state recorder and downlinked later.
The observer on the ground can examine the "raw" images and other data within a few minutes for a quick-look analysis. Within 24 hours, GSFC formats the data for delivery to the STScI. STScI is responsible for calibrating the data and providing them to the astronomer who requested the observations. The astronomer has a year to analyze the data from the proposed program, draw conclusions, and publish the results. After one year the data become accessible to all astronomers. The STScI maintains an archive of all data taken by HST. This archive has become an important research tool in itself. Astronomers regularly check the archive to determine whether data in it can be used for a new problem they are working on. Frequently they find that there are HST data relevant for their research, and they can then download these data free of charge.
Hubble has proven to be an enormously successful program, providing new insight into the mysteries of the Universe.

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Callout

Please note that TinyTIM is no longer maintained or supported. On this page, details regarding parameters, products, and performance, are documented and provided for users as a courtesy. However, we cannot guarantee that the information provided herein is currently accurate. Use at your own risk.

End callout

Tiny Tim (Krist & Hook 2011) is a point spread function modeling tool developed for HST. Details of the code's operation, configurable parameters, and products are given in the Tiny Tim User's Guide (Krist & Hook 2004, version 6.3). Tiny Tim is no longer maintained or supported by STScI; a final version of the source code is hosted "as-is" on Github, for legacy purposes.

On this page we provide notes from some of the HST instrument teams regarding Tiny Tim's applicability and usefulness, in addition to historical resources and information.

HST Active Instruments

Wide Field Camera 3 (WFC3)

Presently, we do not recommend using Tiny Tim for WFC3 data as it relies on outdated optical models (WFC3 ISR 2008-14, published before WFC3 was installed). As an example, WFC3 ISR 2014-10 found significant errors in the WFC3/IR Tiny Tim model. Our team has focused recent efforts on building empirical PSFs and a database of observed PSFs, and we provide a PSF modeling tutorial available from the HST Notebooks Github Repository and HTML Webpage.

Users working with WFC3 data who wish to proceed with Tiny Tim regardless should be aware that while Tiny Tim resources are hosted by STScI, it is not currently maintained.

Space Telescope Imaging Spectrograph (STIS)

For external users, we do not encourage the use of Tiny Tim nor support the software, as it is "available but unsupported." Members of the STIS team currently do utilize Tiny Tim for specific internal purposes.

Tiny Tim may work to convolve against perfect resolution models to compare against STIS disk observations, or to match the structure of PSFs down to ~10^-8 contrast levels (if PSF is adjusted to match focus and jitter evolution). However, we do not know how STIS Tiny Tim PSFs will perform in other science situations, and caution users on that basis.

For coronagraphic observations requiring PSF subtraction, users are advised to follow the recommendations for obtaining contemporaneous PSF calibrators in Section 12.11 of the STIS Instrument Handbook.

Advanced Camera for Surveys (ACS)

We do not support Tiny Tim. For users interested in generating PSFs for ACS, we suggest looking at the effective PSF utility described in the photutils documentation. For additional background on this utility, please see Anderson & King (2000) and WFC3 ISR 2016-12. Additionally, see ACS ISR 2017-08 for comparisons to Tiny Tim generated PSFs.

HST Legacy Instruments

WFPC2, NICMOS, FOC+COSTAR, FOC (Aberrated PSFs), WF/PC-1 (Aberrated PSFs)

Tiny Tim for Legacy instruments is described in the Tiny Tim Users Guide (Krist & Hook 2004, version 6.3)

A summary of the tool's use over the first 20 years of HST science operations is described in "20 years of Hubble Space Telescope optical modeling using Tiny Tim" by Krist, Hook, & Stoehr (2011). This report compares observed and modeled PSFs for FOC, WF/PC-1, WFPC2, & ACS/HRC.

A discussion of WFPC2 photometry using Tiny Tim PSFs is available in the 1997 HST Calibration Workshop Proceedings (Rémy et al. 1997).

Resources

The Tiny Tim source code presents the following input choices:

  1. Camera: You may choose to model the PSF specific to a chosen channel for any of the HST imagers past and present. Available options are given in the menu.
  2. Chip: In some cases you will then be asked to provide the specific detector of interest. Applicable choices are given in the menu. Note that the image file format places ACS/WFC1 in science image 2 and ACS/WFC2 in science image 1. Similarly for WFC3, UVIS2 is in the first image and UVIS1 in the second
  3. Pixel Position: The PSF is field-dependent so the chip location of interest is input. Appropriate range is offered.
  4. Filter: PSF is filter dependent. Only applicable filters are given in the menu.
  5. Spectrum Type / Spectrum Value: Methods for calculating flux over the band are offered. A menu of stellar spectral types, or a field for BB temperatures or power law indices are then offered if applicable.
  6. PSF diameter: Output PSF image extent. ~3" is recommended.
  7. Focus: User-supplied focus value in microns at the Secondary Mirror. The best estimated focus for a time period of interest is provided by the focus model. The focus model output can be used directly in this field without unit conversion. Entering zero for focus in this field will result in the generation of a PSF with no wavefront error in focus.
Output

The Tiny Tim source code produces an output web page (example) containing:

  1. an image illustrating the modeled PSF characteristics
  2. a summary of the input parameters
  3. links to the output data files, including the fits images of the distorted and undistorted modeled PSF
  4. links to the output figures
  5. a link to the tar.gz file that bundles all the output products for download.
    See the Tiny Tim User's Guide for a description of the output products. For ACS and WFC3, PSFs are calculated for both undistorted (tiny2 output) and distorted (tiny3) images. Only the tiny3 distorted image is sampled at the detector resolution. This product corresponds to the images supplied by the pipeline as "_flt" files. For the other instruments, the distortion is much less and has not been considered in Tiny Tim.
    We note that Tiny Tim does not model the PSF found in drizzled (_drz) files.

Performance

The generated PSFs can be useful for image simulation and observation planning, or to indicate the level of PSF variability over the field and with time, or for small aperture photometry. For PSF subtraction where details of the outer portions of the distribution are important, and for other cases where accurate PSF characterizations are needed, these modeled PSFs may lack suitable accuracy, and empirical PSF techniques should be considered when practical.
Refinement of the models may enable their more effective use for other data reduction tasks. PSF fidelity will also be found to vary among instruments. For example, the ACS Tiny Tim PSF is believed to be better modeled currently than the WFC3.
For WFC3, Tiny Tim uses aberration coefficients that are derived from the as-built optical ray-trace model, as a function of field position. While the ground test and on-orbit image quality generally matches expectations, no attempt was made to adjust the aberration coefficients used by Tiny Tim to better describe the observed field dependence. Tiny Tim also uses blur kernels to describe the effects of charge diffusion and, for the IR channel, inter-pixel capacitance. The IR and wavelength-dependent UVIS kernels were updated from those listed in WFC3 ISR 2008-14, to better match pre-launch measurements. These updated kernels are described in WFC3 ISR 2008-40 and WFC3 ISR 2008-41.
Note also that Tiny Tim models for all the Science Instruments do not include the effects of any optical ghosts and halos.
The modeling engine and its modifiable calibration parameters are more fully discussed in the Tiny Tim User's Guide.

HST Help Desk

Please contact the HST Help Desk with any questions.