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Observatory Support
HST Focus Model

May 2012


For a given time, this model estimates the amount of defocus at a particular camera. The model is a function of telemetered temperatures and secular terms. The temperature dependent part of the model is taken from Di Nino et al. 2008 while the long-term secular trend is a more recent determination given in Niemi et al., 2010. In addition to the temperature terms and secular double exponential, the model includes zero point offsets characterizing the focus offsets between cameras and channels.



The interface to the model presents you with the following input choices:

  1. Display:
    You have the option of displaying:
    • the focus position calculated by the model,
    • an actual measured focus position if available,
    • a plot comparing both.
  2. Camera:
    You may choose to model the focus in ACS (HRC,WFC1, or WFC2), WFC3 (UVIS1 or UVIS2), or WFPC2 (PC).
  3. Year:
    Defaults to current year
  4. Date:
    Supply in mm/dd format
  5. Time Period:
    • For the measured or comparison display options, a drop-down menu offers the actual times when measurements are available.
    • For the modeled focus display option, you supply the date and start/stop times in 24 hour hh:mm format (UT, same frame used in HST science image headers). Any time period falling within 1 calendar day can be supplied.

The model produces an output webpage (example) containing the following:

  1. the average focus value over the time period requested (measured and/or modeled values as applicable).
  2. a link to a space-delimited text file containing Date, Julian date, Time, and Model value
  3. an in-line plot, downloadable as a Portable Network Graphic (.png) file.

All focus values are expressed as the number of microns of longitudinal motion of the Secondary Mirror (sometimes referred to as "piston" or "despace"). 1 micron at the secondary induces 6 nanometers of rms wavefront error.


Annual Summary
A summary of model values covering one year at a time may be obtained as a single text file. The values are given as a function of Modified Julian date along with a time in normal date and time format. The focus values given are for a mean camera position. Camera-specific corrections are about 0.25 microns and are listed below in units of microns.

Results are available by selecting one of the years below. A right click allows you to download the file

2003 2004 2005 2006 2007 2008 2009 2010 2011 2012 2013

The results of comparing the model with the actual measurements normally show agreement within a few microns. A fuller discussion of the model's fidelity is given in Cox & Niemi 2011. T he differences between the measured and modeled values are summarized in the diagram below extracted from the ISR. It shows the cumulative fraction of cases which fall within any deviation. 1 sigma accuracy is ~1.6 microns (depending on the specific SI), and 80% of the time the agreement is within 2 microns, though occasional errors of up to 4 microns are seen. Since normal fluctuations within an orbit can be 6 microns or more, the model provides useful improvements to the knowledge of the focus position.


Generating a PSF
The Tiny Tim point-spread function modeling software has been modified recently to accept output from this focus model (Krist & Hook 2011). The application of modeled and empirical PSFs to science analysis is discussed briefly elsewhere.