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 non-thermal terms. The temperature-dependent part of the model is taken from Di Nino et al. 2008 while the long-term secular component is based on more recent fits given in Niemi et al., 2010. In addition to the temperature terms and secular fits, the model includes zero point offsets characterizing the focus offsets between cameras and channels.

Note (May 2014): We have re-posted a revised focus model, containing an updated secular fit to better characterize the focus trend since March 29, 2011. We have added a linear term with slope of -4.232e-03 microns/day or -1.55 microns a year beginning on March 29th 2011. This correction has been applied to the online model and the yearly summary files. Any results measured from this site before May 23rd 2014 will not have this correction applied. If you obtained focus model values from this site prior to May 23rd, for HST observations made after March 29th 2011, then obtaining those values again now will improve agreement with measurements.

Note (February 2020): Studies during 2019 incorporating a more reliable focus measurement method have resulted in an improved model.  Also the linear term referred to above has been modified so as not to apply beyond 29 August 2014. Any values obtained from the model beyond the end of 2014 should be reviewed and replaced by the values produced by a rerunning of the current model.

Because long-term fits must be occasionally performed and added to the temperature-dependent component of the model, the model will over time accumulate a zero point offset. While these are occasionally removed (see the above notes), it is always generally true that the model is more useful and descriptive when applied to understand relative focus changes over shorter periods (i.e., orbits/days).

 

Focus Model Interface

Resources for Hubble’s Focus Model

Please Contact the HST Help Desk with any Questions

https://hsthelp.stsci.edu.