SHARC II Tau fits

The fits located here are least square polynomial fits of the 225 Ghz and 350 micron CSO tau sky dippers. As the tau meters scan the sky every 10 minutes, they can produce noisy data. A single tau measurement can be a poor indicator of the actual tau. Thus, the tau stored in each scan may not be the most apropriate value to use in your reduction. As a helpful tool, these fits show the tau trends during the course of each evening and attempt to produce a tau value that is more realistic than the individual measurements.

The SHARC reduction tool, CRUSH, will automatically look at these fits (if available) to obtain a tau value for use in reduction. The user may opt to override this feature and enter a specific tau value, or just use the tau value recorded in the FITS image header. Please see CRUSH documentation for more information.

In order to obtain the best reduced data possible, these fits must be used with caution. While they do an excellent job of predicting the tau 90-95 % of the time, they are not perfect. Nights with rapidly changing tau produce problems. On this page you will find the analytic fits for both 225 Ghz and 350 micron data along with a readme file for each. Further, plots are available for every fit. To obtain accurate results, observers should inspect the nights of their data and verify that the correct tau value is being used.

A good place to start is the tau fit qualitative analysis page. This page lists all the nights that SHARC II has been online. It reports if 225 Ghz or 350 micron data is available for a particular night and gives a qualitative analysis of the fit (including time ranges that do not have a good fit).

Qualitative analysis of fits
Overview and Readme file for 350 micron tau fits
Overview and Readme file for 225 Ghz tau fits
225 Ghz fits

350 micron fits

The interested reader may also want to look at the JCMT/SCUBA polynomial fits.

To request fits for missing dates or request that a fit be redone contact John Vaillancourt (

This file last updated on