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Hubble Space Telescope
Common Mistakes that Can Ruin Your Data and Your Program

There are several issues related to HST program implementation that are entirely proposer responsibilities, which STScI does not have the resources to verify systematically. They are explained in our documentation, and user tools are provided to address them, but there continue to be failures caused by proposer oversights in these areas. Observations that fail for these reasons will not be repeated. Here are some reminders:

  1. Although proposers must do so initially, STScI checks the count RATES for all targets and fields to be observed with the UV detectors for over-illumination that would damage them. However, we cannot check exposure TIMES, including saturation of the STIS CCD. The Exposure Time Calculators and APT/Aladin provide sophisticated means for doing so. In particular, if a STIS/CCD Acquisition exposure is saturated, the visit is likely to fail. It is essential that proposers specify correct exposure times for all observations, both acquisition and scientific, to ensure useful results.
  2. Specification of correct target coordinates and proper motions are entirely proposer responsibilities and can't be systematically checked by STScI. Again, both APT/Aladin and the improved Target Confirmation Charts provide powerful means to check these specifications, but some users have not consulted them. The incidence of failures caused by the units of proper motion in right ascension has been reduced by the change in APT from a default value to explicit specification, but some incorrect entries continue to occur. Recall that there are three relevant epochs: that of the original image, the one specified in the Phase II proposal, and the approximate observing epoch in the future. The confirmation Charts display the difference between the original image and the epoch specified in the Phase II proposal. The BOT tool within APT allows one to display the difference between the epoch given in the Phase II proposal and the approximate epoch of observation in the future using the "Apply proper motion" button. See http://www.stsci.edu/hst/proposing/apt/old19_new_cycle_p2, for further detail.
  3. Observers may request a repeat of observations that failed for reasons beyond their control, by submitting a Hubble Observation Problem Report (HOPR), now linked directly off the front of the Phase II proposal information page. However, there is a rigid policy, rarely if ever superseded, that such reports must be filed within 90 days of time of data distribution. Thus, it is essential that users check ALL of their data for problems within that period. There has been an instance in which only some of the observations were lost but only some that succeeded had been inspected within the required period; no repeat was granted in that case.
  4. Users who require a particular orientation of the STIS detector or apertures on the sky are reminded of the 45 degree offset of the STIS coordinate system from the angle given in the ORIENT constraint that the user specifies in APT. See section 11.4 of the STIS Instrument Handbook for a detailed explanation of this issue.
  5. Observers are reminded that BUFFER TIME values need to be provided in the Phase II proposal. An incorrect computation of the BUFFER TIME can lead to partial or, sometimes, significant loss of data. Specifications of a correct BUFFER TIME are entirely proposer responsibility and can't be systematically checked by STScI. We recommend use of the COS ETC to compute an accurate value of the BUFFER-TIME. The value returned by the ETC should always be compared against the exposure time as well as the source count rates in order to avoid data gaps and/or data loss. A detailed description of how to derive accurate BUFFER TIME values can be found in Section 5.4 of the COS Instrument Handbook.