Jitter Quality Flag of Individual Exposures
Jitter Flag Type Description
P Processable This is a dataset which has reliable jitter information
i.e., small standard deviations,
and keywords do not denote strange behaviour

The WFPC2 stacking pipeline might decide to use the jitter shifts, provided that:

1: the association leader is also of type "P" (best possible jitter information)

2: the start_time difference between the leader and the member is not greater than about 3 or 4 hours


OR
the involved jitter shifts are less than 0.2 WFPC2 pixels

OR
the jitter shifts coincide (within 0.2 pixels) with the WCS shifts (for observations taken after April 1996).
(See Idiosyncrasies for more details)
G Groupable This is a dataset which has jitter information, but one of the following condition holds:
  • standard deviation on the pointing (ra, dec, roll) is too high
  • the GUIDEACT flag is NOT equal to ``FINE LOCK''
  • the field LOCKLOSS has a value greater than 0

The WFPC2 stacking pipeline won't use the shifts computed using non-optimal jitter information.

B Bad This is a dataset which has BAD jitter information like:
  • Telemtry Format is "FN" (No velocity aberration correction)
  • Right Ascension and/or Declination and or Roll have NULL values
  • the SLEWING flag is on (=`T')
  • the APERTURE has a NULL value
  • the standard deviation of the pointing is unbelievably high
  • Jitter file is missing or empty

The WFPC2 stacking pipeline won't use the shifts computed using non-optimal jitter information.

NOTE: Differences between type A & type B WFPC2 Associations

The main driver for building WFPC2 Associations type B has been the fact that too many WFPC2 exposures had associated bad, or even missing, jitter information.

In the type "A" case was in fact true that:

`P' and `G' exposures can then be part of an association, while `B' exposures, not being able to reconstruct where the telescope was actually pointing, have been left alone, as single exposures, not part of any association.

This restriction is no longer true.

In the type "B" case, all the available science exposures are part of the association, regardless of the availability or the reliability of their jitter files. When the jitter files are missing or are not reliable, other methods will be used to compute the offsets among the exposures in the association.

Comments to Alberto.Micol@eso.org