Some interesting examples of HST Jitter balls
When reducing HST data it is very important to evaluate the effect
of the pointing stability of the spacecraft.
Not doing so, the astronomer can incur in bad surprises.
Since a picture is worth thousand images, hereafter the contents of two WFPC2 jitter tables are shown in a graphical form. While the first case is a "good" case, the second shows an unexpected behaviour which has an important impact on the measured PSF.
A typical `normal' case: stability
A typical jitter ball looks like the plot shown here.
| WFPC2 Dataset: U26K2S02T |
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The standard deviation in both directions (RA and DEC) is lower than 4 mas (milli-arcsec).
For the WFPC2 Association Project, this exposure is flagged as a PROCESSABLE one (`P').
A `strange' case: instability
Sometimes something happens during the observation.
In this case, the telescope started the observation and, after about
half of the exposure time, for some unknown reason, the telescope pointing
got shifted as much as 0.5 PC pixels (25 milli-arcseconds).
The plot shows that in a fairly dramatic way.
| WFPC2 Dataset: U3814G02T |
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For the purpose of the WFPC2 Associations project, this exposure has been flagged as GROUPABLE (`G') because of the high standard deviation of the declination: 14 mas (milli arcsec).
Follow this link to visualise the jitter balls for any HST observations.
Comments to: Alberto Micol (amicol@eso.org)