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What guiding concerns should I have for my ACS observations?

Q: What guiding concerns should I have for my ACS observations?

A: The guiding performance and pointing stability of HST are described in
the HST Primer. The normal guiding mode uses two
guide stars that are tracked by two of HST's Fine Guidance Sensors (FGSs). 
However, sometimes two suitable guide stars are not available. This can 
happen for example when there are stringent scheduling constraints 
(e.g., a special requirement that restricts the orientation or timing 
of the observations). By relaxing the scheduling constraints it may become 
possible to find a scheduling opportunity for which two guide stars are 
available. Alternatively, single-star guiding can be used, with the 
telescope pitch and yaw controlled by the FGS, while the roll controlled by 
the gyros.  The gyros have a drift rate which, when combined with other 
perturbations, produce a total of ~ 1 to 2 mas/sec of roll rates. On rare 
occasions it can be as large as 3 mas/sec. This causes a rotation of the 
target around the guide star, which in turn introduces a small translational 
drift of the target on the detector. The exact size of the drift depends on 
the exact roll drift rate and distance from the single guide star to the target 
in the HST field-of-view. 

For ACS with single-star guiding, the following drift rate of the target on the 
detector are provided for reference.

         per 1000 sec exposure        per orbit (96 min)
         ------------------        --------------
WFC      0.009 arcsec (0.18 pix)     0.052 arcsec (1.04 pix)
HRC      0.011 arcsec (0.44 pix)     0.063 arcsec (2.53 pix)

- 1.5 mas/sec roll rate
- maximum guidestar to target separation (1200" WFC and 1400" HRC)

The drift over an orbital visibility period is easily calculated from
these numbers; the typical visibility period in an orbit (outside the
Continuous Viewing Zone [CVZ]) is in the range 52-60 minutes, depending 
on target declination. (see the HST Primer)

The drifts inherent to single-star guiding have two consequences:

(1) There will be a slight drift of the target on the detector
within a given exposure. For the majority of observations and
scientific applications this will not degrade the data (especially if
the exposures are not very long). The drift is smaller than the FWHM
of the point spread function (PSF). Also, the typical jitter of the
telescope during an HST observation is 0.003-0.005 arcsec, even when
two guide stars are used.

(2) There will be small shifts between consecutive exposures. These
shifts can build up between orbits in the same visit. This is not a
problem if, during data reduction, different exposures are aligned
before co-addition or comparison. The AstroDrizzle software
provides a way of doing this. Also, there is always a slow drift 
of the telescope up to 0.01 arcsec/orbit due to thermal effects, even 
when two guide stars are used. So even when two guide stars are used 
it is often necessary to align exposures before co-addition.

In summary, for most scientific applications single-star guiding will
not degrade the usefulness of ACS data. However, we do not recommend
single-star guiding for the following applications:

(a) programs that require very accurate knowledge of the PSF,
including coronagraphic programs and astrometric programs.

(b) programs that rely critically on achieving a dithering pattern
that is accurate on the sub-pixel scale. (However, note that even with
two-star guiding this can often not be achieved).

Observers who are particularly concerned with the effect of pointing
accuracy on the PSF can obtain quantitative insight using the
TinyTim software package. While this does not have an option 
to simulate the effect of a linear drift, it can calculate the effect 
of a circularly symmetric jitter of a specified RMS value.

For a more detailed discussion of pointing and roll errors, and the HST 
jitter files, see the Pointing Calculations & Source of Error Section of that document.