The accuracy of the target input coordinates, and of the pointing, have a large impact on the instrumental throughput, of the point source flux accuracy but only a minor impact on the wavelength accuracy of SWS for point sources. ISO's random pointing error can also introduce effects in the data if the target coordinates are incorrect or if the target is in a crowded field.
Several documents were written covering the effects of pointing accuracy. `Effects seen in data induced by incorrect target coordinates or crowded fields', by K. Leech & A. Heras, 12 June 1997, gives examples of what can go wrong when incorrect target coordinates are used.
In summary, a star offset from the center of the aperture by 6
in the cross-dispersion direction (y-axis) loses approximately
40% in throughput (example taken in the LW, 17
). Incorrect target
coordinates can lead to the derived flux being incorrect by large factors. An
offset in the dispersion direction (z-axis) affects the
wavelength calibration, but the effect is not of the same order of magnitude as
the effects on flux. A 4
mispointing in the dispersion
direction corresponds to about a 1 LVDT (1/8 grating resolution) wavelength
offset, so the maximum possible offset is about 2 LVDT, or about
1/4 of a grating spectral resolution element. This
error is within the quoted wavelength accuracy.
Also note that problems may occur if there is another object, with a flux
similar or greater than the target, 4 - 10 arcseconds away in the z-direction,
as can happen in crowded fields.
This study was extended in `The effects of the pointing jitter on SWS
observations', by A. Heras, 6 August 1998. This
compares the ratio of on- and off-source observations of a star with the
pointing jitter. It finds that sudden flux jumps are associated with pointing
fluctuations. It also notes that even for nominally on-source observations
(defined to be those where the pointing error is less than about
4
), there is a correlation between noise in an observation and
pointing jitter.
During the ISO mission several stars were observed every few weeks for purposes of wavelength and flux calibration. The flux of one of these targets, Gamma Dra, was seen to have a modulation in it that is suspected to be due to pointing errors. The report `Status of study on Gam-Dra fluxes and related pointing errors' by H. Feuchtgruber, issued on 10 June 1998 can be read for further information on this.
Information on how the spacecraft pointing can affect observations of extended objects can be read in the document `Status of study of AOT02 line profiles and fluxes on NGC6543', by H. Feuchtgruber issued 2 June 1998. In this case the observed line fluxes varied by 10% over the course of several months. This was attributed to the spatial extent of NGC 6543, the SWS slit size projected on the sky, see section 3.3.2, and the change in roll angle between the observations.