The standard ISO observing mode was a 3-axis-stabilised pointing at a selected astronomical target, permitting observations for a period up to several hours. Pointing accuracy was defined according to three terms:
, half-cone;
 |
(5.1) |
Observers may also come across the rarely used term Average Measurement Accuracy (AMA), the angular separation between the actual and the measured orientation of the satellite fixed axis defined instantaneously over a time interval. The requirements for the pointing accuracy in terms of these three definitions are given in Table 5.1, along with the accuracies achieved in-flight.
Pointing and tracking were carried out by the use of one of the two Star-Trackers (STRs) mounted outside the cryostat (the other was not commissioned), the Quadrant Star Sensor (QSS), located on the optical axis of the telescope, and the gyros and reaction wheels controlling movement.
One of the first tasks accomplished during PV was the proper determination of the focal-plane geometry through a series of observations designed to measure the precise locations of the instrument apertures with respect to the QSS boresight.
Any changes in alignment with respect to the nominal focal-plane geometry were expected to be caused by temperature changes of the STR baseplate. It was anticipated that the temperature would vary, and it had been planned to have an update of the pointing quaternion (see Appendix F) at the start of every revolution (on the assumption that the variation within a revolution would be small).
At the beginning of the mission, while the satellite was still cooling down, the measured temperature was far from what was expected, and in the first revolutions SCC finetuned the STR parameters in order to bring any misalignments within the required value.
Before this calibration work was completed in revolution 21 and the data incorporated into the spacecraft's pointing procedures, all pointings were subject to systematic errors in the aperture positions used. Anyone interested in using very early PV observations (few of which contain science data) should therefore do so with caution.
In-flight tests of the satellite pointing during the PV Phase soon revealed a much better performance than that required, with the obtained values of RPE, APD and APE given in Table 5.1. The excellent stability of the ISO spacecraft was mainly due to lower than expected thermoelastic distortions between the STR and the optical axis of the telescope.
In order to check this stability during the mission, any misalignment between the operational STR and the telescope boresight (and by inference the scientific instruments) was determined at the start of every revolution by placing a guide star in the centre of the field of view of the QSS and determining its position in the STR.
A specific calibration programme was designed and executed in revolution 137 to evaluate the ISO pointing performance, by which a number of point-like sources were observed with ISOCAM, since it was suspected that some systematic effects could still be present in the determination of the APE. This was combined with additional observations performed in revolution 264 with a more accurate star selection procedure, based on the Hipparcos star catalogue, and a better determination of the colour-colour relation between visual magnitude (as quoted in the Hipparcos catalogue) and the expected flux in the CAM LW9 filter. As a consequence of this calibration exercise, it was found that there was a clear correlation between pointing offset and position of the guide star in the STR field of view, indicating that there was still room for additional improvements in the ISO pointing accuracy.
| Type of pointing error | Required | After | After | After |
| PV | rev. 368 | rev. 452 | ||
[ ] |
[] |
[] |
[] |
|
| Jitter (RPE) |  2.7 |
0.5 | 0.5 | 0.5 |
| Drift per hour (APD) | 2.8 |
0.1 |
0.1 |
0.1 |
| Absolute Pointing Error (APE) | 11.7 |
 2.5 |
1.4 |
2.0 |
Notes:
for the radius of the cone, i.e. the
angular separation between the actual and the commanded
pointing direction is within these limits 95% of the time.
As a consequence of the analysis performed, the following measures were taken:
. A further refinement took place later during Post-Operations when the guide star proper motions and differential aberration effects were taken into account, together with a refinement of the ISO Guide Star coordinates via the Hipparcos output catalogue (see Section 5.4.3).
In this way, the APE was reduced to 1.4
in the legacy
version of the ISO Data Archive, almost ten times
better than specified (cf. specification of 11.7
)
and the short term jitter was less than 0.5
, about five
times better than the specification of 2.7
(2,
half-cone, over a 30 s period of time).
The excellence of the pointing performance was especially welcome for the use
of small-aperture instruments such as the smaller PHT-P
apertures and the SWS. For
example,
an Absolute Pointing Error of 4
would have both limited the
photometric
calibration accuracy of SWS to about 30% and also compromised its wavelength
calibration via an effective shift of one grating scanner step.
A detailed description of the improvements made to the ISO pointing during the Operational Phase and a quantitative assessment of the ISO pointing accuracy reached at the end of the mission can be found in Salama et al. 2001, [140], and Pollock 2001, [137].
Observations of Solar System Objects (SSO) were implemented as tracked observations using one or more one-dimensional raster observations, although this technique was subject to some limitations of particular relevance to long SSO observations, in which an often complicated trajectory was approximated by a series of straight-line raster operations.
As the spacecraft pointing was stable at each raster position, data from observations using small apertures (e.g. SWS) were often still sensitive to the variable position of the object within the aperture, correlated with changes in both the raster points as documented in the Instrument Reference Pointing History (IRPH) file and the instantaneous pointing position reported in the Instrument Instantaneous Pointing History (IIPH) file.
From revolution
290 onwards an improved SSO tracking algorithm was used, resulting
in smoother SSO tracking. Before then, the spacecraft pointed at the expected
position of the SSO and waited until it was 2
away before
moving to the next pointing. Therefore, the object tended to have an average
offset of 1
along its path, from the spacecraft's pointing.
From revolution
290 the spacecraft was pointed 1
ahead of the
SSO and not moved until it got 1
behind, bringing the
average mispointing down to zero and the maximum offset down from
2
to just 1
.
The properties of the guide stars used as reference for pointing had also an important effect on the pointing accuracy.
A list of guide stars was prepared before the mission based on a pre-release of the Hipparcos catalogue and used for pointing purposes. Guide stars were removed during the mission from the list if the observations performed using them showed pointing problems.
The apparent visual magnitude limits for the guide stars
were between 2 and 8. These limits were set according to the STR sensitivity.
For normal observations this meant that there were always at least one,
and at most five, guide stars in the field of view (3
4) of the CCD detector inside the operational STR.
The accuracy in the determination of the position of guide stars in the STR field of view was in some cases disturbed by the presence of other stars in their vicinity. If a guide star was detected on the same column of the CCD as a bright star, problems could occur due to CCD blooming. Thus, guide stars were avoided if they were predicted to fall near one.
There were also small differences between the Hipparcos coordinates initially used (based on a preliminary version of this catalogue), which were referred to the J1991.25 epoch, and the ISO catalogue coordinates, referred to J1997.00. This resulted in slight differences in proper motions and parallaxes.
To avoid this problem, the ISO guide star catalogue was cleaned and
stars with proper motions larger than 0.5
/yr in the
Hipparcos catalogue were removed. In addition, the ISO guide star
catalogue was updated every three
months with positions of individual stars corrected for proper motion,
calculated with respect to the mid of every period.
The residual effects left were very small (
0.4
)
but still significant in some cases, fortunately only affecting a few
nearby guide stars.
Further pointing inaccuracies were introduced when the guide star was far from the centre of the field of view of the CCD (differential aberration) and when the guide star magnitude was near the lower sensitivity limit.
While aberration was thought to be corrected for ISO observations by the
fact that guide stars are always close ( 2.5) to the observed
target, the differential aberration effect can cause pointing errors up to
0.9
if the guide star is away from the instrument optical
axis (if ISO's velocity is considered as well, this can become even larger).
The final ISO pointing model used in the legacy pipeline considers the effects of differential aberration, proper motion and parallax of the guide stars used and it also makes use of the latest version available of the Hipparcos catalogue. This way the accuracy of the ISO pointing was improved at the end of the mission to the arcsecond level (see Section 5.4.1).