National Aeronautics and Space Administration

EXOSAT ABSOLUTE SOURCE LOCATION

A Progress Report

1. Introduction

2. Coordinate Transformations

Generally, the transformation from the coordinate system (U,V,W) to a new coordinate system (X, Y, Z) involving rotation and no transition can be written as follows:

where X_u, X_v, X_w are the three components of the X unit vector with respect to U, V and W (so called direction cosines) etc.

If the coordinate frames are orthogonal then the transformation matrix T is orthogonal and the inverse of T is equal to the transpose.

Three coordinate systems are defined as follows:

1) The star tracker reference frame (spacecraft axes): (X_ST, Y_ST, Z_ST)

2) The LE detector reference frame (after linearisation): (X_LE, Y_LE, Z_LE).

3) The Celestial Mean Equatorial (ME) 1950 reference frame: (X_ME, Y_ME, Z_ME)

Two transformations are required to establish the relation between the detector reference frame and the Mean Equatorial frame:

A) The auxiliary data on the FOT contain the spacecraft attitude defined as the three components of the roll, pitch and yaw axes with respect to the ME reference frame, ie:

where AUX₁ .... AUX₉ are the values in the Auxiliary data file on the FOT, record 3 bytes 8-124 (see FOT Handbook, Sect. 3.8 p. 11 ) . Note that these values are converted from the requested nominal pointing direction. The actual pointing direction is obtained from the star positions in the star tracker field of view ( refer to next section). RPY is the attitude matrix printed in the automatic analysis output.

A 5" offset is applied to the Y-axis limit cycle to overcome an operational problem of the AOCS electronics (ref. Express No.4 p.34). This affects all observations carried out from 15.9.83 at 07.00 UT (day 253, SHF time key 116924400) and pointing positions must be corrected appropriately by an additional transformation.

where = +5".

B) The relationship between the ST and LE frames is given by the telescope misalignment data in the LE CCF (data type BD) The transformation ST ------ LE is written as:

where MIS1 .... MIS9 are given in the LE CCF (data type BD) bytes 0 - 35 (CMA) multiplied by 10⁹.

The misalignment is the displacement of the LE frame with respect to the ST frame (see Fig. 1) and can be derived in terms of 3 consecutive rotational matrices about the roll, pitch and yaw axes:

where , and are the anti-clockwise rotational angles (misalignment angles) as seen from the respective positive axis in the direction of the origin (fig. 1).

3. Results

EXOSAT RA and Dec. positions are derived using the formula given in the FOT Handbook (Sect. 7.1 p.46). The 5" offset of the Y-axis limit cycle and refinements to the accuracy of the calculation were included. Columns and give the deviations (arcsec) of the EXOSAT position with respect to the known optical/radio position (column 2 and 3).

Secondly, a correction has been applied for the typical limit cycle of ± 2-3 arcseconds maintained during an observation. Image 'deblurring' was carried out by the simple method of calculating the mean offset (pixels) in pitch () and yaw () from the attitude data in the HK records and changing the positions accordingly (see Fig. 1):

X_d = X_b +
Y_d = Y_b -

Two X-ray sources (3C273 and 1156+295) in the sample have twice been observed about half a year apart, with opposite orientation of the spacecraft. Fig. 3 shows the computed 'deblurred' positions for 3C273 together with the orientation of the Ex, Ey plane of the L1 CMA. The source was observed using different filters yielding a number of positions which are within a region of about 4" diameter. A systematic error is clearly present, the two sets of observations each being located opposite the true source position and about 30" apart. Observations of 1156+295 show the same behaviour suggesting a telescope to star tracker misalignment error. This error has been calculated empirically by taking for each set of observations the difference in the E_x and E_y direction:

giving the following errors in the misalignment angles (see Fig.1).

= X/2= -11.83"
= Y/2= +10.18"

If this correction is applied to the misalignment angles, new positions and deviations and are derived. These deviations are typically less than ~7".

In July 1985, improved star tracker calibration reference data was implemented at ESOC (p.2) yielding a better knowledge of the actual pointing direction. Generally, the calculated (requested) pointing direction is within ~1" of the actual one. As a final correction to the EXOSAT positions, this new calibration data gave slightly improved deviations and . , the corresponding distance, is now typically ~6".

There are a few exceptions to this, some of which may be caused by the linearisation of the detector read-out positions. The linearisation for LE1 is based on an in-flight raster scan (Cyg X-2), which is less accurate than the ground-based calibration for the LE2 CMA. However, close to the detector origin, where the linearisation is most accurate, the difference between known RA, DEC and the EXOSAT position is less than 4-5".

In some cases, poor statistics (<~ 50 cts) may yield inaccurate positions, see eg. the third pointing of MKN 421, where Y_b is about 1 pixel less than the two previous values.

4. Conclusions and Status of Analysis Software

• improvement in the calculation accuracy
• 5" offset applied to the Y-axis limit cycle.
• image deblurring of the 2-3" limit cycle.
• possible errors in telescope star tracker misalignment.
• improved star tracker calibration data.

Further work is in progress to:

• implement a more sophisticated deblurring method where each photon is treated individually.
• use a correlation technique in which the source distribution is compared to the PSF based on (in-flight) calibrations.
• analyse several sets of observations separated by half a year to determine the misalignment errors accurately, together with any time dependence.

(A) Interactive Analysis system

• 5" offset of the Y axis limit cycle included.
• improved accuracy of thc calculation.
• new set of misalignment angles.

(B) Auto analysis software

NB:

(1) the new star tracker calibration data is automatically used as from 30.7.85. Observations carried out prior to 30.7.85 have been analysed using the old reference data although any differences in derived pointing are generally small (~1") compared to the other effects described above.

(2) the misalignment angles have not been modified in the CCF, pending further study.

E. Gronenschild

References

Figures

Figure 1

Orientation of EXOSAT coordinate reference frames of the star tracker and the LE detector, showing the misalignment angles , and

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Figure 2

The spacecraft attitude in the mean equatorial (ME) coordinate frame(a). The pointing direction is given by the vector X_LE, the detector origin by and . The north angle is defined as shown (b).

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Figure 3

The observations of 3C273. Two sets of observations were carried out, separated by half a year. The first set yielded positions in the lower right- hand corner, the second set in the upper left-hand corner (numbers refer to the sequence of pointings given in Table 1). The orientation of the detector reference frame E_x, E_y is also indicated. Final source positions (index NS) after deblurring, incorporation of the new misalignment angles and taking the actual pointing positions based on a new star tracker calibration are now close to the true source position.

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Table 1