PHT Interactive Analysis User Manual - (V9.0)

3.3.10) Pointing correction for PHT-S

Due to the small (24"x24") pixel size of PHT-S, and to the sharply peaked footprint shape, already a few arcsecond off-centre positioning may lead to significant intensity changes by the PHT-S spectra, together with alterations of the spectral shape. It was possible to derive correction factors for off-centre point source measurements by using a detailed cross-scan on a calibration point source.

PIA V8.1 offers the user a first implementation of a facility for correcting for off-centre positioning of point sources, based on a report by Péter Ábrahám from MPIA.

The mentioned report is complemented by a 3-dimensional data cube containing the correction factors for all 128 PHT-S pixels. For every pixel a 2D footprint matrix is given, containing the correction factors for a Delta Y - Delta Z offset in 0.2" steps in both directions, covering up to the full PHT-S aperture.

The GUI implementation

Fig. 1 shows the main menu for the pointing correction.

On the upper part the information on actual pointing observation parameters are compared with the intended parameters, as entered by the observer in the proposal. In the rule they should be very close to each other, but in some special cases they can differ up to several arcseconds.
In the central part of the widget are the editable pointing parameters and the derived offsets in the spacecraft Y- and Z-coordinates. By default the intended coordinates are used to derive the offsets. An observer could wish to enter other coordinates as the intended ones, either because the coordinates were wrong entered in the proposal handling or because the exact position of the object was not well known at the time of proposing the observation.
A change of the RA- and DEC-position of the object should be followed by pressing the button for deriving the offsets, prior to the correction calculation. The opposite is also possible (for calibration purposes), calculating RA- and DEC-positions an object should have according to the edited offsets in Y and Z.
All the action and visualization buttons are placed in the bottom of the widget. Once the calculation of the corrections is performed (just by obtaining the most close position in the footprint matrix for every pixel), the amount of the corrections can be plotted, and the corrected spectrum overplotted over the uncorrected one.
Pressing the button "Corr. Data -> buffer" will create a new measurement within the AAP buffer. The header of the measurement contains several keywords giving the RA and DEC coordinates assumed for the correction as well as the derived offsets.

Fig. 1: The widget for the PHT-S pointing offset correction.

Chapter history:
Date  Author  Description 
02/11/1999  Carlos GABRIEL (ESAVILSPA-SAI)  First Version (PIA V8.1) - (Based on  Péter Ábrahám's internal document "Correction for Off-Centre positioning of point sources in the ISOPHOT-S aperture")