The high altitude elliptical orbit of AXAF, combined with the ability to rotate the solar panels about one axis, provides high target visibility over most of the sky. With the exception of avoidance regions associated with the Sun, Earth, and Moon (the latter two of which can be relaxed for special observations), AXAF can observe any point on the sky at basically any time that the spacecraft is above the radiation zones. For any particular target direction, however, the requirement that the Sun be placed in the hemisphere opposite the ACIS radiator, and close to normal to the solar panels, defines an absolute sky orientation for the instrument field of view (FOV) on any given observation date. For many observations, this is of little or no consequence. For others, however, the sky orientation is important. In particular, some observations with the AXAF gratings may require that the spacecraft roll angle be constrained so that spectra from nearby field sources do not contaminate the spectrum for the source of interest. To aid AXAF observers in planning such observations, an Observation Visualizer (ObsVis) tool has been developed. This Java-based tool is accessible from the Proposal Information section of the ASC Home Page.
Figure 34: Observation Visualizer Parameter Form. The user selects the coordinates of the field center, the image size, and the dataset from which to extract the image. Alternatively, the user can supply a FITS image located at a specified FTP site. Catalog overlays can also be selected. Postscript version of the above image.
Figure 35: Sample ObsVis screen illustrating roll geometry for a selected observing field. The user can select an outline of any of the AXAF instruments for overlay, and can select a grating for use in creating dispersion patterns which can be placed over selected sources in the field. The SI FOV can be ``dragged'' to an arbitrary position, and the entire FOV and grating pattern can be rotated to simulate a roll of the spacecraft. The roll corresponding to the selected geometry is displayed. Optionally, the user can plot the nominal spacecraft roll angle as a function of time to determine when particular orientations can be achieved (see Figure 36). In this plot, the selected roll angle of 41° <!IMG WIDTH=21 HEIGHT=12 ALIGN=BOTTOM ALT="tex2html_wrap_inline1396" SRC="img10.gif" > results in undesirable overlap of spectra for two sources in the SI FOV. Postscript version of the above image.
For the seasoned ROSAT and ASCA observer, the functionality of the ObsVis tool might best be described as a merging of the ``Viewing'' and ``Roll'' tools presented with some of the features of SkyView. Specifically, as illustrated in Figure 34, the tool allows the user to specify a region of the sky from which an image may be displayed. Currently, on-line images are available only from the Digitized Sky Survey (which is used for Mission Planning to investigate the spacecraft guide star field). An option for using a user-supplied FITS image is also provided. As illustrated in the example shown here, the user merely specifies a pathname and address from which their image can be retrieved through anonymous FTP. (For users without access to such a site from their own institutions, the site sao-ftp.harvard.edu is available.) Overlays can be selected from the SAO Bright Star Catalog, the HEAO-1 X-ray Catalog, and the ROSAT All-Sky Survey Bright Source Catalog. Future releases will incorporate additional catalogs as well.
Figure 36: Nominal roll angle on sky (curve with error bars) and average target visibility plotted as a function of time. The RA and Dec of the target direction are given at the top along with the date range for which the plot was generated. See text for roll definition. Postscript version of the above image.
Once an image or sky map of the region of interest in generated, the user is presented with the capability of creating an overlay of the AXAF Science Instrument (SI) FOV for the four different AXAF detectors (Figure 35). The SI overlay can be placed at the desired position in the field and rotated to an orientation which provides the desirable coverage of the science target. The roll angle is displayed, and the user can choose to plot the nominal spacecraft roll for AXAF as a function of time to determine dates for which the selected roll can be accommodated (see the curve with error bars shown in Figure 36). (Nominal spacecraft roll corresponds to the orientation which places the sun vector on the normal to the solar panels. For a given observation date and target direction, this translates into a particular orientation of the FOV on the sky. Small deviations from this are possible, depending upon the angle between the telescope boresight and the plane defined by the solar panels. These are indicated by the error bars although the model for this is still preliminary.) The definition of the roll angle on the sky is based upon the spacecraft coordinate system: the positive x-axis, around which roll is defined, is directed along the telescope boresight, toward the target being observed; the y-axis is parallel to the axis of the solar panels (and also to the long axes of the HRC-S and ACIS-S detectors); and the z-axis completes the right handed system with the ACIS radiator in the +z direction (and, thus, with the sun being constrained to the x/-z half-plane). Positive roll is defined with the +y axis rolling toward +z. On the sky, this corresponds to roll increasing west of north - opposite that of traditional position angle. Roll angles are defined in the range 0-360 degrees (hence the discontinuity near day 200 in Figure 36). Also indicated in the plot (as a curve with no error bars) is the average visibility for the target as a function of time. Due to high radiation regions of the orbit, the maximum average visibility is about 83%. Reductions in visibility may occur from Earth, Moon, or Sun avoidance constraints; a Sun constraint can be seen in the plot in Figure 36.
Gratings can also be selected with the SI FOV such that clicking on individual objects in the field will produce patterns which mimic the associated dispersed spectrum (though these are geometric only, and not scaled in any way to brightness). An example is illustrated in Figure 35 where we have shown a ROSAT image of a star field with the ACIS-S FOV overlaid and HETG spectral patterns displayed on a number of the brighter field objects. The roll angle used for this figure is undesirable because it results in complete overlap of some spectral components from the two sources in the detector field. In Figure 37 we present another orientation at the same celestial position which provides uncontaminated spectra for the central source. (The orientation shown has a roll angle of 180° <!IMG WIDTH=28 HEIGHT=13 ALIGN=BOTTOM ALT="tex2html_wrap_inline1398" SRC="img11.gif" > . Similar results could be achieved using a roll angle of 0° <!IMG WIDTH=12 HEIGHT=13 ALIGN=BOTTOM ALT="tex2html_wrap_inline1584" SRC="img72.gif" > , but this would conflict with the Sun avoidance constraint shown in Figure 35, and is thus not permissible.) We note, of course, that other orientations will provide similar results and, more importantly, that some orientations will allow spectral results to be obtained for other sources without impacting the central spectrum. With the results from such investigations, users can specify roll restrictions on their observations and also determine the associated time constraints (which may be useful for planning coordinated observations).
As a final note, it should be pointed out that roll constraints on targets are no different than time constraints; each introduces further complexity into the target scheduling process. Large numbers of tightly constrained observations will ultimately lead to reduced observing efficiency for the observatory. Use of the ObsVis tool will help users identify such observing constraints, and will also give them the capability to investigate the observing orientations that maximize the available time during which the desired observations can be carried out.
Figure 37: As in Figure 35, but with a sky roll ( 180° <!IMG WIDTH=28 HEIGHT=13 ALIGN=BOTTOM ALT="tex2html_wrap_inline1398" SRC="img11.gif" > ) which eliminates contamination of source spectra by those from other objects in field. Postscript version of the above image.