Observational Strategy

Our goal is to map the entire remnant at the highest attainable spatial resolution that can be attained with the ROSAT-HRI. Achieving this ambition is not simple, because the resolution is limited by the intrinsic instrumental properties and photon counting statistics. The large range of surface brightness allows us the luxury of mapping the brightest regions at full HRI resolution, but forces us to seek a compromise between effective resolution and observing time in the dimmest regions. To give a concrete example, the surface brightness observed with the ROSAT yields a count rate in a pixel (essentially an HRI resolution element) of . For the brightest regions along the shell (within of the highest surface brightness) a exposure will net 15 counts (vs. 0.4 background counts). It would take to obtain a similar number of counts in the interior. We therefore adopted the following observing strategy:

1. For the brightest regions, the shell and a few select adjacent regions (mainly the ``carrot'') we can use the full 5'' resolution. This restricts the effective field of view to 8' radius. There are 24 such pointings, each requiring 15-20 ks to obtain 10-15 counts per cell.
2. For moderate surface brightness interior regions (e.g., the northeast quadrant) it is is possible in a comparable exposure time to obtain typically 10 counts per pixel. For this pixel size, the effective HRI field of view has a 12' radius. There are 9 such fields, each requiring 20-25 ks.
3. For the low surface brightness interior regions, we can obtain 10 counts per cell only if we use a 15'' or larger size. For these, we utilize the full HRI field of view. Each of the 10 fields requires a 35 ks exposure.
4. One low surface brightness region along the western edge of the ``breakout'' requires 50 ks.

Fig.1 shows the locations of the survey pointings on the Einstein IPC map.

This project is a collaborative project between U.S. and German observers because of the large total observing time required to complete this program (  s). As a joint project, it displays the level of bipartisan collaboration that has been the hallmark of the ROSAT program since its inception, and in this spirit we have divided the Cygnus Loop fields equally among the two national TAC's. It is our intention to carry out this program for the community at large, therefore we have waived our proprietary rights so as to grant immediate access to the data.

  
Figure: Scheduled and requested Cygnus Loop ROSAT-HRI pointings superimposed on the Einstein IPC image (Seward 1990). Circle size indicates the useful field of view and is proportional to integration time. Solid circled fields have been observed or scheduled; dashed circles are the AO7 request.