FITS Format Output Files

  FITS, or Flexible Image Transport System is a standard format for data transfer which may be read by many data analysis packages. The ability to write FITS output files has been incorporated into this package to permit the possibility of importing simulated Space VLBI data generated by uvplot into a variety of VLBI data analysis packages, such as those developed at the California Institute of Technology, Brandeis University, and the National Radio Astronomy Observatory (NRAO). The format chosen for the uvplot FITS files conform to the Random Groups format used by the Advanced Image Processing System (AIPS) package developed at the NRAO. This FITS format has been chosen because it has been in use for more then a decade and is that standardly used in the radio astronomical community for transporting - data files. The uvplot FITS output files have an ASCII header which is user readable, followed by a section written in binary, followed by a FITS extension file conforming to the proposed BINTABLE extension format. Note that BINTABLE is descended from the A3DTABLE extension used in older AIPS distributions.

uvplot need not have been run previously in order to create a FITS output file. This will save time in the case of simulations involving close sampling times and a large number of stations. In addition, the switch in the uvplot.par file to create a uvplot.dat file need not be on in order to obtain a FITS output file; in fact, is recommended not to write out the uvplot.dat file at the same time as a FITS file, since this causes heavy disk I/O. The user can interrupt the generation of FITS data using the Escape key. The resulting FITS file will only contain the - data generated before the interrupt. In this way, partial data sets can be generated if desired.

During the writing of the FITS file, a number of working files (uvfits.tmp, uvfits2.tmp, antenna.tmp, antenna2.tmp) are created, which are used by uvplot to temporarily store the - data to be written. These files are automatically removed if uvplot completes it run successfully but sometimes are not erased if the program has been interrupted for some reason. It is not usually possible to predict the size of disk space needed for these files and so it is possible to run out of disk space if large amounts of - data is generated. For example, if data is generated for 5 stations observing with Radioastron over a complete orbit with a time delay of 1 minute between samplings, the uvfits.tmp file, which is the largest of the temporary files, will be approxmately 6-8 Megabytes in size. If there is not sufficient disk space for these temporary files, uvplot may seem to crash for no reason.

The size of the final FITS file is very much less then that of the temporary files used to create it. For example, a FITS file which is 560 Kilobytes in size will require about 6.7 Megabytes in temporary file space. Because of this, the generation of FITS files can be considered to be a form of data compression.

It should be mentioned here that generation of FITS data will take quite a long time!. This is because of the generation of visibililties which need to be calculated from a given source model and also the generation and addition of Gaussian noise (if the `F8' key option is chosen) from each baseline. And example is if we use the 10 VLBA stations along with Radioastron and use a 28 hour orbit with a 5 minute sampling interval. This will take approximately 2000 seconds on a 386/33 Mhz PC with 4 MB RAM.

The generated FITS file can then be imported into AIPS via the AIPS task UVLOD and the uvdata can be treated the same as any other uvdata. Samples of actual data generated is included as an option to be added to the end of this manual. Please note that the FITS file created should be treated as a bianry file for the purposes of copy or transport even though the header is ASCII. Transport or copying in ASCII mode will corrupt the file and introduce errors which will render the file unimportable to AIPS.

It should also be mentioned that it is not possible at this time to generate simultaneous FITS data from both spacecraft in the same model run. (i.e. even if the `b' key mode and both indicators for the spacecraft were on) only the last spacecraft uvdata would be written to the FITS file. This is because the dual uv track mode was initially made for visualization of uv coverage and not for the evaluation of simultaneous imaging capabilities using models.