Using TOPCAT Software for Windows to visualise and interpret SDFITS data (NASA single dish data format)

The SDFITS single dish data format (https://safe.nrao.edu/wiki/bin/view/Main/SdfitsDetails#:~:text=Briefly%2C%20SDFITS%20is%20a%20convention%20for%20storing%20single-dish,freedom%20to%20add%20any%20additional%20columns%20as%20necessary):

The SDFITS convention was developed in the late 1980s. An early version was used in the UniPOPS data analysis program and it was modified to work in aips++. The version used by aips++ was chosen as the disk storage format for GBTIDL. The two references above describe that version. Since that version of SDFITS was developed, the FITS community has made considerable progress in describing coordinate axes both in images as well as in binary tables. Other observatories have made some attempt to adjust the SDFITS convention to use these newer descriptions of world coordinate systems. The SDFITS in use in Green Bank has not yet been updated. It is anticipated that an effort will be made in the near future to do that so that it should be easier to share data between the various telescopes and analysis programs. Briefly, SDFITS is a convention for storing single-dish radio-astronomy data in a FITS binary table. It consists of a dictionary of a few required columns, a few more suggested columns and the freedom to add any additional columns as necessary. A column that has the same value in all rows can be expressed instead as a FITS keyword provided the column name is no longer than 8 characters. A constant column expressed as a FITS keyword is called a “virtual” column. There is a DATA column which is multi-dimensional. The shape of the DATA column is given by either a TDIMn keyword (where “n” is the DATA column number) or sometimes by a TDIMn column when the shape may change from row to row. The sdfits program always writes out the DATA’s TDIMn value as a keyword, meaning that the DATA shape is fixed within a single binary table produced by sdfits. There are also several associated columns which describe the physical coordinates associated with each axis of the DATA column (frequency, pointing direction, polarization). All but the first axis (which must be a frequency-type axis) will have just a single pixel. This means that each row in an SDFITS binary table is a single spectra with associated information (often called “the header”).

Further information on the Single Dish FITS (SDFITS) Convention for Radio Astronomy Data (from https://fits.gsfc.nasa.gov/registry/sdfits.html):

• Description: The SDFITS binary table convention is used for interchange of single dish data in radio astronomy
• Submitted: August 2010 by Robert Garwood (NRAO) and Mark Calabretta (ATNF/CSIRO)
• Registered: August 2012
• Date and Place of First Usage: SDFITS was developed at a meeting of representatives from many major radio astronomy observatories at Green Bank, West Virginia in October 1989.
• Extent of Usage: Variants of the SDFITS convention are used by a number of radio observatories, including NRAO, Arecibo, Parkes, and Mopra (Australia). The definition document (below) specifically applies to the NRAO version of SDFITS; the data from other observatories may differ slightly.
• Full Documentation:
  • SDFITS definition document: PDF format, Postscript format
  • see also Garwood (2000)
• Comments and/or critiques:
  • Discussion of differences between the NOAO and Parkes SDFITS formats: sdfits_format.txt
• Sample FITS Files:
  • PParkes_GASS.fits This file is from the Parkes Galactic All Sky Survey (GASS) of the atomic hydrogen (HI) emission in the southern Milky Way. It contains 7-beam, 2 polarization x 2049 channel, 8MHz bandwidth, dual-IF, scanned, frequency-switched Parkes multibeam GASS data. The data table has been truncated to 28 rows to reduce the size of the file. This file differs from the SDFITS documentation in having a vector TSYS column, with one value for each polarization.
  • TREG_091209.cal.acs.fits, This file contains processed (by GBTIDL) Green Bank Telescope (GBT) data.
    Additional comments from Bob Garwood:This shows how we add a new extension when the data length changes. I don’t believe it differs from the convention. For the GBT we always write one spectrum per row. I went that route rather than how I approached things for the aips++ sdfits writer (which is what Mark’s example is closer to – i.e. both polarizations on the same row) after a conversation with Harvey about 6-7 years ago where he said that it was his impression that the sdfits convention was never meant to hold more than one spectrum per row because of the scalar nature of the TSYS column.I’ve used fv on this example file to pare it down from it’s original size of 30 rows in the first extension and 300 rows in the second extension to something more manageable. The first extension is calibrated individual integrations from an observation of a galaxy. There’s 2 polarizations and one spectral window. The raw data is position switched data. The second extension is 2 polarizations and 2 spectral windows of a different galaxy. It’s also calibrated individual integrations and the raw data were 2-beam nodding data (one scan with one beam on source followed by a second scan with the other beam being on source) – it’s shorter integrations so the emission is harder to see in the individual integrations than it is for the data in the other extension.
• Sample Header Listings:
  • PParkes_GASS.txt,
  • TREG_091209.cal.acs.txt,

Using TOPCAT Software to visualise SDFITS data:

To plot a picture in TOPCAT, follow these steps:

1. Load your data

• Open TOPCAT and load your data table (FITS, CSV, VOTable, etc.).
• Go to File > Load Table and select your file.

2. Open the Plot window

• Click on the Graphics menu, then choose a plot type — e.g., Sky Plot, Scatter Plot, Density Plot, etc.

3. Configure axes

• In the plot window, select the columns you want to plot on the X and Y axes from the dropdowns.
• For sky coordinates, you might select RA and Dec.

4. Add an image (optional)

• To overlay an image (like a sky survey background), go to:
  • Layers > Add Sky Image
  • Choose a source like DSS or SDSS (requires internet).
  • If you have a local image, you can use Add Grid/Image instead.

5. Customize appearance

• Use the Form and Style tabs to change markers, colors, size, labels, and more.
• You can overlay multiple datasets using Layers.

6. Export your plot

• Go to File > Export > Save Window as PNG/PDF to save your plot.

Would you like to plot a specific dataset or sky image? I can guide you through more advanced tweaks if needed.