Merging SDFITS Files

Merging SDFITS Files#

This recipe shows how to merge SDFITS files. There is no built-in function to merge SDFITS files, so we will put the files we want to merge in the same directory, and then load them and write them to a single SDFITS file. For this recipe we will use a single SDFITS file to generate two files from subsets of its data.

You can find a copy of this tutorial as a Jupyter notebook here or download it by right clicking here and selecting “Save Link As”.

Loading Modules#

We start by loading the modules we will use for this recipe.

# These modules are required for loading and writing data.
from dysh.fits import GBTFITSLoad

# These modules are only used to download data.
from pathlib import Path
from dysh.util.download import from_url

Data Retrieval#

Download the example SDFITS data, if necessary.

url = "http://www.gb.nrao.edu/dysh/example_data/positionswitch/data/AGBT05B_047_01/AGBT05B_047_01.raw.acs/AGBT05B_047_01.raw.acs.fits"
savepath = Path.cwd() / "data"
savepath.mkdir(exist_ok=True) # Create the data directory if it does not exist.
filename = from_url(url, savepath)

Data Loading#

Next, we use GBTFITSLoad to load the data, and then its summary method to inspect its contents.

sdfits = GBTFITSLoad(filename)

sdfits.summary()
SCAN OBJECT VELOCITY PROC PROCSEQN RESTFREQ DOPFREQ # IF # POL # INT # FEED AZIMUTH ELEVATION
51 NGC5291 4386.0 OnOff 1 1.420405 1.420405 1 2 11 1 198.3431 18.6427
52 NGC5291 4386.0 OnOff 2 1.420405 1.420405 1 2 11 1 198.9306 18.7872
53 NGC5291 4386.0 OnOff 1 1.420405 1.420405 1 2 11 1 199.3305 18.3561
54 NGC5291 4386.0 OnOff 2 1.420405 1.420405 1 2 11 1 199.9157 18.4927
55 NGC5291 4386.0 OnOff 1 1.420405 1.420405 1 2 11 1 200.3042 18.0575
56 NGC5291 4386.0 OnOff 2 1.420405 1.420405 1 2 11 1 200.8906 18.1860
57 NGC5291 4386.0 OnOff 1 1.420405 1.420405 1 2 11 1 202.3275 17.3853
58 NGC5291 4386.0 OnOff 2 1.420405 1.420405 1 2 11 1 202.9192 17.4949

Writing SDFITS Files#

To show how to merge SDFITS files, we will create two separate SDFITS files first. We use the GBTFITSLoad.write method to write the data. We will write scans 51 and 52 to two separate files.

Create an Output Directory#

The approach we will use to merge SDFITS files relies on having the files to be merged in the same directory. We create a new directory ./output/merge_sdfits if it doesn’t exist already.

output_dir = Path.cwd() / "output/merge_sdfits"
output_dir.mkdir(exist_ok=True, parents=True) # Create the output directory if it does not exist.

Write the Intermediate SDFITS Files#

Now we write scans 51 and 52 to the directory we created.

sdfits.write(output_dir / "scan51.fits", scan=51)
sdfits.write(output_dir / "scan52.fits", scan=52)

 ID    TAG    SCAN # SELECTED
--- --------- ---- ----------
  0 c090bf33f   51         44

 ID    TAG    SCAN # SELECTED
--- --------- ---- ----------
  0 278d257ea   52         44

Merge SDFITS Files#

Now that we have two SDFITS files we want to merge, we can do this by loading them using GBTFITSLoad by passing the directory containing the files we want to merge. In this case, that would be ./output/merge_sdfits.

sdfits_merged = GBTFITSLoad(output_dir)

Loaded 2 FITS files

sdfits_merged.summary()
SCAN OBJECT VELOCITY PROC PROCSEQN RESTFREQ DOPFREQ # IF # POL # INT # FEED AZIMUTH ELEVATION
51 NGC5291 4386.0 OnOff 1 1.420405 1.420405 1 2 11 1 198.3431 18.6427
52 NGC5291 4386.0 OnOff 2 1.420405 1.420405 1 2 11 1 198.9306 18.7872

Write the Merged SDFITS Files to a New File#

Now we can write the merged SDFITS files to a new file using GBTFITSLoad.write. We tell it to write to a single file with the multifile=False argument. We write to a new directory to show that this is only one file now.

new_dir = Path.cwd() / "output/merged_sdfits"
new_dir.mkdir(exist_ok=True)
sdfits_merged.write(new_dir / "merged_sdfits.fits", multifile=False)

Load the merged SDFITS file.

sdfits_merged = GBTFITSLoad(new_dir)
sdfits_merged.summary()
SCAN OBJECT VELOCITY PROC PROCSEQN RESTFREQ DOPFREQ # IF # POL # INT # FEED AZIMUTH ELEVATION
51 NGC5291 4386.0 OnOff 1 1.420405 1.420405 1 2 11 1 198.3431 18.6427
52 NGC5291 4386.0 OnOff 2 1.420405 1.420405 1 2 11 1 198.9306 18.7872

You can check that it is indeed a single file by looking at the files attribute of the GBTFITSLoad object.

sdfits_merged.files

[PosixPath('/home/docs/checkouts/readthedocs.org/user_builds/dysh/checkouts/release-0.11.5/docs/source/how-tos/examples/output/merged_sdfits/merged_sdfits.fits')]

The same principles can be applied to more complex examples, like selecting an IF for certain scans, and another for others, and then merging them all in a single file.

Contents