Modifying Metadata

Modifying Metadata#

In this example we will show how to modify the metadata of a SDFITS file using dysh.

Background#

We will use a practical example. For observations with the GBT it is recommended to observe a flux density calibrator (see e.g., Perley & Buttler 2017 for a list of calibrator sources and their flux densities) with the same configuration as that used for the science observations. The reason being that the values of the temperature equivalent power of the noise diodes stored in the SDFITS files can be out of date.

In this example we won’t use observations of a flux density calibrator, but instead we will use the analysis of Goddy et al (2020). They find that the temperature stored in the SDFITS files is on average lower than the measured values, so that the temperature must be corrected by 20%

\[ T_{\rm{CAL,corr}}=1.2T_{\rm{CAL,file}}. \]

Here we use this to correct the temperature of the noise diode.

Example#

We start by importing the relevant modules.

from dysh.fits.gbtfitsload import GBTFITSLoad
from dysh.util.download import from_url
from pathlib import Path

Data retrieval#

We download the data we will use for this example, if necessary.

url = "https://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"
filename = from_url(url, savepath)

Data loading#

We load the data and inspect its contents.

sdfits = GBTFITSLoad(filename)

sdfits.summary()

	SCAN	OBJECT	VELOCITY	PROC	PROCSEQN	RESTFREQ	DOPFREQ	# IF	# POL	# INT	# FEED	AZIMUTH	ELEVATIO
0	51	NGC5291	4386.0	OnOff	1	1.420405	1.420405	1	2	11	1	198.343112	18.64274
1	52	NGC5291	4386.0	OnOff	2	1.420405	1.420405	1	2	11	1	198.930571	18.787219
2	53	NGC5291	4386.0	OnOff	1	1.420405	1.420405	1	2	11	1	199.330491	18.356075
3	54	NGC5291	4386.0	OnOff	2	1.420405	1.420405	1	2	11	1	199.915725	18.492742
4	55	NGC5291	4386.0	OnOff	1	1.420405	1.420405	1	2	11	1	200.304237	18.057533
5	56	NGC5291	4386.0	OnOff	2	1.420405	1.420405	1	2	11	1	200.890603	18.186034
6	57	NGC5291	4386.0	OnOff	1	1.420405	1.420405	1	2	11	1	202.327548	17.385267
7	58	NGC5291	4386.0	OnOff	2	1.420405	1.420405	1	2	11	1	202.919161	17.494902

Metadata inspection#

Now we inspect the current noise diode temperature stored in the SDFITS file.

sdfits["TCAL", "PLNUM"]

	TCAL	PLNUM
0	1.424292	1
1	1.424292	1
2	1.452650	0
3	1.452650	0
4	1.424292	1
...	...	...
347	1.452650	0
348	1.424292	1
349	1.424292	1
350	1.452650	0
351	1.452650	0

352 rows × 2 columns

For polarization 0 the noise diode temperature is 1.452650 K and for polarization 1 it is 1.424292 K.

We will calibrate the data using these values to compare after we update the noise diode temperature. We use position switching calibration, then we time average all the scans and remove an order 1 polynomial.

ps_original = sdfits.getps(plnum=0).timeaverage()
ps_original.baseline(degree=1, remove=True)

Metadata update#

Now we update the temperature of the noise diode by multiplying by 1.2.

sdfits["TCAL"] *= 1.2

/home/docs/checkouts/readthedocs.org/user_builds/dysh/envs/release-0.4.0/lib/python3.10/site-packages/dysh/fits/gbtfitsload.py:2559: UserWarning: Changing an existing SDFITS column TCAL
  warnings.warn(f"Changing an existing SDFITS column {items}")
/home/docs/checkouts/readthedocs.org/user_builds/dysh/envs/release-0.4.0/lib/python3.10/site-packages/dysh/fits/sdfitsload.py:1046: UserWarning: Changing an existing SDFITS column TCAL
  warnings.warn(f"Changing an existing SDFITS column {items}")
WARNING: AstropyDeprecationWarning: The update function is deprecated and may be removed in a future version.
        Use update_header instead. [dysh.fits.sdfitsload]

Now we check that the values were updated.

sdfits["TCAL", "PLNUM"]

	TCAL	PLNUM
0	1.70915	1
1	1.70915	1
2	1.74318	0
3	1.74318	0
4	1.70915	1
...	...	...
347	1.74318	0
348	1.70915	1
349	1.70915	1
350	1.74318	0
351	1.74318	0