Research:Radio Data
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PRO Data Analysis
Data analysis with PRO is done (at UW) in IDL. Many other schools have written their own routines in various languages. Early on in Generation 3 (the current group of students building PRO) it was decided to abandon the idea of using LabView, and instead write our own program in IDL. This was influenced by three main factors:
- None of us knew LabView (or wanted to learn)
- The SRT Kit came with adequate telescope control/tracking/acquisition software
- IDL is a standard in astronomy and ideal for processing large amounts of data
Analysis with IDL
Tracking Constant Sources
The simplest data to reduce with the SRT is tracking a steady source. This is the sort of data we will use to calibrate PRO. The process is essentially as follows:
- Take a long integration (recording) while tracking a steady source
- A longer integration will increase the Signal to Noise ratio, commonly written: S/N
- Read the data into IDL (readcol, etc...)
- Combine each frequency channel over the entire integration period. Due to the horrible S/N of each sub-integration (typically 0.52seconds) a median combine of all sub-integrations can remove a lot of the scatter, and is preferred.
- Plot Wavelength/Frequency/Velocity, which ever is your preference (x-axis) vs. Power/Counts/Kelvin (y-axis). The starting wavelength (in Angstroms, Å), is encoded in the .rad files.
Scanning or Mapping sources
To create a map of the sky (as in the proposed PRO All-Sky-Survey!) we need a data reduction pipeline built to handle a constantly changing source. The proposed survey would park the telescope at a given declination and let the sky drift by in RA for a day, adjusting the dec every day by half the beam-width. During each slice of the sky taken in a day, the telescope would be receiving a constantly changing signal.
Naturally calibration will be key to finding the signal, but it is also important to create software that will read the .rad files and display them as a map. This can be aided if the SRT control software is modified to include the RA and DEC of each sub-integration.
When displaying spectral data as a map, we create an image over time. To make that image most digestible to humans, we need to give it color. Color can be assigned in two ways:
- Color varies with intensity (i.e. blue is weak, red is strong signal)
- Assign colors to wavelengths, and make intensity proportional signal strength.
Because PRO has a small bandwidth, the first option is the best for us.
To create a good map, we need to be able to input multiple files of data and over-plot them, with each data point being an average HI signal strength for each sub-integration. Each point should also median combine with other points within the beam-width, adding to the signal. Creating the proper code for this will be tricky, but should be elegant and hopefully without too many FOR loops.
One difficultly I see is less S/N: during a fixed source integration, you can combine a very long integration to weed out random noise. Each source will only be in the field for a fixed amount of time (around 20 min for point sources). Taking slices at half beam-widths will make progress slow, but will add signal.
Understanding the .rad Files
PRO produces files with the extension .rad for its data output. Understanding what is in these files and how to interpret them is critical to analysis.
The format of the files is (tab delimited)
UT Julian_date Azimuth Elevation Az-offset El-offset RA Dec starting-frequency (MHz) freq-bin-width (MHz) bin-mode# #bins bin1 bin2 .....
Understanding (K Km/s) Units
(K Km/s) is a unit typically found in radio astronomy. It is analogous to a sum of intensity over many wavelengths, or a brightness. These units stem from two equations worth understanding:
- The simple Doppler Shift equation which we can us to relate a wavelength (or frequency) to some rest wavelength (here the 21cm HI line at 1420.406MHz) in terms of a velocity (namely Km/s). This is the x-axis in your spectra.
- The equation for antenna temperature relating a blackbody of temperature T (°K) to the intensity of your spectra and a certain frequency. This employs the Raleigh/Jeans approximation which is (according to Dr. Andrew A. West, Berkeley):
Intensity = 2 k T / λ
Thus if you can determine the gain and response of the system thru calibration (see below), you may convert to °K for each channel of wavelength (λ). Of course, one must convert frequency to wavelength using the equation:
c = λ υ
Calibration
Calibration is key to producing understandable spectra. There are several points to consider when calibrating a radio telescope, many are not obvious.
Firstly, there is some baseline structure inherent to the system. This can go by the name of "system temperature". Essentially it is a known signal contaminating your data, produced by the equipment. This can be identified in many ways
Note: At the time of writing this, PRO has yet to be calibrated yet and it is believed all spectra taken thus far suffer greatly because of it. This is a task to be completed in the Fall of 2006. A suggested beginning to calibration is given at the bottom.
You must also characterize the response of your system: i.e. how much signal corresponds to 100°K increase, etc. For this, one needs to take measurements of objects with known temperatures (A bucket of ice and a bucket of boiling water, for instance). This will yield the gain of your telescope
Calibration Regions
To check your system's efficiency/accuracy, it is important to look at known sources. This can be done with so called "calibration regions", essentially extended sources of HI with well known fluxes. PRO's software has a couple of these identified (S8 from Williams 1973 paper seems popular)
Suggested first steps for PRO's Calibration
- Take a 100s integration with the feedhorn covered with ~8" of foam padding (~290°K)
- Take a 100s integration with a bucket of ice/salt water in front of feedhorn (~270°K)
- Take a 100s integration with a bucket of very hot water in front of feedhorn (~370°K)
- Disconnect the feedhorn cable from the amplifier/electronics package inside the feedhorn. Attach the Signal Generator and feed it a **very quiet** steady signal at 1420MHz. This should be repeated for a large range of integration times, and hopefully over a large range of frequencies (adjusting freq. on the Signal Generator).
Given these steps, you should be able to start to characterize the system temperature, zero-signal structure, and gain. This should be enough to put any data onto a standard system. Your test of this is to take data of a known source in the sky, reduce it (i.e. correct it for all these factors) and compare it to published findings.
Calibration Data for 02/23/07
On Feb. 23, 2007 three sets of data were taken for calibration. Volumes of liquid N2, H2O ice, and CO2 ice measuring greater than 10.5 cm across were placed near the feedhorn for each integration. The .rad files are linked individually in the following table, or can be downloaded as a tar file.
Radio Calibration Data .tar Material Duration File liquid N2 1 minute ln2-1m.rad 5 minutes ln2-5m.rad 10 minutes ln2-10m.rad H2O ice 1 minute h2o-1m.rad 5 minutes h2o-5m.rad 10 minutes h2o-10m.rad CO2 ice 1 minute co2-1m.rad 5 minutes co2-5m.rad 10 minutes co2-10m.rad
Calibration Software
Here is some plotting code for your enjoyment. Just copy and paste this into your favorite editor (vi, of course) and call it proplot.pro. You will also need the median plotting procedure, aka "meplot.pro", to run proplot. Then go ahead and bring up IDL and plot to your hearts content. This program will only work on newer .rad files that have the new data format. Files from 2007 will definitely work and you should check the file if it is from 2006 (accepted format documented in the code). Email davidkr AT astro.washington.edu if you have any questions, comments, accolades, etc.
References
Here are a few helpful links with a lot of what is discussed here, and more!
[Calibrating the SRT with S8]: Note how they talk about looking at an HI source off freq. or looking on freq at a blank source.
[Conducting Observations]: The same site as above, defiantly look thru this site.
[Radio Astronomy Tutorial]: Talking about units and basics of radio-astronomy.
D.R.W. Williams, "Studies of four regions for use as standards in 21-cm observations," //Astron. Astrophys. Suppl.// **8**, 505-516, 1973.
