Determining the DPFU and aperture efficiency of the SRT

Introduction

Another important characteristic of a telescope that one needs to know in planning observations is the sensitivity of the telescope. This can be described in several ways. The amount of radiation detected is a fraction of that which enters the antenna aperture. The aperture efficiency is the ratio of the effective aperture area to the geometric area (pR2) of the reflector. Knowing the effective aperture can be used to convert the measured antenna temperature to the flux density of the radiation entering the telescope. This concept is sometimes described as the degrees-per-flux-unit (or DPFU), i.e. the number of degrees K in antenna temperature that results for every Jy of flux density from an astronomical source).

The sensitivity of a telescope is crucially important for planning observations when you have an idea of the flux density of the source and know the system temperature (measured in lab 1). In this lab, you will measure the antenna temperature when observing a source of known flux density, and hence directly measure the DPFU and calculate the effective area. The known source that you will use is the Crab nebula, whose flux density at 1420 MHz is 875 Jy.

The procedure in this lab will also familiarize you with how to carefully measure the antenna temperature. The receiver noise plus signal from the background is measured, interpolated in two-dimensions, and subtracted to yield the antenna temperature of only the source of interest. This method is called switching. In any single-dish observation in which the flux density of a source is desired, switching must be performed.

You will also learn in this lab about writing and using a command file. Command files are helpful and useful for a number of reasons. Once familiar with the allowed commands, you are less likely to make mistakes in the observation by writing and carefully examining a command file in advance of the observation. As you may have unfortunately discovered in lab 1, mouse clicks and inputs from the keyboard during a long observation are prone to mistakes. Secondly, command files enable one to obtain an observation at a telescope without having to travel to the telescope, as is generally desired when granted time on a national facility far from one’s home.

Get Familiar with SRT_Plotter

Switching Observation

Time to Make the Observation

Command File Guidelines

Lab Procedure Instructions

Analysis of the Data

Switching Observation of Crab Nebula

To determine the antenna temperature due to only the source of interest, a single-dish observation should always include observations of positions aimed away from the source (called the “off” source scans) as well as the position “on” source, and the average of the “off” source scans are subtracted from the average of the “on” source scans. There should be off source positions symmetric about the source.

Because of temporal variations in the gain of a telescope, the on source and off source scans need to switch on a time scale fast enough that the gain fluctuations do not alter the on-off calculations. With the SRT, we should switch on a time scale of order 10 seconds.

The Crab Nebula is in the plane of the Milky Way Galaxy which, itself, emits significant radio wavelength emission. Therefore, we will need to be sure that, in addition to the noise of the receiver, the background of the galactic plane is also subtracted off in our switching observation. This is accomplished by using off positions that contain the same amount of galactic plane emission. The Galactic coordinates of the Crab Nebula are 184.6^o -5.8^o; the first number is the galactic longitude and indicates how far, in degrees, along the galactic plane, it is from the Galactic center and the second number is its galactic latitude and indicates its distance in degrees above the galactic plane (the negative indicating the Crab is below the plane)). For our switching observations, then, we want off positions with Galactic coordinates of 177 -5.8 and 192 -5.8. To ensure we get these positions, instead of using offsets in azimuth and elevation, we can define positions using “galactic coordinates”, using the command “galactic”.

Time to Make the Observation

As always with observational astronomy, you need to take care in choosing the time to make your observation, which depends on the location of your source and the calendar date. In general, you need to know the rise, set, and transit times of the target source. For this observation, your target source is the Crab Nebula. During this month, the Crab transits early in the morning. The Crab is not bright enough to yield as reliable a pointing measurement as the Sun, so you should use the pointing corrections you measured in Lab 1.

Command File Guidelines

The filename should have suffix .cmd
Each command must start with a colon (“:”) followed by a blank space.
A line starting with an asterisk (“*”) is a comment, which is recorded in the data file but does not instruct the telescope to do anything.
Some commands that can be used are:
1. freq ffff m– sets the central frequency and mode number (for this lab, use mode 1)
2. noisecal – perform a Tsys cal using the noise diode
3. record filename
4. mode n (perform NPOINT)
5. Crab (go to the Crab)
6. azel aa ee (azimuth and eleation to move to)
7. offset aa ee
8. radec hh:mm:ss dd:mm:ss (RA and Dec to move to)
9. galactic ll.l bb.b ( galactic longitude and latitude, in degrees, to move to)
10. A colon followed immediately by a number with no space (e.g. “:30”) instructs the computer to collect data for this period of time.
11. roff (stop recording)
12. stow

Here is a simple example of a command file.
* Probing beam of Sun in elevation
* Lab 1, Joe Smith and Betty Jones
: record lab1smithjones
: Sun
: offset 0 10
: noisecal
: offset 0 0
* Do NPOINT
: mode n
* Measure beam in elevation, in steps of 2 degrees, with 30 second integrations
: offset 0 -10
:30
: offset 0 -8
:30
: offset 0 -6
:30
: offset 0 -4
:30
: offset 0 -2
:30
: offset 0 0
:30
: roff
: stow

Procedure Using the SRT:

Well in advance of your observation, prepare a command file to accomplish the following procedure. Email your command file to your instructor, for examination, at least two days in advance of the observation. Write a command file to perform the following.
1. Record your data into a file whose name includes observers’ names
2. Sets the frequency to 1419 and mode 1
3. Slews to the Crab Nebula,
4. Moves to an offset of +10 degrees in elev, measures the Tsys, and then returns to 0 offset.
5. Sets an offset equal to the pointing correction from lab 1.
6. Performs a switching observation using off positions one beam width to either side of the Crab and parallel to the Galactic plane (i.e., same b as Crab but with l that shifts by 7.5^o in each direction), and with integrations at each position of 5 seconds.
7. Contains a total of 240 scans (or a total of 20 minutes of data)
8. Stops the recording
9. Returns to stow.

At the time of the observation:
1. Turn on the computer and ground controller box and click on the SRT icon.
2. Click on “Rcmdfl” and type in the name of your command file. Make sure that you get confirmation in the lower part of the SRT display of this filename. Watch the screen, taking note of the appearances of yellow crosses and the “cmd” positions, to ensure that the program is doing what you expect it to.
3. When it reaches the end of the file, the telescope should have returned to stow. If so, click on “exit”, turn off the ground controller, copy the recorded data file to your flashdrive, and shut down the computer.

Analysis of Data:

1. Open SRT_Plotter, click "Open File" and "browse" to find the data file you just created.
2. Note that in the plot the measured antenna temperature at the very ends is very low and that the antenna temp takes about 8 channels to get up to the proper value. Click "Delete End Channels." This will bring up a window in which you get to select in which data blocks you want to delete the end channels. You might as well as do "select all". And then use the mouse and the left button to select an area containing the data in the end channels on the left and another area for the channels on the right. The selected data points will turn to red diamonds. When you've got all the end channels selected, click on "Delete Selected End Channels".
3. Now click "Average Blocks of Data" and select all the data blocks that correspond to observations with the telescope pointed directly at the Crab. Then click "Select". This will create a new data block at the end of the list, which is the average of your selected data blocks. This new data block, therefore, should be the average of all your "on source" scans of the Crab.
4. Now average all the data blocks of the calibrated observations when the telescope is pointed away from the Crab. This new data block is your average of all the "off source" observations.
5. Select the average on-source data block, which you created in step 3. Click on "Average Over Frequency." A number then appears underneath the list of data blocks. This is your final measure of the antenna temperature when observing the Crab.
6. Select the off-source data block and click on "Average Over Frequency".
7. Subtract your answer in step 6 from that in step 5. By doing this, you have subtracted the background signal and all other radiators in your system. Your final answer is: T_A(source) = T_A(on-source) - T_A(off-source). 8. Now divide your measured antenna temperature by the known flux density of Crab (875 Jy) and write this value in your report as the telescope’s DPFU.
9. Calculate the effective aperture of the telescope by using Equation 4.31 from the textbook. Write this value in your report as the telescope’s effective aperture.
10. Calculate the aperture efficiency by dividing the effective aperture by the geometrical area of the dish, i.e. πR², where R = 1 m. Write your inferred value of the aperture efficiency in your report.