Measuring the Temperature of the Moon

Introduction

Now that we’ve calibrated and characterized our telescope, you can use it for a simple observation. In particular, in this lab you will measure the surface temperature of the Moon. Since the Moon is opaque and has a temperature above 0 K it will emit a Rayleigh-Jeans radio spectrum. So, its brightness temperature is a good measure of its surface temperature. Chapter 3 in the "textbook" discusses the relation between brightness temperature and antenna temperature. So, this lab is merely a careful measurement of the antenna temperature when observing the Moon and converting to brightness temperature.

There is, however, one potential complicating factor, and that is that the surface of the sunny side of the Moon is probably hotter than the dark side, which would make the data analysis complicated. So, it would be good to make a measurement of just the dark side (when it is a New Moon) or just the full side (when it is a Full Moon). The Full Moon side will be hotter and easier to measure, while the Full Moon will not be near the Sun, which can confuse the measurement (if the Sun is within a beam width or two), so it would be best to make this measurement with a Full Moon.

Even the full moon is not bright enough for an NPOINT, so you should use the pointing corrections determined in the Measuring the Beam Lab.

Get Familiar with SRT_Plotter

Lab Procedure Instructions

Analysis of the Data

Procedure Using the SRT:

Time of Observation:

The Moon moves significantly each night, and so the times that it is high enough in the sky changes from night to night. First, use the following web page:
http://aa.usno.navy.mil/data/index.php
to determine the date and time of full moon (note that the chart you get will be for Universal Time, which is 5 hours ahead of Eastern Standard Time and 4 hours ahead of Eastern Daylight Savings Time) and decide on an observing date that is within 2 days of the full moon. Then use the same webpage (scroll down to find “Altitude and Azimuth of the … Moon During One Day”) to obtain a chart of the altitude and azimuth of the Moon for many times throughout your observation day (this chart will be for your local time). For our SRT, we need the Moon to have an azimuth between 90 and 270 degrees. Pick a 1.5 hour period (to be safe) to make your observation.

Command File:

Write a command file to perform the following steps
1. Record the data into a specific filename.
2. Set the frequency to 1420 MHz, mode 1.
3. Use “offset” to input the pointing corrections established in the "Measuring the Beam" lab.
4. Slew to the “Moon.”
5. Go to an offset with an additional +10 degrees in elevation, measure the Tsys, and return to the offset in step 3.
6. Perform a switching observations using “offset” to define off positions at least one beam width to above and below the Moon. (Since azimuth angles equal smaller angles of arc at larger elevations, it takes longer to move one beamwidth away in azimuth than in elevation. So, switching only in elevation allow for slightly faster switching.) Remember to include the pointing corrections in all your offsets.
7. Make a total of 300 scans to create 20 minutes of data. However, taking into account the slewing time between positions, of about 18 seconds, this will take a total of about 100 minutes of telescope time (and your time).
8. Slew to “stow”
9. Stop the recording.
Carefully examine your command file and then send to your instructor.

At observation time:

1. Turn on the radio telescope, computer, and control box, and click on the SRT icon.
2. Copy your command file into the C:/SRTvsrt folder.
3. Run your command file by clicking “rcmdfl” and inputting your command file name
4. When done, exit the program, copy your data file to flashdrive, shutdown the computer, and turn off the radio control.

Analysis of Data:

1. Open SRT_Plotter, Click "Open File" and find the data file you just created.
2. As in Lab 2, delete the bad end channels from all blocks of data. (Click "Delete End Channels," choose "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. When you've got them all click on "Delete Selected End Channels".)
3. Click "Average Blocks of Data" and select all the data blocks of the On-Moon scans and click "Select."
4. Average all the data blocks of the off-source scans.
5. Select the average on-source data block, click on "Average Over Frequency," and read and record the final "antenna temperature" of the Moon (that appears underneath the list of data blocks).
6. Select the average off-source data block, click on "Average Over Frequency," and read and record the final "off source antenna temperature."
7. Subtract your answer in step 6 from that in step 5 to get the net antenna temperature reading of the Full Moon.
8. Convert the antenna temperature to brightness temperature using T_B = T_Ant ( Ω_beam / Ω_source ) where Ω_source=π(θ_moon)² /4 and θ_moon = 0.5^o (remember to convert to radians), and θ_beam = λ²/ A_eff and A_eff was determined in lab 2.
9. What is your measured surface temperature of the surface of the lit side of the moon? Is this reasonable?