One of the most common activities in radio astronomy is the performance of
meridian drift scam observations. The meridian can be defined as a line
that runs from your southernmost horizon point (180 degrees azimuth) through the
zenith (directly overhead) to the northernmost point on the horizon (0 degrees
azimuth). An antenna pointed along this line will over the period of a day
have all objects within a certain range of declination (determined by the
beamwidth) pass through the beam. The time the object passes across the
imaginary meridian line is called the transit time. If we use a sidereal
clock, then by definition the object's Right Ascension (RA) is equal to the
transit time. For example, Virgo A has a RA of
12h:30m:48s and will cross the meridian at 12:30:48 every day at a local
mean sidereal time of 12:30:48. Neat huh?
To perform a meridian drift scan observation, you need to know at what ALT (= elevation) to aim the antenna along the meridian and when the object you are interested in will be passing through your antenna beam. Conversely, you might just set your antenna to a given ALT and record what happens. Now you see a bump of the right shape on your chart recording and you need to determine if there is a radio source that corresponds well with this. In both cases, the Meridian Drift Scan Table can be helpful.
Prepare before Using the Table Tool
The tool is selected from the Tools menu of the main window. Before selecting the table tool, you should aim your antenna beam to the appropriate location on the sky map. This is done by clicking the Beam Characteristics button and inputting an azimuth of either 0 or 180 degrees and the appropriate elevation. You may also want to limit the number of things that show up in the table. For example, you can turn off the pulsar display or set a minimum flux for the other radio source objects you want to display.
The table is generated for 24 hours. You can change the date and time using the drop down calendar. The Time column is the time that the object transits. It will be in UT or Local time depending on how you have opted to display time on the main window. Next to the object's name column you find the Trans ALT column. This will tell you where to set the antenna elevation for a direct "hit" during the object's transit. Enter and leave beam columns give an approximate idea of when the object contributes some percentage of its power to the observed output. Vert. Offset tells you how much the object will miss the center of the beam. Finally the RADEC values appear.
You can right click on any object in the list and select either "Point to Object" or "Show Transit on Sky Map." Below is an example of a transit of Cygnus A displayed by this method.
Or displayed on the Dome View it looks like this:
Buttons provide for printing or saving the table..
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