|Try to finish this activity by 5pm on Tuesday afternoon|
|Name||Common Name|| R.A.(J2000) |
| cz |
|Abell 2634|| ||354.6||27.0||  9400.|
|Abell 2666||Pegasus||357.7||27.2||  8000.|
|N 383||Pisces||16.9||32.4||  4800.|
|N 507|| ||20.5||33.3||  4700.|
|Abell 262|| ||28.2||36.2||  4800.|
|Abell 347|| ||36.5||41.9||  5600.|
|Abell 426||Perseus||49.7||41.5||  5500.|
|We also need a way to figure out the intensity calibration of the system noise. To do this, we inject the noise from a diode (whose noise has been measured by the engineers in the lab) into the system and also record the noise when the diode is "off". This sequence is referred to as the "CAL ON/CAL OFF" sequence; we perform this measurement at the end of every target galaxy position-switched ON/OFF pair. A single observation of a target source then includes both the ON/OFF source observation and the CAL ON/OFF calibration sequence, as summarized by the panel to the right. Sometimes, we observe a source more than once, often on a different day, for example, if there is a marginal detection whose reality we would like to confirm. It is possible to combine multiple observations of the same source to improve the signal-to-noise ratio.|
To the right are the four WAPP spectra obtained from the position switched observations of
the galaxy AGC 310858 displayed with the
horizontal axis in frequency units (top) and then velocity units (bottom). Here, the
ON and OFF-source spectra have been differenced and normalized and the calibration derived
from the CAL-ON/OFF sequence has been applied. In the frequency units plots,
there are interesting positive flux density signals (which we designate by letter)
at 1422.3 MHz (A), 1420.4 MHz (B), 1404 MHz (C),
1381 MHz (D); the 4th spectrum covering the lowest frequency range appears quite different.
Let's try to figure out what these spectra show.     a.   Identify each of the features A, B, C and D in the spectra shown in velocity units.
Click here to see a larger version of the first WAPP spectrum, centered at 1415 MHz.
b.   How can you explain Feature A?
c.   What is feature B associated with and why does it appear to be show both positive and negative flux density?
Click here to see a larger version of the third WAPP spectrum, centered at 1375 MHz.
d.   Feature D arises from well-known radio frequency interference (RFI). Check out Phil Perillat's list of RFI frequently seen at Arecibo and our own ALFALFA team RFI page. What is Feature D?
e.   Why isn't the transmitter responsible for the RFI at 1381 MHz on all the time? Hint: See our ALFALFA team RFI page.
Because the RFI transmitter isn't on all the time, we can discard that portion of the
dataset when it is on to produce an RFI-free difference spectrum; because it contains fewer data records,
it will be noiser that one that contained a full 5 min-ON and 5 min-OFF source but it's better than the one contaminated
with the RFI or having no spectrum at all.
The spectrum to the right shows the result of forming the final spectrum after excising the
records with the RFI present.
Was the transmitter associated with the 1381 MHz RFI on during the ON-source
or the OFF-source portion of the position-switched observations of AGC 310858?
Click here to see a larger version of the fourth WAPP spectrum, centered at 1355 MHz.
g.   What RFI makes the fourth spectrum useless to us?