[evlatests] P-Band sensitivity

[Bryan Butler]

i tend to prefer not to work in SEFDs (i know, it's irrational).  noise 
in the image should be:

             sqrt{2} k Tsys
dI ~ -----------------------------
       n_a n_c N A sqrt{n_p dt df)

Tsys ~ 100 K
n_a ~ 0.4
n_c ~ 1
N ~ 23 (assuming you flagged a few antennas here)
A ~ 490
n_p = 1
dt = 3 s
df = 2.5E5 Hz

dI ~ 50 mJy

looks to me like it's pretty close.  maybe you slipped a digit somewhere.

	-bryan


rperley at nrao.edu wrote, On 12/4/13 23:59 :
> The observations made last month to investigate the low frequency flux
> scale included brief observations of blank fields.  In preparation for the
> upcoming low frequency talk in Pune, I've made 'blank field' maps from
> these observations.  From these, I can determine the noise and from this
> the array sensitivity.  There seems to be a problem.
>
>>From Paul's spreadsheets, the system temperature is measured to be about
> 100K.  If the efficiency of the antenna is ~ 0.4, then the expected SEFD
> is:
>
> SEFD ~ 5.62*Tsys/effic ~ 1500 Jy.
>
> However, the noise measurements give a very different answer.  Using a
> single 250 kHz channel, in an RFI-free subband (#8), on a blank field --
> and after removing the obviously noisy antennas, an image made with 15570
> visibilities, in single ('RR') polarization, and 3 seconds integration
> provided an rms noise of ~40 mJy.  This corresponds to an SEFD of well
> over 6000 Jy -- four times too high.
>
> Just in case the extra noise came from various sidelobes, I made a really
> large image of the entire beam, using almost the entire subband width, and
> both polarizations.  This gave a noise of 6 mJy/beam, corresponding to an
> SEFD of nearly 9000 Jy.
>
> I also made noise maps for three clear channels in each of the 16
> subbands, to see how the sensitivity varies with frequency.  As expected,
> the noise is very high at the lowest frequencies (10 times higher at 220
> MHz), and dropped steadily as the frequency rises, reaching the 'best'
> value in subband 7.  Surprisingly (to me, at least), the noise then stayed
> at about this level all the way out to near subband #15 (440 MHz) before
> rising a bit in the last two, highest frequency subbands.
>
> I had expected a clear maximum sensitivity in the middle, and decreasing
> sensitivity at higher frequencies, due to increasing defocusing.  But this
> appears not to be the case.  I didn't think the decreasing galactic
> synchrotron contribution to be enough to offset the defocussing, but
> perhaps I'm wrong.
>
> On the other hand, the apparent fact that the best sensitivity is four
> times worse than expected indicates either I am doing something very wrong
> in the calculations, or there's something very wrong with the system
> temperature and/or aperture efficiency.
>
>
>
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