The sensitivity of the byu system was measured using a single dipole
fed down to the AO iflo system and recorded on the interim correlator.
Crosses on continuum sources as well as temperature measurements using
sky and absorber.
Tsys using sky and absorber:
On 26aug10 Tsys was measured using sky and absorber.
The setup was:
- data taken with the dual pol array in position A0, focus=0.
- polA dipole 1 and polB dipole 2 were recorded.
- The interim correlator recorded 1.25 MHz bw at 1612. 15 1 second
integrations at each position were taken.
- The sequence was:
- on sky for 15 secs
- move turret, mount absorber
- on absorber 3*15 secs. absorber temp = 84.5F
- dismount absorber, move to tertiary, take 2*15 seconds more of
Processing the data:
- Each 15 second integration was average over time and frequency.
- The data was recorded in correlator units (linear in power). No
cal diode was available.
Analyzing the data:
- Tabs the absorber temp
- Trcv the receiver temp
(including the dipole)
- Tspill the tertiary
spillover and any scattered energy off the platform.
- Tsky the power from
power in correlator counts
power in degK
The plot shows
the Hot,cold measurement (.ps)
-Tcold_C = (Tabs_C + Trcv_C) - (Tsky_C + Tspill_C + Trcv_C)= Tabs_C -
(Tspill_C + Tsky_C)
measured Tabs_K (302 K) with a
- To convert Correlator counts to Kelvins:
- Using Thot: we measured Tabs_K,
estimate Trcv_K, we measured Thot_C
- CorToKelvins= (302.3K + Trcv_K)/Thot_C
- Once we have CorTokelvins we can then compute the Tsky_K+Tspill_K:
- (Tsky_K + Tspill_K)=(Tabs_K - deltaT_C*CorToKelvins
- Top: this shows the ratio Thot/Tcold for the 3 measurements it
was relatively stable.
- the black line is dipole 1 polA, the red line is dipole 2 polB
- Bottom: Tsky + Tspillover in kelvins vs Trcv in kelvins.
- A guess at TrcvK would be 45 K (32K from the amps and 13K
from lines and dipoles)
- This gives Tsky+Tspill
- dipole1: Tspill + Tsky = 53 K
- dipole2: Tspill + Tsky = 60 K
- You might expect Tspill dipole 2 to be larger since it is
offsets from the paraxial ray.
- Tsys for dipole 1 would be : 53
+ 45 = 98K.
These measurements used a single dipole. The phased
array feed will have different values.
Sensitivity measurements using crosses on
Crosses were done on continuum sources using dipole 1
The cross setup was:
- +/- 3 beam widths in azimuth followed by +/- 3 beam widths in za.
- Each leg of the cross took 60 seconds with 1 hz sampling.
- the interim correlator sampled 1.5625 MHz centered at 1612 MHz.
- Crosses were done a the various positioner sites: A0 .. A6
- Most crosses were done at focus = 0 cm. (about 10
crosses (out of 144) were done at focus=-8 cm.
- the data was taken in jun10 and aug10 using the single pol array.
Processing the data
- 2-d gauss fits were done to each cross:
- Fit for: The Tsys offset, Gaussian amplitude, x,y offsets,
major, minor axis widths, and ellipse rotation angle.
- 122 crosses were done in total.
- No cal was available so the SEFD
(Tsys/Gain) was computed.
- As we moved away from the central position A0, the beams become
less Gaussian due to the coma. lobes.
The first plots show the strip
data with the fits overplotted for 7 crosses done on A0..A6 (.ps)
- Black in the az strip, red is the za strip, and green is the 2d
- positions A0-A6 are shown. These 7 strips were taken on 19aug10
with the single pol array.
The next set of plots show
the results of all the fits (.ps)
- Page 1 top: SEFD in janskies versus za for
the 122 crosses.
- The different beam positions are color coded.
- The dotted lines are the median SEFD for each position.
- The outer positions have larger SEFD's
- Page 1 bottom: Ae/Tsys:
- This is the value plotted in most of karls sensitivity maps.
- Page 2: average SEFD and Ae/Tsys for each position
- Top is the sefd, bottom is the Ae/Tsys
- Black is the mean , red is the median.
- Page 3: beam widths vs za.
- Top has the beam widths for each cross.
- * is the major axis beam width
- + is the minor axis beam width.
- the color coding are the positions A0 .. A6
- The units are Arc seconds
- Bottom: The average beam widths for each position.
- * black is the average for the major axis
- + red is the average for the minor axis
- * green is the average of the two.
- The outer positions beam widths are distorted by the
- Page 4: major, minor axis lengths
- The beam widths measured were plugged into germans approximations for the
major,minor axis for an ellipse
- black is the major axis, red is the minor axis, and green is
- the axis lengths for the outer beams look a bit suspicious
(since the tertiary can't illuminate 300 meters).
- this is coming from the coma messing up the beam width fits
for the out beams.
- The lengths are larger than the expected: 237x207.
- position A0 has a major axis mean length of 250 meters (using
- For the
single pixel receivers, the hpbw's were measured back in 2004-2005
- Using hpbw=1.02*lambda/DiamEffective gave a typical
diameter of 225 Meters (depending on the horn taper).
- Using the mean bandwidth of dipole1 = 160 Asecs beam width
- diamEffective=240 meters.
- This is close to the 235 meters measured for the 610 MHz
- The horns have a taper on the tertiary around 15-18 db and a
9 db taper on the edge of the illuminated area (i wonder how the
illuminated area was defined to get this 9db value... need to ask lynn
- To get an idea for the single dipole illumination of the
tertiary, I took a horzontal 1/2 wave dipole lambda/4 above a
ground plane (Kraus antennas pg 461.)
- The plot shows the dipole
illumination pattern (.ps) (.pdf)
- At the edge of the tertiary (60 deg) there is a 6db taper
in the power pattern (assuming i didn't mess something up copying the
- This taper should give a more uniform illumination of the
primary than the horn feeds. So you would expect a smaller bandwidth.
- Conclusions are for the single dipole at A0 (unless other wise
- The paf will have different results since they can play with
phasing the various dipoles to create a beam.
- Note: 2760 m^2=1 K/Jy.
|Assumes Trcv =45K
|Only relies on the source flux
(and the 2d fit).
Ae/Tsys and Tsys
|assumes Trcv=45K, Tsys=100K
- This will increase as Tsys goes up
- the geometric area is from german's "last ray) over the
- major axis: 150 asecs
- minor axis: 170 asecs
- The avg bw is smaller than the lbw receiver at 1610 MHz:
- dipole:6 db
- Using 1/2 lambda dipole lambda/4 above ground plane from
- this compares with the 15-17 db tertiary edge taper for the
single pixel horns
- The smaller edge tape should give a narrower beam width for