update: 19apr21.. the glitches were created
by the sband portion of the signal overdriving things.
On 09apr21 9 crosses were done on the crab
nebula. The setup was:
- 7 172 MHz Bands covering xband were taken
- The crosses consisted of 1.2 deg long strips done in
azimuth (great circle) and then elevation.
- The beamwidth at xb is about 10 Amin (FWHM)
- there were 7.2 beams across a strip
- Data was sampled at 10Hz giving 1220samples/strip and
166 samples across a beam
- PolA,B data was taken separately. The two pols were
combined when fitting the data.
- No cals were taken to scale the two polarizations
- The receiver chain still had glitches and baseline
variations, so the results should not be take as final.
- The crab is an extended 7'x5' while the
beamwidth at xband is about 10'. No attempt was made to
correct for the source extent.
- At xband the crab flux is about 612 Jy.
Processing the crosses
For each cross a 2d gaussian was fit to
the total power data for each 172MHz Freq band.
- PolA and PolB where averaged (no cals were
taken to scale the data).
- The total power was taken across the entire band.
- No attempt was made to remove rfi.
- bands with only partial rfi power (1st and last
band) still computed the total power over the entire
The fit used was:
- z= A0 + A1*exp[-xp^2/sigxp^2 - yp^2/sigyp2] +
- xp,yp have been rotated to align the axis with the
major and minor axis of the ellipse (assuming the
beam is ellipitical)
- Let x,y be the az,el great circle offsets from the
center of the strip.
- x0,y0 be the fitted offset from the center
- xm=(x - x0), ym=(y-y0)
- xp=xm*cos(th) + ym*sin(th)
- yp=-xm*sin(th) +ym*cos(th)
- where th rotated the axis to the major ,minor axis
of the ellipse.
- A7 measures the slope of the baseline with respect to
the change in the elevation encoder
- Note thate x,y are offsets from the center
of the strip. For the az strip , there is no
change in the elevation offset.
- But during the az strip, the elevation encoder changes
since the source is moving, so A7 needs to multiply the
actual encoder value and not the eloffset from the
center of the source.
- The actual encoder used for the A7 coef. is the
elEncoder - ElEncoderAtcenterOfmap
Plotting the results
Over plot the azimuth
strips for the 7 frequency bands from 1 cross (.ps)
- Plot the total power vs azimuth offset for 1
- top frame polA
- bottom frame polB
- The colors show the strips for the 7 172 MHz freq bands
- the dropouts are occurring over all freq bands at the same
- The dropouts in polB are much larger than polA
- PolB also has a baseline variation during the 120 seconds
Plot the azstrips
data and fit at one freq band for all 9 patterns.
- The previous plots showed that all freq bands have the
glitch, baseline variation pattern.
- Each pattern has 2 frames
- frame 1: tp polA (black) polB (red) total power vs
- frame 2: tp =(mean(polA+polB) green . Blue line is
the 2d gaussian fit evaluated for the az strip
- The first few patterns have a larger
polB baseline variation.
- pattens 1-2 do not fit very well because of the
baseline variation of polB.
Plot the fit parameters vs
elevation (.ps) (.pdf)
- Each color is a different 172 MHz freq Band.
- The rightmost + is the first cross, the leftmost + is the
- Page 1 Amplitude and pointing offsets
- Top: src deflection. The different freq bands are pretty
- The src amplitude has been divided by the fit constant
- The first 2 patterns (on the right) did not fit
well. Their amplitude varies more
- If the crab is 612Jy (and we ignore that
it is extended) then SEFD=6120 Jy (this is not a
good measurement of the sefd since the source is
- Middle: azimuth pointing error.
- There is lots of variation (probably because of the
- The median value is .326 arc minutes
- Bottom:elevation pointing error.
- this data was taken with model 3 (2015) installed.
- This data was taken with a .35 deg offset always add
to the elevation. Looks like things have changed since
- the median value is .213 Amin
- Page 2: Beam widths and rotation angle
- Top: azimuth beam width (after rotation of the
ellipse). About 10 Amin FWHM
- 2nd: elevation beam width (after rotation). Again
about 10 Amin FWHM.
- 3rd: BeamWidthAz/BeamwidthEl .. to see if
the beam is elliptical.
- We will have to wait till we stabilize the receiver
before saying anything about this.
- Bottom: rotation angle to rotate x,y axis to the major
minor axis of the ellipse
- Values cluster about 0 , but the fits are not stable
enough to draw any conclusion