We set up for 4 antenna, 6 baseline observing with a set of
3-way splitters at antenna/cpu bulkhead. Baseline check will
be done with Crab long track on May7.
May 2 : Interferometry Planning
A prime target for interfering next week with goal of
making an image is the
Cyg-A/Cyg-X region of the
sky as seen at 8.5 GHz with 20m antenna at Green Bank by
Langston
and colleagues
The most authoritative writeup of how to understand this
data is 2nd edition of Thompson, Moran & Swenson text;
1st edition ok too, but my page refs are to 2nd.
-ch 4: pp 86-94 give intro to geometry of baselines.
-ch 6: pp 168-175 follow signal through heterodyne receiver
defining what happens to relative phase and delay.
-ch 12: pp 467-475 describe process of fitting for positions
and baselines (astrometry & geodesy, resp).
Suggested approaches to get feel for data:
-collect all for given source/baseline in single file
-collect baseline data from earlier email
-understand conversion of time to UTC to LST
-fit contiguous data chunks to (a) a parabolic "amplitude term" across
the 3 lags with center of the parabola defining the relative
delay at that time for that baseline multiplied by (b) "phase terms" of
[+sine(2.pi.rate.time+phase),
+cosine(2.pi.rate.time+phase), -sine(2.pi.rate.time+phase)]
temporal terms and (c) a additive constant ("DC level".
This reduces all data to:
time_0 = (
April 24: I-Day
Paul and Don visit site to fix up code and
launch I-Day. Baselines 2-4, 4-6 and 6-2 used with splitting
now at bulkhead in control room.
All's well by ~11 AM. Lots of fringes obtained
during remote observing from 544 Campbell. The following show
fringe record for the 3 baselines on various sources.
Two teams visit the RPA and try out 3-element
interferometry. One polarization was split at antenna base and sent
down both RF lines to control room. There ant 2-4 and 4-6 and 6-2
signal pairs were sent to cpus 2,4,6. Paul created "xcorr" program to
sample data as in H spectrum activity, but then produce cross
correlations at, nominally, 0,+/-1 lags. Trev proceeded to install
autocorrelations to provide Tsys scale. A hilarious hot/cold
measurement was done duck taping the eccosorb panel to handy
pitch fork, courtesy of the horse barn the RPA lab resides in.
Day ground to a halt as we sorted out exact quantization scheme
of A/D and c-code.
March 20: H-Day
March along the Galactic plane taking spectra Paul's analysis
with gorgeous l-v map and extraction
of tangent velocities in first quadrant.
[Spectra were scp'd from rpa0 to astro account "ay203"; check with
dcb for pwd.]
The spectra are dominated by receiver gain vs frequency, G(nu).
The ratio spectrum removes most of G(nu). The residual changes
can be removed by baseline fit of low order polynomial with
sampling below, in middle and above two lines. This is then
subtracted. Then the spectrum is split, shifted by 1.0 MHz (the
amount of LO shift), and differenced to produce final spectrum.
This calibration of Gain is then followed by transfer of level
with hot/cold and cal on/off readings which indicate the system
temperature on Kelvin scale. In ratio spectrum the system
temperature is 1.00, which is subtracted via polynomial baseline
fit.
Next, we need to calibrate the FFT "bin number" to frequency
via, we think, 20.0 MHz/4096 although my recovery of spectrum
indicates this may be off a bit -- need to check on Monday
with test tone injection. Then add LO2, 192.5 MHz, and then
LO1, which was either 1220.4 or 1221.4 MHz in our frequency
switched observation.
And, calibration is not over! Next we need to subtract
1420.4054 MHz, the rest frequency of Hydrogen hyperfine
transition, and take ratio with same and convert to
"lab" velocity scale. And finally, finally, we need to
determine the motion of the telescope in direction of
source with respect to the "Local Standard of Rest, LSR". The
Solar System moves with about 13.4 km/s wrt LSR. Only after
all of these steps can we make an astronomical calculation
on our spectrum. [We can use "idoppler.pro" from UG Astro
Lab for this calculation.]
March 13: Paul, Alex, Don
Measure hot/cold load with cal on/off
Try out spectroscopy - problems with trigger