Projects a2703 and a2708 ran 20,21 apr14
18:00 to 1:30 22apr14. They reported trouble with their
baselines. I looked at the data from 21apr14.
The on/off data taken
Setup:
- lband wide 1350-1400 MHz range
- a2703 used 1370 hipass with the 1230-1470 filter
- a2808 used the 1320 hipass filter
- liner polarization
- interim correlator
- sbc1 12.5 MHz, 2048 chan polA
- sbc2 12.5 MHz 2048 chan polB
- sbc3 25 MHz polA,B wide band to look at rfi
- sbc4 25 MHz polA,B to look at rfi
- 1 second dumps
- patterns
- 300sec on,off positions switching multiple times per
source . (a few 240 second on/off 's were done).
The data:
- I processed all of the data for both experiments
using pfcorposonoff()..
- The data was scaled to Tsys rather than degK or Jy.
- 1Tsys is about 3 Jy (30K/10K/Jy)
- The multiple on/offs on the same source were averaged
together (i did not scale by the za, gain).
- After averaging, i measured the rms across the
spectrum.
- I also computed the expected rms using:
- expRms = 1.02*sqrt(2)/(bw/nchans *1.208*2.25 *
secondsOn)
- the factors are:
- 1.02 is from 9 level sampling
- sqrt(2) dividing on/off
- 1.208 from the sinx/x filters of the autocorrelator
- 2.25 from hanning smoothing
The
a2703 on/offs (.ps)
(
.pdf)
- this data was taken 18:00 to 20:30 ast
- PolA is black, polB is read
- Page 1:frame 1,2: srce 108103
- the ripple looks a more like two then 1 MHz
- large offset polA,polB.. maybe the cal got messed up?
- Page 1: frame2,3 src 108143
- looks like a detection. measured rms is about 30% >
then expected
- Page 2 frame 1,2 src 108144
- Page 2 frame 3,4 : src 110007
The
a2808 on/offs (.ps)
(
.pdf):
- data was taken 20:30 to 1:30 (22apr14) ast
- page 1:
- src: J0959+0503.. rfi at 1344
- J1104+3141 .. airport radar at 1351..
- Page 2:
- src J1210+3157
- src J1330+3119
- Page 3:
- src J1342+1`844
- detection
- polA ,polB +/- .004Tsys.. is this the cals?
- src J1356+2515
- Page 4:
- src J1357+1704
- src PG1049-006
- detection
- polA,polB again have +/0 .004 Tsys..
What was the platform doing during the observation:
I looked at:
- the platform height (using the distomats.. when the
worked)
- the tiedown position and tensions
- the temperature (measured via the old temp sensor and the
newer weather station temperature sensor)
- The rainfall (from the weather station)
The plots
show the platform motions
(.ps) (
.pdf)
- Page 1: 0 to 24 hours 21
apr14.
- The dashed vertical lines show were a2703 and 2808 started
- Top: air temperature vs hour of day
- black: from old temperature sensor
- red: temperature from the weather station. (only started
at 15:00)
- They differ between 15 and 19 hours, and then they come
together.
- I think the old temperature sensor is shielded from the
sun,rain. i'm not sure about the weather station.
- The dip at 15:00 was the beginning of a rain storm
- 2nd frame: Rain intensity (inches/hour) computed as a 5
minute running average.
- it was raining at .5 inches/hour 15:00 to about 17:00
- 3rd frame: cumulative rainfall (inches) for the day.
- It rained about 1.4 inches.
- 4th frame: how far the platform was out of focus (cm)
- This is the vertical offset of the platform from the
correct height (1256.22feet)
- the feed is out of focus by this value * cos(za)
- the distomat readings were not working 20:00 to about
23:00.
- bottom: average tiedown jack position (inches)
- the tiedowns were turned off 15:00 to 20:00
- they were moved manually at 20:00 and then left off.
- the a2703,a2808 observers requested the tiedowns to be
turned off so there would not be any adjustments between
the on and the offs.
- Page 2: blowup showing a2703 ,
a2808 run 18:00 to 24:00
- Top: air temp vs hour of day:
- the temperature was changing 18: till about
20:00
- 2nd: how var the platform was out of
focus.
- it moved from 4.5 to around 3cm during a2703
run..
- there is not measurements during the a2808
- bottom: tiedown motion
- the tiedowns were pulled down 2.5 inches at
20:18
- This would move the platform down
2.5*/1.7*2.54=3.7 cm (1.7 tdinch/platform inch)
- this should have put the platform pretty
close to focus for a2808.
summary
- rain 15:00 to 17:00 cooled off the main cables, shortening
them, bringing the platform about 5cm out of focus
- with the tiedowns off, the platform drifted lower about
1.5 cm in 3 hours as the air warmed up and the cables came
back to the air temperature.
- The observers requested the tiedowns off, so there would
be no motion between the on and off positions
- unfortunately with the tiedowns off, there was no way to
stop the normal motion of the platform with temperature.
- The observer commented that it wasn't raining during the
observation. The data shows that recovering from a rain
storm may take hours for the cables and air temperature to
come to a stable value that doesn't require active tiedowns.
- My guess is that leaving the tiedowns active would have
done a better job for the position switching
- In the past, when the distomats failed, the telescope
would switch to use a temperature sensor to control the
height (using a model).
- This model was never very accurate because the air
temperature (which was used as input) and the cable
temperatures (which you need to control the height) could
vary by 5 to 10 degF.
- I've disabled this feature. When the distomat's fail,
the tiedowns just stay where they are..
- If the distomats are working the program will move the
platform every 2 minutes as long as the platform has moved
by 1 mm
- this is about 1.7 degrees of phase on the standing wave.
As i recall this should not give a very large
standingwave.
- the observers evaluation of the data for 20,21 apr14 was:
- In summary,
I have been observing 7.5 hours each of the last
two nights (A2703+A2808), and the data are all unusable
- Looking at the on/offs the actual rm is off by 10
to30 %.. in some cases a factor of 2.
- I'd say that they weren't perfect, but completely
unusable may be pushing it a bit...
- For best baselines and if you're going to leave the
tiedowns off, the platform should be thermally stable. This
probably always happens by 21:00.