on,off position switching offsets
On,off position switching is done to remove standing
waves from the resulting spectra. Tracking the same part of the dish
for the off should have the same "ground based" standing waves as the
on position (this will not work as well for energy that comes from the
sky). Typically people will track a source for 5 minutes, and then
spend the next 5 minutes retracking the same portion of the dish (
which is an ra/dec position offset by 5 minutes + the move time).
If the off position does not match the on
position track exactly, how fast does the baseline degrade? This is the
question that is looked at here.
project a2340 tracked a 20 degree dec source rise to
set for multiple days using the cband receiver. The setup was:
- 7 172 bands covering 4200 to 5200 MHz were used. 172/1024 =
172 kHz resolution was used.
- 900 second scans at .9 second integrations were taken.
There were 9 complete scans.
- The mock spectrometer took full stokes data with 32 bit dumps.
Each 900 second scan was used to emulate on,off
The plots show the baseline rms vs
on,off distance offsets (.ps) (.pdf):
- The first N samples were taken as the On position while the
next N samples were the off position. The offset between
the on and the off tracks on the dish where then N*source motion in .9
- The offsets used (N) were:
- 1,5,10,20,25,50,100 samples corresponding to great circle
angular offsets of .004,.02,.04,.07,.09,.18, and .35 degrees.
- Each trial had 450 seconds on and 450 seconds off.
- Since the same source was tracked, the main beam power should
not have changed.
- For each scan:
- compute the on/off for the 7 different offsets.
- Do this for each of the 7 frequency bands.
- Compute the baseline rms for each on/off spectra (throwing out
- Record the measured/expected rms (from the
- Record the acf of on/off to get the power vs delay
to see if the 1 usec standing wave is present.
The image shows the acf's for on/offs
- Page 1: rms vs great circle offset.
- Each frame is a different frequency band
- black is polA, green is polB.
- The rms starts to increase at .2 degrees. Part of the spread is
- Page 2: rms vs little circle offset.
- These are the same plots as page 1 except they are versus
little circle offsets (azdif is not multiplied by sin(za).)
- The offsets do not group as well as the great circle offsets.
This is a little strange since ground based radiation might be more
sensitive to little circle offsets than great circle.
- The 7 different on,off offsets are separated by dashed
lines (the bottom has the smallest offsets).
- Within each set, the acfs for the 5 frequencies (4275 and
4405 had rfi), and 9 scans are shown.
- You start to see a vertical line around 1.03 usecs start in the
4th section (.07 degree offsets). It is strongest at the top
section (.35 degrees).
- A second reflection is seen at .9 usecs in the top section.
- The lower delays (0-.2) start to have power i the 3rd section.
- 450 sec on,offs which track different parts of the dish
were constructed from a 900 sec track of a source.
- great circle offsets of .004,.02,.04,.07,.09,.18,.35 degrees were
- The rms of the on/off starts to increase around .2 degrees.
- The rms increase correlates better with great rather than little
- the 1 usec standing wave starts to be seen in the .07 degree
- a .9 usec standing wave is seen at .35 degrees offset.
- The rms increase is probably also a function of az,za and not
just the offset distance. It probably correlates with the telescope
- standing wave at cband have the same period as lband. The phase
moves 4 times faster as the distance changes. It would be a good idea
to repeat this at lband.