* WORKS: + 100 micron repeatability, with no adjustment between runs.
+ PHASE:
. ~ 1 deg drift over 4hrs, VLBA + PC backend, here
. 5-10 deg in 30min, NIGHT
. upto 60 deg in 30min, DAY
(REMOVED to first order in calibration)
More Details
* BUT: + NOT STRAIGHTFORWORD ~ 12-18hr
job
1-2hr setup + 1hr F/R/P + 4hr raster + 4hr Calibration
+ SATELLITE tracking tricky
. pointing, freq lock, etc. (1 mph tracking window = 40MB/hr
data rate)
+ CALIBRATION like astronomy expt.
* PT DONE. Initially thought main ref. Now looks like subrefl after all.
See here
for surface maps at 2 rotation settings,
corresponding to the 3mm (212) and 1cm (320) feed positions.
Note the astigmatism rotating with subreflector.
Some more small steps:
. Horn patterns @ GB (1deg phase = 70 microns)
. Leave cables @ PT for retest.
. Travel kit: BR or HN next?
* MAIN QUESTION: PICK REALISTIC OPTION FOR SURFACE IMPROVEMENT.
OPTIONS:
1. PHASE CORRECTION PLATE over
3mm feed. Needs to be in far field of
feed. Needs to have subreflector in its near field
(to avoid
converting phase changes into amplitude changes).
2. REWORK subreflector surfaces. Measure
with Faro arm; hand rework
the surface; recoat with reflector.
The following estimate is from Jon Thunborg:
Right now I think it will take about 2 man months to
fix a
subreflector. We will have Andy Vaiza begin working
on the spare
subreflector next week. I will be working with
him for the first
few weeks until we learn exactly how to do it.
As far as cost, I bought one gallon of silver paint.
It cost
$3400/gal. This should be able to cover several
subreflectors but
it has limited shelf life (6 months). I am hoping
it will last
longer than 6 months. The minimum order was 1
gallon. The rests
of the costs should be minimal. We will however,
encounter a large
travel cost when we go to swap out the subreflectors.
3. PANEL SETTING: (typical for
VLA antennas) a 2-day job for 2
people. NOTE: Panel setting will be required after
a good
subreflector is put on an antenna, and the residual
surface errors
are determined by holography. The option considered
here is to get
going quick - see if the errors in BOTH SURFACES are
fixable (by
adjusting the main reflector only) for the 3mm position,
at the
cost of worsening the astigmatism at the 1cm position.
Antennas
without significant astigmatism may be fixed in this
way, saving
subreflector work.
Other considerations:
1. LENS / Phase plate 2. SUBREF. resurf 3. PANEL reset
CHEAP, relatively.
EXPENSIVE, labour int.
SET PANELS, including
subref. errors
DESIGN/DEV required
REQUIRED @ BR e.g.
BLOCKAGE (retractable) SWAP takes many
days,
LABOUR intensive
long-term reliability?
all bands unusable
1 PER BAND - no
ALL BANDS improve
10-15% hit @ 1cm (PT)
impact on others either
way.
ANOTHER ROUND of
WON'T WORK @BR e.g.
CAN'T TEST results w.
panel setting needed (?)
hologr. Can only see
Efficiency improv'mt
CAN RETEST with holgr.
CAN RETEST
LONG term solution,
NOT a long-term
If it works.
solution, but gets going
now.
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SUMMARY: Option 2, subreflector
resurface and swap, was chosen. We already have gone
some way down this path with the purchase
of the Faro-arm measurement setup.
Action:
a. Continue with resurface effort on spare subref. (2-3months)
b. Measure next antenna with holography, in parallel with (a).
c. Swap subref at PT, measure efficiency and redo holography
(early 2003)
d. re-assess in mid 2003 with better idea of effort and expected
improvements
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