Документ взят из кэша поисковой машины. Адрес оригинального документа : http://www.naic.edu/~astro/aotms/performance/sbfoc.ps Дата изменения: Fri Aug 6 02:43:49 1999 Дата индексирования: Sat Dec 22 05:45:01 2007 Кодировка: Поисковые слова: http www.astronomy.ru forum index.php topic 4644.0.html

Sband narrow focus curve July 29, 1999 12
FIGURE 9: za Error versus za before and after height correction.
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Sband narrow focus curve July 29, 1999 11
FIGURE 8:data for fit zaErr(pltHgt)= A(pltHgt) + B(pltHgt)*sin(za
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Sband narrow focus curve July 29, 1999 10
FIGURE 7: data for fit zaErr(jack)= A(jack) + B(jack)*sin(za)
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Sband narrow focus curve July 29, 1999 9
FIGURE 6: za error vs sin(za) for jack positions 7,11,9,13,15,17,19 inches
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Sband narrow focus curve July 29, 1999 8
FIGURE 5: platform height focus peak vs jack extension focus peak.
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Sband narrow focus curve July 29, 1999 7
FIGURE 4: Jack extension focus peak vs za. dec98,jul99 normalized to 71.3F
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Sband narrow focus curve July 29, 1999 6
FIGURE 3 jack extension at focus vs za. solid 23jul99,dash 17dec98
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Sband narrow focus curve July 29, 1999 5
FIGURE 2: Average platform height for focus vs za
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Sband narrow focus curve July 29, 1999 4
FIGURE 1. source strength (% Tsys) at 19,17,15..etc jack inches.
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Sband narrow focus curve July 29, 1999 3
tem. Figure 9 shows the za error before and after this correction. It reduced the rms za
error from 9.6 to 4.5 (it also took the mean error to zero since we removed the mean).
This zaErr term could be added to the current pointing model (we need to make the aver­
age platform height available to the online pointing program). Since the model has sinza,
sinza**2 terms, this new term is probably biased by the current model. Any new models
may have to redo this measurement.
file: /share/obs4/usr/phil/Doc/sbfoc_23jul99/sbfoc.fm

Sband narrow focus curve July 29, 1999 2
The platform temperature was 74 degrees F during the entire measurement.
3.0 Analysis.
The source strength for the 7 points in a ramp were fit with a g
aussian. The jack extensions
were interpolated to find the jack extension at the gaussian peak. The laser ranging data
was also interpolated to find the average platform height at the focus peak. It should be
noted that the laser ranging data is read up to 10 seconds after the minute so the platform
heights may be off by up to 10/60 of a platform inch (since the platform was moving at 1
inch/minute).
Figure 1 shows the 77 measured source strengths (as a percent of Tsys). There are 11 com­
plete tiedown ramps with 7 points per ramp (the first point is 19 ``, 2nd point 17''...etc).
Figure 2 shows the average platform height versus zenith angle for focus. The platform
temperature was 74 degrees F.
Figure 3 has jack extension versus zenith angle. The solid line is 23jul99 at 74 degrees and
the dashed line is 17deg98 at 71.3 F.
Figure 4 corrects 23jul99 to 71.3 F and overplots it with the 17dec98 data. The correction
used .2 platform inches per degree F and 1.74 tiedown inches per platform inch. There is a
1 inch jack offset between the two dates (or .6 platform inches). This difference was also
seen between the 17dec99 sband and 1jan99 cband focus curves.
Figure 5 plots the platform height versus the jack extension for the peak source strengths.
A linear fit gave 1.74 tiedown inches per platform inch.
To correct the pointing for a change in platform height fits were done to zenith angle error
versus sin(za) for each jack position. Figure 6 plots za error vs sin(za) for each of the
seven jack heights. Linear fits of za error versus sin(za) were done for each jack position
(zaErr=A + B*sinza). The offsets and slopes from the fits are plotted in figure 7 v ersus the
jack position. The coefficients were then fit versus jack position to get the change in error
with jack height. The slope crossed zero at 13.523 jack inches so this was taken as the ref­
erence. The fit gave:
zaErr(J) =(J ­ 13.523) * (17.74*sinza ­ 1.605) + 7.76 asecs
Where J is in jack inches. This formula can be used to correct for jack position b
ut it is
sensitive to the temperature (74 deg F when measured) since the cables stretch with tem­
perature.
Figure 8 uses the same data but fits to the platform height in feet above sea level. This is
the absolute position and the pointing error derived from this is not a function of the tem­
perature. The fit gave:
zaErr(ft)=(1256.34986 ­ ft)*(367.9425*sin(za)­32.8925) + 8.5 asecs
Where ft is the average platform height above sea level as read from the laser ranging sys­

Sband narrow focus curve July 29, 1999 1
Sband narrow focus curve
phil perillat
23 july 1999
1.0 Summary
A focus curve was done using the sband narrow receiver on 23jul99. The platform temper­
ature was 74 degrees F.The results are:
1. Averaging over the 11 tiedown ramps the FWHM gain for the sband narrow receiver
was 12.31 tiedown inches with an rms of 1 inch (the last ramp that went well o
ver the
peak was 12.7 inches FWHM). see figure 1.
2. The sband focus went from 8 to 12 inches jack e xtension (see figure 3). The average
platform height varied from 1256.4 to 1256.6 feet above sea level (see figure 2).
3. After correcting for the difference in temperature, the focus curve on 23jul99 had a jack
extension 1 inch lower than 17dec98 (see figure 4). The cband focus curve on 1jan99
also had a jack extension 1 inch lower than the 17dec98 sband narrow value.
4. Plotting the focus platform height versus jack extension for the focus peaks gives 1.74
jack inches per platform inch (figure 5)
5. Moving the platform height mainly changes the zenith angle pointing (see figure 6). It
can be parameterized as:
where JackIn is jack inches and pltHgt is the platform height in feet above sea level as
read from the laser ranging system (see figures 7 and 8). Using this correction the za
pointing error went from mean:7.1asec, rms:9.6 asecs to mean:0, rms:4.5 asecs. see fig­
ure 9.
6. We are currently using 14 inches as the average height of the jacks. We should probably
move this down to 12 inches.
2.0 Setup.
The source J2253+161 (about 14 janskies at sband) was tracked from rise to set. The
pointing procedure pntfocus was used to move the tiedown jacks from 20 inches to 6
inches in 14 minutes (1 tiedown inch per minute). At the same time the dome/turret did a
pointing scan across the source (which takes 2 minutes). During one tiedown 14 minute
ramp 7 source positions and gain were measured. They correspond to 19,17,15,13,11,9,
and 7 jack extension inches. The process was then repeated driving the tiedowns from 6
inches back up to 20 inches. The tiedowns had the standard tilt of 20.44 20.44 19.33 to
compensate for the platform tilt.
zaError(JackIn) JackIn 13.523
--
( ) 17.94 sin(za) ­ 1.605
в
( )
в 7.76 [asecs]
+
=
zaError(pltHgt) 1256.34986 pltHgt
--
( ) (367.9425 x sin(za)­32.8925) +8.5 [asecs]
в
=