The aim of this description is to inform interested people about the
actual
situation at the Effelsberg 100-m telescope concerning pointing and
focussing.
The adopted pointing model is presented by the
equations
used
in the software. The
influence
of the
observer on the pointing and focussing during his session is
described.
It finishes with some general remarks and
advice
for the users. For more information about how to do pointing and
focussing
with the telescope one should have a look in the
OBSE
and the
OBSinp
manuals.
The Pointing constants for the 100-m telescope are based on the theory
by P.Stumpff (
Stumpff, 1972)
and have afterwards
been extended and slightly changed due to observations with the
telescope
in the first years of operation (
Pauliny-Toth,Altenhoff,
1972;
Brosche, 1975).
During a
Pointing-Workshop
held in Bonn (
Altenhoff,1996)
general
problems
of pointing models and the pointing of the 100-m telescope in special
have
been discussed. The actual set of constants for the different foci and
receivers is determined during special pointing observations by a
"pointing-committee"
of the MPIfR. The latest changes in the set of pointing-corrections are
made because of proposals by W.J. Altenhoff. He determined influences
of
the (sometimes high) offset-position in azimut of receivers in the
secondary
focus to the pointing in elevation at high elevation. That is corrected
by an additive term
Phorn
to the correction value in elevation. He also determined a systematic
pointing
difference, if the telescope has moved 360 degrees. That is assumed to
be influenced by the cable-twist in azimut, which is now be corrected
by
another constant with an additive term
Pcableto
the correction value in azimut
The Coordinates of the Telescope
In the drive program for the telescope the following coordinates are
used
for the position of the Effelsberg 100-m telescope:
| Longitude: |
-00 27 32.10 s |
| Latitude: |
+50 31 30 " |
| Altitude: |
366 m |
Pointing Constants of the 100-m
Telescope
- P2:
Collimation of
the telescope ( inclination between antenna-beam and the horizontal
axis)
- P7: Zero-shift
of
the elevation-value readout
Terms changing with
azimut and
elevation
- P1:
Zero-shift of
the azimut-value readout
- P3:
Collimation of
the axes (inclination betwenn the vertical and horizontal axes of
rotation)
- P4,P5:
Shift
between
the instrumental zenit and the real one
- P6:
Errors
due to
geographical latitude and time
- P8:
Bending
of the
telescope
- P9:
Sinosoidal correction
in elevation ( proposed by B.H. Grahl )
- R:
Correction due
to refraction (derived from weather data)
- R3:
Factor
for 3rd-order
term in refraction (today: 0.065)
- HYSA:
Hysteresis in
azimut ( normally 0 ")
- HYSE:
Hysteresis in
elevation (today: +2 ")
- B1-B4:
High-order
correction-terms ( proposed by P. Brosche, today
these values are not used)
- Haz:
Shift
of horncenter in azimut (depending on receiver)
- Hel: Shift
of horncenter in elevation (depending on receiver)
- AZtw:
Constant
eliminating the influence of the cable twist in azimut ( proposed by
W.J.
Altenhoff , todays value is +10 " per rotation of 360 degrees)
- NULE and
COL*are
variabel
additive constants, daily checked by the operator and hopefully by the
observer.
- NULA depends
on cos(elevation)
and will be determined if necessary.
- RXAZ and
RXEL are
intended for variable feed-offsets.
Calculation
of the Pointing Corrections
Azcorr =
Azcorr' / cos(el)
Azcorr'
= P20 + P10*cos(el) + P3*sin(el) + P4*sin(el)*cos(az) +
P5*sin(el)*sin(az)
+ P6*sin(az)
Elcorr =
P70 + Phorn + P8*cos(el) + P6*sin(el)*cos(az) - P4*sin(az) +
P5*cos(az) + P9*sin(el) + R*ctg(el) + R3*ctg3(el)
with the following composed parameters:
- ecc: Eccentricity of a horn in the focus (
actual value
of
today
=0)
- Beamdev: Beamdeviation factor ( different
for the
receivers)
- SecFoc: Constant for secondary focus
- K3: Constant depending on the actual value
of FC3
- POLind,FC1ind: Actual
values of
Polarization(Primefocus)
and FC1
- sign vel.: Sign of the actual velocity (az
or el)
- Azcomo: Commanded
azimut-position
in
degrees
- zd: Zenitdistance of position
- Phorn
= el -
arcsin [cos(Haz) * cos(zd)]
- P20 = P2 + COL* + RXAZ + ecc*sin(POLind
- th)
- P10 = P1 + Pcable + NULA +
HYSA*(sign vel.)
+
ecc*cos(POLind
- th)
- P70 = P7 + NULE + RXEL + FC1ind*Beamdev
+
SecFoc*K3 +
HYSE*(sign vel.)
- Pcable
= (
230o - Azcomo) * Aztw
Today the main contributions come from the constants:
- in azimut: P2(~20") *cos(el) and
P3(~10")*sin(el)
- in elevation: P9(~200")*sin(el) and
P8(~160")*cos(el)
Every receiver in the telescope has its own set of pointing constants,
which is automatically activated, if observations are made in the
secondary
focus using several receivers in paralell or at system startup, where
the
receiver in use is asked for. Because of the great number of available
receivers in the telescope and because the constants are continously
updated,
they are not documented here, but the actual sets can be easily
acquired
at the telescope site.
Correction Constants
for the Foci
They have been introduced and determined by I.
Pauliny-Toth
and W.J. Altenhoff, 1972. Since that time they have not been
changed
(low frequencies mean less than ~5 GHz).
The values are in 10th of a millimeter
|
Primary |
Focus |
|
Secondary |
Focus |
|
| Constant |
|
|
|
|
|
dependency |
|
low |
high |
frequencies |
low |
high |
|
| F1 |
1387 |
1427 |
|
1387 |
1427 |
FC1,elv |
| F2 |
136 |
83 |
|
136 |
83 |
FC1,elv |
| F3 |
-36 |
-36 |
|
-36 |
-36 |
FC2 |
| F4 |
-163 |
-153 |
|
-163 |
-163 |
FC2 |
| F5 |
1205 |
1205 |
|
1205 |
1205 |
FC2 |
| F6 |
0 |
0 |
|
1620 |
1620 |
FC3 |
- FC1: radial shift of the subreflector
("Radialverschiebung")
- Assuming a focal length of 30 m for the telescope a
shift of 1
mm gives
6.88 " in the sky (with a beamdeviation factor of 1).
- FC2: focussing the subreflector
("Fokussierung")
- FC3: tilt of the subreflector ("Kippung")
- POL: Polarization rotation of the receiver
Calculation of the Focus
Positions
FC1 = F1*cos(el) + F2 - [ 19610*(1-cos(K3)) +
32900*sin(K3)]
FC2 = F3*cos(el) + F4*sin(el) + F5 +
[19610*sin(K3) +
32900*(1-cos(K3))]
FC3 = F6*cos(el) + 5.2 '
POL = (POLind - 555) * PI/10000
with the additional definitions:
K3 = 20 * [ 1 + sin ( [FC3ind-2500]*PI/5000)
] -
5.2
(in arc min)
FC3ind = actual value of FC3
PI = 3.142...
User Adjustable
Pointing
Constants
( The Daily Pointing )
For the daily adjustment of the telescope-pointing there
exist the
four
parameters, which can be determined by automatic
pointing and focussing procedures. These are additive to the
built-in
constants:
- NULE >>>
P7: Zero-shift
in elevation-readout ( up to ca. +/- 20 " /day )
- COL*
>>> P2: Collimation
of the telescope ( up to ca +/- 10 " /day )
These can be easily determined by
cross
scans in azimut and elevation
- SFC2
>>> FC2:
Radial focus position of the subreflector ( up to ca +/- 5 mm /day)
- SFC1
>>> FC1:
Radial shift of the subreflector ( normally not neccessary to adjust)
This parameter can in the same way automatically be determined by
focus
scans. The absolute value depends on the position of the receiver
and
varies.
- NULA
>>> P1: Zero-shift
in azimut-readout
To determine this parameter one has to find out the azimut pointing at
different elevations and to calculate the best fitting line in
dependence
on the cosine of elevation.
If the
position-readouts
in
azimut have been newly set, it is strongly recommended to determine
NULA,
because there can be a difference to the before determined value.
- RXAZ and
RXEL
can be used for receivers with more than one feed to specify the
different
offset positions of the feeds.
Other interesting Parameters
- Normal weather-data including wind-velocity are
continously
monitored
and
are automatically used for corrections due to refraction (P9).
- A temperature-profile of the actual temperatures at
different
points in
the telescope-structure is continously monitored and archived in the
observing
data and also seperately archived for further analysis.
- Inclinometer-data of the two telescope-towers are
continously
monitored
during the observation and archived with the defined data-rate, but
also
archived with the temperature-profile of the telescope.
- Some observers determined for special receivers certain
values for the tilting (OFC3) or radial shift (OFC1) in connection with
the primary box-position (OPOS) to optimize the system for their
purpose.
Timescales and actual handling of the pointing
- The so called invariable pointing constants are
determined by
accurate
measurements of some days under good observing conditions in a
timescale
of some months. If in the meantime actual problems arise in the
pointing,
short "ad hoc" observations are scheduled to solve the problems.
- The actual sets of the pointing constants for the
different
receivers
can
be printed out, if the observer wishes that for his documentation.
- Besides the determination of the variable pointing
constants
(NULE,COL*
,SFC2) by the observer during his session, nearly every day the
operators
determine and document these variable constants by observations during
their "system-test".
- The determination or check of the Zero-point
deviation in azimut
(NULA)
is only done if one suspects some deviations or if the azimut-readouts
have been newly set.
- The focus constants F1 - F6 have not been changed
since the early
days
of the telescope.
Actual Problems with the Pointing
From the past we have no regularly spaced data of the development of
the
pointing constants at Effelsberg, apart the data taken irregularly
during
the official pointing sessions, once a year or every two years (using
not
always the same receivers). Since 1992 more and more problems in the
azimut-pointing
and -driving came up. Grinding of the telescope rail (1996) brought
improvements
concerning oscillations in azimut. Another part of the problem is, that
nowadays receivers with wavelengths under 2 or 1 cm wavelength have
more
and more observing time. At these wavelengths the pointing of the
telescope
is very much more critical. Now we use for every receiver its own set
of
pointing constants and also we added new corrections. That again led to
a better pointing than before. On the other hand it may be usefull for
secondary focus observations to take the pointing of receivers at
longer
wavelengths for observations with receivers critical to bad weather
conditions.
Actual problems are as follows:
- At very slow actual velocities the telescope seems to
make little
"jumps"
due to frictional effects ("Sägeeffekt").
- The pointing in elevation sometimes shows significant
residuals
in the
normal range of elevation, specially during not ideal weather
conditions.
( Refraction ?)
- Which values of the monitored weather-data best
represent the
model for
calculation of the refractive index is not yet clear
- The focus (very important at higher frequencies)
changes with
temperature,
but in a manner not yet completely understood.
Some Advice
From the experiences of over 30 years of observations with the 100-m
telescope
one can derive some simple general rules, which unfortunately not
always
are respected by operators or observers, nevertheless they are very
obvious
:
- The flux of the pointing- and focussing-sources
should be as high
as
possible
and of course point-like, otherwise one should make more (than the
normal
4), but faster cross scans.
- Point-like sources at longer wavelength not
necessarily are point
sources
at smaller wavelength.
- Pointing and focus should be checked some times
during a period
(the
higher
the frequency, the more), not only at the beginning.
- The higher the observing frequency , the more on must
be carefull
with
pointing and focussing.
- Daily passages between night and day significantly
change the
pointing
and especially the focus. Very critical seems the passage from night to
day in the early morning.
- Unbalanced sunshine on the telescope structure
significantly
changes
pointing
and focus during daytime.
- Pointing should be done on sources nearby the object
of interest,
if it
is too weak for pointing.
- If the resulting pointing values of an observation
show high
differences
to the normal documented ones, one should be very critical in using
them
as general solution.
- First do a pointing, then adjust the focus and then
use the next
pointing
for flux calibration.
Stumpff P.,
Kleinheubacher
Berichte,1972 ,15,431
Pauliny-Toth I., Altenhoff
W.,
MPIfR-Interner
Bericht,1972
Brosche P., AVN 1/1975,35
Altenhoff W. (Editor), "Workshop
on Pointing
Models", MPIfR Technischer Bericht Nr. 78,1996
Schraml J.,
Neidhöfer J., " OBSE,
A User's Guide to the Observing Program at the 100-m Radio Telescope",
MPIfR Technischer Bericht Nr. 71-2, April 1995
Neidhöfer J., " The Command
Language OBSinp,
Version for the Effelsberg 100-m RT ", MPIfR Technischer Bericht Nr.
77,
July 1995
