Observations in Effelsberg
at a Glance
( A short guide to the observing program )
Version of March 1998 / jn
Updated by AK / 2009
If you plan to observe with the 100-m telescope you are advised to consult the manuals for the observing program OBS E and related reports about OBS inp and the reduction packages. They always are available from the operator at the control desk (``Steuerpult'') at the telescope site. This description intends to inform you quickly about the most common observations done with the telescope. An operator, who is able to help in most cases, is always present at the telescope site. Observers already familiar with observations at the 100-m telescope can use this description as a collection of catchwords to aid their memory. Observers with little experience should take this description as a collection of hints, where to search for in the available manuals. It is not meant to replace any of the manuals. In the following the descriptions of the verbs do not contain all possible parameters or adverbs. In order to avoid confusion only the most important ones are mentioned.
One should prepare the scheduling of the observations, necessary catalogs (of sources or line-frequencies) and command-files some time before the observations start.
At the same time one should try to get familiar with the reduction program that one intends to use for the reduction and analysis of the observed data. Most common at the 100-m telescope are the TOOLBOX-, NOD- and the CLASS/GREG-packages. The observations (i.e. controlling telescope, backends, frontends and data-taking) are made with VAX-computers using the VMS operating system. Thus, it is useful to know something about the VAX command language and file-system. On the other hand, the reduction of the observed data normally is done with UNIX-computers made by SUN. The operator's and observer's input is entered via a PC with MS-Windows,Windows-NT and an X-Windows interface. Some knowlegde about graphical user interfaces like MS-Windows (or the equivalent Window-systems on SUN or VAX) or X-Windows can be a great help for an efficient use of the Effelsberg system.
As input for the control program one can have two different types of catalogs, one for source-positions and one for line-frequencies. Default catalogs are always defined. They also can serve as references for writing own user catalogs. The format of the catalogs is described in the manual.
By the following commands one changes the default-settings:
At system-startup the observing verbs and adverbs and the configurations of the frontend(s) and backend(s) are set to reasonable defaults, so that observations are possible using only the verbs without specifying adverbs or parameters (i.e. POINTING, FOCUS, PSWITCH etc.).
Many verbs start a scan directly, without any interventions of the observer. They set up all relevant observing procedures and supply the frontend(s) and backend(s) with the requested parameters and start the measurement. Maps and rasters also set up the observing procedures, frontend(s) and backend(s), but the observation has to be started with the START-command. So, before starting one can check all map-parameters or start an observation at a special subscan of the map or raster. Using CANCEL one can stop a running observation and by entering QUIT one can leave a higher SIC-level, if the procedure failed or stopped with an error during the observation.
Before starting the observations (and sometimes again during the observations) it is necessary to re-adjust the pointing and focussing of the telescope-system. By doing that one also obtains a new calibration for the subsequent observations.
This is accomplished with the OBS E command verbs POINTING and FOCUS. The results can be accepted for subsequent observations by the command CORRECTIONS, whereas SKYDIP can be used to scan the atmosphere. The parameters and adverbs need not be entered explicitly, because the system-default values allow immediate measurements.
... or if the calculated gauss- or parable-fit corrections shall be assigned:
Some adverbs allow the control of offset- or reference-positions and the dimension of a mapping region in general. If the offsets are not to be used one can add the adverb /NOOFFSET to a verb. A reference position can also explicitely be set with some verbs. Using the adverb MAPSIZE one can define the map dimensions.
Depending on the frontend configuration it may be possible to use frequency- or load-switching in addition to the common OFF-ON observing mode. It is also possible to use an additive noise-balance signal during the offscan, if the source background radiation is very high. One should enquire about the possibilities of individual receivers before planning the observations. Standard single position measurements are (the beginning P stands for position (point-)measurements and the F for frequency-switch observations):
To observe large areas one can use point-raster mapping:
The verbs POINTING, CROSS or CONOFF can be used for observations of point sources. CROSS works in the same way as POINTING with the difference that with CROSS one can use offset positions (see above !). The verb CTRACK is for continuously following a position in the sky.
In the secondary focus of the telescope receivers (spread over a large frequency-range) are installed, which can be used for multi-frequency observations. A special set of macros has been developped to make an optimal use of fast switching from one receiver to the next. These procedures are very useful for multi-frequency observations of point sources. The system must be started with the general receiver for the secondary focus. All SIC-symbols as well as OBS E -definitions and the frontends are initialized by a startup macro running with the symbol:
Performing pointing-, focus- or onoff-scans with the different receivers is very easy by entering only the predefined symbols ( for example: sfk_rx < wavelength > ) with the desired wavelength and then using the normal verbs for observations. The operators are familiar with such observations. For more details one should contact J. Neidhöfer at the telescope site.
An easy way to get a map of a region in the sky is the use of the OBS E -verb
Specialized OBS inp -procedures exist for mapping large regions in the sky in connection with the reduction of the observed data with the NOD-package installed on the computers at the telescope site and in Bonn. In particular these procedures are available for the receivers in the secondary focus, which have excellent polarization properties and multi-channel outputs, delivering data of a multiple of feeds (horns) in parallell.
For more details and how to work with the NOD-package one should contact E. Fürst at the telescope site or P. and W. Reich at the institute in Bonn.
The OBS E command language is based on OBS inp and uses the SIC-interpreter. One can use OBS E - as well as OBS inp -commands in command structures of the SIC-interface layer. By typing OBSINP one enters the OBS inp input-level to use elementary OBS inp -commands or files. To start the observation using OBS inp one must use the verb ST_ONLINE. The procedure SYNCHMESS is useful for synchronizing observations in SIC-command procedures. Using SIC-commands one can define variables and symbols and also automate the observations by loops ( FOR ) and IF ... THEN ... ELSE-structures. More details can be found in the appropiate manuals. With the command ''@'' one calls a SIC-macro.
One should be very carefull by mixing OBS E - and OBS inp commands, because OBS E verbs normally can overwrite OBS inp commands given before !
VLBI/VLBA-observations are generally controlled by dedicated schedule- computers, that communicate with the control-VAX. Special observing modes if using OBS E (called VLBI-,X- and A-mode) have been developped to make these type of observations more effective and faster. Special PC-Programs and manuals exist for scheduling VLBI-observations.
Until now pulsar observation are mainly controlled by special control-tasks, which are not accessible to the OBS E command language. Also the backends for pulsar observations cannot be set via OBS E or OBS inp . Dedicated PC- and special control-software exist for such observations.
For further details one should contact A. Jessner at the telescope site, but also the operators are partly familiar with such observations.
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The translation was initiated by Juergen Neidhoefer on 3/12/1998