Addendum to
OBS _E
A User's Guide to the Observing Program at the
Effelsberg 100-m Telescope

Changes since April 1995

Version of April,22,1998
(J. Neidhöfer)

For a Postscript Version click here:    Addendum

What's New

Since the last update of the OBS _E Manual (April 1995) the hardware at the telescope site and proposals by the observers and operators led to changes and additions in the OBS _E - and the OBSINP-program. The following features may be of special interest:

• Level-Control of the Autocorrelators
  Both correlators (the old 1024-channel and the new 8192-channel) now have the same and more possibilities for doing a level control. While with the old correlator a pause of ca 25 sec is made after that, the new one needs a small time of 3 sec pause. Beside doing a level control before a scan or before each subscan, there is now also the possibility to do it after a given number of subscans. More one can find under the adverbs LEVELCONTR (1024-ch-ac) or NLEVELCONT (8192-ch-ac).
• Optimizing the Radial Focus (FC1)
  In the same way as the optimizing of the axial focus (FC2) with the procedure FOCUS one now can automatically optimize the radial focus (FC1) by a new procedure called RFOCUS, which has the same parameters like the FOCUS-procedure. The fitted value can automatically taken by the CORRECTIONS command and also a certain value can be set by that verb (see section 1).
• Frequency Input
  Often there exist receivers which have different parameters and switches for different possible passbands. For such receivers symbols are defined which automatically set all necessary parameters by giving only the wanted frequency. These symbols are FRQRX and LINRX (see section 3).
• 8192-channel autocorrelator
  A new Correlator-System with more channels and bigger bandwidth has been installed at Effelsberg. That now is the third standard backend at Effelsberg beside the Continuum backend and the (old) 1024-channel autocorrelator. More can be found with the verb KORREL8192 (see section 6).
• 1024-channel autocorrelator
  During observations the backend program sends commands to set the parameters of the correlator only in the case if the OBS _E input commands have changed. In dubious cases or in cases where somebody manually changed the status of the correlator, it is necessary to reset the correlator, i.e. to again send all parameters to the correlator. To do this there now exist a parameter. Details are found under the adverb LEVELCONT of the verb AUTOCORREL (see section 5.1).
• Choosing the line backend
  As now there exist two line backends (the 1024-AK and the 8192-AK), one has to tell the program which backend is the active one for the next observation. More details under the adverb BACKEND_L of the verb L_MODE (see section 4.1).

RFOCUS

  

OBSE  >  RFOCUS   < c_mode  >  
                /DEFAULT                /LENGTH_FC   < mm  >  
                /TIME_FC   < Time(s)  >  /STEP_FC   < no_of_steps  > 

RFOCUS performs a series of on-source measurements where the radial focus displacement (FC1) is stepped back and forth symmetrically about the hitherto best value. A parabola is fitted to the averaged values in order to determine the optimum focus. If parameters are entered as current values, they remain active until the verb is entered again, except the /DEFAULT adverb is used to tranfer the parameters into the default values.

• < c_mode > is the continuum observing mode (this is transferred to C_MODE),
  where T = Total-power, B = Beam-switched, L = Load-switched, C = Correlation mode, V = VLBI and E = External.

ADVERBS:

• LENGTH_FC Total length of movement in mm. As a rule of thumb, for short wavelengths ( < 6 cm) the wavelength in mm is a good choice. For longer wavelengths, the total radial focus movement of 200 mm, and the long travel time required, restricts the length to smaller values.
• TIME_FC Integration time in sec per ON-source measurement.
• STEP_FC Number of steps in each direction. If an even number is used, there will be no measurement at the best-guess value. A value of 3 measures at positions of -LENGTH/2, the best-guess position and +LENGTH/2.

EXAMPLE:

OBSE > RFOCUS /LEN 60/TIM 15/STEP 4

This will give 15 sec integration at -30, -10, 10, 30, 30, 10, -10, -30 mm about the best-guess position held in OFC1.

CORRECTIONS

        

OBSE  >  CORRECTIONS    < az  >    < nule  >    < focus  >  
                /NULA            /RFOC

This VERB allows you to use the results of a calibration measurement to update the antenna correction constants. After a POINTING measurement, the pointing in azimuth and elevation can be improved via the results of Gaussian fits to the measurements. After a FOCUS or RFOCUS measurement, the precise symmetry of displacements along the axial or radial focus direction can be improved via the results of a parabolic fit.

All corrections are parameters of the command, the first parameter being the azimuth correction, the second being the elevation correction, and the third being the focus correction in FC1, if /RFOC is typed otherwise in FC2.

For each parameter of CORRECTIONS, there are three possibilities:

a)

* = no change in the relevant coordinate.

b)

F = the result of the fit is to be the correction applied. The displacement computed from the fit is added to the pre-fit value, and thus applying a given correction several times without a new measurement will always result in the same corrected value.

c)

Any number can be entered in units of arcsec (azimuth and elevation) or mm (focus). The appropriate correction will be set to that value.

ADVERBS:  

• NULA
  If this ADVERB is present, < az > is used to update the zero-point correction in azimuth. In the common case, with NULA absent, < az > is used to update the azimuth collimation (COL*).
  NOTE: If F is used for < az > , and NULA is present, the result of the fit is added to the pre-fit zero-point correction. Otherwise, the result is added to the pre-fit collimation correction (COL*).  
• RFOC
  If this ADVERB is present, < focus > is used to update the zero-point correction in radial focus (SFC1). In the common case, with RFOC absent, < focus > is used to update the the axial focus (SFC2).
  NOTE: If F is used for < focus > , and RFOC is present, the result of the fit is added to the pre-fit zero-point correction in FC1 (radial). Otherwise, the result is added to the pre-fit correction in FC2 (axial).

EXAMPLES:

OBSE  >  CORR * * -1.5            updates the axial focus (SFC2) to -1.5 mm.
OBSE  >  CORR * F                uses the Gaussian-fit result to update the elevation pointing.
OBSE  >  CORR -210/NULA                  sets the zero correction in azimuth (NULA) to -210 arcsec.  
OBSE  >  CORR * * 5.5 /RFOC               updates the radial focus (SFC1) to 5.5 mm.
OBSE  >  CORR * * F /RFOC                uses the fit result to  update the radial focus (SFC1).

NOTE: A transfer of the results of gaussian fits to longitude and latitude is only possible after POINTING with the ORIENTATION in both, azimuth and elevation. Repeated entries of CORR without new measurements will always yield the same corrections. If the fit was not successful, but the correction is requested, an error in the SIC command procedure occurs and has to be resolved by entering QUIT.

Results obtained from the command CROSS are not accepted, even though the procedure might set up identical scanning parameters.

If the CORR command is put in before the POINTING is finished OBS _E waits until the end of the observation and until the fit is done. Then the correction values from the fit are applied. In that state OBS _E accepts no input from the keyboard.

Frequency - Input for Receivers with several options

 There exist many receivers, which depending on the selected skyfrequency have a different configuration concerning switches, sideband or the calculation of the ULO-frequency. For these receivers now exist special OBS _E - symbols, which choose and set the right receiver configuration by typing only the frequency. They can be used instead of the normal OBS _E - commands for frequencies and receiver specification like SET FREQUENCY... or LINE... . If one will set the frequencies for such receivers with the normal parameters one has to be very careful to set all other parameters for the right receiver configuration.

Calling these symbols without parameter delivers a description of the necessary inputs. Certain receivers do not need such definitions.
In this case one gets an error-message saying that the symbol is not defined.

Frequencies from a line-catalog one calls with the OBS _E - symbol:

OBSE  >                  LINRX 

  < name > [ < fixed > [ < catalog > [ < option > ]]] 

• < name > : is the line name in the catalog
• < fixed > : F, if the frequency shall be fixed, otherwise it is corrected
• < catalog > : name of the line catalog with or without directory-path in VAX(VMS)-format (e.g.: [dir.dir2]name.ext)
• < option > : more options, which depend on the receiving system

< fixed > , < catalog > and < option > are optional inputs.

A frequency one can call with the OBS _E - symbol:

OBSE  >                  FRQRX 

  < frequency > [ < fixed > [ < option > ]] 

• < frequency > : is the observing frequency in GHz
• < fixed > : F, if the frequency shall be fixed, otherwise it is corrected
• < option > : more options, which depend on the receiving system

< fixed > and < option > are optional inputs.

Both symbols always set both possible frequencies (for line and continuum) to the same value.

Choosing the right line backend

L_MODE

  

OBSE  >                  L_MODE   < l_mode  >  
                /DEFAULT                /BACKEND_L    < backend_id  > 
                  /PHASES_L   < number  >  /RECEIVER_L   < rx_nr  > 
                 /CYCLETIME_L   < t  >    < blank  >  /FREQUENCY_L   < SCF  >    < side  > 
                 /SWITCHES_L   < static1  >    < static2  >    < switch-nr. for CAL-Signal  >   < NB  > 

ADVERBS:

• BACKEND_L < backend_id >
  Sets the name of the line backend.
  < backend_id > = A(Kold) The 1024-channel-autocorrelator is selected.
  = N(ewAK) The 8192-channel-correlator is selected.

EXAMPLE:
SET BACKEND_L AK selects the old correlator

Changes for the 1024-channel Correlator

AUTOCORREL

  

OBSE  >                  AUTOCORREL
                /DEFAULT                /LEVELCONTR   < contr  >    < Set AK  > 
                 /SPLIT   < split_mode  >    < doppler_mode  > 
                /AC_OPTIONS   < IF_conn  >    < quantization  >    < output_func  >  
                /CHANNELS_L   < IF_distribution  >  /PREINTEG_L   < dump_time  >  
                /BANDWIDTH_L   < f1  >    < f2  >    < f3  >    < f4  >  [unit]
                /FRQ_DOPPLER   < o1  >    < o2  >    < o3  >    < o4  >  [unit]
                /VEL_DOPPLER   < v1  >    < v2  >    < v3  >    < v4  >  [unit] 

Connects the 1024-channel autocorrelation spectrometer with the frontend, which is defined by the command L_MODE. If parameters are entered as current values, they remain active until the verb is entered again, except the /DEFAULT adverb is used to tranfer the parameters into the default values.

• LEVELCONTR < contr > < Set AK >
  < contr > : automatic level control for quantization
  -1: level-control before every subscan
  0: no level-control at all
  +1: level-control before the beginning of a scan
  contr > 1: level control again after < contr > subscans
  < Set AK > : if set to SET , the next measurement will again set all parameters of the AK independent of whether they have changed or not.
  That is only valid for the next measurement. After this the parameter is automatically reset to normal.

EXAMPLE:
SET LEVELCONTR * SET will send all commands to the correlator before the next scan.

The new 8192-channel Autocorrelator

  The new 8192-channel-autocorrelator normally uses the VLBA-IF ( 500 - 1000 MHz ) at Effelsberg, but also it is possible to use the standard smallband IF of 150 MHz. Depending on the splitmode the maximum bandwidth in one IF-channel is 160 MHz and the minimum is 10 MHz. There exist an option using external filters to reduce the minimum bandwidth to for example 5 MHz. Splitting the correlator in 8 channels with a bandwidth of 160 MHz each, one can arrive at a maximum bandwidth of about 1 GHz.

As the new 8192-channel-autocorrelator can use the standard (old) Effelsberg-IF of 150 MHz as also other higher IF's there is one parameter in OBSINP to tell, which IF is used. That is important for the calculation and setting of the frequencies. That parameter is:

OBSINP AIF3 < if > [in MHz]

At the moment there exist the 150-MHz-IF and the 750-MHz-VLBA-IF.

KORREL8192

  

OBSE  >                  KORREL8192
                /DEFAULT                /NLEVELCONT   < contr  >  
                 /NSPLIT   < split_mode  >    < doppler_mode  > 
                /NAC_OPTIONS   < offset  >    < quantization  >    < output_func  >  
                /NCHANNELS_L   < IF mode  >    < IF unit  >  
                /NPREINTEG_L   < dump_time  >  
                /NBANDWIDTH   < f1  >  until   < f8  >  [unit]
                /NFRQ_DOPPL   < o1  >  until   < o8  >  [unit]
                /NVEL_DOPPL   < v1  >  until   < v8  >  [unit] 

Connects the 8192-channel autocorrelation spectrometer with the frontend, which is defined by the command L_MODE. If parameters are entered as current values, they remain active until the verb is entered again, except the /DEFAULT adverb is used to tranfer the parameters into the default values.

ADVERBS:

• NSPLIT < split_mode > < doppler_mode > :

< split_mode > (number) How to split the correlator bands

                1:  1x8192 (10 or 20 MHz bw)            2:   2x4096 (10 or 20 MHz bw)
                3:  4x2048 (10 or 20 MHz bw)            4:   8x1024 (10 or 20 MHz bw)
                5:  8x512  (40 MHz bw only)             6:   8x256 (80 MHz bw only)
                7:  8x128  (160 MHz bw only)            15:  1x8x512 (40 MHz bw only)
                16:  1x8x256  (80 MHz bw only)          17:  1x8x128 (160 MHz bw only)
                25:  2x4x512  (40 MHz bw only)          26:  2x4x256 (80 MHz bw only)
                27:  2x4x128  (160 MHz bw only)                 35:  4x2x512 (40 MHz bw only)
                36:  4x2x256  (80 MHz bw only)          37:  4x2x128 (160 MHz bw only)
                250: 2x2048  (40 MHz bw only)           260: 2x1024 (80 MHz bw only)
                270: 2x512  (160 MHz bw only)           350: 4x1024 (40 MHz bw only)
                360: 4x512  (80 MHz bw only)            370: 4x256 (160 MHz bw only)

< doppler_mode > Frequency shift within a correlator band:
Defines, if and how the values entered in NFRQ_DOPPL and NVEL_DOPPL should be used to calculate a shift of the correlator analyzing band with respect to the center of the IF band. These option might be handy especially when the correlator is split in more bands. Option 2 and 3 shift the correlator band by a fixed amount, typically used in frequency switching or testing for bad channels. Option DOVE centers the main line frequency (FREQUENCY_L) on different velocity windows, option DOFR uses the source velocity and centers the bands at different line frequencies (ex. isotops), the option DOFV allows the different lines and different velocities. In the latter case the FREQUENCY_L should be set to optimize the IF band.

NONE: No frequency- or velocity-offset is used
OFFR: Offset in frequency

the following modes are not yet implemented:
OFCH: Offset in channels
DOVE: Use velocities given below for sky frequency recalculation
DOFR: Use frequencies given below for sky frequency recalculation
DOFV: Use both, the frequencies and the velocities given below.

• NLEVELCONTR < contr > : automatic level control of the incomming power
  -1: level-control before every subscan
  0: no level-control at all
  +1: level-control before the beginning of a scan
  contr > 1: level control again after < contr > subscans
  
• NAC_OPTIONS < special > < quantization > < output_func > :

< special > = OFSET (only first letter is used)
defines the next measurement as an offset measurement in order to determine the offsets of the internal total-power-counters of the correlator by switching off the incomming IF.
< special > = SETNEW (only first letter is used)
will newly set the autocorrelator also if no input parameter has changed. That is normally useful, if in the meantime one has manually changed something at the autocorrelator (e.g. a hardware reset).

This parameter is only valid for the next observation. After that it is automatically reset to NORMAL.

< quantization > defines the bit-quantization for the correlator , where 1 = 1-bit quantization, and 2 = 2-bit quantization).

< output_func > defines the type of function, which is archived (AC = autocorrelator):

0: AC-function without clipping correction
1: Spectrum

• NCHANNELS_L < IF_mode > < IF_unit >
  < IF_mode > : Sets the IF-mode for the the autocorrelator:
  -1: Internal noise generator to all AC channels
  0: use incomming IF-channels as define bei the split-mode.
  1: override split-mode and distribute first if-channel to all correlator channels.
  2: override split-mode and distribute two if-channels to the correlator channels.
  3: override split-mode and distribute four if-channels to the correlator channels.
  4: override split-mode and distribute eight if-channels to eight correlator channels.
  

< IF_unit > : set the smallband IF-unit distribution for the autocorrelators.
0: unit not connected
1: receiver-IF A connected to outputs 1,2,3,4
2: receiver-IF A connected to outputs 1 and 3
receiver-IF B connected to outputs 2 and 4
3: receiver-IF A,B,C,D connected to outputs 1,2,34
4: internal noisediode connected to outputs 1,2,3,4


• NPREINTEG_L < dump_time > : Sets integration-time < dump_time > in [millisec] for each data transfer to archive. The number of transfers per subscan depends upon the integration time, which is relevant for a subscan.
• NBANDWIDTH < f1 > until < f8 > [unit]:

Bandwidth of the video input in GHz, MHz (default) or kHz. This is dependent on the splitmode and can vary between 10 to 160 MHz. Special modes have certain bandwidthes. If there is only one possibility for a special splitmode, the bandwidth is automatically set.

• NFRQ_DOPPL < o1 > until < o8 > [unit]: Frequencies or frequency offsets for the autocorrelator-channels in [unit] of GHz, MHz (default) or kHz, The < doppler-mode > in /NSPLIT determines the way in which the frequencies are used.
• NVEL_DOPPL < v1 > until < v8 > [unit]: Velocities for the autocorrelator channels in [unit] of km/sec (default) or m/sec. The < doppler-mode > in /NSPLIT determines if and how the velocities are used.

About this document ...

Addendum to
OBS _E
A User's Guide to the Observing Program at the
Effelsberg 100-m Telescope

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