In the Time-Estimator, we restrict ourselves to basically 2 modes :

• "raster" mapping
• on-the-fly mapping

 
 1- "Raster" Mapping

1-2 Presentation

What we call Raster is somewhat misleading, because we refer to it as opposed to an on-the-fly method : a succession of points on which the telescope stays some time, to integrate. The confusion may arise since there is an OBS command OBS\RASTER , which does not allow you to use the classical switching modes (see note below). This is the reason why we propose more than 1 raster modes, but in fact 4.
The first 3 modes (PSW, FSW, WSW) refer to what you would do if you were to make a map with this observing mode : a loop on the different points of the map.

In the present form, the sampling is supposed to be less than 24", the distance between 2 successives pixels on the array. So, for example, if one wants to sample at 24", then, do not enter any sample (see default values below). In such a case, the time given is the time of one integration on the sky. You can specify the number of such "points" you want to make.

3 modes with one ON per REFerence :

FSWITCH

PSWITCH

WSWITCH

 

1 mode where you can adjust the number of ON per REFerence (we advise not to use more than 4 ON per REFerence) :
 

 RASTER

1-2 Default parameters

• nb of subscans : 20
• integration time per point in switched mode : 30s
• time between 2 CALibrations : 15 min
• integration time per point in RASTER mode : 30s
• size : 120 x 120 "
• sampling : 0"
• 1 ON per REFerence

Note :
Please note that these 3 observing modes are not compatible with the OBS\RASTER observing mode. We precise that this latter one is the only one which allows you to combine several ON integration per REFerence. Thus we emphasize that the 3 "integration" modes we just mentionned are quite time-consuming.
 
 
 

2- On-The-Fly Mapping

2-1 Presentation

Three SL-OTF modes are presented, which we think are the essential ones. They differ only by the derotator's angle. In some way, this angle decides for the sampling in the direction orthogonal to scanning. The sampling in the scanning direction is determined by both the dump time and the velocity of the telescope. We thus propose 3 different derotator angles, giving 3 different modes :
 

angle = 0 degree : this is the most basic mode. It is not optimized in time, in that sense that one point on the sky will be "seen" many times (3, 6 or 9 times). The sampling is then completely determined by the step in the orthogonal direction.

angle = 9.6 degrees : in this configuration, the map is over-sampled (sampling = 4" = 1/3 beam).

angle = 18.4 degrees : this is the less time-consuming mode. The step in the orthogonal direction is fixed (67"). The price is that the map will be slightly under-sampled (8">1/2 beam).

2-2 Default parameters

• dump time : 2s to allow the use of all the auto-correlator channels

3- General default parameters

1- Tuning

• We assume that 20 minutes are enough to make the tuning of HERA.
 
2- Pointing
• We consider that a pointing session is done every 2 hours. Of course, this depends on the quality of the pointing model, the atmosphere and on the project. Observations of small sources (e.g. protoplanetary disks) require frequent pointing's sessions, while some extended sources (molecular clouds) can allow the observer no to check every 2 hours the quality of the pointing.
• A pointing session is composed of 2 OBS\POINTING scans, composed of 4 subscans of 40 seconds each (including overhead due to slew motions and data dumping). The time to move the telescope from the source to the pointing source is fixed to 30s.
 
3- Focus
• The Time-Estimator assumes that a focus session is done every 4 hours. This is a typical value. Nevertheless, during sunset or sunrise, the frequency of focus' sessions can be shorter.
• A focus session is composed of 2 OBS\FOCUS scans, composed of 6 subscans of 10s each. To allow for overhead, we take a focus scan to last 70s (instead of 60s). The time spent to move the telescope from the source to the focus source is included in the pointing time, since a focus is always (I think !) preceded by a pointing scan.

4- Others

1. to open and close a new scan
2. time between 2 subscans
3. blanking times in WSW and FSW

3- How to fill-in the form ?

3-1 Definitions

1. What we will call a position, referes to the position of telescope on the sky. Accordingly, one position gives 9 points (from the 9 pixels).
2. The sampling is the sapcing between two successive points.

3-2 Using the Time-Estimator

• Suppose I want to make a coarse map, in position-switching, with points spaced by 24". In that case, I will integrate with HERA on one position. Then I will move HERA by 72" (in order not to redo some points, make a drawing). This Time-Estimator is not yet able to manage such a simple case. But it allows you to compute the time needed to make N such positions, and give you the rms you reach on each point. These information must be enough for you to make a time estimation. The Time-Estimator can not manage such a case for the RASTER mapping. It is foreseen.
• Suppose I want to make a fully-sampled map in frequency-switching. Then, I just enter the size of the map, the sampling (at most 6"). The other parameters are meaningless, since PSW does not allow you to make several ON between 2 REFerence's positions. The size of the region effectively mapped will not, in general, match exactly that you wanted. It will be at most ~30" less or more.
• If the baseline quality is not the decisive criterium for your observationnal strategy, then you can make your map in RASTER, which is less time-consuming, for several reasons : first, one submap is only one scan, and you save the time needed by the control system to load a scan (in PSW for example, each position on the sky is a scan). Second, you can combine several ON with one REFerence position. In that case, the Time-Estimator assumes that the time spent on the REF is t_REF = sqrt(nb_of_ON)*t_ON.
• If the map I want to make is big (more than 4 arcminutes) then, OTF mapping is interesting. The parameters are the same as for Raster maps : size and sampling. The Time-Estimator will compute the number of ON (fly scan) you can make between 2 REFerences (the maximum time between 2 REF is taken to be 300s).
• Finally, we advise you to always try the results given with the 2 methods : RASTER and OTF.

4- General remarks

If you need more details or explanations (because I can understand that this is not clear at all !) about :

• HERA : please consult the on-line  HERA documentation , or contact Albrecht Sievers
• Time-Estimator : contact Rainer Mauersberger

All your remarks are welcome.