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The Optopus session

After the initial setting up of the Optopus session, the user is now ready for the ``real thing'', that is to use of the main commands of the Optopus package:

HOLES/OPTOPUS converts the RA and DEC coordinates in the MIDAS table created by CREATE/OPTOPUS (and precessed by PRECESS/OPTOPUS) into :X and :Y positions of the holes to be drilled on the Optopus starplate. It outputs the following information:

objects or guidestars falling outside the plate area;
objects or guidestars falling in the so called ``forbidden area'', that is the thicker part of the plate used to fix it to the spectrograph;
objects which are too close to a guidestar (big or small);
objects of only which are in competition because of their proximity.
The plate is needed. The user is offered two alternatives: either to enter pre-determined center coordinates (using SET/OPTOPUS CRA=value1 CDEC=value2, where value1 has the format HH,MM,SS.sss, and value2 the format +/-DD,AM,, and SET/OPTOPUS ACFLAG=N, or to use the command to compute them automatically (SET/OPTOPUS ACFLAG=Y. The automatic determination of the center simply uses the arithmetic mean of the :RA and :DEC columns. The result is not always optimal. To choose the ``best'' center (that is the one which permits to keep the maximum number of objects inside the plate limits) from this guess, the user uses the MODIFY/OPTOPUS command. This command displays graphically the position of the holes on the starplate and, if required, permits modifications of the RA and DEC of the center using SET/OPTOPUS CRA=value1 CDEC=value2. The user should re-run HOLES/OPTOPUS followed by MODIFY/OPTOPUS to verify the improvements.

An important point is that both the center of the plate and the :RA and :DEC coordinates in the input table must be corrected to the same equinox. If you decide to input your own pre-calculated center coordinates, either precessed or not, you also have to remember to set the value of the parameter PFLAG accordingly. In case of automatic determination of the center, the center is calculated by averaging the :RA and :DEC columns in an already precessed table, so PFLAG is by default set to N.

The output table created by the command HOLES/OPTOPUS contains also a column called :CHECK. A letter N in this column identifies objects or guidestars with location problems of any kind (they will be indicated by a square, in the graphic output produced by MODIFY/OPTOPUS and ZOOM/OPTOPUS).

The task of MODIFY/OPTOPUS is very simple and twofold:

Some users like to start their Optopus session with an populated field of candidate source. They then proceed to eliminate objects until a suitable number is reached. However, care should be taken to avoid eliminating more objects than necessary in cases where several targets are closely grouped together. In fact, even if the minimum separation between adjacent pairs is large enough to pass all the overlap checks performed by HOLES/OPTOPUS, once at the telescope it may become problematic to physically introduce the fibres into extremely close holes. It then may happen that one is forced to a late rejection of more scientific targets than one would have liked. However, this might turn out less harmful than expected if one had careful enough to have some ``backup'' holes drilled in the starplate.

In case of very close groups of objects, the command ZOOM/OPTOPUS may also be helpful. If the resolution provided by MODIFY/OPTOPUS is not enough, this command permits to actually blow up a section of the Optopus starplate plotted on the graphic screen by MODIFY/OPTOPUS. The user only has to choose the center of the section she/he wants to be enlarged with the cursor. In most cases the default zoom factor of 5 is sufficient to resolve close groups or pairs. However, should this resolution not to be enough, the possibility exists to enter the command ZOOM/OPTOPUS again, with a new zoom factor, the center remaining unchanged.

When all inacceptable objects have been removed, it is time to use the command
REFRACTION/OPTOPUS to correct the X and Y position of the holes on the starplate for the effect of atmospheric refraction. For a detailed description of the correction algorithm and an estimation of such effects in the particular case of La Silla, we refer to G. Lund, 1986, OPTOPUS, ESO Operating Manual No. 6, pag. 17-18. Here, we summarise that from coordinated of the plate center coordinates, the specified temporal observating window and the wavelength range of interest, REFRACTION/OPTOPUS determines:

Note, that the coordinates of the plate center must be the same as the ones already used with HOLES/OPTOPUS. It is not necessary to reset these since REFRACTION/OPTOPUS will get the (precessed) values from the keyword PLATECEN that have been saved by HOLES/OPTOPUS.

In general, the observer will try to observe his/her fields at the smallest possible overall hour angle (airmass). This optimalisation has to be made in advance. The window in sidereal time for each of the plates which will be observed during a single night can be easily computed knowing that for the date entered by SET/OPTOPUS DATE=value, the command REFRACTION/OPTOPUS outputs the sidereal times at the beginning and end of the night on La Silla. Not more than 4 (in summer) or 5 (in winter) Optopus starplates can be used in one night. So, just to run REFRACTION/OPTOPUS using the default value for the sidereal time slot (ignore any error messages you may get, as in this first run you are only interested in the first line of the output, which will be correct anyway) and divide the night into 4 or 5 exposures (allowing for some start-up time at the beginning, approx. 20 minutes). An example of the output of REFRACTION/OPTOPUS can be found in Table H.3.

Table H.3: Output of REFRACTION/OPTOPUS command
Darkness will begin at ST: 20.37
and end at ST: 5.16
Sidereal time for observation: 21.00
Hour angle: -27.49 degrees
Zenith distance: 24.84
Maximum refraction correction: 0.23 arcsec
Position angle of correction vectors: -106.56 degrees
Chosen length for exposure: 60 minutes
Approx. optimal obs. slot (ST): 20h 30m to 21h 30m
Approx. optimal obs. slot (UT): 24h 6m to 25h 6m
Corresp. range of corr. vectors: from -99 to -116 deg.
Wavelength range for optimisation: 3800 to 5500 Ångstroms
Optimal correction at wavelength: 4329 Ångstroms
Chromatic correction needed in X: -46. microns
Chromatic correction needed in Y: -14. microns

The sidereal time for which the corrections are finally calculated can either be enforced by the user, by setting the parameters ASTFLAG=N and OST=value, or automatically determined by the command. In the latter case ASTFLAG must be set to Y.

REFRACTION/OPTOPUS produces an output table quite alike the one created by
HOLES/OPTOPUS. The most obvious differences are that now the :X and :Y columns contain coordinates corrected for the atmospheric refraction effects, and the column :NUMBER, has been added. This new column will later be needed to identify the holes on the starplate by a sequential number.

Another important characteristic of the table produced by REFRACTION/OPTOPUS is that, being the final table generated in the Optopus session and the one which the observer will presumably bring along to the telescope, it contains all relevant output information (e.g. see Table H.3) in its descriptors. Besides, as already remarked, the user has the possibility to save all parameters used in the session as well, by using the command SAVE/OPTOPUS tablename.

next up previous contents
Next: Closing down Up: Using the Optopus Package Previous: Starting up
Petra Nass