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:
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:
In case of very crowded fields, the limited physical dimensions of the outputs of some graphic devices can make it difficult to read the identification labels of the objects, and hence making the task of deleting the ``right'' objects a really tricky one. To avoid undesirable results, some auxiliary information is displayed whenever the user clicks on an object: :RA, :DEC, :IDENT and content of the :CHECK column. The user is prompted for substitution the ``N'' already present in this later column with a ``D'' or ``d'' (for delete).
In case the wrong object has been selected, that is the :CHECK column is empty, it is sufficient to hit return to keep everything as it was. It may also happen that a ``wrong'' object is selected twice, that is the :CHECK column already contains a ``D'' or ``d''. In this case one has to type the same letter (``D'' or ``d'') again, otherwise the object wlll not be rejected. Note, that in case of close pairs of objects, both of them are surrounded by the square symbol which means ``candidate for deletion''; however, both squares will disappear after having deleted only one of the two object.
Finally, if for any reason you decide you would rather keep one of the objects marked by a square, click on it and type anything but a ``D'' or a ``d'' when prompted. In order to see what the starplate looks like after this editing, the user first has to deactivate the graphics cursor (by hitting the spacebar or pressing the right button of the mouse of your workstation) and then rerun HOLES/OPTOPUS and MODIFY/HOLES. Reverting HOLES/OPTOPUS is not compulsory, since it would be enough just to rerun MODIFY/HOLES to have a new plot of the Optopus starplate. However, it is also useful to check to review outliers and close pairs repetetively.
The command HOLES/OPTOPUS is reasonably fast, so we advise the users to frequently switch between HOLES/OPTOPUS and MODIFY/OPTOPUS and vice versa.
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
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.
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
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.