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The Slit-Positioning Algorithm

Field of View

The algorithm sorts and discards the objects as follows:

The algorithm will drop objects listed in the catalog that are not in the field of view - regardless of their priority. The field of view is rectangular in shape, with the corners cut off; similar to the shape of the FITS image.
It will then determine if any of the acquisition objects overlap and, working from left to right, discard those that do overlap.
The position of the slit is shifted according to the value of slitpos_x and slitpos_y for each object. If the object is too close to the edge such that (in length or width) the slit falls even partially outside the field of view, then that object will also be discarded.

Acquisition objects

The acquisition objects are sorted according to their `y' pixel positions. The object that has the smallest y-value (i.e. is lowest in the field of view) is selected first. The next object which does not overlap the first and whose lower slit y-value is at least 2 pixels away from the upper slit y-value of the first is then also accepted. If any overlap occurs, or if the slits are within 2 pixels of eachother, the next object is discarded. This process is repeated until all acquisition objects are sorted and either kept or discarded. Before submitting the mask, you must ensure that at least two acquisition objects are present in the mask, and that these are away from the GMOS CCD gaps.

Priority and position sorting of priority 1,2 and 3 objects

The algorithm then sorts the remaining objects according to their priority. Since it must preferentially include a priority 1 object in a mask over a priority 2 object, the objects must be assigned a relative weighting. For instance, is there was a choice between picking 99 priority 3 objects, or 1 priority 2 object, then the algorithm will choose the priority 2 object to put in that space. The relative weighting of object to priority is:

One priority 1 object = 1000 priority 2 objects,
One priority 2 object = 10000 priority 3 objects.

Once the algorithm has divided the objects up based on their priority, it will then sort the remaining objects according to their `x' pixel position, in order from from left to right. The constraints on the object selection are as follows:

For slits that overlap in the x-direction, a minimum of at least 2 pixels must separate the slits in the y-direction. Currently, the size of the pixels in the pre-imaging is used, e.g. if the pre-imaging was binned 2x2 then the mask making software will put 2 binned pixels between the slits.
If the central wavelength is adjusted such that two-tiered slits are possible, then there must be at least a separation of at least 2 pixels between the ends of the spectra in the x-direction.

Within a single priority, the method used to decide which objects should be selected is to choose strips of varying widths. The objects would then be weighted based on whether or not they fit within that strip. The object with the highest priority number and the highest weighting based on the "strip-test" is then selected. For objects with the same `x' coordinate and same priority, the one that appears first in the original catalog will be chosen. To force the algorithm to choose objects that are required in the mask, it is often useful to re-label a lower priority object "Ignore Object" if it is preventing a higher priority object from being selected.

Back to GMOS Mask Making Software Instructions

Last update April 30, 2004; Inger Jørgensen
In original form February 10, 2003; Dione Scheltus