It was decided quite early on in the project that the addition of a
dichroic and (undispersed) camera, centred at the H-α wavelength
corresponding to the appropriate [O III] wavelength, would have
considerable scientific return. As well as yielding additional
information about the PNe (and possibly helping to discriminate against
HII and Ly-α galaxies) the "third image" from the non-dispersed
arm will certainly improve the determination of the position and
velocity of the PNe found.
Finally, there is the fact that we will
obtain very deep images of the
field, in which serendipitous discoveries could be made.
The option of having an H-α arm was only possible because the optimization
of the optics to the [OIII] wavelength was restricted to the final
sections - prior to that point we used broad-band AR coatings and of course the
[OIII] filter itself is down near the grating assembly at the end of the collimator.
This left the required space for the dichroic, splitting the light
from the telescope between the two bands.
We had asked Damien to include the H-α option from the very
start, though of course in practice there were complications, such as finding
a practical position of the dichroic. This delicate component shares the
space which we need to use during the exchanging of the [OIII] filters,
and solving the mechanical problems raised the costs.
We started to collect funding for the project.
last modified by NGD 11/2007