A PN.S Design Study for WHT and TNG
The PN.S consortium has agreed to design and construct
a PN Spectrograph to operate at La Palma, making use of two 4-m
class telescopes. These are the
4.2m William Herschel telescope operated by the ING (historically
run by the U.K., Netherlands, and Spain), and the 3.5m TNG, owned
and operated by the Italian community.
The general design philosphy is as follows:
- The instrument will be optimised around the
5007Å [O III] wavelength.
The instrument should have as few `moving parts' as possible.
There will be no filter wheels or adjustable gratings, and
the telescope will have to provide all A&G and image
derotation facilities, as well as (hopefully) calibration lamps.
There will be a multi-slit mask which is necessary for
calibration, and a shutter.
Other calibration and
alignment must be done off-line and/or with astronomical objects.
The instrument is designed for use with two CCDs where
available but ideally should be operable with one CCD. Detectors are not included
but are provided by the relevant observatory (2k x 2k minimum).
The same optics are used at the two telescopes. This would seem
trivial as both are f/11 telescopes, but the WHT employs a
classic paraboloid/hyperboloid mirror combination, while the
TNG has a dual ellipsoid optical design. This leads to completely
After conducting two design studies (PNS-1 and PNS-2) we
are proceeding with a modified PNS-1 design in which the
collimator achieves the required compromise between
the TNG and WHT.
Ideally, the two
images required for PN velocities can be generated by
rotating the spectrograph between exposures. Alternatively
the pupil can be split as in the PNS-1 design study.
Compared with the original PNS-1 study for the AAT:
- The third order 600g/mm gratings were changed to
first order 1200g/mm gratings, as the original
dispersion was unnecessarily large
- The collimated beam diameter was reduced from 160mm to
120mm. If the camera is adjusted to the same spatial
scale then this change lowers the resolving power
further. This is not a problem in our case, and the
advantage is that the camera optics become smaller.
- The camera is changed from Fcam=350mm to Fcam=287mm.
This choice gives
slightly undersampled images at the TNG in good conditions
- Following a stray light study the gratings
were changed again, this time to first order 600g/mm.