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 different characteristics.

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:

1. The third order 600g/mm gratings were changed to first order 1200g/mm gratings, as the original dispersion was unnecessarily large
2. 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.
3. The camera is changed from Fcam=350mm to Fcam=287mm. This choice gives slightly undersampled images at the TNG in good conditions
4. Following a stray light study the gratings were changed again, this time to first order 600g/mm.

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