http://www.aao.gov.au/local/www/pns/docs/phaseB.letter.html

To:
H. Butcher, NFRA
M. Capaccioli, Osservatorio Astronomico di Capodimonte
R. Gilmozzi, European Southern Observatory
J. Mould, Research School of Astronomy and Astrophysics, ANU
P. van der Kruit, Kapteyn Institute

Dear

I am pleased to be able to send you a short, and positive, report on the state of affairs in the construction of the Planetary Nebula Spectrograph, to which project your institute is a contributor. This letter may also be viewed as a hypertext document at the URL given above, in which case you will be able to follow the links directly.

During the study stage of the project we arrived at two interesting, but ultimately unfeasable end-points. One was a spectrograph in which the gratings were tilted in such a way as to produce counter-dispersed images side-by-side on a single detector
(PNS-2). The other was a design which could be used both at a 4-m and 8-m telescope (the VLT in particular) with only a minor modification. These were valuable studies which might as yet bear fruit. However it was felt necessary, in order to keep the instrument highly competitive, to optimise its performance at the primary stations, the WHT and the TNG, and both of the designs mentioned resulted in an undesirable compromise.

Accordingly a two arm design was decided upon (PNS-3) and RSAA took up the contract to make a final design study. They did this with remarkable professionalism and zeal. Along the way they identified several stray light problems, mostly arising by chance due to our choice of grating geometry. We have now modified the design to avoid most of these effects. The task of fabricating the actual optics has been given to RSAA and a commercial company INOAE. We expect the uncoated optics to be ready in October of this year, after which they will be tested and coated.

As part of the Dutch involvement in this project we had obtained approximately 7 man months of time in the mechanical workshop at NFRA. As the final drawings are produced by RSAA they are made available to NFRA, and most of the instrument housing will be completed in the coming months.

Close liaison is taking place between the Project and the Observatories at which we expect to use the PN Spectrograph. The areas of discussion include telescope access and the use of CCD detectors (which are not included in the PN Spectrograph budget). In addition, we have designed the Spectrograph to make use of the available calibration lamps, where possible. We have found these discussions very cordial and encouraging. There have been no formal agreements with either observatory as yet. Our plan is to have the instrument available by mid-2000.

The two-arm spectrograph in which the gratings are mounted in a concave orientation, or can be replaced by a single grating if desired. The light comes from the telescope off to the left and passes through the collimator and [OIII] filter before being split by the gratings.

The budget for the project was prepared on a speculative basis, well before the final design was available. However it has proven to be quite robust and so far there is no serious overrun. Unfortunately we only have funds for one narrow-band filter, as this turned out to be large (200 mm) and expensive. During the meeting in Leiden which was held in July to discuss the interim Phase B report we identified areas of research and chose the parameters of this single filter such that we could use the PN Spectrograph for a great many projects before new funding would need to be sought.

As early as June 1997, during a progress meeting at ESO it was realised that a considerable increase in scientific return could be obtained from the PN Spectrograph by the addition of a dichroic mirror to reflect an undispersed Halpha image to a third camera. As the [OIII] images alone enable the emission-line objects to be identified and located on the sky, the image would provide the corresponding intensity in the Halpha line, which is likely to be a useful measurement in itself as well as, possibly, providing confirmation of the identification. Not least, the integration times typical for this program (12h) would yield unusually deep Halpha images. Although the additional camera falls completely outside our present budget, it was decided to commission the RSAA team to modify the mechanical design such as to allow for the possibility of adding the Halpha arm later. As can be seen from their sketch, the idea is in principle simple.

Again the telecope is off to the left. This time only the collimator
is shown. The dichroic is placed ahead of the [OIII] filter and
reflects Halpha light to a third camera, actually in a plane perpendicular
to the [OIII] cameras.

The main impact of the addition is that it made it more difficult to achieve stringent degree of stiffness required of the rest of the instrument. In addition, the space required by the dichroic had some effect on the placement of other components. The design change cost about $11,000, but enthusiasn for the idea of a third camera has steadily grown. It is our intention to prepare a budget and then to open a discussion with the project participants soon about possible funding of the option.

At the time of writing, NFRA has nearly completed the manufacture of the collimator housing and RSAA is ready to supply the drawings for the camera housing. Some important details still outstanding include the details of the focal plane mask, and of the grating mount. The current status of the project can be seen on the project page.

Regards,

 
 
 
 
 
N. Douglas (Kapteyn, Project Manager)

Cc:
M. Arnaboldi (OAC)
T. Axelrod (RSAA)
B. Boyle (AAO)
K. Freeman (RSAA)
J. de Haas (NFRA)
R. Kudritzki (U Munich)
K. Kuijken (Kapteyn)
P. Merluzzi (OAC)
K. Taylor (AAO)

Document date: August 27, 1999