A design study was performed by NGD while
at the Anglo-Australian Observatory in 1994-1995
and worked out in detail by
Damien Jones of Prime Optics. Instead of having
an instrument which rotates as shown above we decided
to split the light by using a rooftop construction
consisting of two gratings.
These gratings diffract the
light to the left and to the right, splitting the pupil,
and thus an image of the galaxy is formed by
each of the two cameras. Wheras the orientation of
the field is the same for both, the dispersion
directions differ by 180o.
A summary of the specifications:
| Optics | ||
|---|---|---|
| Design Study | stick with Prime Optics design | (cost approx $4000) |
| Fabrication | ICOS quote | 58,000 (adjusted for inflation) | Assembly and Test | 3 months in house | 17,500 |
| Grating | Milton Roy | 7,000 |
| [O III] Filter(s) | ? | 7,500 |
| Mechanical | ||
| Design, Drawing | 4 months in house | 23,000 |
| Fabrication | 6 months, mixed in house and under contract | 40,000 |
| Materials | 10,000 | |
| Detectors | ||
| Controller (x1) | SDSU/Leach | 25,000 |
| Cryostat and Chip (x2) | Tek | 45,000 |
| Project Manager, Administration | 30,000 | |
| Total ($A) | 263,000 |
COLLIMATOR OPTIONS
Here we explore some implementations of the PNS-I design as they might appear at specific telescopes - the 4.2-m William Herschel Telescope (top) and the 8-m ESO Very large telescope.
In PNS-1 the diffracted beam is separated from the incoming beam by about 20 degrees. The design would be modified to the latest specifications and for use with a 2048² CCD with 15um pixels. Note that the collimated beam size is about 120mm, as shown here. The sketches are to scale.
The `classic' PNS-I design calls for pupil-splitting to give two simultaneous images -- this is shown in the lower sketch -- but for economy we might build the instrument with only one camera instead. It is better to assume this while designing the optics since the dual camera version can always be built later by making a second camera, while the minimum camera for the split-pupil design has a slightly smaller aperture than that required for the one-arm design.
The field of view of the instrument is slightly rectangular owing to
the anamorphic effect of the grating. For the f/15 VLT the long side is
about 260mm in the focal plane. For comparison the corresponding value
for the f/8 AAT, for which the design study was done, is 100mm.
Accordingly the VLT collimator will need to be much larger and possibly very
expensive. To capture all the field, the first lens (`Field Lens') of a
VLT collimator would have to be 350mm in diameter. For the f/11 WHT the
Field Lens has to be 260mm in diameter.
These two circular apertures are shown circumscribed on the corresponding
apertures in the smaller drawings on the left.
In the case of the VLT, the inscribed circle is
also about 260mm in diameter. If this defined the aperture,
the corners of the field would not be imaged but the cost of
the optics would be comparable to that of the WHT collimator.
Perhaps even the same Field Lens could be used.
The f/15 collimator
would no doubt be close to the PNS-II design just arrived at
by Damien Jones of Prime Optics although the F=238mm camera
would probably be more similar in design philosophy to that
used in PNS-I.
EFFICIENCIES:
The success of the PNS depends critically on the efficiency
achieved. Because the instrument will be optimised for a small
wavelength range, the efficiency will far exceed that of general-purpose
instrumentation. We expect the
instrument to have
about 62% efficiency, giving just about 36% total efficiency into both
arms as shown here. For comparison, ISIS at the WHT has 10-20% efficiency.
| Filter | AR coated narrow-band | 0.85 |
| Detector | Loral LR2048 or equivalent | 0.95 |
| Telescope | Cassegrain focus (two reflections, loss at central obstruction) | 0.722 |
| PNS | instrument efficiency | 0.622 |
| Total | 0.363 |
This shows the performance of the PNS-1 design at a number of different observing sites (with appropriate change in collimator).
Assumptions: 2000² 15um pixels, 160 mm collimated beam,
efficiency 36%, sky 21.4 mag/arcsec², galaxy 21.8 mag/arcsec²,
40Å bandpass, 28,000s integration time
| Dtel | f | seeing (") | image scale (pxl/") | PSF (pxl) | F.O.V.(') | resoln (km/s) | S (e-) | SNR |
|---|---|---|---|---|---|---|---|---|
| 3.9m | 8 | 1.0 | 2.76 x 2.31 | 2.76 | 12.1 x 14.4 | 5.9 | 5363 | 36 |
| 4.2m | 11 | 0.8 | 2.97 x 2.49 | 2.38 | 11.2 x 13.4 | 5.2 | 6220 | 41 |
| 8.0m | 15 | 0.5 | 5.66 x 4.74 | 2.83 | 5.89 x 7.04 | 5.9 | 22568 | 114 |
Scale and FOV given as (spatial) x (dispersed) values.
Resolution is here defined as that corresponding to a centroiding accuracy of 0.25 the PSF FWHM in the dispersion direction, or 0.4 pixels, whichever is larger. After determining the displacement between two images this centroiding is improved by sqrt(2) in velocity units.
The background level (B) is 16,473 counts per pixel in all cases.
The signal (S) is for the brightest PNe in a galaxy at D = 20 Mpc (these
PNe have a flux of 1.3e-15 at CenA (assume 3.3 Mpc) and
SNR = S/(sqrt(S+B))
For convenience we give for one case the corresponding
values for the PNS-2 design in which the collimated beam diameter is
reduced to 120mm (this is the only important change):
| Dtel | f | seeing (") | image scale (pxl/") | PSF (pxl) | F.O.V.(') | resoln (km/s) | S (e-) | SNR |
|---|---|---|---|---|---|---|---|---|
| 8.0m | 15 | 0.5 | 5.13 x 5.07 | 2.57 | 6.50 x 6.57 | 13.6 | 22568 | 114 |