Instrument capabilities
CCDs currently in use
Current info on the new DBS Red and
DBS Blue.
Autoguider
CCD D7 and CCD D9 were the DBS Blue and Red CCDs respectively till early 2005.
The full chips took about 70 seconds to read out, but for single object spectroscopy the chip was generally windowed to half of this or less.
For details of individual devices, see the
detector lab.
Four gratings are available for each arm. This table contains approximate figures.
For more detail, see the
DBS grating angle calculator.
Plane mirrors are also available, converting the
spectrograph into a focal reducer that images the 6·7 arcmin field at f/1·5.
|
Order |
Blaze angle |
Resolution
(Å) |
Resolution
(km/s) |
Range
(Å) |
| 300B |
1 |
3° 38' |
4.5 |
334 |
3800 |
| 600B |
1 |
6° 54' |
2.2 |
166 |
1900 |
| 1200B |
1 |
14° 14' |
1.1 |
84 |
970 |
| 1200B |
2 |
28° 49' |
0.5 |
40 |
470 |
| 158R |
1 |
3° 37' |
8.1 |
337 |
7150 |
| 316R |
1 |
6° 48' |
4.1 |
171 |
3600 |
| 600R |
1 |
13° 00' |
2.2 |
92 |
1900 |
| 1200R |
1 |
26° 45' |
1.1 |
45 |
950 |
All gratings give dispersions of approximately two pixels per resolution element.
Five different dichroics are available for the beam-splitter. Each switches
from transmission to reflection at around 6000Å, though this may be varied
over about 1000Å depending on your choice of dichroic. The transition
takes between 400Å and 1000Å for the various dichroics. There is a
plane mirror (labelled D6 on the mounting cell) for using the blue arm alone.
Leaving the beam splitter empty will send all the light to the red arm.
Although curves for six dichroics appear in the original printed manual,
the one named D5 in the manual is not currently available
in the instrument. If for any reason you think you require it, contact us
well in advance of your observing run.
Transmission/reflection curves
have been observationally obtained. The small scale features are in good
agreement with the manufacturers' data, but the system
sensitivity seems to be very significantly reduced redwards of 7000Å
for reasons that are currently under investigation.
The unvignetted field of the telescope and slit is 80 mm or 6·7 arcmin.
The remotely controlled bi-parting slit opens to 4mm (20 arcsec) with an
accuracy of approximately 20micron. A remotely driven, tapered decker
may be used to obscure up the 5mm (25 arcsec) at the centre of the slit.
The entire slit assembly is tilted by 10deg, allowing a CCD
camera to image the reflective slit jaws and is used for offset guiding.
(TBD) Width
Slit width range
Resolution formulae
(TBD) Length
Spatial scale
Chip size / readout window
(TBD) Decker
Telescope scale is 5·02 "/mm and the f/ratio is 17·9.
(TBD) Determined by seeing, guiding, spectrograph etc
(TBD) Typical values
A filter wheel located before the slit carries five neutral density filters of
48mm diameter and a large clear aperture of 120mm. The clear aperture
allows unvignetted use of the full 80mm field at the slit.
The five neutral density filters are Melles Griot metal-on-glass filters
and allow approximately 50%, 10%, 1%, 0·1% and 0·01% transmission at
5000Å. The 48mm filter diameter, restricts the usable slit length to
26mm or 2·2 arcmin. Plots of
transmission curves
are available for some filters.
For details of the dichroics, see above.
We do not
currently have a full throughput analysis of each separate component,
but the experimental exposure
calculator should tell you what you really need to know. i.e.,
how faint you can go. Please note that this version refers to the
previous CCDs and controllers, therefore the results should only be considered as a broad guideline.
TO BE DONE:
Some example data (standards)
Count rates at various magnitudes
S:N calculator
Atmospheric transmission = from AAT RGO manual
CCD QEs
Slit losses
Diego references
example tables
The use of aperture plates on the DBS enables access to a 6·6'
diameter field. Using a 76mm diameter aperture this allows for up to 55
individual objects within this field.
DBS aperture plates can be constructed easily from a reference
image of known plate solution. The scale within the focal plane is 4·9640"/mm.
Ensure at least 2 resolution elements exist between each slit/aperture
(i.e. 0·6mm). Slits/apertures can be made to any appropriate size (appropriate
for SSO seeing!)
The use of aperture plates on the DBS has a serious limitation: acquisition is
made from light off the plate surface that is scattered towards the autoguider
camera. To overcome this, ensure that at least one moderately bright star
(V<13) is present within the central 3'×2' visible with the autoguider. Place
a hole in the aperture plate for this reference star (a hole of 1" works well)
Aperture plates are loaded into the plate turret in the focal plane of
the DBS housing. Align by first back-illuminating the plate and recording the
image on the autoguider. Then move the image of the reference star to its
allotted hole. One can track either on the scattered light (annulus) around
the reference star or preferably use the TV slide to the side viewing mirrors
(±180 on the TV slide).
More description of this mode of operation are contained in the
autoguider manual.
See section on gratings.
Last modified: Wed July 20, 2004 by Marilena Salvo