|
The Ohio State University
College of Mathematical & Physical Sciences
Department of Astronomy
|
LBT Optical/UV Spectrograph Working Group
Meeting Summary
8 - 9 March 1999
Columbus, Ohio
Following its 1998 October meeting in Tucson, the LBT Scientific Advisory
Committee (SAC) asked all consortium members to form a working group for
concept definition of the first-light optical/UV spectrograph that is to be
built by Ohio State. The SAC instructed us to adopt the proposed Ohio State
Multi-Object Double Spectrograph (MODS) as a strawman design, and consider
carefully what additions or refinements might be made to better accommodate
consortium-wide science goals while keeping the cost of the instrument as
low as possible. In addition to this general charge, the SAC had several
specific questions for the MODS team to address.
In response to this charge, the LBT Optical/UV Spectrograph Working Group
(LBTOSWG) was formed by open invitation to all LBT consortium members. The
LBTOSWG met in Columbus on 8-9 March 1999. Present were Bruce Atwood (Ohio
State, MODS project manager and director of the Ohio State Imaging Sciences
Lab), Ralph Belville (Ohio State, Imaging Sciences Lab), Paul Byard (Ohio
State, MODS team), Darren DePoy (Ohio State, representing the LBT SAC),
John Hill (Arizona, representing the LBT Project Office), Mike Lesser
(Arizona), Jerry Mason (Ohio State, Imaging Sciences Lab) Klaus
Meisenheimer (Heidelberg), Tom O'Brien (Ohio State. MODS team), Pat Osmer
(Ohio State, MODS PI), Roberto Pallavicini (Palermo), Dan Pappalardo (Ohio
State, Imaging Sciences Lab), Brad Peterson (Ohio State, representing the
LBT SAC), Rick Pogge (Ohio State, MODS Project Scientist), Jesper Storm
(Potsdam), Giampaolo Vettolani (Bologna), R. Mark Wagner (Ohio State,
representing the LBT Project Office), and Rogier A. Windhorst (Arizona
State). Jill Bechtold (Arizona) participated in part of the meeting by
telephone. The following is intended to provide a summary of the results of
the meeting.
The major conclusions of the LBTOSWG meeting are the following:
- There was consensus that there are strong science drivers for
retaining high UV throughput to atmospheric cutoff (about 320
nm). The LBTOSWG notes that this also provides a unique niche for
LBT, as the comparable facility instruments for the other 8-10m
class telescopes will not work below 380-400 nm. The LBTOSWG also
notes that the overall response of the UV side of the MODS
spectrograph can be significantly better than it appears to be in
the MODS proposal if a more appropriate grating and CCD coating are
used. The MODS team pointed out in answer to the specific question
from the SAC about the cost of UV response to atmospheric cutoff
that extending the response from 400 nm to 320 nm is a small
incremental cost because MODS is using reflective rather than
refractive optics.
- MODS will produce high-quality (D_80 < 0.25 arcsec, where D_80 is
the diameter that encompasses 80% of the light in a point-source
image) images over a 4x4' field, and this was deemed to be
acceptable given that an "extended field" option can be implemented;
with reduced image quality (D_80 < 0.8 arcsec), a 5x6' field
(i.e., a factor of 1.9 larger) is usable.
Given that many of the proposed science drivers for the spectrograph
are essentially "survey" type observations, it is obvious that, all
other things being equal, multiplexing greatly improves
efficiency. For the very faint quasars and galaxies that are MODS
targets, increasing the field size over the originally proposed 4x4'
size thus has a dramatic effect. However, there are a number of
important barriers to implementation of a wider field at the f/15
Gregorian focus. First, the Gregorian focal surface is curved, not
flat. Slit masks larger than 5-6' (or thereabouts) would have to be
curved. Second, implementation of a significantly wider field would
require a completely different spectrograph from MODS. Collimator
aberrations will cause poor images far from field center, though as
noted above, images comparable to those obtainable in median
uncorrected seeing can be obtained throughout a 5x6' field.
Two important points must be noted:
- The f/4 focus is really the wide-field focus for LBT. A wide-field
spectrograph ought to be designed for the f/4 focus, but not at the
present as the f/4 focus will not be implemented at first light.
- The niches for MODS are high throughput, broad wavelength coverage,
and excellent image quality. An affordable instrument at the f/15
focus is simply not going to be competitive in terms of field of
view. Relative to the field sizes of other wide-field optical
spectrographs for 8-10m class telescopes, the MODS field is
competitive only with Gemini (5' x 5').
- It would be desirable for MODS to have a high resolution mode in the
R = 15,000 to 20,000 range. However, very high resolution (R =
30,000 to R = 50,000) will require an echelle-type spectrograph
optimized for single-object spectroscopy at very high resolution;
attempting to meet the MODS science goals and achieve very high
resolution will result in a design that does not serve any science
goals well.
There are at least two ways that the current MODS design can achieve
higher resolution. One possible solution is to simply use a
higher-dispersion grating. However, this will overfill the camera
and thus decrease efficiency, though the design will need additional
study to quantify this. A cross-dispersed mode will also be
investigated. A second possible solution is to design a
longer-focal length camera, either as an upgrade option, or as the
baseline camera. The latter would be possible if in the R = 8000
mode on-chip binning (2 x 2) is used; a positive aspect of such a
design is that the required design change would eliminate an
obstruction (the detector) in the beam, and a negative aspect would
be an increased cosmic-ray problem (4 times as many cosmic rays per
binned pixel) and a gap between two sections of the spectrum that
would result from the necessity of using two CCDs butted
together.
We also note that the higher resolution mode also provides an
upgrade path to work with the adaptive optics (AO) system. The
possible adaptive correction might range from rapid tip-tilt
guiding, to partial (low-order) adaptive correction. In the
unbinned mode, the camera pixel scale is 0.15 arcsec/pixel.
- It should be possible to implement in the future an integral field
mode. The LBTOSWG envisages an integral-field module that can be
inserted between the telescope and the spectrograph. This is
regarded as a future upgrade path, and the MODS design will preserve
the integral-field capability as a future option. We note that in
any case a conventional long-slit mode (4 arcmin) will be
available.
The LBTOSWG has identified the following specific action items for the MODS
team. The MODS team has agreed to provide their responses to these items by
no later than 31 March, at which time they will be circulated to the
LBTOSWG for further consideration.
- Evaluate an unobstructed longer focal length (approximately 700 mm)
camera for both sides (red and blue) and performance in both binned
and unbinned modes.
- Elaborate on the "extended field" (5' x 6') concept. Include
commentary on the likely availability and cost of the atmospheric
dispersion corrector for this larger format.
- Provide updated throughput and quantum efficiency curves based on a
range of realistic gratings and CCD coatings.
The LBTOSWG has also requested descriptions of science programs that will
require "extended field" capabilities (from Klaus Meisenheimer) and
R=20,000 resolution in both the red and blue channels (from Jim
Liebert).
When the MODS team response to the action items outlined above is received,
the LBTOSWG will continue discussion via electronic mail. Our goal is to
have written recommendations on the MODS proposal and possible
modifications and/or upgrade paths ready for the next LBT SAC meeting in
early May.
Compiled by Brad Peterson
peterson@astronomy.ohio-state.edu
March 26, 1999
Return to: [
MODS Project Page] |
OSU LBT Page] |
OSU Astronomy Home Page
]
Updated: 1999 March 26
[rwp]