This paper describes the operating parameters and initial design of a new spectrograph proposed for the 200-inch Hale Telescope at Palomar Observatory. The instrument, whose working name is THuCIDIDES (Two Hundred-inch Cassegrain Image- Deblurred Interchangeable-Disperser/Echelle Spectrograph), will feature high system efficiency and multiple modes of operation, including low- and intermediate-resolution long slit and multi-slit capability over 12.5 X 3 arcmin fields, and a cross-dispersed echelle mode covering 3800 - 8500 angstrom at R equals 20,000 (with a 1.2 arcsecond slit) up to R equals 60,000 (with an image slicer). A 4096 X 4096 pixel CCD will serve as the detector. The quasi-Littrow echelle configuration and use of a prism cross-disperser will result in high system efficiency, estimated at approximately equals 14%. The compact design will permit mounting in the Cassegrain ring plane, to reduce susceptibility to flexure. An optional fast-guiding tilt mirror provides modest improvement to seeing FWHM and slit throughput

Behr, Bradford B.

Bradford B. Behr

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PROCEEDINGS OF SPIE
SPIEDigitalLibrary.org/conference-proceedings-of-spie
THuCIDIDES: a high-efficiency
multimode spectrograph design for
the Hale Telescope
Bradford B. Behr
Bradford B. Behr, "THuCIDIDES: a high-efficiency multimode spectrograph
design for the Hale Telescope," Proc. SPIE 3355, Optical Astronomical
Instrumentation,  (9 July 1998); doi: 10.1117/12.316801
Event: Astronomical Telescopes and Instrumentation, 1998, Kona, HI, United
States
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THuCIDIDES: a high-efficiency multimode spectrograph design
. for the Hale Telescope
Bradford B. Behr
Department of Astronomy, California Institute of Technology
Pasadena CA 91125, USA
ABSTRACT
This paper describes the operating parameters and initial design of a new spectrograph proposed for the 200-inch
Hale Telescope at Palomar Observatory. The instrument, whose working name is THuCIDIDES (Two Hundred-inch
Cassegrain Image-Deblurred Interchangable-Disperser/Echelle Spectrograph), will feature high system efficiency
and multiple modes of operation, including low- and intermediate-resolution long slit and multi-slit capability over
125 x 3 arcmin fields, and a cross-dispersed echelle mode covering 3800—8500 A at R = 20000 (with a 1.2 arcsecond
slit) up to R = 60000 (with an image slicer). A 4096 x 4096 pixel CCD will serve as the detector. The quasi-Littrow
echelle configuration and use of a prism cross-disperser will result in high system efficiency, estimated at 14%. The
compact design will permit mounting in the Cassegrain ring plane, to reduce susceptibility to fiexure. An optional
fast-guiding tilt mirror provides modest improvement to seeing FWHM and slit throughput.
Keywords: optical instrumentation — echelle spectrograph — lens design — fast tip-tilt guiding
1. INTRODUCTION
The facilities for high-resolution spectroscopy at the Palomar 200-inch telescope are presently rather limited. The
only instrument capable of spectral resolutions around R = 20000 over a wide wavelength range is the East Arm
Echelle Spectrograph1 , which utilizes a fiber-optic conduit running from prime focus to a fixed-orientation optical
bench, in order to eliminate fiexure issues and maximize the wavelength stability of the spectrograph. Consequently,
due to lossy fiber coupling, it suffers from poor overall efficiency (i < 2%) , and the single fiber does not permit a sky
background channel. Low order grating and slit devices like the Double Spectrograph2, although efficient, are limited
to R < 5000. Many of the scientific pursuits of Palomar observers — stellar abundance work, quasar absorption
lines, and globular cluster dynamics — would therefore benefit greatly from the availability of a high-efficiency, high-
resolution Cassegrain echelle spectrograph, similar to those currently in operation on the LCO 100-inch3, Palomar
60-inch4, and the McDonald 2.1-meter5 telescopes.
The underlying design philosophy for the THuCIDIDES echelle configuration emphasizes simplicity and maximum
throughput. Specific design choices stemming from these priorities include use of a single lens system in double-pass
as both collimator and camera, a prism (instead of grating) as the cross-disperser, and an optional fast tip-tilt guider.
Details of these subsystems will be discussed below.
Although this proposed instrument was originally conceived solely as a cross-dispersed echelle, it became apparent
that other modes of operation could be incorporated into the design at no penalty to the echelle performance. The
large format of the CCD detector and the f/16 -+ f/3.3 reimaging configuration will permit fields as large as
12.5 x 3 arcminutes (300 x 75 mm at Cassegrain focus) with either a single long slit or multi-slit mask. In order to
accomodate the low-resolution requirements of most long/multi-slit work, the spectrograph follows a modular design,
with interchangable 'bolt-on' disperser, slit, and reimaging packages, which are selected according to the spectral
and spatial coverage desired for a given observing run.
This paper focuses on the optical layout of the spectrograph. The mechanical structure, CCD electronics, control
systems, and other such facets of the design are still under development, and will be discussed in a later publication.
Partof the SPIE Conference on Optical Astronomical Instrumentation • Kona. Hawaii • March 1998 675
SPIE Vol. 3355 • 0277-786X/98/$1O.OO
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2. PERFORMANCE SPECIFICATIONS
2.1. Basic Optical Parameters
Collimator EFL: 2180 mm
Camera EFL: 455 mm
Collimated beam diameter: 136 mm
Estimated efficiency: = 14% at 6000 A
2.2. Echelle Mode
Grating: 12 x 6 inch 79.1 gr/mm R2 grating
Cross-disperser: LF5 prism, 45° apex angle
Spectral range: 3800—8500 A, continuous coverage to 7500 A, 33 orders
Resolution: R = 22400, 5.4 pixels/arcsec
nominal R = 18700 (16 km s ) with 1.2 arcsec slit (6.5 pixel res element)
maximum R = 60790 (4.9 km s') with 0.37 arcsec slit (2 pixel res element)
Slit length: 8.4 arcsec (45 pixels) nominal
Order spacing: 6 pixels minimum interorder gap
2.3. Long Slit Mode
Gratings: 300 gr/mm, 600 gr/mm, 1200 gr/mm from Norris Spectrograph
Spectral resolution and range: &\ = 7.1 A over 4490 A with 300 gr/mm and 1.2 arcsec slit
zA = 1.7 A over 1070 A with 1200 gr/mm
Maximum slit length: 12.5 arcmin (300 mm at f/16)
Spatial scale: 0.19 arcsec/pixel
2.4. Multi-slit Mode
Spectral resolution and range: same as long slit above
Field size: 12.5 x 3 arcmin, 300 x 75 mm masks at f/16
Slit width: 1.5 arcsec = 0.6 mm on mask
3. BASIC LAYOUT
Figures 1(a) and 1(b) show top-view and side-view plans of Thucidides, depicting the system in its short-slit echelle
mode. Light at f/16 enters through the Cassegrain aperture and is reflected by feed mirror FM (optionally tip-tilt
guided) into the Cassegrain ring plane and brought to a focus at the slit focal plane SF. Reflective slitjaws feed
a video guider camera GC via a fold mirror (not shown), providing feedback to the tip-tilt system or indigenous
telescope guiding. The f/16 beam is then collimated by a 240 mm Nikon lens CL, bent by Ml and then reimaged
at f/3.3 onto an intermediate focal plane IF by a 50 mm Nikon lens RL. Additional masking of stray light can take
place at IF. After passing through IF, the light is reflected by a small fold mirror MS to the main collimator/camera
lens CCL and is collimated for entry into the disperser unit, which contains the 79.1 gr/mm R2 grating EG and
double-pass cross-dispersing prism DP (45° apex angle of LF5 glass). Exiting the disperser, the light is refocused by
CCL on to the detector plane, where the spectral image is formed on the detector CCD.
Using the collimator/camera in double pass in this fashion (a technique shared by many other echelles) affords
a number of advantages. Most notably, the small size of the reimaged field at the intermediate focus permits the
angle between disperser input and output beams (2'y in echelle nomenclature) to be very small, for smaller prism
size and improved efficiency, and the camera can be placed immediately following the disperser exit, before the
highly-dispersed output beam has grown to enormous proportions. The design is also relatively compact, since the
2-meter EFL of the collimator does not have to be physically realized, and will mount on the uppermost Cassegrain
ring plane, simplifying the mechanical structure of the device and keeping flexure to a minimum.
For long-slit or multi-slit mode, new reimaging and disperser packages, represented by the broken-line boxes, are
inserted into the system in place of the echelle reimager and grating/cross-disperser units. A larger feed mirror FM2
illuminates a long slit or slit mask at SF. The beam is reflected by M2 immediately after the slit plane and sent
to a large off-axis spherical collimating mirror CM of focal length 510 mm. Collimated beams from CM converge
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towards a pupil, where the lens system RL2 reimages them at f/3.3 onto IP. RL2 will have to be a custom optical
system (not yet designed) to handle the wide field requirements, and a field lens FL at IP will be necessary to
maintain telecentricity downstream of IF. The echelle disperser package will be exchanged for a low-order grating C,
including a flip-in mirror FIM for slit alignment imaging. We may borrow gratings from Palomar's multifiber Norris
spectrograph, mounted in one of the spare motorized grating mounts originally intended for the Double Spectrograph.
Other mode variations are possible. An image slicer can be placed immediately after the slit plane for increased
spectral resolution, since the pixel sampling permits effective slit widths as small as 0.4 arcsec. A coronagraphic mode
might be implemented by placing an occulting disk at SP, a Lyot stop at the pupil position in the small collimated
beam between CL and RL, and a slit at IP. With an interference filter just before RL2 and an alternative echelle
disperser package without the prism, a single-order long-slit echelle mode could also prove useful.
4. SUBSYSTEMS
4.1. Fast Tip-tilt Guiding on Slit
The Palomar telescopes are equipped with a standardized video guider system which will keep stars or other targets
centered on the slit against slow secular changes and pointing drift, but some advantage might be gained by also
including a separate fast-guiding subsystem just upstream of the spectrograph slit. As part of a separate project,
a team under J. K. McCarthy has placed a CCD device on a piezo-driven x-y stage, translating the chip in the
focal plane under closed-loop feedback from a guide star on a photodiode quad cell. Tests of this system at the
200-inch Cassegrain station indicate that fast guiding at 20 Hz will remove a significant amount of the global tip-tilt
component of the atmospheric seeing and wind shake, thereby improving the seeing disk FWHM by 20% under
typical conditions. If such a system were to be implemented by driving mirror FM via feedback from the slitjaw
image, the slit throughput could be improved by a significant amount, and the extent of the target image along the
slit length (the spatial axis) would be reduced as well, minimizing contamination of the sky background channels
and pixel readout noise. Such a system could be easily added after the instrument is first commisioned with a fixed
mirror at FM.
4.2. Collimator/Camera Lens
The lens system labelled CCL in Figure 1 is used in double-pass, as both the collimator and the spectrograph camera.
This six-element all-spherical design was adapted from the Epps camera for the Palomar Double Spectrograph Red
Camera upgrade, simplified and optimized for this application using the Code V raytracing package. The EFL of the
composite lens is 455 mm. Lens element parameters are listed in Table 1. Glass types are from the O'Hara catalog.
The calcium fluoride element (#3) is made the center element of a triplet in order to isolate it from atmospheric
humidity. The field flattener (#6) doubles as the CCD dewar window.
Table 1. Lens element parameters for the collimator/camera system.
—
center
thickness
element R1 (mm) R2(mm) (mm) glass
1 231.94 706.37 30.00 FPL 51
107.73 air
2 307.61 135.49 10.00 LAL 7
3 135.49 —212.37 70.00 CaF2
4 —212.37 —1651.08 10.00 LAL 12
244.64 air
5 225.75 —491.02 30.00 FPL 51
99.39 air
6 —179.48 186.05 5.00 Si02
20.00 air
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This lens system produces good spot sizes from collimated light over the entire (61 mm)2 area of the CCD for
most of the desired spectral range. Polychromatic spot sizes (down to A =4000 A) are 9 (RMS diameter) at chip
center and 21t at the corners. Raytraced spot diagrams for field center, edge, and corner are shown in Figure 2, and
the variation of center spot size as a function of wavelength is plotted in Figure 3. Performance degrades sharply
below 4000 A, unfortunately, but an additional CCL lens element and/or more sophisticated design revisions may
be able to remedy this weakness.
4.3. Grating and cross-disperser
A 79.1 gr/mm echelle grating, measuring 12 x 6 inches, is already in hand (currently in use with the East Arm Echelle),
and will serve as THuCIDIDES' primary dispersing element. The collimated beam diameter will completely fill but
not overfill the projected grating surface area, so as not to lose light of the edge off the grating. The echellogram
image format is well-matched to the available detector size. Thirty-three orders, from m =27 to m = 59, will fit
on the chip along the cross-dispersion axis, for a spectral coverage from 3800 A to 8500A. The redmost part of the
spectrum (A > 7500A) will suffer from some wavelength coverage gaps, as the length of the free spectral range for
orders m 29 exceeds the chip width, but key spectral lines such as the 0 7770 triplet will still be observable. By
tilting the grating, orders further to the red or to the blue can be selected, but the camera's chromatic performance
will deteriorate rapidly outside of the design range.
A Schott LF5 prism with apex angle 45° cross-disperses the echelle orders. We chose a prism rather than a
grating for this task in order to achieve greater system efficiency, and to have more regular spacing of echelle orders
on the detector. LF5 is an excellent glass for this purpose, as it combines low index, high dispersion, and good
throughput even in the blue. The cross-dispersion has been chosen to fit the entire wavelength range of interest on
to the detector, while still providing sufficient inter-order separation in the blue to prevent order overlap.
4.4. Detector
A 4096 x 4096 CCD with 15 pixels, manufactured by Lockheed-Martin Fairchild, will be used as the THuCIDIDES
detector. The device will be thinned and backside-illuminated for maximum quantum efficiency in the blue. The
small pixel size will oversample the normal slit width (1.2 arcseconds) by a factor of three, so for work at R 20000,
on-chip binning may be desired for lower read noise and faster readout. (Alternatively, for higher-resolution tasks
where some efficiency hit is acceptable, a narrow slit or image slicer may be employed.) A standard Palomar dewar
will probably house the detector, and as mentioned in the introduction, readout electronics and control systems are
yet to be decided upon.
5. ACKNOWLEDGEMENTS
Special thanks to J. K. McCarthy, for many patient tutorials on spectrograph design theory, as well as specific
guidance and assistance with this project.
6. REFERENCES
1. K. G. Libbrecht & M. Pen, "A Fiber-Fed Echelle Spectrograph for the Hale 5-rn Telescope" ,PASP 107, p. 62,
1995.
2. J. B. Oke & J. E. Gunn, "An Efficient Low- and Moderate-Resolution Spectrograph for the Hale Telescope",
PASP 94, p. 586, 1982.
3. S. A. Shectman, unpublished, 1988, but see Section 2 of A. McWilliarn, G. Preston, C. Sneden, & S. Shectrnan,
"A Spectroscopic Analysis of 33 of the Most Metal-Poor Stars" , A.J. 109, p. 2736, 1995.
4. J. K. McCarthy, "Echelle Spectroscopy of the Central Stars of Planetary Nebulae" , Ph.D. Thesis, Caltech, 1988.
5. J. K. McCarthy, et at., "The Sandiford 2.1-rn Cassegrain Echelle Spectrograph for McDonald Observatory: Optical
and Mechanical Design and Performance" , PASP 105, p. 881, 1993.
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Figure 1(a). THuCiDiDESCassegrain ring layout (top).
\
/
Figure 1(b). THuCIDIDES Cassegrain ring layout (side).
NN
CM
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680
Figure 2. Polychromatic spot sizes for CCL single pass.
o.oo,ioo field •:
o.00o5340 DG corner 4-
0.00.0.70
field
0.000,3.905 DG edge
0.00,0.00 field :
0.000,0.000 DG center
pixel0
0.00500 CM
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35
30
25
Figure 3. Chromatic performance of collimator/camera lens (CCL) system
681
4000 5000 6000 7000 8000
A
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THuCIDIDES: a high-efficiency multimode spectrograph design for the Hale Telescope

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THuCIDIDES: a high-efficiency multimode spectrograph design for the Hale Telescope

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