ARCES: an echelle spectrograph for the Astrophysical Research Consortium
(ARC) 3.5m telescope

Shu-i Wang[^1]^a,^, Roger H. Hildebrand[^2]^\*b,\ c,\ d^, Lewis M.
Hobbs^b^, Stephen J. Heimsath^a^, Gary A. Kelderhouse^a^, Robert F.
Loewenstein^b^, Solly Lucero^c^, Constance M. Rockosi^b,e^, Dale B.
Sandford^a^, Jeffrey L. Sundwall^a^, Julie A. Thorburn^a^, Donald G.
York^b,c^

^a^University of Chicago Engineering Center; ^b^Department of Astronomy
and Astrophysics, The University of Chicago; ^c^Enrico Fermi Institute,
The University of Chicago; ^d^Department of Physics, The University of
Chicago; ^e^Department of Astronomy, The University of Washington

# ABSTRACT

A new echelle spectrograph was commissioned in 1999 for the ARC 3.5
meter telescope. The key features of the instrument are that it has a
resolution of 9 km/sec, limited by the pixel size of the CCD; has no
moving parts behind the slit during observation; provides complete
spectral coverage from 3200A to 10000A, limited by the prism cross
disperser material on the blue side and by the CCD sensitivity on the
red side; provides blazeless spectra; achieves S/N\>3000; and is
remotely operable. The instrument is being used for studies of
abundances in stars and for a large survey of diffuse interstellar
bands.

# 1. OVERVIEW

The ARC Echelle Spectrograph, ARCES, is a high-resolution echelle
spectrograph designed as a facility instrument for the Apache Point
Observatory (APO) 3.5m Telescope. (APO is owned and operated by the
Astrophysical Research Consortium, ARC.) The spectrograph covers a
wavelength range of 320-1000 nm in a single exposure. The spectrum is
recorded in about 130 echelle orders. The separation between orders
ranges from about 9\" (9 arc-seconds) in the blue, to 5\" in the near
IR. The method of dispersion is an echelle grating, which is combined
with double-prism cross-dispersion in an in-plane arrangement. The large
echelle grating is fed with a 200mm diameter beam from the f/10
telescope and a matching, off-axis, paraboloidal (OAP) collimator,
which, when used with the f/2.7 Schmidt camera, permits a wide entrance
slit of 1.6\" on the sky. With reasonable seeing and careful focusing,
ARCES should have minimal light lost at the slit. The two, very large,
UBK7 prisms give relatively uniform spacing of the echelle orders and
high throughput. The resolution is about 9 km/sec using a SITe Tk2048E,
2048x2048 CCD array detector with 24 µm pixels. The spectrograph
characteristics are shown in Table 1. ARCES was commissioned in 1999^1^.
Since commissioning, ARCES has enabled the observations of many
scientific programs. Examples of the spectra may be seen in Snow et.al.
^2^.

# 2. OPTICS

ARCES is located at the f/10 Nasmyth 1 port of the ARC 3.5-m telescope.
The main spectrograph optics consist of an off-axis paraboloidal (OAP)
collimator, an echelle grating, and two cross-dispersing prisms, located
in the rough-vacuum upper tank, and an f/2.7 Schmidt camera with
achromatic correctors, located in the low-vacuum camera tank/cryostat.
Figure 1 shows the layout of the spectrograph optics. Light from the
telescope enters through the spectrograph aperture into the upper tank,
and a 45° mirror folds the beam onto the OAP collimator. The collimator
reflects a collimated beam of 200mm diameter to the echelle grating,
oriented in a conventional in-plane mounting. Two UBK7 prisms with 45°
apex angles are mounted at the \"elbow\" of the upper tank to separate
the echelle orders. The blue cutoff of the system sensitivity is
primarily determined by the UV transmission of these prisms. The top
view of

Table 1. ARCES system specification.

  ------------------- ---------------------------------------------------
  **SYSTEM**           

  Spectral Range      320 - 1000 nm

  Resolving Power     33,000 with 1.6\" slit, 24 µm pixels, or a velocity
                      resolution = 9 km/s

  Sensitivity         V = 16 yields 0.2 count/resolution element/sec

  Entrace Aperture    1.6\"x1.6\" or 1.6\"x3.2\", manual change

  Platform            ARC 3.5m Telescope Nasmyth Port without Focal Plane
                      Rotator

  Cooling Scheme      LN2 with autofiller

  Volume               

  Upper Tank          970 liters

  Camera Tank         210 liters

  Weight               

  Upper Tank          900 lbs

  Camera Tank         700 lbs

  **SUBSYSTEMS**       

  **TELESCOPE**        

  Type                Ritchey-Chretien

  Aperture            3.5 m

  Focal Ratio         f/10

  Platescale          5.86 arcsec/mm

  **CALIBRATION**      

  Calibration lamps   Thorium Argon (Th-Ar) hollow cathode lamp, Quartz
                      Tungsten Halogen (QTH) lamp

  **GUIDE CAMERA**     

  Camera System       Photometrics SenSys 1600 Camera

  Detector            KAF 1600, 1536x1024 array with 9x9µm pixels

  Platescale          0.05 arcsec/pixel, normally binned 3x3 yielding
                      0.151 arcsec/binned pixel

  FOV                 1.35 arcmin x 0.9 arcmin

  Operator Options    Manual, auto guide, auto trail

  **SPECTROGRAPH**    

  Magnification       0.27x

  Final Platescale    21.8 arcsec/mm = 0.52 arcsec/pixel, perpendicular
                      to the dispersion

  Collimator          f=2,032 mm, D=225mm (200mm beam size), f/9 Off-axis
                      Parabola, quartz substrate

  Echelle Grating     B&L, 204mm x 408mm, 31.6 grooves/mm, 64.6º blaze
                      angle, 69.5º incident angle

  Cross-disperser     two 45º UBK7 Prisms, minimum deviation; total
                      deviation through both = 52.7º

  Camera              Schmidt type with LLF2 (12.7mm thick) + UBK7
                      (19.05mm thick) doublet corrector

  Schmidt mirror      f=540mm, D=381mm, f/1.4 Zerodur mirror

  Field Flattener     UBK7 Plano-convex lens, f=839mm, D=75mm (matched to
                      CCD curvature)

  Detector             

  Type                SITe TK2048E back illuminated 2048x2048 CCD with 24
                      µm x 24 µm pixels

  Operating           160K (-110C)
  Temperature         

  Read Noise          7 e-

  Dark Current        7.35 e-/pixel/hour

  Full well           150,000 to 200,000 e-

  Gain                3.8 e-/DN

  Nominal curvature   1581 mm, convex @ 300K; 1535 mm convex @ 160K with
                      stiffener
  ------------------- ---------------------------------------------------

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Figure 1. ARCES mechanical layout on left, and raytrace on right.

Figure 1 is a projection in the plane of the echelle dispersion, and the
bottom view is in the plane of the cross-dispersion. The Schmidt camera
tank is mounted to the elbow, or spherical joint, of the upper tank. The
spherical joint allows rough adjustment of the cross-dispersed orders on
the CCD. The Schmidt corrector design is an airspaced doublet
configuration consisting of a LLF2 aspheric plate and an UBK7 aspheric
plate with their plane sides facing each other. The corrector plates
double as the vacuum window for the Schmidt camera. The f/2 spherical
camera mirror reflects the light to a SITe Tk2048E 2048x2048 CCD array
with 24 µm pixels located about halfway between the corrector plates and
the camera mirror. The spectrograph should also work well with a
2048x4096 pixel format CCD with 15 µm pixels. The SITe CCD has a room
temperature bow of about 1581mm convex curvature^3^. To reduce and
control the change in the bowing of the CCD upon cooling, a kovar
stiffener plate is attached to the back of the CCD. The bow becomes more
convex by about 10 µm when the CCD and stiffener are cold, giving us a
cold radius of 1535mm. A field flattener lens is mounted in front of the
CCD to match the curved field.

The mirrors in the upper tank and the Schmidt camera mirror were coated
with a bare aluminum coating. The fused silica entrance window for the
upper tank and the UBK7 CCD field flattener have been coated with a
wide-band AR coating by QSP Optical Technology Inc. (Santa Ana, CA). The
AR coating has an average reflectivity of 1% over the 320 -- 1100nm
spectral range, with a maximum reflectivity of 3%, and a maximum
absorption of 0.5%.

The spectrograph optics have been described in detail by Schroeder^4^.
The current system differs in that a small \"wedge\" prism, used in the
original design, no longer exists. The prism was inserted in different
apex positions to shift the orders perpendicular to the dispersion on an
800x800, 15 µm pixel CCD. The motion of the grating was originally
needed to shift the long, red orders parallel to the dispersion, but has
now been disabled. The 800x800 CCD has since been replaced by a larger
CCD.

# 3. MECHANICS

1.  **Mechanical Layout**

The spectrograph is mechanically separated into 2 sections: the upper
tank contains the collimator and dispersing elements in a low vacuum
(200 milliTorr, for thermal and pressure stability and cleanliness), and
the camera tank contains the camera optics and detector cryostat
components in a high vacuum. The camera tank is the enclosure and
mounting structure for the Schmidt camera optics. The entire 210 liters
volume of the tank also functions as the cryostat for the CCD, and so
contains the liquid nitrogen reservoirs for cooling the detector and a
cold shield, as well as the CCD preamplifier and clock voltage
conditioning electronics. The CCD is mounted at the focus of the Schmidt
Camera, not kept in a separate dewar. This setup permits faster camera
optics but makes access to the CCD a time consuming process. An enclosed
front-end is attached to the entrance port of the spectrograph upper
tank. The front-end contains the spectrograph aperture, guide camera,
and calibration subassemblies. Section 5 details the front-end elements.

The overall design makes the spectrograph extremely stable. The
stationary Nasmyth telescope mount, with kinematic x, y and tip/tilt
adjustments, eliminates flexture with on-sky positions. A \"foot\"
support under the camera tank located at its center of gravity further
enhances stability. The CCD is held fixed with respect to the Schmidt
mirror with an invar frame, so the camera focus is not subject to
short-term drifts. The location of the star/object and quartz
calibration orders drift through the night by 0.5 pixels over 10 hours,
but return to the same position the next night. The drift is most likely
due to the thermal changes of the prisms with temperature. The stability
in the grating dispersion direction is excellent. The shift is
systematic through the night and amounts to 0.35 pixels in 10 hours, or
0.18 pixels (0.7 km/sec) per 5 hours. While this drift is twice our
design goal of 0.1 pixels in 5 hours, it occurs mostly in the first half
of the night. In the second half of the night, the drift is less than
0.3 km/sec/5 hours. As a practical matter, a 5 hour exposure would not
be taken on a bright source, and the drifts in the first half can be
easily calibrated out by interpolating between calibrations made every
hour. In the second half of the night, even a 5 hour integration could
be done.

**3.2 Camera Cryostat**

The function of the cryogenic system for the ARCES camera is to hold the
CCD at its operating temperature of 160K; the rest of the optical and
mechanical components in the tank remain at room temperature. The
detector is cooled by attaching the CCD package to a copper block, which
is connected through copper cold straps to a liquid nitrogen reservoir
in the camera tank. The operating temperature of the detector is then
regulated to better than 1˚ by controlling the current in a 40Ω resistor
in the cold block. The CCD is mechanically mounted in a Kel-F fixture,
which is in turn suspended at the focus of the Schmidt camera by a
4-vaned spider assembly; the vanes for the spider were made out of a
glass filled polyimide (Kapton) sheet. The low thermal conductivity of
Kel-F and small physical cross-section of the spider vanes and mounting
fixture where it touches the cold CCD package thermally isolates the
160K CCD from the rest of the room-temperature camera tank. The spider
vanes are mounted to two concentric rings for remote CCD tip/tilt
adjustments. The rings are mounted to the camera mirror assembly via two
precision screws that allow remote CCD focus adjustments. The maximum
obscuration ratio of the Schmidt camera as a result of the size of the
CCD mount and the CCD tip/tilt/focus assembly is 0.2.

Table 2 shows the factors involved in controlling the CCD temperature.
The dimensions of the cold straps are chosen so that a heater input of
\~ 2.5 watts (in addition to radiation from the room temperature walls
of the cryostat) is sufficient to balance the heat flow through the cold
straps to the LN2 reservoir when the CCD is at the operating
temperature. The joints in the cold straps are constructed of thin
copper and silver straps folded in such a way as to permit nearly
torque-free tip, tilt, and focus adjustments to the CCD. Two screw
joints must be disconnected whenever the CCD block is removed.
Cryogenic-application grease mixed with aluminum nitride powder is used
at the joints to ensure good thermal contact.

Table 2. Temperature control.

+-------------------------------------------------------+--------------+
| Acceptable operating range for CCD                    | 160K ± 10K   |
+-------------------------------------------------------+--------------+
| Equilibrium CCD temperature (no regulating current)   | 136K         |
+-------------------------------------------------------+--------------+
| Temperature variations (regulated)                    | \< 0.1K      |
+-------------------------------------------------------+--------------+
| Power into control heater                             | \~ 2.5 watts |
+-------------------------------------------------------+--------------+
| CCD Cold Straps                                       |              |
|                                                       |              |
| Parallel straps                                       |              |
|                                                       |              |
| one silver, 0.010\" X 2\" X 6\"                       |              |
|                                                       |              |
| two high-purity oxygen-free copper, 0.005\" X 2\" X   |              |
| 6\"                                                   |              |
|                                                       |              |
| Each strap in two sections: one 4\" & one 2\"         |              |
+-------------------------------------------------------+--------------+

In principle, the cryogenic system would require no other components. In
practice, as explained in the next paragraph, the large room-temperature
interior surface area of the camera tank surrounding the cold CCD
necessitates a second LN2 reservoir and an ion pump to overcome the
effects of outgassing. The interior surface area of the camera tank is
\~ 2 square meters and must be kept at room temperature because of the
camera tank\'s structural role in supporting the Schmidt camera optics.
We have found that despite adsorptive pumping by charcoal getters and
efforts at the use of good vacuum practice, outgassing from this large
interior surface area will slowly freeze out on the cold surfaces. The
first indication of frosting is an increase in the unregulated
equilibrium temperature of the CCD due to a decrease in the emissivity.
On careful inspection frost became visible on all cold surfaces after a
day or two.

The solution to this dilemma that we settled on was to place an ion pump
in the camera tank that pumps continuously on the tank while the CCD is
cold. We also added a second LN2 reservoir of larger capacity, to cool a
gold-plated, copper, cold shield and a plate of charcoal getter mounted
to the reservoir. The getter consists of a 0.06\" oxygen-free, copper
plate of 10\" diameter. The entire getter surface is covered with
coconut charcoal (Calgon) epoxied on with Eccobond 286. The cold shield
sits inside the camera just outside the beam path to trap stray gas
molecules. This shield is colder and much larger (\~ 0.14 square meter
per side) than the CCD and is cooled for at least two hours before
filling the CCD LN2 reservoir. The two reservoirs are sized so that if
the LN2 supply is cut off, the CCD reservoir empties first and the
reservoir that cools the shield and getter will stay full long enough
that the detector is at or near room temperature before the getter warms
up and releases its accumulated gas into the tank. Table 3 shows
procedures for the cooling and warming cycles such that no frost forms
on the CCD. A sensor system is in place to alert the local staff in case
an unscheduled warm up of the CCD occurs, so the status of the two
reservoirs can be checked and monitored.

The LN2 reservoirs are automatically refilled every 4 hours with an
autofill system^3^ which connects the reservoirs to an LN2 transfer
dewar via vacuum-jacketed lines. The transfer dewar is an 80-liter,
Cryofab dewar, which is refilled about every two days by the observatory
daystaff. Automatic refills of the reservoirs while observing do not
adversely effect the wavelength stability of the instrument.

Table 3. Cooling and warming cycles.

+---------------------------------------+--------------------+--------+
| **Cool Down Procedure**               | ## Pressure        | #      |
|                                       |                    | # Time |
+---------------------------------------+--------------------+--------+
| 1\. Start pump (16 cfm dry scroll     | 1 atm → 5x10^-5^   | \~ 12  |
| pump; 300 l/s turbo pump).            | torr               | hrs    |
+---------------------------------------+--------------------+--------+
| 2\. Leak check dewar.                 |                    |        |
+---------------------------------------+--------------------+--------+
| 3\. Fill shield reservoir^a^.         | 5x10^-5^ torr →    |        |
|                                       | 5x10^-7^ torr      |        |
+---------------------------------------+--------------------+--------+
| 4\. Start ion pump (300 l/s diode     | 5x10^-6^ torr →    |        |
| type ion pump).                       | 1x10^-7^ torr      |        |
+---------------------------------------+--------------------+--------+
| 5\. fill CCD reservoir^b^. Read out   | 1x10^-7^ torr →    | 2 -- 3 |
| CCD while cooling to keep thermally   | \<1x10^-7^ torr    | hrs    |
| generated electrons from freezing     |                    | after  |
| into traps.                           |                    | step 3 |
+---------------------------------------+--------------------+--------+
| 6\. Reach CCD operating temperature.  | 1x10^-7^ torr      | 2 -- 3 |
|                                       |                    | hrs    |
|                                       |                    | after  |
|                                       |                    | step 5 |
+---------------------------------------+--------------------+--------+
| 7\. Enable the autofill system to     |                    |        |
| automatically refill reservoirs.      |                    |        |
+---------------------------------------+--------------------+--------+
| **Warm up Procedures**                |                    |        |
+---------------------------------------+--------------------+--------+
| 1\. Let CCD reservoir go empty and    |                    | ≤ 8    |
| let CCD warm up to 0C, keeping shield |                    | hrs    |
| reservoir cold^c^.                    |                    |        |
+---------------------------------------+--------------------+--------+
| 2\. Turn off ion pump.                |                    |        |
+---------------------------------------+--------------------+--------+
| 3\. Let shield reservoir go empty^c^. |                    |        |
+---------------------------------------+--------------------+--------+
| 4\. Let shield and shield reservoir   |                    | \~ 12  |
| warm up.                              |                    | hrs    |
+---------------------------------------+--------------------+--------+

^a^Shield reservoir: capacity = 15 liters; hold time = 22.5 hrs. A
coconut charcoal getter is in thermal contact with bottom of shield
reservoir.

^b^CCD reservoir: capacity = 3 liters; hold time = 8 hrs.

^c^Because shield reservoir has larger capacity, it will automatically
stay cold for \> 14 hrs after the CCD reservoir is empty. Process can be
speeded up by backfilling the camera tank with dry nitrogen.

# 4. DATA AND ELECTRONICS

The CCD electronics are slightly-modified versions of the camera
electronics for the Sloan Digital Sky Survey (SDSS) Photometric
Camera^3^. The conceptual layout for the electronics is shown in Figure
2. A Macintosh computer functioning as the Instrument Control Computer
(ICC) resides inside the observatory computer room. It communicates with
the CCD controller saddlebag and the Motor Control Computer (MCC) via
fiber links. The saddlebag, mounted to the camera tank, contains an
analog circuit board, which provides the CCD voltages and does the
temperature regulation, processing and digitization of the pixel data,
and a Forth microcontroller which communicates with the ICC. The Forth
microcontroller provides the CCD clocking signals and runs the autofill
system. Attached to the saddlebag is another box which contains the
fiber converters for the CCD data and the shutter controller. The MCC
resides at an intermediate level rack in the telescope dome, along with
the UPS and calibration lamp power supplies. The MCC controls the motor
driver electronics located in a Pit Rack next to the spectrograph
camera. Motorized parts include the CCD tip/tilt/focus, filters for the
guide camera and calibration lamps, the calibration flip mirror, and the
spectrograph aperture focus. Only the calibration flip mirror and the
calibration filters are used routinely. A separate Macintosh computer
resides inside the computer room for controlling the guide camera. It
communicates with the guide camera with a fiber SCSI link. The ARCES ICC
is connected to the Observatory Control Computer, which passes commands
and status information between the instruments and the remote observing
stations^5^. Details of the software user interface and helpful hints
for observing with the Echelle spectrograph can be found at:
<http://astro.uchicago.edu/echelle/>.

**5.** **ECHELLE SPECTROGRAPH FRONT-END**

**5.1 Overview**

The Echelle Front-end is a box located between the telescope port and
the Echelle Spectrograph Upper Tank. The Front-end attaches to the Upper
Tank entrance window assembly, and holds the spectrograph aperture,
guide camera and calibration optics subassemblies. The subassemblies are
mounted on a focus slide such that the subassemblies can be moved along
the telescope optical axis as a unit. This arrangement allows focusing
of the aperture during alignment.

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Figure 2. Echelle electronics layout.

**5.2 Spectrograph Aperture**

The spectrograph aperture is mounted on a light-tight fixture located in
the middle of the front-end focus slide. The front side of the aperture
box holds the spectrograph entrance aperture. A baffle made of a large
bellows extends from the back of the aperture box to the shutter. The
shutter is mounted on the entrance window port of the upper tank. Two
apertures are available: 1.6\"x1.6\", 1.6\"x3.2\". The larger aperture
is normally preferred, as it does not limit operations in good seeing
and it allows more light through when seeing is poor. Switching between
apertures is a manual operation. Each aperture is mounted in an aperture
holder assembly which mounts to the aperture box with 2 screws.
Alignment pins on the aperture box are used to align the aperture holder
assembly with respect to the spectrograph dispersion axes.

**5.3 Guider**

The spectrograph aperture is made from a reflecting substrate. The
aperture is tilted at an angle of 15 degrees. A fold mirror sends light
reflected from the spectrograph aperture onto a slit viewer camera
subassembly. A Nikon camera lens (105mm, f/2.8) is used to relay a 1:1
image of the aperture onto the CCD camera. A 6-position filter wheel is
located at the pupil image plane between the lens and the camera. The
six filters currently installed in the filter wheel are ND 0.04
(equivalent to open position, filter used for path length compensation),
ND 1, ND 2, ND 3, red (Schott RG665, 2mm thick), and blue (Schott BG12
(1mm) + BG39 (1mm)).

The current guide camera is a Photometrics (Roper Scientific) SenSys
1600 Camera. The camera uses a KAF 1600, 1536x1024 CCD with 9x9µm
pixels. The field of view (FOV) of the CCD is 1.35 arcmin x 0.9 arcmin.
The apertures are on 1.24 arcmin diameter substrates. The slit viewer
camera sees a FOV larger than the reflective aperture substrate in the
prism dispersion direction, and a FOV smaller than the substrate in the
grating dispersion direction. During "normal" operation, the CCD camera
is usually binned 3x3 to give 0.15\"x0.15\" pixels. The guider CCD is
free of dead areas. However, the CCD is only cooled to +10C, and the
guider is therefore limited by hot pixels and dark current to viewing a
15th magnitude object without dark subtraction. With dark subtraction of
an image of the same integration time (a software option used with faint
objects), the guider can view the wings of a 16th to 17th magnitude
object on the slit with a two-minute integration. We plan to replace the
Guide Camera with a cooled unit during the year 2002.

The user interface (REMARK) allows the operator to guide manually, in
0.1 arcsecond minimum steps; to use an autoguider that uses the
reflected light from the aperture substrate to implement a quadrant
balance of the wings of the image of the guide star; and to auto trail
along the long dimensions of the aperture.

**5.4 Calibration**

Two calibration lamps are available: a quartz-tungsten-halogen (QTH)
\"white\" lamp and a thorium-argon hollow cathode lamp. The lamps are
mounted to a 2\" integrating sphere located on the spectrograph
front-end. The integrating sphere has three 0.5\" input ports, and a
0.5\" exit port. Light exiting the integrating sphere is transmitted
through an aperture stop with a central obscuration to simulate the f/10
beam from the telescope. An achromatic doublet images the exit port of
the integrating sphere onto the spectrograph aperture, and forms a
virtual image of the aperture stop of about the same size and distance
as the telescope pupil. A fold mirror is flipped into the beam path
during calibration to direct the calibration beam onto the spectrograph
aperture. The fold mirror is flipped out of the beam path during
telescope observations. This mirror and the two filter wheels are the
only moving parts used during normal operations. Aperture focus,
collimator focus, and CCD focus are checked, but have not required any
changes for three years.

The QTH lamp has a 6-position filter wheel between the lamp assembly and
the integrating sphere. Position 2 of the filter wheel currently has a
blue filter identical to the guide camera blue filter (Schott BG12
(1mm) + BG39 (1mm)). The blue filter is useful for obtaining the blue
end of the spectrum since the red end tends to saturate the spectrum,
unless it is attenuated by the noted filter combination.

**6. DATA ANALYSIS**

The ARCES is capable of complete wavelength coverage from 3200Å to
10,000Å in 130 spectral orders. A typical extraction includes about 107
orders. Shorter wavelengths can be observed in very blue objects. Figure
3 shows plots of an ARCES spectrum of HD 183143. HD 183143 is a B7Ib
star with E (B-V) = 1.4 and V=6.2. Interstellar atomic and molecular
lines, diffuse interstellar bands, stellar lines, and telluric
absorption lines are visible in the spectrum. Note the strong diffuse
bands at λ = 4430 Å, 5780 Å, and 6284Å.

Due to the exceptional number of recorded orders and the relatively
large size of the detector pixels, the ARCES poses a data reduction
challenge for standard data reduction packages. Reduction of ARCES data
is performed using the echelle package in the National Optical Astronomy
Observatory's (NOAO) Image Reduction and Analysis Facility (IRAF).
Because certain ARCES data characteristics vary significantly from a
typical echelle spectrograph, data reduction procedures are slightly
modified from the conventional methods. Aliasing is easily introduced by
IRAF routines, which do not perform fractional pixel weighting. A paper
detailing the data reduction techniques is in preparation^6^. A data
reduction guide to aide the user in using IRAF routines to extract
maximum information from ARCES data can be found at:
<http://www.apo.nmsu.edu/Instruments/echelle/ARCES_data_guide.pdf>.

Flat fielding in particular requires different methods than are most
often used in IRAF. The first difficulty arises from limited slit length
in the ARCES, which causes QTH lamp orders to be approximately the same
width as those in a typical object exposure. Forming a two-dimensional
quotient of two similar width orders leads to improperly weighted sums
and noise amplification unless aperture mask profiles are independently
imposed upon the data. For this reason, two-dimensional flat fielding is
not recommended for ARCES. More detrimental to the data integrity
however are the problems inherent in dividing one narrow order by
another: the aliasing present in a narrow-order ARCES exposure
(discussed below) is greatly increased in severity when a quotient is
formed. To avoid these difficulties and in lieu of

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Figure 3. ARCES Spectrum of HD 183143 from 3700Å to 9700 Å, assembled
with 93 echelle orders.

two-dimensional flat fields, one-dimensional flat fielding is performed
after spectral extraction and before assignment of a dispersion
function. This technique permits signal-to-noise ratios of up to 3000 in
summed ARCES data.

ARCES data extraction itself is complicated by mild aliasing in the
prism dispersion direction. Originally designed for a detector for
smaller pixels, the ARCES has narrow (full width at half maximum \~ 3
pixels) and closely spaced orders which, as is the case in all echelle
spectrographs, do not run perpendicular to the isowavelength contours,
or the grooves of the echelle grating. Prismatic cross-dispersion
further exacerbates aliasing in minimally sampled ARCES data by
introducing order curvature which varies as λ^-3^. If ARCES data are
extracted in the conventional manner without special measures to
minimize the effects of aliasing, particularly in high-signal data, a
semi-regular "ripple" pattern is observed in the extracted spectra.
Similar effects are well documented in other instruments with minimally
sampled spectral orders (e.g. the Space Telescope Imaging Spectrograph
(STIS) as documented in Leitherer, C., et al. 2001, \`\`STIS Instrument
Handbook\'\', Volume 5.1, (Baltimore: STScI)). Noise due to
undersampling is reduced in STIS by dithering the source light along the
length of the entrance aperture. Similarly, ARCES observers pursuing
high signal-to-noise spectra are encouraged to trail objects along the
slit length when possible. Despite these precautions, generic echelle
spectral extraction using the \'apall\' task in IRAF will cause aliasing
ripples of moderate amplitude (\~0.8%) and low, slowly varying frequency
to appear in extracted orders. Although \'apall\' does include
provisions for fractional pixel weighting in the wings of aperture
masks, it does not do so in areas of high signal and thus does not
retrieve maximum information from ARCES data.

To minimize the deleterious effects of line spread function
undersampling, individual ARCES object spectra are increased in scale by
a factor of four in the prism dispersion direction: rather than 2048
pixels, spectra are linearly interpolated to 8192 pseudopixels in the
reduction process. Aperture traces are then calculated in the
conventional way and used as a guide for \`apscatter\' to remove
scattered and background light in the expanded scale images. Final
spectral extraction is then performed on the interpolated, background
subtracted data. A similar strategy is employed to convert summed QTH
flat field exposures to a one-dimensional format. Extracted object
spectra are then divided by the one-dimensional flat field and then
normalized. Wavelength calibration is derived from two-dimensional
Chebyshev polynomial fits to arc line positions in extracted
thorium-argon lamp exposures. Typically, a fourth order fit in both the
grating dispersion and prism dispersion directions is sufficient to
remove residuals and leave rms scatter of 0.025 A. After dispersion
correction, individual echelle orders are weighted and summed in
wavelength space to generate a single "blazeless" spectrum covering the
entire spectral range of ARCES.

7\. **COMMISSIONING**

ARCES was taken through a series of tests that constitute science
commissioning during the first quarter of 1999. Several stars were
observed with well known interstellar spectra. The data showed excellent
agreement with observations using other instruments. The resolution, as
measured by the FWHM of calibration lamp lines, is uniform over the
field. For single line measurements, the FWHM is 2.2 pixels, with a 2σ
error of 0.16 pixels, over the entire spectrum. This performance
translates to a spectral resolution of about 33,000, or a velocity
resolution of 9 km/s. The wavelength scale of ARCES can be determined at
any one time to very high precision, using the Th-Ar lamp or using the
telluric lines near 700nm and the dispersion constants of the echelle.
This last technique gives a scale with an accuracy of 0.1 pixel. The
read noise for the CCD and the electronics was measured to be 7
electrons 1 sigma, with a gain setting of 3.8 electrons per data number.
The dark current was measured to be 7.35 electrons per pixel per hour
averaged over the CCD. This level of dark current, 0.002 electrons per
second, or 29 electrons per pixel in 4 hours, contributes less than read
noise for practical exposure times with ARCES. This value of dark counts
seems to be high by a factor of 30 compared to lab results on the same
device. Although the dark current is not a factor currently, it does
have a dominant effect on projections of S/N ratios in future upgrades
planned to allow observations of fainter objects with a lower read noise
CCD. We plan on reworking the CCD cold straps in the future to run a new
CCD colder.

We checked the spectrograph\'s ability to obtain absolute velocities by
taking a few radial velocity standards at different positions in
altitude and azimuth, ranging over the entire sky. One order near 496nm
was cross correlated with a daytime sky spectrum, assumed to have 0
radial velocity. Corrections for the motion of the Earth and the Sun
were made for each stellar spectrum obtained. Otherwise, no corrections
were made. There is a 3.6 km/sec shift that is systematic, but with that
correction, the rms scatter from a linear regression is 1.1 km/sec.
Similar results were obtained for the few other orders tried. For
precise work, the zero point and the dispersion in data points can be
improved upon.

Stray light and crosstalk may appear between the orders due to
scattering out of the orders, ghosts from intensely illuminated parts of
the chip, light generated internal to the spectrograph, or from light
entering the spectrograph through light leaks. Various light leaks were
found and plugged during commissioning, and we are not aware of any
internal sources of light remaining within the spectrograph. Figure 4 is
a stacked plot depicting two adjacent echelle orders in the vicinity of
a strong telluric absorption complex. The nearby order shows no apparent
sign of cross-contamination by its neighbor. The background subtraction
scheme used functional fit to interorder light. The 2-d functions used
to approximate the interorder light pattern are smoothly varying in the
grating dispersion direction and they cannot introduce or remove high
frequency events in this axis. This performance is further evidence that
the actual crosstalk between orders is indeed very low.

![](media/image8.png){width="5.375in" height="3.7284722222222224in"}

Figure 4. Stacked plot of HD 183143 depicting two adjacent echelle
orders in the vicinity of a strong telluric absorption complex. The
telluric lines in one order produce effects in the adjacent order of \<
1.0 mÅ, 5σ.

Cosmic rays appear as sharp, bright spikes in the ARCES data during long
exposures. The number of pixels affected is about 1 in 75 in one hour.
When binning 2x2, the number of affected pixels would be 1 in 20. This
number of radiation hits reduces the advantage of binning for increasing
the signal to noise ratio in long exposures. Clearly, for such long
exposures, median filtering of multiple long exposures is sound practice
for identifying cosmic rays. There does not seem to be an excess of
radiation attributable to the echelle spectrograph itself (i.e.,
radioactivity from glass or metal.), because instruments using
completely different configurations of CCDs at APO show the same
relative effects. Cosmic rays provide the current limit on the length of
exposure time. Replacing the current CCD with a CCD of smaller pixels
would help, since most of the events affect only a single pixel.

About 2% of the photons entering the atmosphere for an APO echelle
target reach the computer disk as data numbers on a good night.
Estimates of the telescope efficiency alone suggest that the peak
efficiency of the spectrograph is 8%±2%, against a goal of 9%. Exposure
times can be estimated by assuming that a signal-to-noise ratio of 100
per resolution element (two pixels) is reached in one hours on a star of
magnitude V=13, in good seeing with no trailing, for the region of peak
sensitivity (600 - 680 nm). The detected number of photons decreases
continuously to the red and the blue of the peak, dropping by a factor
of two by 820nm and 460nm, respectively, for an unreddened FO sub-dwarf.

The bright object limit is set by saturation of the CCD at the peak
count rate (between 600 -- 680 nm for most sources). Assuming a shortest
exposure of 1 second, for practical reasons, a 0 magnitude object would
just be observable on a good night, without saturating. The faint limit
is set by the performance of the guider, the read noise of the chip, the
rate of radiation hits and the mechanical stability of the spectrograph.
In practice, because of the need to use short exposures to permit median
filtering to remove cosmic rays, the limiting magnitude is V=15. With a
guider that permits observing an 18^th^ magnitude object and
installation of a new CCD with 2 electrons read noise and negligible
dark current, the limiting magnitude of 18 in 20 hours (S/N=10, per
resolution element) would be set by photon noise.

# 8. CONCLUSION

ARCES has been shown to be a stable facility instrument enabling many
science programs. The resolving power is limited to 33,000 by the large
(24 µm) pixel size. Analysis indicates that with 15 µm pixels, the
resolving power would be limited at 54,000 by zonal errors on the
aspheric corrector plates. The combined telescope/spectrograph system
efficiency is \>2.2% at 647nm. With assumptions about the telescope
mirror reflectivity, we estimate the spectrograph efficiency as 8%±2% at
630nm. The throughput can be further improved by applying wide-band
anti-reflection (AR) coatings to the prisms and corrector plates. The
spectrograph bright limit is 0 magnitude V at 630 nm. The radiation hits
are 0.013 /pixel-hour, severely limiting the time length of a single
exposure. The faint limit is 16.5 V, S/N \> 20 (the S/N refers to one
resolution element, or two pixels) in 20 hours, limited by cosmic rays
and the repeated read noise from ½ hour integrations. The faint limit
can improve to 18^th^ magnitude with a cooled guider sensor, and an
improved CCD with lower read noise and with smaller pixels, to decrease
the effects of cosmic rays. The CCD uniformity is good enough that no
flat fielding is needed for S/N up to 500 at wavelengths less than
700nm. At longer wavelengths, fringing fluctuations may be as high as
20%. The spectrograph is mechanically stable to shifts of 0.2 pixel in
wavelength per 5 hours. Only seasonal checks of the spectrograph
internal focus are needed.

# 9. REFERENCES

1\. Based on observations obtained with the Apache Point Observatory
3.5-meter telescope, which is owned and operated by the Astrophysical
Research Consortium.

2\. T. P. Snow, D. E. Welty, J. Thorburn, L. M. Hobbs, T. Oka, B. J.
McCall, D. G. York, and P. Sonnentrucker, \"Unusually Weak Diffuse
Interstellar Bands toward HD 62542\", Astrophysical Journal, 573, p.
670, 2002.

3\. J. E. Gunn et. al., \"The Sloan Digital Sky Survey Photometric
Camera\", AJ, 116, pp. 3040-3081, 1998.

4\. Daniel J. Schroeder, \"Optical Design of the ARC Echelle
Spectrometer\", *Instrumentation for Ground-Based Optical Astronomy,
Present and Future*, Editor L. B. Robinson, The Ninth Santa Cruz
Workshop in Astronomy and Astrophysics, pp. 39-46, Springer-Verlag, New
York, 1988.

5\. R. F. Loewenstein and D. G. York, \"Instrument control and user
interface for the ARC 3.5m observatory\", *Instrumentation in Astronomy
VI*, Editor David L. Crawford, SPIE Vol. 627, pp. 162-166, Tucson, 1986.

6\. Julie A. Thorburn et al., manuscript in preparation.

# 10. AKNOWLEDGEMENTS

We gratefully acknowledge the work of many people in making this a
working facility instrument: Al Harper, Doug Duncan, Bob Pernic, Bob
Hirsch, and Ellen LaRue, of the University of Chicago; Mark Klaene,
Craig Loomis, Russet McMillan, Dave Woods, Karen Loomis, Jon Davis,
Camron Hastings, Gretchen Van Doren, and Bruce Gillespie of the Apache
Point Observatory; Daniel Schroeder (Beloit College), Sean Callahan
(University of Arizona), Thomas McMahon (University of Arizona), Fred
Mrozek (Recon Optical), and Paul Gettings (University of Utah).
Financial support was provided by the Astrophysical Research Consortium
and by the University of Chicago.

[^1]: ^\*^ Current Affiliation: <swang@fltvis.com>; phone 1 630
    466-4343; fax 1 630 466-4358; <http://home.fnal.gov/~swang/>; Flight
    Systems, Inc.,43W752 Route 30, Sugar Grove, IL 60554-0250

[^2]: ^\*\*^ <roger@oddjob.uchicago.edu>; phone 1 773 702-7581; fax 1
    773 702-8212; University of Chicago, Department of Astronomy and
    Astrophysics, 5640 South Ellis Avenue, Chicago IL 60637