GROND - a 7-channel imager

0.94 MB·

by  J. Greiner

GROND - a 7-channel imager
J. Greiner, W. Bornemann, C. Clemens, M. Deuter, G. Hasinger, M. Honsberg, H. Huber,
S. Huber, M. Krauss1 T. Kru¨hler, A. Ku¨pcu¨ Yolda¸s, H. Mayer-Hasselwander, B. Mican, N.
Primak, F. Schrey, I. Steiner2, G. Szokoly3, C.C. Th¨one4, A. Yolda¸s
8
Max-Planck-Institut fu¨r extraterrestrische Physik, 85740 Garching, Germany
0
0 jcg, wab, cclemens, grh, hhuber, shuber, kruehler, ayoldas, H-M-H, b.mican, prima, fzs,
2 [email protected]

n and
a
J

1 S. Klose, U. Laux, J. Winkler
3 Thu¨ringer Landessternwarte, Sternwarte 5, 07778 Tautenburg, Germany

laux, klose, [email protected]
]
h
p
-
o
r
t ABSTRACT
s
a
We describe the construction of GROND, a 7-channel imager, primarily designed for rapid
[
observationsof gamma-rayburstafterglows. It allows simultaneous imagingin the Sloang′r′i′z′

1 and near-infrared JHK bands. GROND was commissioned at the MPI/ESO 2.2m telescope at
v
La Silla (Chile) in April 2007, and first results of its performance and calibration are presented.
1
0 Subject headings: instrumentation: detectors, techniques: photometric
8
4
1. Introduction ate between physical state changes and changes
.
1 induced by geometrical variations, like eclipses;
0 Simultaneous imaging in different filter-bands
(2) monitoring of AGN to understand the phys-
8 is of interest in a variety of astrophysical areas.
ical origin of the observed variability; (3) deter-
0
The primary aim is to measure the spectral en-
: mining the inclination of X-ray heated binaries
v ergy distribution or its evolution in variable ob-
(Orosz & Bailyn 1997) (4) mapping of galaxiesto
i jects in order to uncover the underlying emis-
X study the stellar population; (5) multi-color light
sion mechanism. Examples are, among others,
r curves of supernovae (Taubenberger etal. 2006);
a (1) monitoring of all kinds of variable stars (flare (6) differentiate achromatic microlensing events
stars,cataclysmicvariables,X-raybinaries)tode-
(Paczynski 1986) from other variables with sim-
termine the outburst mechanisms and differenti-
ilar light curves; (7) identifying objects with pe-
culiar spectral energy distributions, e.g. photo-
1present address: ATV Technologie GmbH, D-85591
metric redshift surveys for high-z active galactic
Vaterstetten, Johann-Sebastian-Bach-Str. 38; email:
[email protected] nuclei, or identifying brown dwarfs; (8) follow-
2present address: Ruhr-Universita¨t Bochum, As- up observations of transiting extrasolar planets
tronomischesInstitut,Universita¨tsstr. 150,44780Bochum; (Jha et al. 2000); or (9) mapping of reflectance
email: [email protected] of solar system bodies as a function of their ro-
3present address: Institute of Physics, Eo¨tvo¨s Univer-
tation to map their surface chemical composition
sity, Pa´zma´ny P. s. 1/A, 1117 Budapest, Hungary; sz-
(Jewitt 2002).
[email protected]
4present address: Dark Cosmology Center, Niels Bohr A new need for multi-band imaging arose with
Institute, Univ. of Copenhagen, Juliane Maries Vej 30, the observation of a large number of gamma-ray
DK-2100Kobenhavn; [email protected]
burst (GRB) afterglows with the Swift satellite
1
(Gehrels etal. 2004). With its much more sen- infrastructure which were implemented to use
sitive instruments it detects GRBs over a very GROND for rapid follow-up observations (§5).
wide redshift range. Since intermediate to high-
resolution spectroscopy to measure the physi- 2. Instrument Design
cal conditions of the burst environment (e.g.
2.1. Scientific requirements
Vreeswijk et al. 2007) is constrained to the first
few hours after a GRB explosion, a rapid deter-
TheprimarygoalofidentifyingGRBafterglows
mination of the redshift became important. This
andmeasuringtheirredshiftledtotheconceptofa
is best done with multi-band photometry (until
camerawhichallowsobservationsinseveralfilters
integral field units have grown to several arcmin
throughouttheopticalandnear-infraredregionat
field-of-views)andderivingaphotometricredshift
thesametime. Thesimultaneityisdictatedbythe
basedonthe Lyα break(Lamb & Reichart 2000).
fact that a typical GRB afterglow fades by about
Previousandcurrentinstrumentswithsimulta- 2–3magbetween5–10minaftertheGRB,andby
neous imaging capability in different filter bands another 4 mag in the following 50 min (e.g. Kann
include ANDICAM (A Novel Double-Imaging etal. 2008). Rapidly determining the photomet-
CAMera; two channels, one for visual, the other ric redshift of GRBs at z >5 requires multi-band
for near-infrared (Depoy 1998), presently oper- photometryinatleast3–4bands. Inordertohave
ated at a 1.3m telescope), BUSCA (Bonn Uni- a stable lever arm to determine (i) the intrinsic
versity Simultaneous CAmera; four visual chan- power law slope of the continuum emission and
nels (Reif et al. 1999), presently operated at the (ii) the galactic foreground as well as GRB host
2.2m telescope at Calar Alto), HIPO (High- intrinsic extinction, near-infrared bands up to K
speed Imaging optical Photometer for Occulta- are essential. Extending the wavelength coverage
tions; two visual channels (Dunham et al. 2004), beyond K by including, e.g. an L channel, was
to be operated on SOFIA, the Stratospheric Ob- seriously considered during the design phase, but
servatory For Infrared Astronomy), MITSuME both the prospects of detecting a GRB in the L
(Multicolor Imaging Telescopes for Survey and band as well as the substantial, additional techni-
Monstrous Explosions; three channels with fixed calconstraintsdue tothe requiredlowertempera-
′
bands g, RC and IC (Kotani et al. 2005), oper- turesledusto dropthispossibility. Goingasblue
ated at a 50 cm telescope), TRISPEC (Triple aspossibleiswarrantedbydetectingtheLy-break
Range Imager and Spectrograph; three channels down to redshifts of ≈3; however, also including
with one CCD and two near-IR detectors and a U or u′ band turned out to be difficult due to
wheels for filters, grisms and Wollaston prisms space and logistics constraints. Leaving out one
(Watanabe etal. 2005)), SQIID (Simultaneous or two bands within our wavelength band would
QuadInfraredImagingDevice;JHK andnarrow- increasetheerrorinthephoto-z determinationby
bandLfiltersinfrontofindividual512X512quad- a factor of three at least (for the redshift range
rants of an ALADDIN InSb array, designed for covered by that band). Thus, we decided to use
the f/15 Cassegrain foci of the KPNO 2.1-m and four bands in the visual, plus the standard JHK
4-m telescopes (Ellis etal. 1992)), ULTRACAM bands in the near-infrared (NIR).
(ULTRAfast, triple-beamCCD CAMera for high-
The field-of view (FOV) of the camera should
speed astrophysics (Dhillon et al. 2007); portable
belargeenoughtocoverthetypicalerrorboxesof
instrumentwhichhasbeenused,amongothers,at
gamma-ray bursts, but on the other hand have a
theVeryLargeTelescopesatESO,ortheWilliam
pixelscalelessthanthemeanseeingtoallowaccu-
Herschel Telescope, Canary Islands).
rate photometry. Given the typical brightness of
Here we describe the design (§2) and perfor- GRBafterglows,areasonablylargetelescopewith
mance (§6) of a 7-channelimager, called GROND permanent access is another important require-
(Gamma-Ray Burst Optical and Near-Infrared ment. Finally,asGRONDwouldactaspathfinder
Detector), which was specifically designed for topre-select“interesting”GRBsformoredetailed
GRB afterglow observations. We also mention follow-up observations,a nearby 8–10m telescope
some basics of the operation scheme (§3), related was consideredadvantageousin the searchfor the
software (§4), and the changes to the telescope final host telescope.
2
Fig. 1.—LayoutoftheofGRONDopticswiththe
top panel showing a cut through the visual arm,
andthebottompanelacutthroughthe NIRarm.
Fig. 2.— 3D structure of the visual and NIR
beams with most of the components labeled.
2.2. Optics
2.3. General considerations
The separation of the different photometric
The finalchoice for the telescope wasthe MPI- bands has been achievedusing dichroics, whereby
owned 2.2m telescope, operated by ESO on La theshortwavelengthpartofthelightisalwaysre-
Silla (Chile); see §5. This telescope is a f/8.005 flected off the dichroic, while the long-wavelength
Ritchey-Chretien telescope (Ritchey 1928) with a part passes through. Thus, the first dichroics are
focal length of 17600 mm, on an equatorial fork used to split off the visual bands which is done
mount; the intrinsic image quality is 0′.′4. in the converging beam before the telescope fo-
cus. The relatively short back focal distance of
The prime goal for GROND is to investigate
550mmimpliedverytightconstraintsonseparat-
the high-redshift universe, thus emphasis was put
ing the beams and still having enough space for
on the NIR channels, and not on the visual ones.
the detectors. This, unfortunately, also prohib-
Consequently, it was decided to develop a focal
reducer in the NIR with a 10′× 10′ field-of view ited using larger-formatCCD detectors, since the
consequently largerbeams couldnotbe separated
in order to cover GRB error boxes with a typi-
further.
cal error radius of a few arcmin (anticipated to
occur about twice per week). In contrast, in the After passing a diaphragmatthe nominaltele-
◦
visual channels, no lens system was planned and scope focal plane, the beam is folded by 180 for
the plate scale provided by the telescope optics space constraints, and fed into a collimator (Fig.
was used. 1, 2). The splitting of the three near-infrared
channels occurs in the parallel beam between col-
3
limator and camera objective of the NIR focal re- 2.3.1. The four visual channels
ducer.
For the visual channels the incoming 1:8 tele-
One might expect different ghosting effects in
scope beam is split using 4 dichroics(top panel of
thevisualarm(dichroicsinconvergingbeam)and
Fig. 1). The field of view is determined by the
the near-IR arm (parallel beam). While it is
telescope focal length (17.6 m) and the size of the
true that reflections from the back surface of the CCDs (20482 × 13.5µm). For each CCD the field
dichroicintheparallelbeamproduceghostswhich of view is 5.4 × 5.4 arcmin2, and the plate scale
are not offset relative to the main image, the ef- is 0′.′158/pixel.
fectoftheseghostsisnegligible. Boththedichroic
All four CCDs are backside illuminated E2V
aswellastheanti-reflectioncoatingshaveefficien-
devices without anti-blooming structures and are
cies of about 99.5% in their transmitting regions.
operated in inverted (AIMO) mode. The chips
Since backsurfaceghostsinvolvetwocoatingpas-
come on nickel plated ceramic base plates. Since
sages, they arrive with <0.0025% (corresponding
each of the detectors serves in one and only one
to 11 mag fainter) of the original intensity, undis-
filterband,differentsensitisationsofthedetectors
cernable in data analysis.
havebeenapplied: g′: astro-broadbandonnormal
Another typical concern of dichroics is their silicon, r′: mid-band on normal silicon, i′ and z′:
polarisation dependent location of the cut wave-
basic-NIR on deep-depletion silicon.
lengths, in particular since gamma-ray burst af-
Theuseofdichroicsimpliesthatadjacentbands
terglow emission shows variable optical/NIR po-
have identical 50% transmission wavelengths,
larisation. We have therefore embarked on a ded-
makingtheSloanfiltersystem(Fukugita etal. 1996)
icated study of this effect prior to the coating of
the obvious choice. Thus, the dichroics were de-
the optics. While standard cut wavelengths dif-
signed such that the combination of their cut-off
fer by 7% (3-4%) for dichroic cubes (plan-parallel
wavelengths defines bands identical to the Sloan
plates)betweenthe perpendicularpolarisationdi-
system, with the exception of the i′ band. Since
rections, the final GROND coatings only have a
in the Sloan system r′i′z′ overlap at their ∼70%
difference of 1% (e.g. 15 nm for the JH dichroic
transmission values, we decided to compromise
coating). For a 100% polarised white light beam
the i′ band in favour of standard-width r′ and z′
this leads to a <∼0.1% change in the transmitted
flux, which in turn implies a <∼10−3% change in bands.
the color terms (see §6.2), and thus is tolerable. Thetelescopebeamisfirstsplitbyreflectingoff
the g′ andr′ bands,thenthei′ andz′ bands. The
Theentiresystemhasbeendesignedforawork-
longwavelengthpartofthebeam(theNIRpart)is
ing temperature of 80 K in vacuum, because de-
then passed through the primary cold stop in the
viations from the nominal temperature by more
focal plane. Two further dichroics are placed into
than 20 degrees would lead to refractive index
each of the two visual beams to separate g′ and
changes large enough to cause noticable image
r′, and i′ and z′, respectively. The dichroic plates
qualitydegradation,andweconsideredittoorisky
are wedge shaped by ∼13–19 arcmin so that they
topredictanequilibriumtemperaturegradientbe-
compensatefortheastigmatismofaplane-parallel
tweenthevisualandNIRpart. Sincethemechan-
plate in the converging 1:8 beam of the telescope
ical assembly was done at room temperature, all
(Woche etal. 2000). The curvature of the focal
optics(andmechanical)systemparametershadto
plane of the telescope has a radius of 2205 mm,
be calculated for a temperature of 80 K. For ex-
which corresponds to a focus difference of 43µm
ample, the focal length of the focalreducer differs
between the center and edge of the CCD. This
by 3 mm between 293 K and 80 K, compared to
is smaller than the depth of focus of ∼60µm at
a depth of focus of 25 µm. We imposed a high
theshortestwavelengthused,butofcoursecauses
accuracy requirement on the mechanical support
astigmatism (see Fig. 3).
structure, so that an alignment of the optics sys-
Originally,no otheroptics wereplannedexcept
tem is only required along the optical axis.
forthedichroics. Afterfirstlightitbecameappar-
ent that the g′ and i′ bands suffered from single
reflections of r′ and z′ light, respectively, off the
4
backside of the dichroics, at a strength of about
0.5% of the incident intensity. Thus, each of the
CCD channels has been equipped with a short-
pass filter to block these single reflections. The
backsides of those 3mm glass filters have anti-
reflectioncoatingsat>99.5%efficiency,exceptfor
the g′-band (see below).
Another post-commissioning change has been
implemented for the g′ band: originally, no short-
wavelengthcutoff was foreseen, since we expected
the transmission of the telescope mirrors and the
entrance window as well as the efficiency of the
CCD detector to fall off rapidly enough below
∼400 nm. In fact, the ill-defined cut-off led to
a photometric uncertaintyofabout 5%whichwas
deemed unacceptable (see §6.1). We thereforeim-
plemented a short-wavelength edge-filter with a
cut-off wavelength of 380 nm.
2.3.2. The near-infrared channels
The NIR part is designed as a focal reducer
system allowing for an optimized adjustment of
the focal length, or equivalently the field of view.
′
In this way the required 10 FOV is imaged onto
a 1024 × 1024 Rockwell HAWAII-1 array (pixel
size 18.5µm, plate scale of 0′.′60/pixel). The fo-
cal reducer system (5 collimator lenses with a re-
sulting Fcoll=360mm and 6 camera lenses with
Fcam=129.6mmforeachNIRchannel)hasapupil
reduction factor of 48.89 and a pupil diameter of
45mm. For the f/8 telescope the resulting effec-
tive focal length is 6336 mm.
Adiaphragmisplacedinthetelescopeandcol-
Fig. 3.— Spot diagram of the four visual chan-
limatorfocus,includingblockingoftheM2mirror
nels at different off-axis angles (rows): the num-
and M2 spider structure. The two beam splitters
bersontheleftgivexandycoordinatesindegrees
for reflecting off the J and H bands are located
within the field of view, the top row correspond-
in the parallel beam of the focal reducer, between
ing to the optical axis. It shows the geometri-
collimator and the camera objectives (Fig. 1, 2).
cal abberations (without diffraction) in the focal
In addition, each channel has a cold pupil stop
plane. Thewavelengthrangeofeachbandissimu-
near the dichroic, as well as a filter in front of
latedwiththreeindividualwavelengths. Thenon-
the first camera lens defining the standard JHK
symmetricalimagesaredue tothewedgeshapeof
the dichroics. Note that the r′ channel only in- bands (which also prohibit long-wavelengths sin-
gle reflections to reach the detector). The JHK
volvesreflectionsfromthedichroics,andtherefore
filters differ from canonical NIR filters in their
is symmetric (and thus shows the image quality
much reduced blocking range which is sufficient
provided by the three mirrors of the telescope).
forGRONDbecauseoftheactionofthedichroics.
Each panel is 27µm×27µm, corresponding to 2x2
Thus, the mean transmission in the JHK bands
pixels of the detector.
(including dichroics) is >∼98%.
Because of the parallel beam, the dichroics
5
could be manufactured as true plane parallel
◦
plates and mounted in a compact 60 box which
allowsforaspace-savingsplittingofthebeaminto
theJ,H andK channels. TheK channelincludes
an additional flip mirror for dithering purposes.
ThefocalreducersystemdesignedforGROND
differs substantially from that of other optical fo-
cal reducer systems (e.g. the Calar Alto Faint
Object Spectrograph CAFOS1, the Multi Ob-
ject Spectrogragh for Calar Alto MOSCA2, or
the Potsdam Multi-Aperture Spectrophotometer
PMAS (Roth etal. 2006) at Calar Alto), since no
optical glasses could be used. Basically only four
media are available for transmission in the 900–
2400 nm range: CaF2, BaF2, OH-free Silica and
undoped YAG. Because of its transmission char-
acteristics, YAG is particularly suited for NIR
optics (but also difficult to grow in large dimen-
sions). Using these four optical media, and after
measuring their refractive indices at the foreseen
operational temperature of 80 K, the focal re-
ducer system could be designed with a high per-
formance (Fig. 4) using software developed by us
(Lauxetal.1999). The 3DvisibleandNIRmodel
as well as the optics components CAD files were
created using the ZEMAX software package3.
2.4. Cryo-Mechanics
2.4.1. Cryostat
The GROND cryostat (Fig. 5) consists of the
followingcomponents: a)GROND-cylinder,made
of titanium, and electron beam welded; b) the
front and back plates made of aluminum; c) 4
spacer rods; d) 8 Belleville-spring systems; and
e) O-ring seals (Viton).
The components inside the GROND cryostat
have different thermal requirements. In addition, Fig. 4.—Same spotdiagramasFig. 3but forthe
in the case of single component failures, no sensi- threeinfraredchannels. Thefocalreducerwasde-
tive components should be exposed to dangerous signedtooptimizetheimagequality(pointsource
thermal conditions. The complete internal struc- shape), but still contains image distortions, i.e.
tureismountedonatitaniumalloy(TiAlV)plate plate scale changes depending on the position in
that provides thermal insulation. This plate is the focal plane. These geometrical distortions are
mounted on the front plate of the cryostat, which corrected during the data reduction process (see
accordingly becomes the nominal reference point §4.3). Each panel is 37µm×37µm, corresponding
of the system. As the different components (front to 2x2 pixels of the detector.
1www.caha.es/CAHA/Instruments/CAFOS
2w3.caha.es/CAHA/Instruments/MOSCA
3www.zemax.com
6
To reduce the thermal input, the optical
benches are thermally insulated from the cryo-
stat. In addition, thermal shields are installed to
reduce the radiative heat transfer. The main ra-
diation shield is between the optical benches and
the vessel. It is thermally insulated from both.
The radiationshield is constructedof three round
plates (two at the front plate, one at the back
plate ofthe vessel), four half cylinders aroundthe
spacing bars and six dismountable cylinder seg-
ments – all made of 1.5mm thick highly reflective
aluminum plates. The cylinder segments are ad-
ditionally covered with multi-layer insulation foil.
Fig. 5.— Removing the cylindrical shell of the
cryostat allows easy access to all internal compo-
nents.
plate,titaniumplate,opticalbench)havedifferent
temperatures, the mounting must allow for ther-
mal contractions. This is achieved by using long
holes and centering pins to mount the titanium
plate. The internal components (optical benches)
areconnectedtothebackplateofthecryostatus-
ing a metallic membrane that allows contractions
only along the optical axis of the instrument.
The cooling system is based on a two-stage
Sumitomo Closed-Cycle Cooler (CCC) with a
Fig. 6.—GROND structuralchassis with the two
SRDK-408-S cold head which provides a cooling
separate optical benches (visual in orange, NIR
power of >∼35 W down to 45K at the warm stage,
in blue) and the main aluminum alloy front plate
and >∼6.3 W down to 10K at the cold stage. The
(grey spider plate).
cold head is mounted using a bellow (to reduce
vibrations) on the back plate of the vessel.
The first stage of the CCC (higher tempera- 2.4.2. Structural Chassis
ture and higher cooling power) is used to cool the
Theimplementationoftheopticalrequirements
optical benches (visual and near-IR) and all op-
with the geometrical and mechanical constraints
tics components to 80K (temperature-controlled).
such as tight space, minimum weight and differ-
The second stage (lower temperature and lower
ent temperature regions within the main housing
cooling power) is used to cool the near-IR detec-
resulted in substantial complexity which in turn
tors to 65K (optimum temperature to maximise
required most sub-units and their interrelations
the signal-to-noise ratio). The CCD detectors
to be treated simultaneously. The development
have an intentionally bad thermal connection to
of the optical/mechanical configuration included
thevisualopticalbench(80K).Theyarestabilised
considerations such as: (i) space requirements for
at 165K using resistor heaters. Thermal connec-
the 4 CCD cameras and 3 NIR-detector units,
tions between the two baseplates and the com-
(ii) use of structural components, such as opti-
ponents are realised using copper flanges, copper
calbenches,mountinginterfaces/connections,and
ledges and flexible copper lines. Galvanic isola-
cold connections,(iii) assembly and handling pro-
tion of the cooling system is achievedby isolation
cedures,(iv)cabling,(v)low-temperatureinduced
foils being mounted between the thermal copper
contractions interacting in 3 dimensions.
flanges and the CCC stages.
The final structural chassis (Fig. 6) consists
7
Fig. 7.— Visual baseplate with nearly all optical and mechanical components marked.
mainly of two separate optical benches. The top mirror (adjustable in 3 degrees of freedom); (2)
bench(Fig. 7)containstheopticalandmechanical collimator lens unit; (3) two NIR beam splitters
groupsforthevisuallightdetection,inparticular: to separateJ, H,andK;(4)one cameralens sys-
(1)the entrancelightbaffle(notshowninFig. 7); tem (split into two mechanical lens holders, Fig.
(2)principallightsplitter unit withtwodichroics; 1); for each of the 3 NIR channels; (5) dithering
(3) small dichroics splitting the g′, r′ and i′, z′ mirror unit for the K-channel; (6) NIR-detector
channels, respectively; (4) two VIS-shutter units units (motorised for focussing); (7) Zeolith boxes
forCCD-channelsg′r′andi′z′,respectively;(5)all neartheCCCcoldstageaswater(humidity)sinks
fourCCD-boxesforg′r′i′z′;(6)NIRchannelshut- (not shownin Fig. 8). The NIR light path is only
ter to close the NIR sector completely; (7) mirror open at the focal plane, but otherwise completely
carrierunitforfoldingmirror1(alsonotshownin closedbyadedicatedlight-tighthousingcooledto
Fig. 7). The lower bench (NIR-area; Fig. 8) con- 80K. Cabling is done through light traps.
tains the following components: (1) 2nd folding
8
Fig. 8.— Infrared baseplate with all optical and mechanical components.
2.4.3. Lens Mounting ter to a square of 50x70 mm). Consequently, the
mechanicsalsohadtobe re-designedfromscratch
Within the focal reducer, 23 lenses are main-
and the opportunity was taken to implement two
tained at precise position during the cooling to
new features: (i) motorised positioning of the de-
80K.Aspecialspring-basedlensmounting design
tector along the optical axis, and (ii) a gimbal
wasdeveloped(Fig. 9)whichkeepsthelensescen-
mounting to allow manual alignment of the de-
teredandinthecorrectaxialposition. Duringthe
tector plane perpendicular to the optical axis in
cooling the radial flexibility of the lenses is kept
both degrees of freedom (Fig. 10).
small enough to ensure the required centricity.
AcoldfingeronthebacksideofeachNIRdetec-
2.4.4. IR Unit tor is directly connected with the second stage of
the CCC, thus allowing the NIR detectors to cool
The tight space constraints also made it nec- belowthetemperatureofthebaseplateandoptics.
essary to develop a new, much smaller detector Since the CCC temperature is load dependent, a
board (shrinking from originally 220mm diame-
9
Fig. 9.— Exploded view of the lens mounting
block for the 4 collimator lenses in each of the
three NIR channels.
heater is installed on the coldfinger to control the
temperature and keep variations below ±0.1 K.
Fig. 11.— Schematics of the CCD detector unit.
The arrows denote the alignment possibilities.
• NIR channel Shutter: This door closes the
near-IR arm of the instrument, so one can
takedarksinJ,H andK. Itisimplemented
Fig. 10.— Schematics of the NIR detector unit.
using a stepper motor.
2.4.5. CCD detector unit • Visual Shutters: Due to the constraints of
the optical design of the instrument, the
The CCD detector units have no motorised fo- CCD shutters must be cryogenic (as the
cusingstages,becausethedepthoffocusislarger. shutter must not block the infrared chan-
Still, they can be aligned in 4 degrees of freedom nels). Due to the tight space constraints,
(Fig. 11). Since the minimum allowed opera- one shutter per CCD was mechanically im-
tionaltemperatureoftheCCDs(∼155K)ismuch possible;thus twoshutters havebeen imple-
higher than the inner cryostat temperature, the mented, one for the g′r′ and one for the i′z′
CCDs are equipped with a heat finger and ther- channels,respectively. Eachshutterisathin
mally controlled via the PULPO electronics (see metal disc with two quadrant shaped open-
section 2.6.2) to 165K within ±1 K. ingsatoppositesidesthatareslightlylarger
than the beam at this position. Thus, each
2.5. Cryo-Motors
disc has two ’shutter closed’ and two ’shut-
There are severalmotorizedmechanisms in the ter open’ positions, i.e. 4 possible positions
◦
instrument which operate at 80 K. All motors are intotalwhichare90 apart. Eachshutteris
from Phytron, but use different gears. equipped with a stepper motor and a posi-
10