Elmer performance after characterization and
pre-shipping tests
M.L. Garcı́a-Vargas1,2, J.M. Martı́n-Fleitas2,3, A. Cabrera-Lavers2,3, J.M.
Rodrı́guez-Espinosa2, P.L. Hammersley2,3 ,, and R. Kohley3
1
2
3
FRACTAL S.L.N.E.,C/Castillo de Belmonte 1, bloque 5, Bajo A, E-28232, Las
Rozas de Madrid, Madrid, Spain. marisa.garcia@fractal-es.com
Instituto de Astrofı́sica de Canarias (IAC), C/Vı́a Láctea, s/n, E-38200, La
Laguna, Tenerife, Spain.
GTC Project Office, C/Vı́a Láctea, s/n, E-38200, La Laguna, Tenerife, Spain.
Summary. Elmer has been exhaustively tested at laboratory and it is ready to be
shipped to the Observatorio del Roque de los Muchachos, where will be installed at
the GTC Nasmyth focus. This talk, together with other two contributions presented
in this conference, summarize the acceptance tests performance results.
1 Introducing Elmer
Elmer is a workhorse instrument for the GTC. The observing modes - all simultaneously available - are Imaging, Long-slit and Mask multi-object Spectroscopy, Slitless multi-object spectroscopy, Fast Photometry and Fast shortslit spectroscopy. The pupil elements are a full set of conventional broad-band
and narrow-band filters as well as a set of prisms, grisms and VPHs, which
allow resolving powers of 200, 1000 and 2500 between 365nm and 1000nm. We
have intensively tested the instrument at laboratory and each of the observing modes has been fully characterized. The high throughput and excellent
image quality, in combination with the GTC, guarantee a powerful scientific
return. Its general scientific capabilities are summarized in Table 1. Imaging
and Spectroscopy performance has been described in two poster contributions
(Cabrera-Lavers et. al. 2006, hereafter CL-I and CL-II 2006). We complete
here the information about the Imaging and the Fast modes.
2 Imaging Mode
Imaging Mode operates through the use of a filter wheel near the pupil that
contains a set of broad and narrow band filters, specifically designed by the
Elmer’s team for the large pupil (filters have 135mm φ size and 128mm φ
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M.L. Garcı́a-Vargas et al.
Table 1. Elmer General Capabilities
General Capabilities
Focal Station
Wavelength range
FOV
Scale at detector
Detector Focal Plane
Pixel Size
Detector capabilities
Nasmyth B and Folded-Cassegrain
365 to 1000 nm
4.2’ φ Imaging and Slit-less Spectroscopy
3.0’ x 3.0’ Long Slit and Multi-object Spectroscopy
0.194”/pixel
2048 x 4096 EE2V CCD44-82
15µm x 15µm
Charge shuffling and Frame transfer
Imaging Modes
Broad band Filters
Narrow band Filters
SDSS set: g’, r’, i’, Z’
[SII]wide 672.6nm, [SII]narrow 671.7nm
Hα wide 656.3nm, Hα narrow 656.3nm
[OI]630.0nm, [OIII]495.9nm+500.7nm
Hβ 486.1nm, [OII]372.7nm
Neutral density filters ND2 and ND3
Direct Imaging
Over 4.2’ φ FOV
Fast Photometry Mode Aperture: 3’ x 12.48”
Duty cycle
> 98% at 1 Hz
Charge Shuffling
Yes (at 50µs/line)
Spectroscopy Modes
Spectral resolution
Dispersive elements
Long Slit
Fast short slit
Charge shuffling
Mask multi-object
Positions for masks
R = 50-500 (2 prisms)
R = 1000 (2 grisms)
R = 2500 (6 VPHs)
2 prisms , 2 grisms and 6 VPHs
3’ x 0.6”, 1.2”, 2.0” and 5.0”
20” x 0.6”, 1.2”, 2.0” and 5.0”
3’ x 0.6”, 1.2”, 2.0” and 5.0”
3’ x 3’ FOV
Up to 4 depending on configuration
clear aperture) and manufactured by the company OMEGA Optical. The
filters have an excellent transmission, see CL-I (2006), and are within the
specification tolerances, as shown in the data that have been measured with
independent methods by the manufacturer and by us with the use of the VPHs
spectral calibration. Filter holders at the wheel are tilted to prevent ghosts
due to the main Optics configuration. However, the ghosts associated to the
filter’s manufacturing process, based on multi-layer deposition, are specially
strong for the narrowest filters and they are unavoidable. During the characterization process we took images through each filter in its optimum focus
configuration to measure, on the detector, the position, intensity and orientation of ghosts images with respect to the main image of a pinhole. Figure 1
of CL-I (2006) shows the detected ghosts for the different filters. We strongly
Elmer performance after characterization and pre-shipping tests
3
recommend taking into account this effect in the case of high dynamical range
images with weak point-like sources. Imaging Mode performance has been reported in CL-I (2006). In summary, over the whole FOV, image quality gives
EER80 < 15.0µm, less than 1 pixel; distortion values are smaller than 0.4 %
at the corners; plate scale is 0.194”/pix being temperature-independent, and
flexure is smaller than 1.3 pix over 180◦ , which can be considered a record
for an instrument without any active mechanism for stability compensation.
In the following we report some effects that can help to select the adequate
observational strategy when using Elmer in Imaging Mode.
FOV on detector: Elmer 4.2’ φ FOV is imaged on the CCD upper half
(1293 pixels) while the CCD has 2K x 4K pixels. This implies that the CCD’s
lower half is not used for Imaging. For this reason the readout mode by default
is Frame Transfer, in which the upper half can be exposed while the lower one
is being read. Three identical exposures for each image are recommended to
be taken, in order to remove the cosmic rays with common reduction routines.
Fiducial origin: It is important to know that each filter produces the image
of a point-like source in a slightly different position on the CCD.
Table 2. Filters Fiducial Positions of the FOV center with respect to r’
Filter
g’
r’
I’
z’
Hα broad
Hα narrow
Hβ
[OI]
[OII]
[OIII]
[SII]broad
[SII]narrow
Focus (mm) X offset(pix) Y offset (pix)
10
12
13
13
12
15
10
11
10
9
17
11
+2.037
+0.000
+3.116
+3.086
+2.038
+4.184
+0.368
+0.091
+1.102
+0.192
+2.498
+0.399
−0.925
+0.000
−0.606
+0.494
−0.749
+1.223
+2.002
+1.476
+0.327
−1.797
+0.166
+0.432
Since the GTC Acquisition and Guiding system will guide with the filter
r’, by pointing to the center of the field viewed through that filter, we have
chosen the center of the FOV through r’ as the zero pointing reference. Table
2 gives, for each filter, the X and Y offset values (in pixels) for the FOV center,
with respect to the center viewed by r’ filter. During the commissioning phase,
it should be tested that the center of Elmer’s field is aligned with the telescope
optical axis and the table should be corrected accordingly to those results. The
observational strategy will be one of the following: (a) the center of the FOV
through r’ will be defined as the instrument zero pointing reference and the
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M.L. Garcı́a-Vargas et al.
telescope will correct by making a pointing offset for each filter. In this case
all the images on the CCD will be well centered on sky co-ordinates and the
same extraction routine will be applied; or (b) the telescope will point the sky
object position to the center of the FOV through r’ and then the instrument
will take the image. In this case, each image will be taken centered in a slight
different sky position and the extraction reduction routine will correct from
this effect by taking the offsets on detector from the fiducial position table
and extracts a squared image on the detector for each filter, well centered
at the real pointing sky co-ordinates. In this later case, if the user does this
extraction by himself the effect has to be considered before subtracting images
through different filters (e.g. to give colors). To subtract images on real-time
then the first observational strategy has to be selected.
Narrow Band Imaging: One of the most important specifications for a narrow band filter is the pass band. However, two effects shift the wavelength and
affect the transmission performance. These are (a) the temperature change
and the incidence angles of the rays at the filter position. Regarding the first effect, the central wavelength of an interference filter shift linearly with changes
in ambient temperature (to larger wavelengths for a temperature increase and
viceversa). The average value for our set of filters is 0.018 nm/◦ C. The GTC
nominal operation temperature ranges between -2◦ C and +19◦ C. The total
operation range (the telescope can operate but with degraded performance) is
-6◦ C - +35◦ C. Therefore, the maximum wavelength shift will be +0.27nm (at
+35◦ C) and -0.47nm (at -6◦ C). We remind the reader that both, image quality and plate scale in Elmer are temperature-independent thanks to Elmer’s
athermal optical design. In general, the amount of wavelength shift is dependent upon the incidence angle and the effective index of the filter. The larger
the telescope focal plane plate scale is, the larger the shift. In particular, this
effect is very strong for instruments at the GTC, with a large plate scale, and
fast cameras. Elmer filters are placed near the pupil in the collimated beam.
In that position, the effect is unavoidable and the only thing the user can do
is to know the instrument performance when using the narrow band filters
and take it into account at the time of preparing the scientific observations.
Since Elmer filters are in the collimated beam, for each position of the entrance FOV the rays (all parallel) come with a different incidence angle. This
produces a wavelength shift that is equivalent to tune the filter (or to transmission variation for a given λ within a given filter). The following formula is
used to determine the wavelength shift of a filter in a collimated light (which
is the case in Elmer) with incidence angles up to 15◦ .
s
2
Ne
λ = λ0 1 −
sin2 θ
(1)
Ni
where λ is the wavelength at angle of incidence θ, and λ0 is the wavelength
at normal incidence, N e is the refractive index of the external medium (1 for
Elmer performance after characterization and pre-shipping tests
5
the air) while N i is the filter index. In Elmer, the nominal position for the
central FOV is 6◦ tilt to avoid ghosts and the maximum angles between the
two extreme positions of the FOV (+2.1’ and -2.1’) are 4.3◦ (corresponding
to incidence angles of 1.7◦ for -2.1’ and +10.3◦ for +2.1’ position). Figure 1
shows the shift effect for Hα narrow filter (the worst case).
Fig. 1. Hα narrow filter band shift due to the incidence angle.
One of the problems is the determination of Ne. We have measured it
thanks to the use of the VPHs. We took spectra of a pinhole mask, covering
the FOV. We put a given filter plus the appropriate VPH. After spectral
calibration, we derived the central λ at each spatial position. From λ and the
formula we got Ne. Results are shown in Table 3.
Table 3. Measured Central wavelength, band-pass and effective refraction index
Filter
Hα broad
Hα narrow
Hβ
[OI]
[OIII]
[SII]broad
[SII]narrow
λ (Å)
6577
6572
4862
6310
4993
6737
6727
±
±
±
±
±
±
±
∆ λ (Å)
1
2
2
2
3
3
1
66.3
16.7
25.5
42.4
56.2
69.0
24.2
±
±
±
±
±
±
±
1.3
0.5
1.2
5.8
1.3
0.6
3.7
Ne
1.72
1.83
2.23
1.86
1.86
1.82
1.71
±
±
±
±
±
±
±
0.11
0.02
0.15
0.20
0.27
0.17
0.03
Figure 2 illustrates how the shift effect in Hα narrow filter makes that
different observed lines through the filter have very different transmission ac-
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M.L. Garcı́a-Vargas et al.
Fig. 2. Transmission at 656.3, 654.8 and 658.4 nm respectively when observing with
the Hα narrow filter as a function of the FOV. Lines correspond to Hα and the two
[NII] lines at z=0. The line at 658.4 is really out of the pass-band for the whole FOV,
but the line at 654.8 becomes important for fields beyond +0.5 arcmin, reaching the
same weight than Hα narrow around this value. Hα can be detected with good
transmission between -1.5 and +1.0arcmin. These final curves calibrated with the
GTC and Elmer) should be taking into account when observing with Elmer.
cording to the target spatial position in the FOV. The final flux calibration
shall be done during the commissioning at the telescope, by observing a bright
planetary nebula, with known and flux-calibrated emission lines, in different
positions within the FOV. This will allow producing calibration maps to be
used as reference to correct this effect. Also, specifically observational strategies are recommended according to the scientific program’s goals.
3 Fast spectro-photometry capabilities and performance
As we have mentioned above, Elmer’s detector has a surface larger than required for imaging the FOV. This allows the implementation of some observing
modes like charge shuffling and fast spectro-photometry, which can use the
remaining area as charge storage, reaching very high duty cycles. These capabilities open a wide range of scientific possibilities for time-resolving spectrophotometry. In Elmer these modes are simultaneously available with the others since only a movement of the slit wheel is required to accommodate the
appropriate mask (Fast Photometry or Fast Slit Spectroscopy apertures) at
the telescope focal plane. The sequence for the observation is: (1) Open the
shutter (400ms) during CCD clean; (2) Make a loop of exposures and charge
Elmer performance after characterization and pre-shipping tests
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movement (usually to move the sub-image outside the aperture) until CCD
is filled with sub-images. This loop is specified by the parameters Nshifts
(number of shifts or sub-images) Shiftlen (displacement or number of lines to
move each sub-image) and Exptime (exposure time of each sub-image); (3)
Close the shutter (400ms) and (4) Readout the CCD. The highest temporal
resolution attained is 100µms (with a goal of 50 µms to be tested). In this
mode the charge is continuously moved and exposure time is zero (Exptime=0,
Nshifts=1, Shiftlen=2700, corresponding to the maximum available number
of CCD lines for sub-image storage.). The time between blocks of observation
can be couple of seconds depending on the area of the CCD to be read. Fig.
3 shows a real image taken with Elmer fast spectroscopy mode.
Fig. 3. Left: Elmer Fast Imaging Mode. Figure shows an image of a central circular
aperture’s spectrum, continuously exposed (Ne lamp + VP660- 2500), with 40 subimages of 7 ms of exposure time each, and a displacement of 50 pixels between
consecutive sub-images. The smear among images can be seen due to the continuous
exposure, and the lines are not continuous because the lamp intensity is changing at
a lower frequency (100 Hz) than the line-to-line sampling (10 kHz).Right: Detail at
other scale and zoom of the same image showing how the lamp frequency is sampled
with exposure and shift times (83 kHz).