| INSTRUMENT_DESC |
Instrument Overview
===================
EMMI is a flexible instrument which allows a wide range of observing
modes, from wide-field imaging to high-dispersion echelle
spectroscopy, including long-slit and multi-object spectroscopy.
This manual also describes SUSI (SUperb Seeing Imager) which is
mounted in the other Nasmyth focus of the NTT and complements the
imaging capabilities of EMMI. A brief description of the active
optics system of the NTT and its basic operational principles are
also provided in this manual.
The driving concepts in the instrument definition were the high
image quality foreseen for the NTT, the need to complement or
improve on the instrumentation on the 3.6m telescope, and the need
to minimize instrument change-overs. The concept which was adopted
is that of a dual-beam instrument, fully dioptric, and based on the
white pupil principle. CCD detectors were foreseen for the two arms
with the possibility to adapt to the geometric characteristics of
future detectors by changing the cameras only. The main advantages
of this type of design are the high efficiency in both channels and
the easy conversion from wide field imaging to grism and grating
spectroscopy.
After the first observations with the NTT, it became clear that the
telescope and the atmosphere at La~Silla could provide stellar
images with diameters as good as 0.3 arcsec. Images of this quality
could not be sampled adequately with EMMI, which scale had also to
be adapted to the spectroscopic modes of observation. Thus, SUSI
was designed and built for the other Nasmyth arm of the NTT. The
design is such that switching between EMMI and SUSI is done in a
matter of minutes, so that they can be considered as different parts
of the same instrument.
Optics
======
A detailed description of the optical design of EMMI can be found in
Dekker et al. (1986).
It has two 'arms' with separate detectors. One arm ('EMMI blue arm')
is coated for high efficiency in the region 300 to 500 nm, the other
('EMMI red arm') for 400 to 1000~nm. Each arm supports several
different observing modes. The capabilities of the blue and the red
arm are not identical, e.g. Multi-Object Spectroscopy (MOS) is only
available in the red arm.
Each of the two arms has two possible light paths: one is used for
grating spectroscopy and the other for imaging and (only in the red
arm) low-resolution spectroscopy using grisms. In grating
spectroscopy, a dichroic can be inserted into the beam allowing the
use of both arms at the same time. Each of the five possible light
paths (two per arm plus the dichroic mode) is called an 'Observing
Mode'. The five modes are called:
BIMG: Blue Imaging
For wavelengths less than 500 nanometer, 0.37 arc sec/pixel
resolution and a field of view of 6.2 arc min by 6.2 arc min.
Cameras and Detectors
=====================
EMMI works with two scientific CCD cameras, one at the red arm and
one in the blue arm.
The image scale at the F/11 Nasmyth foci of the NTT is 5.36
arcsec/mm or 186 micron per arc sec. This is also the scale of the
direct imaging with SUSI. The actual field size and scale depend on
the detector and camera being used and are given in
Image scale for EMMI and SUSI
EMMI BIMG #31 TEK 1024 F/4 24 micron/0.37 arc sec
CCD characteristics for EMMI and SUSI
# 31 slow mode 1.7e/ADU conversion 2.5ADU RON 280.2ADU BIAS 8e
DARK
fast 3.4 1.6 275.2 8
Saturation is in most cases defined by the ADU converter, at 65
kADU. The actual well depth is around 2 x 10^6 e so that the
linearity is good up to digital saturation. The exceptions are EMMI
red and SUSI when read out in fast mode. For EMMI red in the fast
mode, exposure levels should be kept below 40 kADU, and for SUSI
below 24kADU. Otherwise, the linearity of the CCD is better than
0.5%. The measured linearity curves can be found in the CCD test
reports.
The fast readout mode has as main advantage a reduced readout time.
This becomes important on EMMI red where fast readout saves two
minutes. It is much less important in SUSI. The disadvantage is
increased readout noise and digitization noise, and sometimes
increased electric interference. For broadband imaging and many
spectroscopic programmes, the readout noise is not important
compared to the photon background, and the fast readout mode would
be recommended. Electronic interference could be larger in fast
readout: when in doubt, it is worthwile to take a few biases in the
afternoon to check on the readout noise and on the presence of
pattern noise. Remember that, in general, calibration frames such
as bias, flat fields, and darks taken with slow readout cannot be
used for correcting fast exposures, and vice versa.
CCDs have an electronic bias that can be evaluated by averaging
several 0s dark exposures and subtracted from the science images to
take it out. By using these exposures the observer can check the CCD
readout noise and possible pick-up patterns in the electronic
background. At least one, but if possible more, long (at least 1
hour) dark exposures are important to monitor the dark current of
the CCD (if possible, take a dark longer than the longest science
exposure).
Filters
=======
EMMI has four filter wheels: the blue and red imaging filter wheels,
and the blue and the red below-slit wheels. The last two are only
used for grating spectroscopy and usually contain neutral density
filters only. Each of the two filter wheels used for imaging has 9
positions of which 8 are available for mounting filters and one is
kept free. The R-filter is usually needed for focusing. Both red
and blue filters have a free circular diameter of 80 mm and an
outside diameter of 85~mm. Adapter trays are available for filters
of other instruments (e.g. EFOSC) but use of smaller filters will
produce vignetted images only useful in the centre of the CCD. In
such cases it might be better to use SUSI.
The SUSI filterwheel has 8 positions of which 7 are available for
filters. In contrast to EMMI, SUSI uses 60-mm filters which are the
same size as the ones used for EFOSC. EFOSC filters can therefore
also be mounted in SUSI (if they are not required by the EFOSC
observer!). There are also a large number of 60-mm filters which
are not allocated to a particular instrument and which can be
requested.
All filters are permanently mounted in special cells which make
replacement very easy. EMMI red-arm filters, which are inserted in
a parallel beam, are mounted at 5 degree inclination to avoid
reflections between the CCD and the filter. Blue arm filters, used
in the converging beam in front of the blue camera, do not show this
effect and hence are mounted with no inclination. Using a red-arm
filter in the blue will result in a slight change of the central
wavelength and will cause some astigmatism. If a blue filter is used
in the red arm, every object in the field produces a ghost due to
the mentioned reflections, which is about 5 magnitudes fainter than
the original object. Thus, although it is possible to use blue
filters in the red and vice-versa (one might want to do this in the
overlap region, 400 to 500 nm), filters should normally be used in
the wheel they are intended for.
Filters with very narrow bandwidths are not really suitable for the
red arm. Because the filters are mounted in the parallel beam, the
central wavelength will change across the field. In extreme cases,
the central wavelength may be outside the filter response near the
edge of the CCD. The tabulated wavelength corresponds to the centre
of the CCD. The effect is further described in Chapter 11. As a
guide line, avoid filters with wavelength change <5 nm for
wide-field imaging. This effect also affects wide-field photometry
if using narrow filters.
The ESO Image Quality Filters Catalogue (Gilliotte, 1992) contains a
list of available filters and transmission curves. More recently
(1995), a number of new filters have been acquired and all
transmission curves re-measured. This new data can be viewed using
the MIDAS graphical user interface (GUI) {\tt FILTERS}, available in
MIDAS version 94NOV or later. The most recent version is always
available in La Silla. Lists of standard EMMI filters are also given
in Chapters 5 (RILD) and 6 (BIMG), respectively (Tables 5.1 and
6.1), and a list of standard SUSI filters in Chapter 11 (of the
manual).
Calibration
===========
There is a system of calibration lamps associated with the
adapter/rotators at the NTT which can be used for most of the
wavelength calibrations required for the EMMI data. The main
component of the calibration system is an integrating sphere mounted
on the side of the adapter. Light from the output aperture of the
integrating sphere passes a lens and is reflected to the center of
the focal plane by a 45 degree mirror which is moved to the optical
axis. On the integrating sphere He, Ar, and ThAr lamps are mounted,
while the light of flatfield and other spectral lamps that are
mounted in a rack on the floor is fed to the sphere through an
optical fibre. The fiber induces some broad absorption features
around 724 micron and 880 micron which do not occur in the
scientific data. The angular size, location, and shape (including
central obscuration) of the NTT exit pupil are approximately
simulated. The illumination is homogeneous and unvignetted in a 3
arc min by 6 arc min field and is still usable in a field of 5 arc
min by 8 arc min which is the maximum field size for MOS.
Processing
==========
The data from the EMMI and SUSI detectors are simultaneously
transmitted to IHAP and MIDAS databases. MIDAS runs on a Unix
workstation equipped with a DAT tape unit. IHAP uses standard 1/2
inch 2400~foot tapes at 6250 BPI with a total capacity of 45 by 1024
times 1024 images in FITS format.
The FITS headers of CCD files contain all the information necessary
for the scientific use of the data, that is all the telescope,
instrument, and detector parameters. Most of these parameters are
stored in so-called hierarchical keywords. MIDAS can read these
keywords, but some other packages may not since these are an
extension of the FITS standard. If you are not using MIDAS, it is
worth to check the actual FITS header for further information which
may be useful.
Electronics
===========
The NTT is controlled by two HP1000/A900 computers, one for the
telescope (called NTT) and one for the instruments (called NTI).
The control software of EMMI is organized in such a way that EMMI is
presented as five sub-instruments called RILD, REMD, BIMG, BLMD, and
DIMD. Depending on the type of observations, the user selects one
of these modes and the control software automatically moves the
functions to be set for this mode. This leaves only the parameters
of the particular type of observation to be defined. For instance,
when setting up an exposure in RILD, the required mirror unit and
the upper red folding mirror are automatically set. The observer
must only specify the camera focus, the choice of slit, filter
and/or grism, and exposure parameters (see section Getting
started).
The user interface (UIF) consists of a RAMTEK monitor where mouse
driven menus and forms are displayed, and a CRT (LU:53) monitor
where messages from the system about the instrument are given and
commands may be entered. Parameters are entered by filling in forms
on the RAMTEK screen.
Once all optional optical elements are installed by the operation
group, according to the observer's request, a setup form is
produced. A printout of this form is left in the control room so
that the observer can verify the setup and can use it as reference
during the night. The positions in the wheels of filters,
grisms,and slits, and the gratings in the housing will be displayed,
on the RAMTEK UIF in sofar as it is used in the chosen mode,
whenever a setup in that mode is defined.
System Performance
==================
The pointing of the NTT is better than 1.5 arcsec rms. Guiding the
telescope is normally done by the night assistant by centering a
star on one of the two guide probes in the adapter and starting the
autoguider.
The camera on the blue arm of EMMI is at F/4 and the detector
presently used is a TEK CCD of 1024x1024 pixels, 24 micron in size
(ESO No 31). This gives a scale of 0.37 arcsec/pixel and a field
of view of 6.2 by 6.2 arcmin.
Ancillary Data
==============
Bias and darks
--------------
It is not safe to assume the bias to be always a scalar and
therefore it is recommended to take many bias exposures.
It has proven to be extremely difficult to isolate the CCD
electronics from electrical interference from components in the NTT
adapters/rotators. Therefore to some extent, the EMMI CCDs show
pick-up patterns in the electronic background (the bias). This
noise is minimized in SLOW readout mode, but may be rather strong
in FAST readout frames. The patterns are not stable, but change from
one exposure to the next, so it is difficult to remove them
completely by substracting bias frames. However, some reduction can
be achieved and, therefore, it is recommended to take a good number
of bias frames throughout the observing run. Should strong patterns
(i.e. more than a few ADUs) appear on SLOW readout bias frames, call
the NTT coordinator (93-50). Note that spurious patterns are
introduced if images are displayed with demagnification factors.
At least one, but preferably more, long (1 hour) dark exposures
should be taken to monitor the dark current and any exposure
dependent features.
Flat fields
-----------
The linearity of CCD No 31 is very good up to 160,000 e/pixel. The
most accurate results for flat fields in broad band imaging are
obtained using sky flats. This may be achieved by median filtering
of science images, if they are not too densely populated with stars
and do not contain very extended objects, or by doing multiple
exposures of sparsely populated fields, using spatial offsets. A
list of such fields is available in the control room. Another
approach is to use morning and evening twilight.
Shutter timing
--------------
A time delay of 0.80 seconds has been measured for the shutter in
the blue F/4 camera. Because of the location of the shutter in the
optical path, the exposure time over the field is constant and equal
to the chosen time plus the average shutter delay. If critical for
your application, it is recommended that you check the shutter
timing by either taking exposures of increasing duration on a star,
or using one of the internal lamps and a pinhole in the aperture
wheel.
Typical count rates
-------------------
In blue imaging the efficiency of EMMI is the product of the
transmission of the atmosphere, three reflections in the telescope,
the transmission of the blue coated optics of mode BIMG, filter, and
quantum efficiency of the CCD. The efficiency in B and U was checked
for the F/4 camera and TEK CCD No 31 in 1993. The count rates in
e/sec deduced for a 15th magnitude A star with approximately zero
colour are U: 2200, B: 16,900 at unit airmass.
Wallander A.: 1993, Remote Control of the 3.5m NTT User Guide, ESO
Operating Manual No 17.
Dekker. H., Delabre, B.: 1987, Applied Optics, 26, 8, 1375
Dekker, H., Delabre, B., D'Odorico, S.: 1986, SPIE, 627, 339
Gilliotte, A.: 1992, Image Quality Filters Catalogue, Internal ESO
publication
Melnick, J., Dekker, H., D'Odorico, S.: 1989, ESO Operating
Manual #4
Prieur, J. L., Rupprecht, G.: 1990, Efficiencies of EMMI, ESO
internal report.
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