**2.3 Spectrographs**

***Introduction***

The SNAP visible and IR spectrograph must produce confirmation spectra
and redshifts of candidate SNe events and host galaxies quickly and
accurately. This requires high throughput and broad wavelength coverage.
In addition, the capability to measure simultaneously the spectra of the
SNe and of the host galaxy would significantly boost the observatory
performance. These lead to the baseline specifications outlined in Table
1.

To meet these goals, we are proposing an integral field spectrograph
based on an image slicer. Integral field spectroscopy is now a mature
technique and has been used in many ground based instruments, with the
latest generation being developed for 8-meter telescopes like Gemini and
VLT (see e.g. Bacon et al., 1999; Le Fèvre et al., 2000; Davies et al.,
1998). The principle is simple: take spectra of each resolution element
in a contiguous sky area. In this way it is possible to measure at the
same time the spectrum of the SNe candidate and that of the host galaxy,
without slit losses and without strong requirements on the pointing
capabilities of the spacecraft.

The heart of the proposed system is an image slicer of the same type as
we have studied in the context of the NGST near-IR spectrograph
(Allington-Smith et al., 1999; Le Fèvre et al., 1999). This technology
has been ranked at a NASA readiness level already of 5 on a scale of 7
(6 being required to be \"space qualified\") by a panel of NASA experts
in the context of the concept appraisal of pre-phase A NGST studies.
Continuous prototyping activities are on-going at Laboratoire
d'Astrophysique de Marseille (LAM) and in collaboration with other
European institutes to validate this technology both for large
ground-based telescopes and for space applications, under funding with
various agencies including ESA, CNRS and CNES. The R&D effort necessary
to adapt this concept to the SNAP requirements therefore meshes in
nicely with on-going activities.

*baseline design*

The proposed technical solution for the spectrograph is an integral
field spectrograph based on a slicer unit (e.g. NGST IFMOS study, Le
Fèvre et al., 1999, ASP conference series, Volume 207, 313). The basic
principle is identified in Figure 1: the 2-D field of interest is
\"sliced\" in several strips, with the slices rearranged to enter the
spectrograph as the equivalent of one single long slit. This concept has
several major advantages compared to a long slit spectrograph: all
resolved spatial elements of a contiguous area on the sky have spectra
taken, there are no slit losses, and the telescope pointing is relaxed
to a fraction of the observed field, rather than being constrained to a
fraction of a slit width, allowing for quick acquisition.

![](media/image1.wmf){width="3.4652777777777777in"
height="4.038888888888889in"}

Figure 1: Image slicer principle (courtesy J. Allington-Smith, Durham
U.)

The main requirements for the visible and IR spectrograph are listed in
Table 1. The visible and infrared spectrographs are similar. We present
below the baseline for the infrared spectrograph.

Table 1: spectrograph main characteristics

  --------------------------- -------------- -------------- --------------
  Item                        Visible        IR             Spec

  Wavelength coverage μm      0.35-1         0.6-1.7        0.35-1.7

  Field of view               2.03\" x       2.03\" x       2\"x2\"
                              2.03\"         2.03\"         

  Spatial resolution element  0.07           0.12           0.07-0.12
  (arc-sec)                                                 

  Number of slices            29             17             \-

  Spectral resolution, A      15/30          30/50          15-30-50

  Throughput w/o slicer       75%            75%            As high as
                                                            possible

  Throughput w/ slicer        65-71%         65-71%         As high as
                                                            possible
  --------------------------- -------------- -------------- --------------

High level requirements will be iterated in subsequent phases, based on
the science specifications. The detailed technical specifications and
the instrument configuration will then be established, including the
field-of-view, spatial resolution, spectral resolution, channel count,
etc. The concept discussed below represents a first step in this
process.

There are several main components for this instrument:

***Front Optics***

This unit is the interface between the telescope beam and the
instrument. The optical solution is highly dependent on the
implementation of the instrument. The definition of this optical system
requires the knowledge of the spectrograph position with respect to the
telescope focal plane. The beam can be picked off wherever it's most
convenient for the overall instrument. It will be beneficial to correct
some telescope aberrations (ex: astigmatism) within this optical system.
A 2- or 3-mirror configuration should be sufficient to satisfy
requirements, as has been designed during the IFMOS study for NGST.

***Slicer Unit***

The slicer unit acts as a field reformator. The principle is to slice a
2-D field of view in long strips and optically align all the strips to a
long spectrograph entrance slit.

The slicing mirror comprises a stack of slicers. Each slicer has an
optically active surface on one edge (see Figure 2, the active spherical
surface is the first edge).

A line of «pupil» mirrors does the reformatting. Each mirror sends the
beam to a slit mirror, which does the pupil adaptation to the entrance
of the spectrograph.

![](media/image2.jpeg){width="4.174305555555556in"
height="2.770138888888889in"}

Figure 2: picture of one silica individual slice (dim: 400mm x 0.9mm)

The long, thin, active surface of each individual slicer will produce a
large diffraction effect.

In order to minimize flux loses to a few percent, the spectrograph
entrance pupil has to be oversized. A combined theoretical and
experimental approach is underway at LAM to define the optimum entrance
pupil (in the infrared bands 1-5µm).

The baseline requirements on the slicer unit are λ/10 rms on the optical
surfaces and 5nm rms for the surface roughness (existing prototypes are
fully compliant with these numbers).

***Spectrograph options***

Thanks to the low beam aperture and field-of-view, the spectrograph will
be very simple. Some possibilities are:

**Classical spectrograph (baseline)**: a classical spectrograph with one
collimator mirror, plus one grating, plus one camera mirror, should work
well. Using spherical shape for all the mirrors will provide sharp
enough image quality, but having aspherical mirrors will make possible
to a very compact spectrograph.

**Spectrograph with a spherical grating**. The sphere will "image" the
long slit on the detector and the grating groove on it will disperse the
light. First simulations show that this option is possible, but will
require more volume.

**Spectrograph with a flat grating or a prism or a grism plus a
mirror**. This option would give good image quality for lower
dispersion. For the required dispersion of 30/50Å, it is difficult to
reach a satisfactory image quality. At this stage we have developed a
complete design for the infrared spectrograph. This is the more
demanding because of the diffraction effects at the higher wavelength. A
visible spectrograph will be in all respects similar to the IR
spectrograph, although it will need to be larger to accommodate more
slices (see Table 1).

Figure 3 shows the classical spectrograph configuration. In this
configuration, a dichroic is placed in order to cover two bands
simultaneously (e.g. 0.6-1.1 μm, 1.1-1.7 μm) with a small overlap around
1.1µm. This arrangement can cover the two requested octaves. The
resolution will be changed with a grating exchange unit. A complete
analysis of wavelength-coverage possibilities and spectral resolution
will be carried out during the conceptual phase.

![](media/image3.jpeg){width="5.3277777777777775in" height="2.925in"}

Figure 3: The infrared Spectrograph

The pupil has been oversized in order to catch \>95% of the flux at
0.6µm, and at worst 85% of the flux at 1.7µm. The optimization will be
performed during the conceptual phase.

***Detectors***

In the visible, thinned back-side illuminated low noise CCDs of at least
1024x1024 pixels will be the baseline detectors. *Explain why thinned
CCDs rather than LBNL CCDs.*

We have been considering a 1024x1024, 18.5-μm pixel HgCdTe array from
Rockwell for these first designs. Other detectors like the InSb Raytheon
arrays will be investigated.

The baseline spectrograph design provides spectra about 400 pixels long,
which could even fit on a 512x512 array. This will be investigated
further.

We are planning to coordinate our visible and IR detectors needs with
the work of the teams involved in the visible and IR cameras.

***Overall instrument layout***

***Figure 4*** presents the complete instrument.

![](media/image4.wmf)

Figure 4: The Infra-Red Integral Field Spectrograph optical design
(scale \~1)

During subsequent phases, we will conduct an in-depth analysis of these
designs and will explore alternatives. A trade-off analysis will be
performed, balancing performance and budget, in order to select the best
possible design.

Development plan:

***R&D activities***

The R&D activities related to the SNAP spectrograph fit in a logical way
within the on-going R&D activities of our team (VLT-VIRMOS,
NGST-NIRSPEC, NGST-IFMOS, and VLT- second generation instruments).
Specifically, we have an on-going program to qualify image slicers for
space instrumentation, the core component of any integral field
spectrograph.

***Slicer R&D***

The main R&D activity identified is the image slicer unit development.
We are now in the process of developing a realistic prototype for a
space-qualified unit, based on silica-glass slices. Several slices have
been successfully manufactured to specifications (Figure 2). This
prototype will adequately represent the SNAP case. The main goals of the
R&D are:

-   Demonstrate a reliable optical surface quality on slices, within the
    image quality required specification

-   Demonstrate that the stacking of slices is possible within the
    overall unit tolerances

-   Qualify a test unit under simulated space environment conditions

Development planning for the image slicer R&D:

-   Fabrication process of each individual slice (silica): proven

-   Fabrication of 15 slices: on-going

-   Assembly of a 15-slice prototype: completed by February 2001

-   Testing of prototype on cold chamber: by end 2001

-   Vibration testing of prototype: by end 2001

-   Full space-qualified prototype: ready by the end of 2002

This activity will continue until the end of 2002. We are planning to
finish all R&D activities for the SNAP spectrograph prior to the CDR.

While we are engaged in the first phases of a prototype based on
ESA-NGST IFMOS pre-phase A studies funds, we expect ESA to fund a study
dedicated to the manufacture a slicer prototype in 2001 in the context
of NGST instrumentation.

***Performance analysis and trade-off***

The ultimate performance of the spectrograph depends on

-   The detector performance, including readout noise and dark current.
    The cutoff wavelength of the array needs to be as close as possible
    to 1.7 μm, while keep noise figures low. The operating temperature
    will have an impact on the systems engineering.

-   The overall telescope and spectrograph operating temperature: the
    thermal background at 1.7 μm should be negligible

-   The ability to perform accurate spectrophotometry to better than 1%

We will address these issues with a full simulation including all
effects, and packaged into an exposure-time calculator. The output of
this activity will be a recommendation for the detector specifications
and spectrograph operating temperature.

Analysis shows that no other component of the spectrograph requires R&D
activity.

***R&D Deliverables***

The table below lists deliverables generated during the R&D effort.

  -------------------------------------------------- --------------------
  **Deliverable**                                    **Completion Date**

  Science and Technical trade study                  1-Jul-2001

  Requirements document                              1-Jul-2001

  System conceptual design                           1-Sep-2001

  Slicer Prototype report                            1-Oct-2002

  Construction cost and schedule                     2-Oct-2002
  -------------------------------------------------- --------------------

***Risk Assessment***

***Manpower:***

The instrument development will be carried out under the responsibility
of the Laboratoire d\'Astrophysique de Marseille (resp. Olivier Le
Fèvre). The LAM has experience in large space and ground projects
directly applicable to the SNAP spectrograph. Laboratory facilities
include optical benches and metrology, large cold vacuum chambers,
launch load tests. The LAM will coordinate a team of institutes
belonging to INSU and IN2P3, the astronomy and particle physics branches
of the CNRS (National Research Council).

Table 2 shows the manpower estimate that have been derived from the
upper planning

Table 2

+-------------------+-------------------+----------+------------------+
|                   | **2001**          |          | **2002 Manpower  |
|                   |                   |          | Estimate**       |
|                   | **Manpower        |          |                  |
|                   | Estimate**        |          |                  |
+-------------------+-------------------+----------+------------------+
|                   | FTE               |          | FTE              |
+-------------------+-------------------+----------+------------------+
| Project Scientist | Olivier Le Fèvre  | 0.1      | 0.1              |
+-------------------+-------------------+----------+------------------+
| Science team      | See below         | 0.3      | 0.3              |
+-------------------+-------------------+----------+------------------+
| Project Manager   | Eric Prieto       | 0.3      | 0.3              |
+-------------------+-------------------+----------+------------------+
| Slicer Specialist | Christophe        | 0.2      | 0.2              |
|                   | Bonneville        |          |                  |
+-------------------+-------------------+----------+------------------+
| Optical Designer  | Eric Prieto       | 0.3      | 0.3              |
+-------------------+-------------------+----------+------------------+
| Mechanical        | Serge List        | 0.4      | 0.6              |
| Designer          |                   |          |                  |
+-------------------+-------------------+----------+------------------+
| Thermal Designer  | Christelle Rossin | 0.1      | 0.1              |
+-------------------+-------------------+----------+------------------+
| Electronics       | Patrick Levacher  | 0.1      | 0.2              |
| Designer          |                   |          |                  |
+-------------------+-------------------+----------+------------------+
| Control Hardware  | Patrick Levacher  | 0.1      | 0.2              |
+-------------------+-------------------+----------+------------------+
| Detector          | R. Barbier        | 0.2      | 0.5              |
| specialist        |                   |          |                  |
+-------------------+-------------------+----------+------------------+
| Software          | In discussion     | 0.1      | 0.1              |
+-------------------+-------------------+----------+------------------+
| Performance       | E. Pécontal       | 0.1      | 0.1              |
| analysis          |                   |          |                  |
+-------------------+-------------------+----------+------------------+
| Project Assistant | In discussion     | 0.2      | 0.3              |
+-------------------+-------------------+----------+------------------+
| Quality Engineer  | In discussion     | 0.1      | 0.1              |
+-------------------+-------------------+----------+------------------+
| **total**         |                   | **2.6**  | **3.4**          |
+-------------------+-------------------+----------+------------------+

People listed besides scientists are experimented engineers in the field
of space optical instrument who have worked for different space and
ground based instruments (Rosetta, COROT, FIRST, NGST, VIRMOS...).
Christophe Bonneville is a Ph.D. student focused on slicer development.
In addition, we have a team of astronomers available to define and
review specifications and design (Giraud, Le Fèvre, Malina, Tresse,
Treyer).

***Schedule Summary***

The first milestone is the CDR (end of 2002) at the end of phase A. The
planning for the next two years has been phased with this milestone.

Figure 5 shows the development planning to go to the CDR.

![](media/image5.png){width="7.0055555555555555in"
height="2.6395833333333334in"}

Figure 5: Development planning