May 25, 2001

SNAP

Supernova/Acceleration Probe

# Draft Optical Specification

Based on optical and structural/mechanical studies a preliminary
specification for the optical telescope assembly (the 'telescope') has
been prepared and is described in this specification. The basic optical
design developed for SNAP is found in Appendix II. Other pertinent
documents including the 'Optical Telescope Assembly Definition and
Requirements' which serves to highlight the factors that led to the
optical design described here can be retrieved from the SNAP website
([http://snap.lbl.gov)]().

In brief, the SNAP telescope is to be a 2m aperture, f/15 optical system
with a nominal 1.26 degree annular field of view (FoV) enabling it to
cover 1 square degree with diffraction limited performance at 1 µm over
a flat field. This very large FoV telescope will be mated with a billion
pixel camera, referred to as the GigaCam. The GigaCam, whose focal
surface is an annulus whose dimensions are 0.66m OD x 0.3m ID, is far
larger than the cameras used on Hubble or planned for NGST. Information
about this camera, for reference purposes, can be found in documentation
that is retrievable from the SNAP website. SNAP will be launched by a
Delta III or IV-M, an Atlas-III or a Sea Launch vehicle into a very
19x57 Re high elliptical orbit. The telescope (mirrors, structures,
baffles, forward structure) will have an estimated mass not to exceed
720 kg; however, lower masses are preferred owing to considerations of
total system cost. It is anticipated that the telescope will be operated
at or near the temperature at which it was manufactured and assembled,
to avoid the need for figuring the 2m optics for some colder
temperature. The GigaCam will be in a thermally isolate enclosure and
cooled to nominally 140K by a passive radiator located on the anti-sun
side of the Observatory. SNAP will be designed so that it can be
optically and thermally tested in the laboratory prior to flight.

The SNAP telescope comprises the following subassemblies:

a)  OPTICS, METERING STRUCTURE, and BAFFLES: The metering and support
    structure (1) determines the orientations and spacings of the
    mirrors, (2) provides the attachments for the GigaCam and other
    science instruments, (3) provides attachments to the spacecraft bus,
    and (4) supports the internal optical baffles.

b)  MECHANISMS: We anticipate that the telescope will be fitted with a
    motorized focus and alignment system, an emergency solar shutter,
    and a main aperture "door" release. Presently the filter wheel and
    shutter assembly is part of the GigaCam and is not included here.

c)  OUTER BAFFLE: This is a shell-like structure which encloses the
    primary mirror (PM) and the secondary mirror (SM). This structure
    provides stray light control and offers some degree of thermal
    control for the telescope. All science target fields are at least 60
    degrees away from the sun. This structure also includes a
    contamination protection door or equivalent jettisonable cover and
    supports the Observatory's solar array subsystem (interfaces TBD).

d)  INTERFACES: The interfaces between the telescope and the GigaCam and
    the other instruments have not yet been fully defined. For the
    purposes of this specification only the GigaCam instrument needs to
    be considered for space allocation and mounting and for thermal
    control purposes. The GigaCam consists of the large annular detector
    assembly, a dewar and radiative cooler assembly, and the
    filter-wheel assembly.

There are currently several unresolved parts of the telescope
specification denoted by the 'gray' areas in the Appendix. It is
anticipated that the telescope optics will be temperature controlled to
remain at the manufacturing temperature by heater/thermostat systems,
similar in principle to what was used for the 2.4m Hubble telescope. It
will be required that whatever these temperatures are, that they can be
simulated during the test phase.

The optical design described in the Appendix represents a concept that
has evolved but may not be the only approach that should be considered.
This specification also does not imposing any constraints on detail
design or material choices. ULE, Zerodur, and silicon carbide are
considered candidates for the optics material.

Appendix

# Preliminary SNAP Telescope Specification

![](media/image2.wmf)

# SNAP Optical Design 

![](media/image3.png){width="4.944444444444445in" height="3.26875in"}

## 

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## Optical Prescription

![](media/image4.wmf){width="5.991666666666666in"
height="3.082638888888889in"}[Performance Summary]{.underline}

#### Sky coverage: 1.0 sq. degree

> R~outer~: 11mrad = 0.63 deg = 33 cm
>
> R~inner~: 5 mrad = 0.286 deg = 15 cm
>
> Geometrical Aberration Total:

### Average RMS blur, full field: 2.5 microns

> Worst case (edge) FWZ blur: 18 microns

##### Compare with diffraction limit, 2 meter aperture

> 1 micron wavelength: FWZ first ring = 36.6 microns
>
> 0.5 micron wavelength: FWZ first ring = 18.3 microns