The MODS baseline configuration will have 1 imaging flat, 1 grating, and
1 double-pass prism installed in each of channel.
Instrument Hatch
- Function:
The instrument hatch covers the entrance to MODS from the telescope
light path. It consists of two sliding doors that meet in the middle
with an overlap seal. The opening is rectangular to admit light into
the central science and offset guide fields.
The hatch is open while MODS is in use, and closed to keep foreign
material falling onto the MODS focal plane when not in use. In
addition, the hatch is closed when making wavelength and internal
flat-field calibrations. This prevents MODS calibration light from
leaking back into the LBT environment, and similarly prevents outside
light from entering MODS (allowing calibrations during daytime or with
the enclosure lights on).
| Number: | One per MODS |
| Mechanism: | Dual-Direction Belt Drive sliding doors |
| Drive: | NEMA 34 drive stepping motor |
| Sensors: | P&F 8mm precision home switches |
Calibration System:
- Function:
The calibration tower is located immediately below the hatch,
mounted on a 1-axis linear slider. The tower carries a projection
optics that direct light from the calibration system integrating sphere
into the MODS. The projection optics consist of a pupil mask at the
exit port of the integrating sphere, a pickoff mirror at the base of the
tower, and a projection lens located at the output end of the tower
(located just above the slit). The lens projects a pseudo-pupil image
onto the grating, mimicing the central obscuration of the LBT secondary
mirror. During science observations, the calibration tower is stowed
out of the MODS science beam.
| Number: | One per MODS |
| Mechanism: | THK Linear Slide and stepper motor (pickoff) |
| Drive: | NEMA 34 drive stepping motor & electric brake |
| Sensors: | P&F 8mm precision home switches |
AGw Camera Stage:
- Function:
X-Y stage to carry the pair of LBT facility off-axis acquisition/guide
and wavefront sensing cameras (known collectively as the "AGw"). The
system includes a pickoff mirror (prism), collimator optics, a beam
splitter to divide the light between the AG camera and the off-axis
"slow" WFS camera), the AG camera filter wheel, reimaging optics and a
focus mechanism. The patrol field of the AGw Camera Stage is shown in
this diagram (coming soon).
| Number: | One per MODS |
| Mechanism: | X-Y stage and drive motors, 4-position
rotary filter wheel, focus mechanism |
| Drive: | NEMA 23 drive microstepping motor & electric brake |
| Sensors: | P&F 5mm precision home switches |
Slit Mask Select/Insert:
- Function:
24-slot slit mask magazine carrying the slit masks with an
insert/retract system to deploy masks in the focal plane. These masks
include a suite of fixed long-slits curved to match the focal plane, and
observer-designed, custom-made multi-slit masks machined off site and
loaded into MODS each night. The unit may also carry a future
(post-baseline) Integral Field Unit (IFU). The unit also includes an
embedded barcode reader to scan barcode labels on the masks to verify
their identity before inserting them into the science aperture.
The changer removes a slit mask from the cassete and deploys it at the
focal plane, and then removes the mask and stows it in the cassette
magazine when no longer in use.
| Number: | One per MODS |
| Mechanisms: | THK Linear slides, Bishop-Wisecarver rails, &
drive motors |
| Drives: | NEMA 34 drive microstepping motors, harmonic reducer & electric brakes |
| Sensors: | P&F 5mm & 8mm precision home switches |
| | MicroScan CCD-3 barcode scanner |
Dichroic Beam Selector:
- Function:
The beam selector is located below the slit plane and post-slit
AG stage. It consists of an indexed rotary drum carrying a dichroic beam
splitter, a silvered imaging flat, and an open position. Selecting
a position sets up one of three main beam-receiving modes of MODS:
- Dual Mode: inserts a dichroic beam splitter for Blue+Red channel operation
- Red-Only Mode: inserts a flat mirror directing light into the red
channel and bypasses the blue channel.
- Blue-Only Mode: an open position that allows light to pass straight
into the blue channel, bypassing the red channel.
| Number: | One per MODS |
| Mechanism: | Drum with bull gear, pinion drive gear, spindle bearings, and drive motor |
| Drive: | NEMA 34 drive microstepping motor & electric brake |
| Sensors: | P&F 8mm precision home & code switches |
Collimator Mirror Tip/Tilt/Focus:
- Function:
The MODS collimator mirror cell is mounted on three (3) linear actuators
to provide focus (piston), and tip/tilt beam steering motions. Blade
and torsion flexure constrain motion in translation perpendicular to the
beam and torsion about the optical axis. The tip/tilt motion is used
for both coarse alignment (to compensate for the dichroic being in/out
of the beam), and in real-time beam-steering mode to compensate for
gravity-induced flexure of the spectrograph.
| Number: | Two (2): one each per red & blue channel |
| Mechanism: | Three (3) THK linear slides, harmonic
reducers, and drive motors |
| Drive: | NEMA 17 drive microstepping motor & harmonic reducer |
| Sensors: | P&F 5mm precision home switches |
Grating Select Turret:
- Function:
Rotary turret consisting of a large bull gear and pinion drive gear that
can select one of four (4) positions that carry up to three (3) gratings
and (1) fixed imaging flat mirror. Each position is kinematically
docked for high repeatability. Gratings are tilted using the Grating tilt drives.
Note that because of the cable wraps for the grating-tilt drives, this
mechanism cannot move through an arbitrary number of rotations, and is
constrained to move 0-270° CW and CCW (i.e., it cannot rotate
through the 270-360&\deg; quadrant). Thus, while a rotary system, the
effective drive topology is the same as that of an indexed linear
drive.
| Number: | Two (2): one in each channel |
| Mechanism: | Custom bull gear, bearing, drive motor, pinion drive gear |
| Drive: | NEMA 34 drive stepping motor & Bayside
reducing gear |
| Sensors: | P&F 5mm precision code switches |
Grating Tilt Drive:
- Function:
Pre-loaded worm gear driving a 15-inch 120° sector gear to tilt the
gratings with high precision and repeatability to select the central
wavelength and/or order of interference used. Each grating in the Grating Select Turret has its own independent tilt
drive (the imaging flats are not driven, but are mechanically tilted
during optical alignment and then locked down). Torque-limited
couplings are used to provide safetly against overdriving the grating.
Each gratings is individually counterweighted to provide precise balance
about the tilt axis.
While technically a "rotary" mechanism, because the sector gear is
constrained to 120° the effective drive topology is the same
as that of a 1-axis linear mechanism.
| Number: | Three (3) per Grating Select
Turret per channel |
| Mechanism: | 15-inch 120° sector and worm gears, and drive motors |
| Drive: | NEMA 23 worm drive stepping motor, harmonic
reducer, & torque-limited coupling.
reducing gear |
| Sensors: | P&F 5mm precision home & limit switches |
- Note: In the baseline deployment of MODS1 and MODS2, one of
the grating cells in each grating wheel will be occupied with
double-pass prisms. Since these do not require active tilt drives, we
will use an imaging flat cell, so the mechanism count is reduced by 2
mechanisms/MODS.
Camera Shutter:
- Function:
The shutter for each camera is located on the inside flange of the
camera corrector lens cell. This locates the shutter near the pupil
plane, resulting in very little field-dependent "shading" due to finite
shutter open/close time. The shutter is a 2-bladed, belt-driven
mechanism. The shutter blades are composed of a graphite epoxy skin on a
foam core, with foam seals are used to make the shutter light-tight. The
shutter blades require 0.3-sec to open and close, giving a 0.27-second
minimum exposure. The shutter opens along the slit axis, so that all
wavelengths at a given slit position are exposed simultaneously (no
wavelength-dependent shutter function), while there is a slight shading
of 0.4% along the slit for a 1-second integration, up to 1.5% for the
shortest 0.27-sec exposure. The shutter aperture is 430mm x 230mm.
Note that while the motion of the shutter blades is rotation about one
side, because this rotation is constrained to between 0 and 90°, the
effective drive topology is the same as a 2-position linear
mechanism.
| Number: | Two (2): one in each camera (red & blue) |
| Mechanism: | Belt-and-gear drive with a single drive motor. |
| Drive: | NEMA 23 drive microstepping motor with gear reduction
and a timing belt drive |
| Sensors: | limit switches on each blade for open/closed |
| Other: | Knob on the motor shaft for manual override |
Camera Focus:
- Function:
Moves the camera primary mirror to focus the camera. Because the
camera is an off-axis design, this mechanism uses three (3) coupled
linear actuators geared slightly differently to provide the required
coordinated translate/tilt motion to keep the mirror optically aligned.
| Number: | Two (2): one in each camera (red & blue) |
| Mechanism: | Belt-and-gear drive with a single drive motor. |
| Drive: | NEMA 23 drive stepping motor |
| Sensors: | P&F 5mm precision home & limit switches |
Camera Filter Wheel:
- Function:
8-position rotary gear-driven filter wheel with radially-mounted filter
cells, one per camera. The filter wheel is driven using a pinion gear
and a custom bull gear. Filter wheel positions are kinematically docked
for high precision and repeatability. Filters are mounted in permanent
metal cells that are inserted/removed radially and kinematically dock
into position.
| Number: | Two (2): one in each channel |
| Mechanism: | Custom bull gear, spindle bearing, drive motor, pinion drive gear.
Removable machined filter cells are radially inserted and
kinematically dock. |
| Drive: | NEMA 23 worm drive stepping motor |
| Sensors: | P&F 8mm precision code switches |
