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Second Design Review
(May 12, 2010)
Optical Design
J.J. González, A. Kutyrev, M. Wilson, E. de Leon, F. Cobos, A. Watson
(+ others at GFSC, IA-UNAM and elsewhere)
RATIR: 4-arm system with
(2-arm Visible plus 2-arm NIR channels)
Visible channel (u to i bands): dual-arm under direct imaging of the F/13 beam of
the OAN 1.3 m telescope (no correcting, field-flattening or reimaging optics)
–
blue-arm (u-r):
•
•
–
i-arm:
•
•
–
–
–
Commercial (FLI) camera (Fairchild 20402 15µm pixels) and focuser
Commercial (FLI) 10-slot filter wheel (2” round filters)
FLI camera (Fairchild) and focuser
Fixed i-band filter
Field of view: 5.3x5.3 arcmin2 (Φ ≈ 7.5 arcmin)
Plate scale: 10.54 arcsec/mm => Sampling: 0.16 arcsec/pixel (0.32 with 2x2 binning)
Future upgrade with demagnification
NIR channel (Z to H bands): dual-arm with reimaging optics
–
–
–
Demagnification (m=0.66): for larger field of view and better pixel-sampling
Pupil: for a Cold-Stop to control thermal background and scattered light
ZY-arm
•
•
–
JH-arm
•
•
–
–
Split-filter (half field for each Z and Y band)
H2RG detector (cut-off at ~1.7 µm), 20402 18µm pixels
Split-filter (half field for each J and H band)
H2RG detector (1.7 µm cut-off ?), 20402 18µm pixels
Field of view: 9.8x9.8 arcmin2 (Φ ≈ 14 arcmin)
Sampling: 0.29 arcsec/pixel
For rapid identification of high-z GRB and general-science programs
2
1.5m
Mirror
Instrument
flange
Instrument
Window
1st (r/i) & 2nd (i/Z)
dichroics
F/13 beam
(10’x10’FoV)
Blue (u-r)
CCD Camera
i-filter &CCD camera
JH split-filter
& H2RG
Cold
box
ZYsplit-filter
& H2RG
Cold-stop
Cold Y/J
dichroic
u-r filters
(10-slot wheel)
Tel. focal surface
Field-lens group
(3 lenses)
Fold mirror
Camera (3 lenses)
& dewar window
3
Beamsplitters
Arms: separated via 3 dichroic mirrors, placed in
series along the converging beam
Wedged dichroics: being tilted-glass slabs in a
non-parallel beam, dichroics are wedged for a
partial compensation of their own-induced
aberrations
Visible
channel
Near-Infrared cold
channel
CCD1 CCD2
LWP beam-splitters: all three dichroics are reflect
blue light and transmitted the red (WFE an
other effects α 1/λ)
Inclinations:
–
–
the first two dichroics are inclined ±45º
3rd low-incidence dichroic (20º) has an
orthogonal inclination
Operation conditions:
–
–
First two dichroics operate at ambient conditions
(T=-10º-16º C, P≈0.74 ATM)
Last dichroic is within the NIR dewar (T≈-185º C
in vacuum)
4
Wedged dichroics in converging beams
5
Visible channel
(direct imaging)
Performance essentially determined by the image quality
and field-curvature of the telescope
Field-curvature (Rc=540 mm): images increase by ≤0.1”
over the flat CCDs
RATIR on an idealized-telescope
(curved field)
1” and Airy
circles
For maximum throughput and lower aberrations at shortest
wavelengths, the blue arm only goes through the
instrument window before being reflected by the first
dichroic (r/i)
Conventional 2”-filters located not far from FLI cameras to
avoid vignetting
u-r band
r/i & i/Z dichroic cut-offs limit red camera to the i-band only
The wedge of the (i/Z) dichroic was optimized to minimize
aberrations within the i-band only
The Z/Y-dichroic wedge optimized for the Y to H bands
i/Z wedge => residual lateral color within i-band. For longer
λs, prism-dispersion ~ balanced by the opposite
orientation of 2nd (Z/Y) dichroic
i band
6
NIR channel
(focal reduction + cold-pupil)
Larger FoV (m=0..66 plus 18-µm pixels), better sampling,
field-curvature correction, background control
“Field-lens” group (3-lenses, all spherical surfaces)
Just after the telescope focal surface (~ 20 mm)
Controls the pupil location
Camera
Group
Cold-box:
Stop, Dichroic,
Split-filters & H2RGs
Dewar
window
Focal Surf..
Fold
mirror
Field-lens Group..
Field-lens Group
S-FTM16
2nd (Z/Y)
dichroic
BaF2 S-TIM6
“Camera” group (3-lenses, all spherical surfaces)
Power for image inversion & demagnification
Both groups at ambient conditions (-10º to 16º C)
Pupil diameter: ≈ 36 mm
Pupil walk: < 1 mm (RMS)
Cold-stop: ~ 42 mm
Telescope Focal Surface
(after window & 2 dichroics)
JH
Split-filter
Y/J
Dichroic
Cold
Stop
Dewar
Window
Cold-dichroic (Y/J): 5-mm thick
20º incidence
⅛º Wedge: aberrations for longest bands only (J&H)
Split-filters: 4.8x9.7 arcmin2 on each band
5-mm thick & 19.6 mm from detector
0.1% ghosts Φ=110” – 145” (over 380 pixels)
Gap Field-loses: 36.5” +16” Δ[mm] (~9%)
Camera
Group
(S-FTM16, BaF2, S-FPL53)
Back Focus compensates beyond operation ΔT
7
NIR channel
(Design Performance)
Image-quality performance not far form diffraction limit
Residual Astigmatism/Coma from inclined dichroics
reasonably compensated up to 14-arcmin field
“Prismatic” residual lateral color present but OK
8
NIR channel (Performance vs. T)
EE at 3ºC Z+Y bands (0.83-1.14 µm)
EE at 20ºC Z+Y bands (0.83-1.14 µm)
Back-focus, moving split-filter and detector together,
compensates over the operation T interval (~30µm/ºC)
Thermal gradients are very small: ΔT<0.2ºC
(lenses have very similar thermal mass & dimensions)
9
Optical Prescription (20º C)
10
After-construction Budget
Field and Camera groups (lenses cells and barrels) to be subcontracted as independent units
Subcontractor manages his own manufacture tolerances and compensators, but using
the same overall-system merit and prescription
Integration of the barrel units can be carried out under relaxed tolerances (nominally 100
µm displacements and 3-arcmin tilts). Compensators: fold-mirror, back focus and oversized cold-stop
11
Different glass-combos are possible
Allows for interaction with contractors to evaluate:
Blank and material properties, availability and delivery timescale
Polishing and manufacture difficulty
Manufacture sensitivities and compensations
AR-coating optimization
Cost and delivery of finalized units
Glass Combination
System ID
FT1
FT2
FT3
CT1
CT2
CT3
Demerit
BAF2_BAF2
S-TIH6
BAF2
S-FTM16
S-FTM16
BAF2
S-FPL53
δ (%)
S-FPL53_BAF2
CAF2_BAF2
CAF2_CAF2
S-FPL53_CAF2
"
"
"
"
S-FPL53
S-FPL53
CAF2
S-FPL53
"
"
"
"
"
"
S-NBM51
"
"
"
CAF2
CAF2
"
"
"
"
2.41%
7.31%
8.67%
10.02%
S-FPL53_S-FPL53
"
S-FPL53
"
S-NBM51
S-FPL53
"
12.06%
Pupil Blur [mm]
Axial
50% EE Diameter ["] 10´x10´ FoV
Pupil Radius
Min
Mean
Δ
Worst
Focus
Plate Scale
[µm/"]
System
JH
ZY
Mn
Mx
Δ
JH
ZY
JH
ZY
JH
ZY
[mm]
JH
ZY
BAF2_BAF2
0.23
1.13
17.64
18.78
1.14
0.24
0.19
0.36
0.27
0.45
0.37
0.00
62.59
62.49
S-FPL53_BAF2
CAF2_BAF2
CAF2_CAF2
S-FPL53_CAF2
0.22
0.30
0.21
0.30
1.30
1.39
1.32
1.28
17.46
17.43
16.75
17.09
18.73
18.76
18.45
18.75
1.26
1.33
1.70
1.67
0.24
0.24
0.24
0.24
0.20
0.21
0.20
0.20
0.36
0.36
0.39
0.37
0.29
0.29
0.30
0.30
0.49
0.51
0.49
0.51
0.40
0.41
0.40
0.42
4.15
2.14
-6.92
2.90
62.59
62.59
62.62
62.62
62.49
62.49
62.47
62.47
S-FPL53_S-FPL53
0.18
1.25
16.76
18.95
2.19
0.24
0.23
0.38
0.32
0.51
0.40
-1.69
62.60
62.48
Nothing is for free, but even a system without crystals can be considered
12
Expected Thermal Background
Worst scenario considered:
• Warm night (17ºC)
• Oversized cold-stop (plus 2 to 8
mm)
• Detector cut-off at 2.5 µm
• Filters blocked to 10-4 in K
• Cryostat K-emission not blocked
Instrument background (unmasked
structure and RATIR optics):
• Out-of-band dominated in Z, Y & J
• Mostly in-band emission in H
=> RATIR NIR-arm is always sky-limited
(Sky/Inst >> 1)
13
10-σ limiting magnitudes (AB)
GRBs after 20 minutes
General Science in ½ hr integration
14