Transcript Instrument-Reuse-NIRC2-121206

NGAO Science Instrument
Reuse
Part 1: NIRC2
NGAO IWG
December 12, 2006
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Presentation Outline
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NGAO requirements for thermal NIR imager
Comparison with current NIRC2 specifications
NIRC2 upgrades required to meet NGAO goals
Instrument configuration
Cooling AO and instruments
Constraints placed on NGAO system by using NIRC2
Alternative to NIRC2- starting from scratch
Questions for NGAO science team
Comments from PI to NGAO
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NGAO requirements for thermal
NIR imager
Wavelength coverage
3 to 5.3mm
(NGAO proposal value)
Field of view
25” X 25” (NGAO proposal value)
but solar system science case
requires only 2”, galactic center
science requires 10” but seems to be
a lower priority capability
Plate scale/s
25 mas
(NGAO proposal value)
Sensitivity and instrument
background
AO system + instrument background
less than 10 to 20% of sky
background emission in K-band
(SRD 1_v3)
Residual Wavefront error
50 to 80 nm
(Estimated)
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NIRC2 comparison
NGAO thermal NIR imager
NIRC2
Wavelength coverage
3 to 5.3 mm
0.9 to 5.3 mm
Field of view
25” X 25”
1k by 1k InSb detector with three plate
scales results in:
10” X 10”
20” X 20”
40” X 40”
Plate scale/s
25 mas
10 mas, 20 mas, 40 mas
Sensitivity and instrument
background
AO system + instrument background less
than 10 to 20% of sky background
emission in K-band
NIRC2 itself is cryogenically cooled except for
outside surface of CaF2 window so contributes
little to background. Background is understood to
be telescope and AO system dominated.
Residual Wavefront error
< 50 to 80 nm
10 mas scale is100 nm
20 mas is ~200 nm
40 mas is “low” and limited by camera
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Instrument configuration
• In discussion with the PI it is stated that:
•NIRC2 could operate on its side
•NIRC2 would probably cope with vertical rotation for field derotation if required
•NIRC2 is equipped with handling features that make it relatively easy to
move
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Cooling AO and NIRC2
•NIRC2 could be placed inside the proposed cooled enclosure of the
NGAO system
• In theory this temperature could be as low as -40 ºC but there are a few
concerns that would need to be addressed, most important are the
mechanism feedthrus and brakes.
• Feedthrus were tested to -25 ºC, torque doubled
•Brake is unspecified but minimum temperature probably limited by
bearings
•Various external components are a combination of commercial and
industrial temperature range, these can be enclosed in a “warm” box if
required.
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NIRC2 upgrades required to meet
NGAO goals
• Residual wavefront error may need to be reduced, especially for the 20
mas scale
• Cause for high residual error in the 20 mas scale identified as an optical
manufacturing defect (results in significant field dependent focus shift), this
can probably be reduced with new camera optics (ROM cost est. $500K)
• NIRC2 readout system is obsolete (transputer), detector is no longer
state of the art
• Remember that NIRC2 will be an old instrument come commissioning
time of NGAO (first light was July 29, 2001)…
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Constraints placed on NGAO
system by using NIRC2
• Input f-ratio is ƒ15
• 3 available plate scales (10 mas, 20 mas, 40 mas) but not 25 mas
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Alternative to NIRC2: start from
scratch
• Build only what is required: L and M band imager, one plate scale.
• Might re-use NIRC2 portions of the NIRC2 opto-mechanical
design.
• Very rough estimate from Keith Matthews is $5.5M
• NIRC2 offers much more functionality (coronagraph mode,
spectroscopy etc.) than what is currently required by the NGAO
science cases for L and M band
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Comments and Questions
• We recommend changing the name of the instrument from “Thermal
Near-IR Imager” to “L and M band Imager” to avoid confusion.
• The requirements for this instrument are sketchy, very limited treatment
so far in the current SRD.
• 25″ x 25″ FOV is based on simply scaling from the nominal plate scale
with a 1K x 1K detector, what are the real science driven FOV
requirements?
• Further work is required to define the acceptable range of residual
wavefront error for this instrument
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Comments from the NIRC2 PI
• Current combined emission from telescope and AO system is nominally
acceptable in L band but is terrible in M. We may want to study the telescope
background further in M band.
• The current K-mirror causes lots of problems due to non-uniformities in the
mirrors and pupil wander due to wobble. Adds background and this
significantly reduces contrast in K-band (H-band acceptable).
• A K-mirror near the focus is a bad idea (from a contrast point of view), but a
large K-mirror away from the focus is more expensive.
• If one really wanted to do this right it should be a Cassegrain instrument with
an adaptive secondary.
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