1 NGAO Science Instrument Reuse Part 1: NIRC2 NGAO IWG December 12, 2006

NGAO IWG 12/12/062 Presentation Outline 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

NGAO IWG 12/12/063 NGAO requirements for thermal NIR imager Wavelength coverage 3 to 5.3  m (NGAO proposal value) Field of view25 ” 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/s25 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 error50 to 80 nm (Estimated)

NGAO IWG 12/12/064 NIRC2 comparison NGAO thermal NIR imager NIRC2 Wavelength coverage 3 to 5.3  m0.9 to 5.3  m Field of view25 ” X 25 ” 1k by 1k InSb detector with three plate scales results in: 10 ” X 10 ” 20 ” X 20 ” 40 ” X 40 ” Plate scale/s25 mas10 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 CaF 2 window so contributes little to background. Background is understood to be telescope and AO system dominated. Residual Wavefront error< 50 to 80 nm10 mas scale is100 nm 20 mas is ~200 nm 40 mas is “low” and limited by camera

NGAO IWG 12/12/065 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 de- rotation if required NIRC2 is equipped with handling features that make it relatively easy to move

NGAO IWG 12/12/066 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.

NGAO IWG 12/12/067 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)…

NGAO IWG 12/12/068 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

NGAO IWG 12/12/069 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

NGAO IWG 12/12/0610 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

NGAO IWG 12/12/0611 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.