NAHUAL REVIEW AND COMMENTS IAN MCLEAN UNIVERSITY OF CALIFORNIA

NAHUAL EXTERNAL REVIEW REPORT FROM ULLI KAEUFL 16092008 TO
NAHUAL REVIEW AND COMMENTS IAN MCLEAN UNIVERSITY OF CALIFORNIA





NAHUAL Review and Comments

NAHUAL Review and Comments

Ian McLean

University of California, Los Angeles


Summary:

The NAHUAL team has done an excellent job on the conceptual design. NAHUAL is a powerful and competitive instrument that will serve the GTC well. I was impressed to see the level to which the team had carefully reviewed many other instrument designs and approaches in order to understand feasibility issues, identify risk and reduce costs. After reviewing the design and scientific case in detail I became somewhat concerned that too much was being attempted in a single instrument. You have a very strong scientific case for one mode and there is real risk involved by adding multiple modes. My advice is to “keep it as simple as possible”. Adding more mechanisms will mean creating more places where mechanical problems can occur, with the potential loss of stability and perhaps loss of performance, increased cost and greater likelihood of a longer or even a delayed schedule. I believe that the NAHUAL optical design is very good in principle. Clearly however, some more mechanical analysis will be needed to be sure of the stability issues. Your goal of 5 m/s for radial velocities in the infrared has not been achieved before and even small thermal or pressure effects could be critical depending on the detailed design of the NAHUAL dewar and mechanisms. Nevertheless, with the caveat of making the instrument a simple as possible to achieve the primary scientific goal, I strongly recommend moving on to the preliminary design phase and congratulate the team on a successful conceptual design.


Comments and Recommendations

Top Level Scientific Requirement 4.2.3 defines the Main Mode as High Resolution Echelle Spectroscopy with natural seeing, and it further states that the other science modes “cannot compromise” this main mode. However, Requirement 4.2.4 then list four Optional Modes with diverse sub-requirements which could easily compromise the main mode. Building four additional optional modes into the Top Level Requirements creates a higher level of risk and complexity for the instrument. In my view it would be better to establish the best design for the Main Mode and then show that this design can accommodate additional modes without risk to the main mode. If you make it a requirement then the design of the Main Mode will always been influenced by the need to incorporate the “optional” modes.

Recommendation: Simplify the design by eliminating some or all of the optional modes.


The optical design is very powerful, with the use of the “white” pupil and all-reflective metal optics. Folding the beam with small flat mirrors near the entrance slit seems like a good idea to allow other modes, as long as these don’t compromise the Main Mode. My one concern is with the Beam Stabilization mentioned in 5.3.2.1.8.1 where the option is to mechanize a mask and place it directly on the echelle surface. I would be very concerned about having any mask mechanism so close to the echelle grating surface. Recommendation: Consider whether or not a mask on the echelle is really necessary.


All the optical components and mechanisms will be mounted on an optical bench cooled down to 77 K using a liquid nitrogen can that is thermally attached to the optical bench, or forms part of the bench as in GIANO. I am not sure that this is really going to work. Hopefully you can learn from the GIANO experience. I strongly recommend that the LN2 cold surface be as mechanical de-coupled as possible from the optical bench in order that no mechanical stresses are transmitted to the bench. That is, I don’t like the idea that the bench is part of the cold face. Maybe it will be fine, but I think a detailed thermal and mechanical study is needed, especially because of the exceptional stability that you are demanding. In NIRSPEC we used an LN2 can that was bolt to our optical bench as an auxiliary cooling system to support the pair of CTI cryo-coolers that were also coupled to the bench but via flexible copper straps. When the dewar was under vacuum the cold face of the LN2 can bowed inwards slightly and exerted pressure on the optical bench which caused a small distortion of the bench compared to the “warm” set-up condition when there was no pressure difference. We could even detect spectral line shifts using the NIRSPEC echelle caused by atmospheric changes of pressure inside the LN2. In the end we completely abandoned the use of the LN2 can and NIRSPEC is cooled entirely by the CCRs. Thus, if your system uses LN2 cooling only, then it would be wise to find a way to transfer the cooling power using straps – as if you were using a CCR – between the LN2 cold face and the optical bench. Or, at the very least, carry out some FEA models to see what level of effects are caused by atmospheric pressure changes on the LN2 with the dewar under vacuum.

Recommendation: Study the impact of pressure changes when using an LN2 can whose cold face is in direct contact with your precision optical bench.


Pulse-tube coolers have been having some problems. Have you identified a vendor?


As a general rule it is very important to consult vendors and suppliers as early as possible to be sure that gratings, filters, prisms, lenses, mirrors, detectors, cooling systems and other specialized parts are available in the sizes that you need.

Recommendation: Involve potential vendors early in the PDR phase.


You discuss selecting the HAWAII-2 HgCdTe array, but you might consider the use of the H2-RG device together with the SIDECAR ASIC chip as a way to simplify your detector electronics and also as a means to achieve a lower dark current – critical for a high resolution spectrometer – and less charge persistence too.


Conclusions:

The conceptual design of NAHUAL is excellent. Congratulations to the team. Some simplifications and a reduction in scope by eliminating or at least deferring “optional” modes would greatly assist in reducing cost and getting the project moving forward into the PDR phase. More thermal and mechanical design analysis is needed to fully understand the stringent stability issues and how to mitigate any problems. However, I do not believe there are any real show-stoppers at this stage. I recommend that you move on to the PDR phase.





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