memo_KJW-03-01 CSA - SRON

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Spot diagram aligned on corridor with chopper angles +10.1 resp. +9.5 deg Conclusions: • There is a potential optical interface error between the COA and CSA. • Assuming nominal chopping angles of +9.0 deg the 4w clearance criteria is violated in the corridor between the COA and CSA optics and SMIR inside the CSA is not seen. • In this situation the chief ray into the CSA optics is making an angle of 1.8 resp. 2.1 deg with respect to the z-axis instead of the specified 1.0 deg in the interface drawing from source 2. This angle will be mixer band dependent. Therefore for each band a different chopper angle is needed to look into the CSA optics. • The difference in angle of almost 1 deg also suggests that the pupil alignment is not maintained when rotating the chopper mirror. This could have an impact on focal plane chopping and the alignment budget for the FPU-telescope alignment. • Apart from this error another obvious error exists in that interface drawing. The angle of incidence and angle of reflection should be the same at PMIR. In the drawing of source 2 angles of 32 resp. 33 deg are mentioned, which can’t be true. We furthermore believe that PMIR should be called CMIR, because as the drawing is suggesting, a Cylindrical rather than Parabolic mirror is required. • Because of the detected interface error mentioned above the optics inside the CSA have not been further analysed. • The required chopper angles in order to align on SMIR are +11.3 resp. +10.6 deg. These angles are outside the limits of the chopper mechanism. 12
• The required chopper angles in order to align on the corridor between the COA and CSA are +10.1 resp. +9.5 deg. Again these angles are outside the limits of the chopper mechanism. • In any case / configuration it is not possible to look to a cold blackbody without severe 4w clearance violations or excessive / impossible chopper angles. Final conclusions Provided the sources of information are right we came to the following conclusions: (meanwhile an experiment has confirmed our findings) • Rotation around an offset pivot point is significantly different from a rotation around the mirror vertex. This is currently not taken into account. • The assumed angle between chief ray entering the CSA and the telescope z- axis of 1.0 deg is rather 1.8 or 2.1 deg, depending on the chosen chopper rotation mechanism. • In the zero-wavelength limit there are several 4w clearance violations. At the back of M4, at the edge of M5 and at the corridor between COA and CSA. • Currently the SMIR of the CSA is not seen by the Mixer-Sub Assemblies. • Although the CSA internal optics have not been further analysed, there exists an obvious error at PMIR. First of all PMIR should be a cylindrical mirror (CMIR), but also the angle of incidence should equal the angle of reflection, which is currently not the case. Consequences Given the limited rotation range of the current design of the chopper mechanism the following consequences can be identified: • Without increasing the angle of rotation required for looking into the CSA significantly, the CSA can not be seen without severe 4w violations. • Even in case the CSA can be seen, the optical design of the CSA needs to be changed in order to couple to the HBB. In specific the position of mirror SMIR needs to be changed. • Using the current design it is not likely that a CBB can be seen by changing the chopper angle. In the present situation the chopper mechanism will have to be operated at its limits in order to look to the HBB. • To chop with 1.5’ at the sky, the chopper mechanism probably needs to rotate by more than the current +/- 2.4 deg, due to the offset pivot mechanism. 13
Page 1 and 2: memo CSA/COA Interface Aan CC Van :
Page 3 and 4: memo CSA/COA Interface 2 Fact findi
Page 5: memo KJW-03-01 COA/CSA Interface Bi
Page 8 and 9: Doc. Code: I_F_check_COA-CSA-Choppe
Page 11 and 12: memo KJW-03-01 COA/CSA Interface Bi
Page 17 and 18: Last line: scan angles M6 (scan mir
Page 19: COA / telescope focal plane optical
Page 25 and 26: COA CSA interface analysis moeten w
Page 27 and 28: CODE V> rsi f1 r1 s25 z^z Telescope
Page 29 and 30: CODE V> rsi f1 r1 s25 z^z Telescope
Page 31 and 32: CSA Optical Analysis Willem Jellema
Page 33 and 34: Graphs showing differences between
Page 35 and 36: Layout for +2.4 deg rotation Spot d
Page 37 and 38: Conclusions: • There is a clear d
Page 39 and 40: Conclusions: • In all cases, with
Page 41: Spot diagram aligned on SMIR with c

memo_KJW-03-01 CSA - SRON

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