TSpec4 Technical Report 1 ID: TSpec4TR #02 Subj: The TripleSpec Design Date: February 17, 2012 (v2.0) From: Terry Herter, John Wilson, Chuck Henderson To: The TSpec4 Team Summary: This report briefly summarizes the original design of TripleSpec (TSpec) as relevant to TSpec4. The performance is discussed in TSpec4TR #1 (TSpec4 Requirements) while the design changes required for TSpec4 are given in TSpec4TR #3 (Design Changes). OptoMechanical Layout: The basic design of TripleSpec consists of three key components: reimager, slit viewer, and spectrograph. The reimager transforms the incoming telescope fnumber to match that of the spectrograph. It also contains a field stop, Lyot stop, and at its output is the slit. A roughly 4’x4’ field is reflected from the slit plane to the slit viewer camera which is used for acquisition and guiding. The light passing through the slit enters the spectrographic portion of TripleSpec where a prismatic crossdisperser provides order separation ahead of the reflection grating and 7element camera. The slit viewer uses a 1024x1024 NIR array while the spectrograph employs a 2048x1024 section of a separate NIR array. Data acquisition occurs independently for the two arrays. All versions of TSpec (including TSpec4 for Blanco) have identical spectrographs. The changes are in the reimaging and slit viewer sections. Figure 1 shows a toplevel solid model of the TSpec design. The main (clam shell) LN2 cryogen tank cools the internal optics, mechanical structures and slit viewer detector. TripleSpec is designed to operate at any orientation so that the 120 liter main tank is only halffilled giving roughly a three day hold time. A twoliter secondary cryogen tank (not shown) provides a stable temperature base for the spectrograph detector obviating the need for active thermal control of the focal plane. The hold time of the secondary tank is many weeks. Three rigid bulkheads (Figure 2) section off the instrument volume and provide dimensional stability and mount points for the optical components. The resulting dimensional and thermal stability are excellent.
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ID: TSpec4-‐TR #02
Subj: The TripleSpec Design
Date: February 17, 2012 (v2.0)
From: Terry Herter, John Wilson, Chuck Henderson
To: The TSpec4 Team
Summary:
This report briefly summarizes the original design of TripleSpec (TSpec) as relevant to TSpec4. The performance is discussed in TSpec4-‐TR #1 (TSpec4 Requirements) while the design changes required for TSpec4 are given in TSpec4-‐TR #3 (Design Changes).
Opto-‐Mechanical Layout:
The basic design of TripleSpec consists of three key components: re-‐imager, slit viewer, and spectrograph. The re-‐imager transforms the incoming telescope f-‐number to match that of the spectrograph. It also contains a field stop, Lyot stop, and at its output is the slit. A roughly 4’x4’ field is reflected from the slit plane to the slit viewer camera which is used for acquisition and guiding. The light passing through the slit enters the spectrographic portion of TripleSpec where a prismatic cross-‐disperser provides order separation ahead of the reflection grating and 7-‐element camera. The slit viewer uses a 1024x1024 NIR array while the spectrograph employs a 2048x1024 section of a separate NIR array. Data acquisition occurs independently for the two arrays. All versions of TSpec (including TSpec4 for Blanco) have identical spectrographs. The changes are in the re-‐imaging and slit viewer sections.
Figure 1 shows a top-‐level solid model of the TSpec design. The main (clam shell) LN2 cryogen tank cools the internal optics, mechanical structures and slit viewer detector. TripleSpec is designed to operate at any orientation so that the 120 liter main tank is only half-‐filled giving roughly a three day hold time. A two-‐liter secondary cryogen tank (not shown) provides a stable temperature base for the spectrograph detector obviating the need for active thermal control of the focal plane. The hold time of the secondary tank is many weeks. Three rigid bulkheads (Figure 2) section off the instrument volume and provide dimensional stability and mount points for the optical components. The resulting dimensional and thermal stability are excellent.
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Figure 1: Simplified solid model view of TSpec showing the cryogen tank and optical components. The other shell, cryogen tanks, mounting bulkheads, baffles, etc. are not shown.
Figure 2: Image of TSpec during assembly showing bulkheads and spectrograph camera (black). The array mount (not yet installed) is in the foreground.
Dewar Window
Collimator
Slit Viewer
2 Folds for packaging
Grating
Detector
7-element camera
Clam Shell LN2 Tank: 60 liter ‘half-load’ ~ 3 day hold time
Fits inside ~30” dia, 46.5” long dewar. 700 lbs
Optics Mount to inner 3 bulkheads
Bulkheads resist ‘closing’ of LN2 tank
clam shell horns
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Re-‐imager and Slit Viewer:
Figures 3 and 4 show the front end of TripleSpec. Light enters through a ZnSe entrance window, passes through a field stop at the telescope focus and is collimated by an off-‐axis paraboloid (OAP1). The beam proceeds through a Lyot stop to another off-‐axis paraboloid (OAP2) which focused the light onto the entrance slit of the spectrograph. The spectrograph slit consists of an gold coated silicon wafer which reflects the field surrounding the slit towards the slit viewer optics (Figures 4 and 5). A fold mirror directs the light into the slit viewer camera which in the Palomar version consists of two lenses that focus light onto a 1024x1024 Hawaii-‐1 detector. A K-‐band filter is placed at the beam waist which serves as another Lyot stop. The slit is located about 100 pixels (25”) from the edge of the slit viewer field. This serves two purposes: this is the location of best image quality for the re-‐imager, and additional reference/guide stars can be found by rotating the field (accomplished by rotating the instrument).
Figure 3: Optical layout of TSpec re-‐imager which relays the telescope focus to the slit plane matching the f-‐number of the spectrograph.
Dewar Window
Telescope Focus
Off Axis Paraboloid 1
Off Axis Paraboloid 2
Lyot Stop
Reflective Slit
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Figure 4: Opto-‐mechanical layout of the re-‐imaging and slit viewer (SV) sections of TripleSpec. Numbers track the passage of the optical beam through the system. See text for additional explanation.
Figure 5: Optical path of TSpec slit viewer (SV) which receives light reflected from the slit plane. The Palomar system has a two-‐lens camera while the APO SV camera has four.
5. Camera (OAP2)
1. Dewar Window
8. Slit Viewer
9. SV focal plane (not shown)
2. Field Stop
4. Lyot Stop
3. Collimator (OAP1)
6. Spectrograph slit & mirror defining SV field
7. Fold Mirror
Reflective Slit Plane
Fold Mirror
Lens 1 (ZnSe, aspheric)
Lyot Stop + Ks Filter
Lens 2 (ZnSe, aspheric)
Hawaii-1 detector
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Spectrograph:
Figures 6 and 7 show the spectrographic stage of TripleSpec. After passing through the slit the light is collimated via an off-‐axis paraboloid. Two fold mirrors redirect the light which passes through two ZnSe prisms and an Infrasil prism which provide the low-‐resolution cross-‐dispersion for order sorting. Next a grating diffracts the light into the 7-‐element camera which focuses onto a 2048x1024 section of a Hawaii-‐2 HgCdTe array. The camera includes elements of CaF2, Infrasil, Cleartran (ZnS) and ZnSe. One surface is aspheric and one is a conic; the balance are spherical.
Figure 6: Two views of the optical layout of the spectrograph portion of TSpec. Light passing through the slit is collimated, reflected off two fold mirrors and passes through three prisms before reaching the reflection grating. A 7-‐element camera images the spectrum onto a 2048x1024 pixel detector area.
Reflective Slit Substrate
Collimator (Off-axis Paraboloid)
3 Prisms (in series)
Detector 7-element Refractive Camera
Reflection Grating
Fold mirrors
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Figure 7: Opto-‐mechanical layout of the spectrographic (post-‐slit) section of TripleSpec. Numbers trace the passage of the optical beam through the system. See text for additional explanation.
4. Grating
1. Collimator 3. Prisms
2. Fold mirrors
6. Detector
5. Camera
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Component Hardware:
Table 1 lists vendor-‐provided major components of TSpec with delivery times updated for TSpec4 using vendor supplied quotes. (The TSpec4 schedule adds margins to the procurement time to established schedule contingency.) Although there is a necessary change in the precise optical prescription of the re-‐imager OAPs and the slit viewer lenses, the specifications (i.e. surface accuracy requirements) are expected to be the same. (A full tolerancing analysis of the TSpec4 reimager and slit viewer will be conducted prior to ordering these optical components.) Note that the spectrograph camera is procured as a fully assembled and tested subsystem from a single vendor, as was the case for prior versions of Tspec.
Figure 8 shows the order layout for TSpec. The array is oriented (rotated about the optical axis and translated) to place the third order (K-‐band) roughly parallel to the “top” of the array. TSpec uses a 110.5 lines/mm grating replica grating fabricated from a GNIRS master. Figure 9 shows the variation of the spectral resolution as a function of wavelength and order. The as-‐designed spectral resolution is achieved with the Palomar TSpec. Moreover, the spectral resolution increases for the (physically) smaller slit size used with the APO TSpec (as expected, the resolution is limited by the slit and not any optical aberrations).
Figure 8: As-‐designed order layout for TSpec showing the wavelength range of the orders. Order widths (slit lengths) are 30” (Palomar) and 43” (APO).
1.88 2.46
1.48 1.235
1.06
0.93 0.829
1.415 1.13
0.945
0.81
0.74 0.77
1.85
0.78 0.8
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Figure 9: As-‐designed spectral resolution of Palomar TSpec as a function of order and wavelength. This spectral resolution is achieved by the Palomar TSpec. For APO TSpec the spectral resolution improves to ~ 3700, as expected given its slit size.
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Summary
The Palomar and APO versions of TSpec have met there their original design specifications. They have been acquiring science data for over three years. The changes required to adapt the TSpec design to the Blanco telescope are relatively straightforward and present no significant technical challenges. The required changes are given in TSpec4-‐TR #3 (Design Changes).
