All TripleSpec instruments must include adjustments to fore-optics and slit viewer to accommodate the telescope’s f/#. TS4 will be mounted at the 4m Blanco telescope’s f/8 Ritchey-Chreti´en focus. A Lyot mask is placed at the image of the primary mask formed by the first OAP to minimize the thermal emission from the telescope supports and central obscuration (which is set by the DECam instrument). Additionally, we will be adding some new features to the slit substrate, improving the short wavelength performance of the optical coatings and changing the slit viewer to J band.
A.3.1
Slit Substrate Changes
The TripleSpec spectrograph will contain a cold slit that accepts a small area of the sky for spatially isolating a science target, minimizing the amount of back- ground radiation and increasing the spectrograph’s resolution. The rest of the 4’×4’ field is reflected to the slit viewing optics, shown in Figure A.2. As with the Palomar and APO TripleSpec instruments, TS4 will have an etched silicon wafer coated with a gold that results in a very low surface roughness surface (< 2nm) (Vandervelde et al., 2006). The slit substrate, fabricated by Michael Cabral (Virginia Commonwealth University), is produced by wet etching the substrate from the front and rear surface and removing the remaining silicon with a dry etch.
put and thus increase the instrument’s sensitivity but have lower resolution. Narrow slits have lower throughput but their higher resolution improves radial velocity precision and resolves individual telluric (Earth’s atmospheric) airglow lines for easier background subtraction. For the seeing conditions at CTIO (with a median of 0.9” Full Width at Half Maximum (FWHM) and the science needs of the spectrograph, 1.1” is the optimal width.
Figure A.4: Previous slit design compared with two different slits being fabricated for TS4. The silicon slit substrate (pictured in Fig- ures A.1 and A.2) accepts a small region to the spectrograph and reflects the rest of the 4’ ×4’ field to the slit viewer. TS4 will have 1.1” × 1.1” outrigger boxes for easy identification of airglow OH lines, which are used for wavelength calibration. Additionally, a “Dumbbell” option includes two 3.5”×3.5” boxes for high throughput, lower resolution observations of exoplanets and asteroids. One of the two slit options shown here will be chosen after testing.
Previous slit designs were rectangular in shape, seen in Figure A.4, but TS4 will have added features. Two outrigger boxes are placed on the ends of the slit which will make square-like sources of the telluric OH airglow lines on the detector for easy identification and location. These airglow lines can be used for wavelength calibration and tracing the spatial and spectral directions on the spectrograph.
tion. The dumbbell slit contains two boxes 3.5” each towards the ends of the slit. Point sources such as exoplanets and asteroids placed in these boxes will have minimal slit losses, but with lower resolution spectra. The large slit widths within the boxes reduce the spectral slopes created by differential slit loss, such as caused by the variation of stellar FWHM with wavelength due to seeing and position due to refraction. Two boxes are used to enable sky subtraction when nodding the 25.2” distance between the box centers. The remaining 21.7” portion of the slit can be used in the conventional TripleSpec observing mode with sufficient spatial room remaining to nod point sources or extended sources along the slit and obtain higher resolution R ∼ 3200 spectra. In all, four or- thogonal slits are cut into the substrate, with the final choice selected by the orientation of the substrate. The three unused slits are outside the 4’×4’ field of view so they do not appear on the slit viewer detector.
A.3.2
Improved Coatings
The Palomar and APO TripleSpec instruments had about half the expected transmission for wavelengths 1.0µm and shorter because the coatings’ short wavelength cutoff was mistakenly specified to be 1.0 µm. As seen in Figure A.5 Top, the ZnSe prism transmission drops off sharply at these wavelengths. The new ZnSe TS4 coating shown in Figure A.5 Bottom has less than 1.5% re- flectance across the entire 0.8µm to 2.45µm wavelength range. The new coatings are expected to improve the short wavelength sensitivity of TS4 because they are correctly extended to 0.8µm.
Figure A.5: Top Transmission of various components of the Palomar TripleSpec system suggests that the ZnSe prism coatings may be responsible for the lower than expected sensitivity at short wavelengths. Bottom Reflectance for the new coatings from II- IV, which may improve the overall short wavelength perfor- mance.
A.3.3
J Band Slit Viewer
TS4 will have a J band slit viewer for imaging the field and aligning a science target on the spectrograph slit. The J band slit viewer will have lower thermal
backgrounds than the APO and Palomar KS band imagers, which require sky
subtraction for all but the brightest guide stars. As with the Palomar TripleSpec, an imaging solution was found with two aspherical lenses, shown in Figure
A.2.4 Due to their gradual slope of index of refraction as a function of wave-
4The slit viewer must be modified for each telescope (despite working to the same f/#) be-
length across the J band, CaF2 lenses were found to have the best performance. The smaller downstream surface has two curved surfaces and was constructed with a flat ring surrounding the clear aperture on the upstream surface for sim- pler mounting. The ray-traced spot size FWHM of 0.48” is well below the me- dian CTIO seeing of 0.9”. The filter’s transmission is above 50% from 1.17µm to 1.33µm.
A.3.4
Detector Upgrade
Previous TripleSpec instruments had Hawaii I and Hawaii II Teledyne arrays which will be replaced in TS4 by Hawaii-II RG HgCdTe arrays, also from Tele- dyne. The Hawaii-II RG detectors have reference columns, which allow one to subtract and reject common-mode noise sources. The substrate-removed HgCdTe sensors also have enhanced J-band quantum efficiency. The array mounts and cabling were re-designed to accommodate the new arrays, but their mounting schemes are similar.
The slit viewer detector is an engineering-grade array for the purposes of aligning and orienting the slit and guiding. Only a 1024×1024 portion of the 2048×2048 array will be read out for a field size of 907× 926 pixels. A 1024×2048 portion of the spectrograph detector (science-grade) will be used for science exposures.
Both the slit viewer and spectrograph arrays will be read out with the current generation Leach controllers. The user interface will be written in ArcVIEW
the option to read out a small sub-array window at a faster cadence (a few Hz frame rates) for guiding or monitoring of the sources on the spectrograph slit while simultaneously acquiring data on the full array. The fastest expected full frame rate for the guider will be ∼1Hz. The slow read out mode is selected over the fast mode to minimize read noise, which is important for faint astronomical sources.
Initial tests of the slit engineering grade viewer detector indicate perfor- mance within manufacturer specifications. The read noise for correlated double
sampling was 14e−/px with a noise floor of 4.5e−/px for Fowler 16 sampling.
Inter-pixel capacitance was measured to be ∼3% and 95% of the pixels are linear
to within 5% up to counts of 90,000 e−. The spatial and temporal gain were an-
alyzed independently and converge on a value of ∼1.8 e−/ADU (analog digital
unit). The science grade detector for the spectrograph is expected to have better performance.