The Dark Energy Spectrometer (despec): a multi-Fiber Spectroscopic Upgrade of the Dark Energy Camera and Survey for the Blanco Telescope

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4.F The Blanco Telescope

The Blanco 4m telescope was named after Dr. Victor M. Blanco (1918-2011), an astronomer from Puerto Rico, who was the second Director of CTIO (1967-1981). The Blanco is an equatorial mount telescope with a 150” diameter primary mirror. The primary mirror and prime focus cage are mounted on a Serrurier Truss (Abdel-Gawad, et al. 1969). The telescope was commissioned in 1974 (Blanco 1993). Until 1998, it was the largest telescope in the Southern Hemisphere. It has a high-quality Cervit primary mirror that provides excellent image quality. An extensive set of improvements made a decade ago included replacement of the passive primary mirror supports with an active system and alterations to the telescope environment to improve the air flow and remove heat sources. More recent improvements made for the Dark Energy Camera include replacement of the primary mirror’s radial supports, which has substantially reduced motion of the primary in its cell, and an upgrade of the telescope control system. Figure 4.12 shows a photograph of the Blanco 4m telescope with the new DECam prime focus cage installed.

Figure 4.12: Installation of the new top end of the Blanco 4m telescope with DECam. The same cage, optics, and hexapods would be used for DESpec. Image credit: T. Abbott & NOAO/AURA/NSF.

4.G Technical Summary

We have described technical solutions to the DESpec design and demonstrated that there are no significant technical risks. For the major new systems, we have provided examples that have already been designed and built for existing or near-term instruments. While these are not final technical choices, they do represent solutions that satisfy our project scope and goals with minimal R&D. The technical, schedule, and cost risks of the project appear to be manageable.

We have estimated that this instrument is approximately the same scale project as DECam and can be built at roughly the same cost and on similar timescale. Members of this team have completed the $35M DECam Project on budget and on schedule. Our experience with DECam has guided our estimate for the DESpec cost and schedule. The reference design of DESpec has 4000 fibers, 10 double spectrographs, and no ADC. An optical design for the corrector exists that allows an ADC to be added later, which would be a good upgrade option in the future (e.g.. to observe LSST fields at higher airmass). There are a number of plausible descope paths, for example a smaller number of fibers (and spectrographs) and single-arm spectrographs (for which we would require no additional CCD’s). For the double-arm spectrograph reference design, our DESpec cost estimate includes materials and construction at $27M, plus 47% contingency ($13M) (relatively high, not because of perceived risk, but because the design and scope are still under development and will be optimized based on the scientific case). Including the contingency, the total DESpec construction cost is expected to be no more than $40M. Adding an ADC would increase this number by about $1M, and adopting single-arm spectrographs would decrease the total by about $9M, including contingency. We expect that substantial costs will be defrayed through the contribution of university and international collaborators, as was the case with DECam.

5. Project Summary

DESpec offers an extraordinary opportunity for advances in cosmology and fundamental physics, and the instrument enables community access to wide-field, deep, multi-object spectroscopy in the southern hemisphere in the LSST era. Much of the required hardware is already in place with DECam. The minimal scope of the DESpec project is specified by the technical requirements to constrain cosmological parameters, but higher performance can be achieved with straightforward enhancements to the reference design. As this white paper was being finalized for distribution, the reports of the NSF Astronomy Division Portfolio Review Committee and of the Rocky III committee appeared, which both endorse the importance of wide-field, highly multiplexed spectroscopy for cosmology and galaxy studies.

Appendix: DESpec and BigBOSS

As noted above, the combination of DESpec in the south and BigBOSS in the north would enable spectroscopic surveys over the entire sky, maximizing the DE constraints that could be pursued from the ground. Here, for context we briefly compare and contrast the DESpec and BigBOSS designs. BigBOSS is a proposed 5000-fiber spectrograph system for the Mayall 4-meter telescope at Kitt Peak National Observatory in Arizona (Schlegel, et al. 2011). The Mayall and Blanco telescopes are essentially identical mechanically, and both observatories are operated by NOAO. The BigBOSS design calls for a larger field of view (FOV) than DESpec (7 vs. 3.8 sq. deg.), requiring several larger optical corrector elements and entailing an entirely new prime focus cage, active alignment system, and set of corrector lenses for the Mayall. The BigBOSS robotic fiber system would be similar to that used for the LAMOST project in China; it has a larger pitch (inter-fiber physical separation) that is technically less challenging to achieve than DESpec’s. BigBOSS would employ multi-arm spectrographs to span a broader range of wavelengths (extending down to 340 nm, to access the Lyman-alpha forest along QSO sight-lines) at slightly higher spectral resolution than currently envisioned for DESpec. BigBOSS is optimized to probe Baryon Acoustic Oscillations (BAO) to redshifts z>1 and would select spectroscopic targets mainly from WISE (infrared), Palomar Transient Factory (PTF), and PanSTARRS imaging, which are shallower than DES. Given its location at relatively high latitude, BigBOSS would be able to target only ~600 sq. deg. (~1/8) of the nominal DES survey area and up to ~several thousand sq. deg. of the LSST footprint, so it would not take full advantage of the synergy between weak lensing (DES, LSST) and redshift-space distortions from the same volume (Secs. 2, 3) nor would it be able to follow up the majority of the DES and LSST target area, nor overlap with South Pole Telescope survey fields. The deep, multi-band, precisely calibrated photometry from DES and LSST should in principle enable more efficient spectroscopic targeting, a topic requiring further study. On the other hand, the DES and BigBOSS collaborations have jointly explored the possibility of increasing the area overlap between DES/DECam imaging and BigBOSS spectroscopy through increased DES survey time in the celestial equator regions; an overlap of up to several thousand sq. deg. is possible but would require DECam imaging time beyond the 525 nights allotted to the DES project.

In order to reach the density of galaxy targets optimal for BAO studies, BigBOSS would revisit each field several times. DESpec, with a higher density of fibers on the sky, would reach the same target density in fewer visits. However, since DESpec has a smaller FOV, the two instruments would in fact have comparable speed (number of targets per unit area per unit time) for a BAO-optimized survey. BigBOSS proposes to measure ~20 million galaxy redshifts over 14,000 sq. deg. in 500 nights. More conservatively assuming longer cumulative exposure times to ensure redshift success, we estimate that DESpec could measure ~8 million redshifts over 5000 sq deg. in ~300 nights and by extension ~24 million redshifts over 15,000 sq. deg. with DES+LSST imaging in ~900 nights. DESpec would enable many other follow-up programs over the entire 20,000 sq. deg. LSST survey area as well. Given the site conditions, DESpec would be expected to have a higher fraction of useable nights than BigBOSS (~80% vs. ~65%) and better seeing (median delivered to the Blanco 0.9” compared to 1.1” for the Mayall).

The BigBOSS proposal of Oct. 2010 responded to an NOAO Announcement of Opportunity for a new instrument for the Mayall telescope; in Jan. 2011, the Large Science Program panel that reviewed the proposal recommended that NOAO work with the BigBOSS team to further develop the proposal, retire key risks to the project, and bring it to a state of readiness for submission to DOE. As indicated above in Sec. 1, NOAO currently has no plans to issue an announcement of opportunity for a new prime-focus instrument for the Blanco, but we note that DESpec already has many of its optical and mechanical parts installed on the telescope, and the construction phase is likely to be relatively fast once it begins.


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