FY13 Cosmic Frontier Experimental Research Program – Lab Review Argonne National Laboratory Background Material Program Status & Plans

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6.a Progress Report (FY11, 12, 13)

6.a.1. DECam Accomplishments 2010-2013

Argonne made crucial contributions to DECam mechanical engineering, construction, and commissioning at CTIO. The DECam project funded more than $1.3M of work at Argonne, more than 20% of the entire mechanical construction part of DECam. The DECam project funded five trips to CTIO for ANL scientists to commission major pieces of the project, detailed below.

f/8 Secondary Mirror Handler: This 6-ton, 15’ tall secondary mirror installation platform was designed, built, and tested at ANL. The platform must align and install the 1-ton precision mirror to within 300 microns, and be strong enough to withstand earthquakes which are common in Chile. Figure 1 (left) shows the platform assembled and tested first at Argonne, mated with the DECam cage at CTIO (middle), and the mirror is shown mounted in front of DECam (right).

Argonne scientists made four trips to CTIO for the commissioning of this platform.

Figure : Argonne engineer Allen Zhao is standing next to the partially assembled f/8 secondary mirror platform in the ANL/HEP assembly building (left). The mating of DECam (on the Blanco telescope) and the f/8 secondary mirror handler is pictured during tests at CTIO (middle). The f/8 mirror is shown mounted on the DECam cage and ready for science testing (right).

DECam Instrument Control System: The DECam Instrument Control System controls the shutter timing, CCD temperature, DECam vacuum, and many other pieces of DECam. It was designed and tested by ANL engineers and scientists, including on-site commissioning at CTIO. A key element of this system is the dozens of alarms that protect DECam systems.
Other DECam/ANL L3 Manager Projects: In addition to the two projects described above, ANL led three other DECam mechanical projects and L3 manager positions. These included the DECam electronics cooling system, one of the telescope simulator rings, and the filter changer platform.

6.a.2. PreCam and DECam Calibrations

Our work on DECam CCDs naturally led us into a leadership position for the PreCam calibration project, since the dewars that were previously designed and built at Argonne were almost exactly the same size and shape needed for PreCam. Without the PreCam calibration project, less than 1% of the DES fields would have any standard stars; with PreCam the DES will hit a PreCam field with about 1000 standard stars every 20 minutes. Argonne designed and built PreCam, and our post-doc Kyler Kuehn led the commissioning and hardware operations through the science run. The initial survey results and hardware description was published in 2013, with Kuehn as lead author [1]. The PreCam standard star catalog of more than 200K stars is being used to improve DES global calibrations. These standards help the absolute and relative calibrations, reduce flat-field gradients, and since the camera used mini-DECam filters, improves calibrations without color terms that plague other standards.

Figure 2: The PreCam camera is shown installed on the Curtis-Schmidt telescope at CTIO, and Argonne post-doc Kyler Kuehn is working with CTIO staff on the electronics crate installation (left). The PreCam i-band coverage vs RA/DEC is shown as light-blue dots (middle), with the DES Science Verification (SV) data shown in red and DES supernova fields shown as dark-blue dots. In the SPTEast region, the comparison of PreCam standard star photometry and DES SV measurements, in i-band, are shown as a function of DES i-band (right).

The current data of the PreCam survey does not complete the ultimate grid of standard stars in all filters in the DES survey area. The survey completeness is about 67% in g,r,i while z and Y standards (which require much longer exposures) are only available along SDSS stripe 82. Kuhlmann has led discussions and possible planning of additional PreCam runs, and is the main contact with the owner of the CTIO Curtis-Schmidt telescope which was used previously. The key question is the incremental improvement in calibrations these additional data would provide. We are using the DES SV data to address these questions, and DES year-1 data will be added to these studies.

In summary, the PreCam survey has provided a unique source of >200K standard stars throughout the DES footprint, and is the only source of standards using mini-DECam filters to reduce the effects of color terms and spectral uncertainties.

[1] “PreCam, a Precursor Observational Campaign for Calibration of the Dark Energy Survey”, K. Kuehn et al., Publ.Astron.Soc.Pac. 125 (2013) 409-421

6.a.3. Simulations for Surveys


Figure 6.A.3.1 Top panel: Two BOSS footprints fit into one simulated synthetic sky. Bottom panel: BOSS galaxy clustering data compared to predictions from the simulations (solid curve) [1].
Galaxy surveys constrain dark energy in several ways. BOSS exploits a geometrical probe of dark energy, using a spatial feature in the two-point galaxy correlation function (or corresponding ‘wiggles’ in the power spectrum). Measuring the apparent size of this ruler as a function of redshift allows one to constrain the expansion history of the Universe. Because of its low level of susceptibility to systematic errors, the BAO method has become one of the key probes of dark energy. It is the primary focus of the Baryon Oscillation Spectroscopic Survey (BOSS) and of its follow-on mission, the Dark Energy Spectroscopic Instrument (DESI). Surveys also constrain the rate of growth of structure in the Universe through the measurement of redshift space distortions. This technique is complementary to the BAO method as it is sensitive to dynamical effects of dark energy, and can help distinguish new fields from modifications to general relativity.
Simulations play an essential role in enabling these measurements, calibrating the impact of nonlinear evolution, estimating the systematic errors inherent in observations and analysis methods, and, finally, in quantifying the error distributions of the various estimators that may be considered. To carry out these tasks requires running a large number of simulations that can produce synthetic surveys with excellent fidelity. In recent work (in collaboration with Nikhil Padmanabhan’s group at Yale), we have shown that a suitably simplified version of the Argonne HACC framework can accomplish this task [1] (Figure 6.A.3.1). These results formed the basis for a proposal to the ASCR Leadership Computing Challenge (ALCC) competition for a 48M core-hour simulation campaign targeted to BOSS (PI: Heitmann) in collaboration with UC Berkeley, Harvard, and Yale; the proposal has recently been approved at its full request.

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