Results from the LSST Site Monitoring. Spain

Transcription

1 Results from the LSST Site Monitoring Sebastian Els Gaia DPAC ESAC Madrid Spain Jacques Sebag NOAO Tucson USA

2 8.4 M Primary Aperture 3.5 Degree Field Of View 3.2 Billion Pixel Camera ~40 Second Cadence Two 15 second exposures Full sky coverage every few nights Public Data Top Ranked in Decadal Survey First Light in 2018 More Details at LSST.ORG 2

3 View of the Pachon ridge from Cerro Tololo in 2008 (image courtesy T. Abbott) Image from the LSST website

4 Site monitoring stations on the Cerro Pachon ridge Penyon Pachon LSST Gemini-S SOAR DIMM, AWS, Tower MASS-DIMM, AWS, Tower AWS

5 The site monitoring equipment on El Penyon - Since 2003 Automated weather station (T, RH, WS, WD, AtmP) - Since 2006 DIMM telescope: Meade LX 200 on a 5m tower - Since m tower equipped with ultrasonic anemometers Metek 5m, 12m, 20m, 30m micro-thermal probes every 2m up to 30m - Since 2008 LuSci campaigns Since Dec 2009: Comparison campaign with the TMT T1 MASS-DIMM system

6 Seasonal windroses at El Penyon and SOAR between Prevailing wind direction changes by 90 deg from Northern to Southern edge of the Pachon mountain ridge. Consistent with wind comparison at Pachon summit (Gemini) and SOAR location, see Baldwin (1999)

7 Large differences of the atmospheric conditions (wind direction) along the Pachon ridge which could also affect astronomical observing conditions * local terrain drives the airflow close to the ground might affect ground layer, thus overall (DIMM) seeing * how does the high atmosphere behave; is there anything peculiar going on above Pachon? MASS seeing

8 Site Testing Instrumentation - the combined MASS-DIMM instrument * DIMM is one of the site testing standard tool and delivers the total seeing through the entire atmosphere * MASS is - a compact device, even when combined with a DIMM (Kornilov et al. 2007) - robust - precise (Tokovinin 2007, Els et al. 2008) MASS delivers Cn^2 dh at 0.5, 1, 2, 4, 8, 16 km > seeing above 500m > isoplanatic angle > tau_0 estimate MASS electronics DIMM CCD segmentator optics

9 Comparison of MASS seeing measured at El Penyon, Pachon and Tololo Monthly median values of MASS seeing measured at all available stations: - Pachon MASS - Tololo CTIO MASS - Tololo T3 MASS - Penyon T1 MASS Statistics of Dec09-Aug10 Simultaneous (20min) data Conclusion: MASS seeing in this region is - on average - the same, no matter which mountain (2200m asl, 2722m asl or 2650m asl); It is really a 'free atmosphere' seeing

10 DIMM seeing at El Penyon In January 2006 a Meade LX-200 with a DIMM was installed on a ~5m tall tower Statistics of all collected data between Jan 2006 May 2010 => median DIMM seeing = 0''84 => comparable to the old Tololo analysis 0''88?!

11 Reminder: Site conditions at Cerro Tololo > DIMM installed in 2001 (Boccas 2001) > first MASS/DIMM data in Tokovinin et al. (2003) > upgraded to MASS-DIMM in early 2004 => Tololo median DIMM seeing : 0''88

12 The TMT-T3 system operated over several months ( ) on Cerro Tololo only 10m away from the CTIO site monitor (details in Els et al. 2009)

13 The DIMM seeing measured by the CTIO monitor IS compromised during northern winds. Meaning: quite a lot of the time! But can be corrected using simultaneous MASS measuements: ==> Tololo turbulence parameters A similar effect at the LSST DIMM?

14 Findings of the TMT-Tololo DIMM comparison led to the decision to conduct a comparison with LSST DIMM. The TMT T1 system was lended by the TMT project to conduct this campaign. T1 history: Oct Aug 2007 on Cerro Tolar (TMT site testing) Aug Nov 2009 on Vizcachas (E-ELT site testing framework) Dec Nov 2010 on El Penyon (LSST site monitoring framework) => Nov 2010 begining of site preparation for excavation!

15 Both systems were operational mainly between Dec09 - May10 misconfiguration occured in Jan10 configuration control!!! only 5000 measurements to calibrate correction of DIMM data obtained since 2006 close to 9000 measurements to compare to an opened dome Before end of January 2010 After end of January 2010

16 Ratio of DIMM seeing [asc/asc], the corrective term solid line - northern winds (crosses #data) dashed line - southern winds (asterisks #data) Under Southern winds the offset is constant Under Northern winds offset affect by dome Application to DIMM data since 2007 ( samples) 94% of those data could be corrected

17 Comparison between dome half closed open and fully opened

18 Measured with TMT-T1 solid lines indicate the median behavior dotted lines 10%, 90%iles dashed lines 25%, 75%iles dots indicate the amount of data per bin El Penyon DIMM seeing dependence on wind direction and wind speed Maybe a slight (<0''1) increase of the DIMM seeing under Northern winds A drop (~0''15) of the DIMM seeing for non-zero wind speeds * this is consistent with observations at the chilean TMT candidate sites (Skidmore et al. 2009)

19 MASS-DIMM computed ground layer strength at El Penyon (6 months using T1) compared to Armazones, Tolar and Tolonchar from the TMT survey Figure after Els et al. (2008)

20 The TMT MASS-DIMM telescopes have been used for a number of corrcalibrations, which demonstrated seeing biases due to abberations seeing biases due to objects close to the DIMM itself Shall the TMT DIMMs be considered as the standard DIMM? => they are getting close to optimal, but...

21 Conclusions 1. The LSST DIMM in its present form shows a seeing bias similar to what was found for the old CTIO DIMM. Correcting for this bias results in a median seeing at El Penyon of 0''7 2. No strong dependence of DIMM seeing on wind direction; maybe some slight increase under northern winds 3. No significant difference in the free atmosphere seeing in the Pachon-Tololo region 4. El Penyon shows similar ground layer seeing behavior as the chilean TMT sites 5. Even the TMT DIMM seems to show a seeing bias Recommendations: use proper hardware: saving at the wrong end might compromise everything ''...I believe one or two thousand dollars could well be spent in such a (seeing) survey.'' H.D. Curtis in US$ (1909) = US$ (2009) rigorous configuration control: better safe and bored than sloppy and sorry

22 LSST is a public-private partnership. Funding for design and development activity comes from the National Science Foundation, private donations, grants to universities, and in-kind support at Department of Energy laboratories and other LSSTC Institutional Members. Without the T1 site testing station this work would not have been possible and we are greatful to the TMT Project to provide this system to conduct the presented study.