arXiv:astro-ph/0412692v1 30 Dec 2004 h asswihmil otiuet erdto npoint e in degradation gain a and to ing contribute mainly which causes The Introduction 1. aevleframliee-aeeghtlsoeo 4mete 64 sur- of rms telescope is maximum millimeter-wavelength a the for (1) value Hoerner face von to According meters. i environment the when severe. degraded and be conditions, conditions, benign with environmental on depends clearly accuracy, wi and gradients fects). (thermal errors non-repeatable an critical, deformations) gravity alignment, mechani initial example during (for errors errors repeatable classes: main two in qaeacrc fterflco ufcs hudb of be should surfaces, reflector the of accuracy square of rule aito,n rcptto,artmeauebetween arcsec sola temperature 1 air without i.e., precipitation, means no width, radiation, which beam the conditions, of environmental tenth good a in be should GHz, 100 at λ/ coe 2h1t 04 oeo Spain Toledo, 2004, (ed 12th-15th P. 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Cagliari, of Observatory Astronomical ntteo ai srnm CR,vaP oet 0,40129 101, Gobetti P. via (CNR), Astronomy Radio of Institue 190 min hc en a means which , o oipoeteHg rqec aaiiiso SRT of capabilities Frequency High the improve to How ∼ µ m 16 ffi n h rcso o o-eetbepointing non-repeatable for precision the and , h R Srii ai eecp)i eea ups,ful purpose, general a is Telescope) Radio (Sardinia SRT The σ inyo ai eecp,cnb divided be can Telescope, Radio a of ciency trtracker star . eprtr probes temperature ffi inya 0 H,teroot-mean- the GHz, 100 at ciency < λ .Pisanu T. min 10 o of C ytmfrassigpointing assisting for system / ∼ h n nhumidity an and 9 1 GHz anemometers .Morsiani M. , sn h empirical the using ewr o mea- for network ae iha with laser ff − rn sub- erent 10 < n pres- and 1 .Pernechele C. , 90%. o more d def- nd and C s.) ǫ tter ffi cal rs, shge hn2 GHz. 22 than higher es ∼ dna-Iay.I ilb natnawihcudipoealo a improve could which antenna an be will It Italy). - rdinia re akTlsoe S) M LreMliie Telescop Millimiter (Large LMT USA), Telescope, Bank Green inyi rqec ag rm30Mzu o10Gz This GHz. 100 to up MHz 300 from range frequency a in ciency s r - taa5,002CptraC) Italy Capoterra(CA), 09012 54, strada H ihafl opeeto instrumentation. of complement full a with GHz et fpitn n e and pointing of nents r ai-eecp nMdcn Blga,bfr ftheir of before (Bologna), Medicina in radio-telescope ers ar ae ntecasclprbl-lis rgra c Gregorian parabola-ellipse classical the on based pair, m gaddvlpn di developing and ng Fr). , o2 H,adte prd ta uuedt ooeaeat operate to date future a at it upgrade then and GHz, 22 to p nen prtoa oe,oeu o2 H ihaminimal a with GHz 22 to up one modes, operational antenna uepoe o esrn ide wind measuring for probes sure nen o sitn h erlg eeomn n o pre for deformations, thermal and and development wind metrology dicting the assisting for antenna oi tt ae,wt S rmDm Optronics Duma from PSD a th 10 two case, has this with sensor In head PC. laser, AlignMeter model the state (www.duma.co.il), solid a 1 arc-second. lin- of 1.0 a accuracy of meters, straightness accuracy a 25 angular p.p.m, to 1 up of range accuracy linear ear Laser a 6D with API System, the Measuring (www.apisensor.com), Apisensor by duced important more its measure to freedom. able of sensor, degrees head away far ters solution. technical ideal the surface. reflecting main accurate the be of vertex must the subreflector to relative the known of position 64-met the SRT the telescope, for accuracy pointing required the achieve To alignment Sub-reflector 2. a and deformations track rail ranging and aliadade measuring rm30t 10nm. 1100 to 300 from ae emdrcinadteohroe hc si h focal the in deviations. is angular which the measure one, lens, other a th of the to plane and perpendicular direction translations beam the laser measuring for one PSD, eouinis resolution O Italy BO, eaeas etn esepniesse,using system, expensive less a testing also are We pro- that is requirements, our satisfy could which system A me- 20 a and laser stationary a of consist shall is system freedom The of degrees five with system metrology laser A h oiinrslto sbte than better is resolution position The 2 yserbe ciesraeeupe,6 eesantenna, meters 64 equipped, surface active steerable, ly .Bu F. , ytmfrmauigsrcua dimensions. structural measuring for system ff ± a 2 ff ffi 2 rn ye fsrtge,instrumentation, strategies, of types erent arcsec n .Vargiu G. and inyerr,tkn datg lofrom also advantage taking errors, ciency × h prtoa pcrlrnegoes range spectral operational the , 10 mm ulai iio aea E Lateral silicon axis dual 1 ff cs a ects, w inclinometers two ± E model FEM 1 µ m n h angle the and , imple- onfig- the t µ m e, n an and fthe of laser ff for ect ly er e e - 294 T. Pisanu, M. Morsiani, C. Pernechele, F. Buffa and G. Vargiu: How to improve the High Frequency capabilities of SRT

3. Temperature monitoring 6. The Finite Element Model A network of temperature probes distributed on the whole The improvement in the predictions from FEM software sim- structure, (alidade, backstructure of the primary mirror, pri- ulations, involved us to use it as a diagnostic tool for knowing mary panels, quadrupod and secondary mirror) has been also the more critical points in the antenna structure and for know- foreseen. ing the thermal and wind effects on it. Because we have abandoned the originally planned thermal There are many studies related to the use of FEM simu- insulation and forced ventilation of the reflector surface, as is lations for predicting and correcting non-systematic errors in at the 30 m IRAM telescope(2), this network of temperature radio-telescopes. sensors is very crucial. We already have at home the FEM model of the antenna, The optimal position of each probe will be set through a developed for us by BCV consulting with the ANSYS soft- FEM analysis software which, according to the measured tem- ware, for evaluating the compatibility between the preliminary peratures, will drive the active surface actuators in a open loop project and the executive project. fashion. Once that the structural critical point have been selected, The sensors which we acquired and tested in the laboratory, we can use the FEM together with the measured temperature are Negative Thermistors NTC from YSI, model GEM 55036 and wind pressures, for predicting and correcting the pointing with a Resistance Ω at 25oC of 10kΩ and a tolerance inter- and efficiency errors. changeability of ±0.1oC from 0 to 70oC. We performed few laboratory test, introducing our sensors 7. Inclinometers in a cooler and measuring their responses from -20 up to 70oC. The results are in a very good accordance with predictions. The tilt meter subsystem will consist of two bi-axial tilt meters, with one tilt meter located near each of the elevation bearings. The purpose of the tilt meters is to measure changes (due to 4. Optical Star Tracker wind and temperature) in the orientation of the elevation axle The pointing model and the tracking, will be supported by an relative to the encoders. Once these rotations are measured,a optical star tracker (OST), which we are planning to develop portion of the pedestals pointing error can be calculated and and to use also in daylight situations. then removed. In order to have enough sky coverage it will be realized A described before, the metrology system, uses wind pres- by using a commercial Maksutov-Cassegrain optical telescope sure sensors, thermal sensors, and subreflector laser metrology with a primary diameter of 180 mm and an aperture of f/10. to compute and correct for pointing errors. The CCD will be a peltier cooled 512x512 pixels with 20 mi- The information from the tilt meters will be used mainly as cron of pixel size. The covered field of view results of 19 x 19 a sanity check and redundant back-up system. arcminutes, while the plate scale is of 2.3 arcsec/px. The lat- ter seems to be well compatible with the typical atmospheric 8. Laser RangeFinders conditions of the site. Initially we are planning to use the OST only for monitor- Finally, the possibility to install laser rangefinders for monitor- ing purposes, of the pointing model and of the antenna servo ing the primary reflector surface deformation, and if necessary system. The last step will be one of realize the auto-guide dur- the pointing with the help of triangulation algorithm, as fore- ing day time, using a IR filter able to cut off the visible wave- seen also for the GBT, is under study. lenghts. 9. Conclusions 5. Pressure probes The possibility to use SRT up to 100 GHz, will depend from the performances of the metrology system mounted on it. The wind pressure sensor system hardware is required for good We are involved into the study of different sub-systems, for pointing accuracy whenever the wind is in excess of ”preci- measuring and correcting the structural deformations produced sion” wind conditions. by gravity, temperature gradients and wind. The accuracy of this hardwarewill greatly impact the point- Few of those sub-systems are redundant, because of their ing, focus, and surface accuracies of the telescope. unknown performancesand so we are experimentingmore than The wind pressure sensor hardware shall operate at full ac- one in such a way that we could use the more performing. curacy when subjected to ”Normal” operating conditions, i.e., an ambient temperature range of -10 C to +40 C, with humid- ity 90%. When operating in ”Extreme” conditions, i.e., -15 to References +50 oC, degraded accuracy is permitted. All components shall o Von Hoerner, S. 1967, in Ap. J. Vol. 72, No. 1, February 1967, pp. survive (non-operating) in temperatures down to 30 C. 35-47 From the FEM model,we havefoundthat for havinga good J.W.M Baars., A. Greve, B.G. Hooghoudt, J.Penalver, ”Thermal con- are necessary a total of 152 sensors are used. This number of trol of the IRAM 30-m millimeter radio telescope, Astronomy & sensors gives significant redundancy and hence high availabil- Astrophysics 195, pp. 364-371, 1988 ity to system