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Thu, Oct 11th, 2012 Click on the icon to download and view this issue

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November <strong>and</strong> December <strong>the</strong> averagevalue decreases <strong>to</strong> 1-1.5 mm. From<strong>the</strong> his<strong>to</strong>grams of all <strong>the</strong> ZPWVobservati<strong>on</strong>s it is c<strong>on</strong>cluded that :15% observati<strong>on</strong>s indicate < 1 mm50% observati<strong>on</strong>s indicate 1-2 mm27% observati<strong>on</strong>s indicate 2-3 mm8% observati<strong>on</strong>s indicate > 3 mrn(iii) PHOTOMETRIC QUAL17'Y OFSKY: E%TlNC"BIONThe extincti<strong>on</strong> coefficients in U,B<strong>and</strong> V filters were determined using<strong>the</strong> 20-inch telescope at Skara-Lehsite by <strong>the</strong> IIA teams, during <strong>the</strong>irnormal stellar pho<strong>to</strong>metricobservati<strong>on</strong>s <strong>on</strong> 68 nights during1985-3,988.The extincti<strong>on</strong> coefficient Kv, Kb<strong>and</strong> Ku have been found <strong>to</strong> varybetween 0.1-0.3, 0.2-0.4, <strong>and</strong> 0.4 -0.6respectively, <strong>and</strong> <strong>the</strong> average valuesobtained are:K, = 0.07 i 0.04 mag (s.d.)Kb = 0.27 zt 0.01 mag (s.d.)Ku = 0.51 0.04 mag (s.d.)A seas<strong>on</strong>al variati<strong>on</strong> of <strong>the</strong>extincti<strong>on</strong> coefficient was als<strong>on</strong>oticed. During July August, <strong>the</strong>extincti<strong>on</strong> was higher compared <strong>to</strong>observati<strong>on</strong>s made in o<strong>the</strong>r m<strong>on</strong>ths.Extincti<strong>on</strong> coefficient in <strong>the</strong> Infraredb<strong>and</strong>s of J,H, <strong>and</strong> K was alsomeasured <strong>on</strong> a few nights in August1988.These observati<strong>on</strong>s indicate <strong>the</strong>following :KJ = 0.05 1 0.01 magKH = 0.04 i 0.01 magKK = 0.04 i 0.01 mag(iv) IR SKY RADlATlON AND IR SKYNOISEThe IR sky radiati<strong>on</strong> emissi<strong>on</strong> <strong>and</strong>sky noise measurements are veryimportant for an IR observa<strong>to</strong>ry siteselecti<strong>on</strong>. In <strong>the</strong> <strong>the</strong>rmal infraredb<strong>and</strong> both <strong>the</strong> emissi<strong>on</strong> from <strong>the</strong> sky<strong>and</strong> absorpti<strong>on</strong> due <strong>to</strong> <strong>the</strong>atmosphere, c<strong>on</strong>tribute <strong>to</strong>wardlimiting <strong>the</strong> quality of ground basedIR observati<strong>on</strong>s. The emissi<strong>on</strong> <strong>and</strong>absorpti<strong>on</strong> by <strong>the</strong> terrestrialatmosphere are directly related. In<strong>the</strong> case of atmospheric windows,where <strong>the</strong> water vapour absorpti<strong>on</strong> isrelatively weak but where <strong>the</strong> watervapour is a major absorber, <strong>the</strong>relati<strong>on</strong>ship between emissi<strong>on</strong> anasky absorpti<strong>on</strong> is m<strong>on</strong>ot<strong>on</strong>ic (loweremissi<strong>on</strong> means less absorpti<strong>on</strong>).Variati<strong>on</strong>s in <strong>the</strong> emissi<strong>on</strong> orabsorpti<strong>on</strong> within <strong>the</strong> atmosphericwindows at 5,lO, 20 micr<strong>on</strong>s <strong>and</strong>l<strong>on</strong>ger wavelengths are dominated-by<strong>the</strong> variati<strong>on</strong>s in <strong>the</strong> water vapourc<strong>on</strong>tent of <strong>the</strong> atmosphere. Merrill(refer <strong>to</strong> NNTT report) has measured<strong>the</strong> IR sky background radiati<strong>on</strong>s withNOAO's IR Sky Background M<strong>on</strong>i<strong>to</strong>r(IRSB m<strong>on</strong>i<strong>to</strong>r) in 11, 18 <strong>and</strong> 25micr<strong>on</strong>s wave b<strong>and</strong>s <strong>and</strong> tried <strong>to</strong> drivea correlati<strong>on</strong> between <strong>the</strong> precipitablewater vapour <strong>and</strong> IR sky radiati<strong>on</strong>. InFigure 2 of Merrill's paper "APreliminaryAssessment ofNOAO/NNTTIR Sky BackgroundM<strong>on</strong>i<strong>to</strong>r Performance", shows arelati<strong>on</strong> between <strong>the</strong> emissivity at -1 8.5micr<strong>on</strong> <strong>and</strong> <strong>the</strong> precipitable watervapour (PWV). For low PWV <strong>the</strong> IRemissivity is also low.For <strong>this</strong> site selecti<strong>on</strong>, it was notpossible <strong>to</strong> acquire an IR SkyBackground M<strong>on</strong>i<strong>to</strong>r, <strong>the</strong>refore <strong>the</strong>actualvalues of IR sky emissivity arenot available. However, <strong>on</strong>e is able. <strong>to</strong>derive some estimates of <strong>the</strong> IRemissivity from <strong>the</strong> PWVobservati<strong>on</strong>s. Generally, <strong>the</strong> zenithPWV value at Mt. Nimmu remains lassthan 1.5-2 mm, <strong>the</strong>refore it may bepresumed that <strong>the</strong> IR sky radiati<strong>on</strong>would be also quite low.

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