1 | ===================== |
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2 | Scantable Description |
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3 | ===================== |
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4 | |
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5 | Introduction |
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6 | ============ |
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7 | |
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8 | The Scantable is the ASAP single-dish data container. It was designed |
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9 | based on the data abstraction of the PKSreader API. |
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10 | |
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11 | ASAP data handling works on objects called scantables. A scantable holds your |
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12 | data, and also provides functions to operate upon it. |
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13 | |
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14 | The building block of a scantable is an integration, which is a single row of |
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15 | a scantable. Each row contains just one spectrum for each beam, IF and |
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16 | polarisation. For example Parkes OH-multibeam data would normally contain |
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17 | 13 beams, 1 IF and 2 polarisations, Parkes methanol-multibeam data would |
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18 | contain 7 beams, 2 IFs and 2 polarisations while the Mopra 8-GHz MOPS |
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19 | filterbank will produce one beam, many IFs, and 2-4 polarisations. |
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20 | |
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21 | All of the combinations of Beams/IFs an Polarisations are contained in separate |
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22 | rows. These rows are grouped in cycles (same time stamp). |
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23 | |
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24 | A collection of cycles for one source is termed a scan (and each scan has a |
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25 | unique numeric identifier, the SCANNO). A scantable is then a collection of |
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26 | one or more scans. If you have scan-averaged your data in time, i.e. you |
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27 | have averaged all cycles within a scan, then each scan would hold just |
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28 | one (averaged) integration. |
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29 | |
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30 | Schema |
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31 | ====== |
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32 | |
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33 | The Scantable consist of a main (casacore) Table and several sub-tables which |
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34 | are referenced via **ID**. The following descriptions use the following syntax |
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35 | |
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36 | **COLUMN/KEYWORD NAME** - *DATA_TYPE(N)=DEFAULT_VALUE* |
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37 | |
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38 | where *DEFAULT_VALUE* is optional and *(N)* indicates a Vector of that type |
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39 | |
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40 | ---------- |
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41 | Main Table |
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42 | ---------- |
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43 | |
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44 | The main table consists of global data (keywords) and row-based data (columns) |
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45 | |
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46 | |
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47 | Keywords |
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48 | -------- |
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49 | |
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50 | * **VERSION** - *uInt* |
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51 | |
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52 | the version of the scantable for compatibility tests |
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53 | |
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54 | * **POLTYPE** - *String=linear* |
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55 | |
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56 | the polarisation type, i.e. *stokes*, *linear* or *circular* |
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57 | |
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58 | * **DIRECTIONREF** - *String=J2000* |
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59 | |
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60 | the reference frame of the direction (coordinate) [redundant?!] |
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61 | |
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62 | * **OBSMODE** - *String=""* |
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63 | |
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64 | no idea... |
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65 | |
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66 | * **UTC** - *String* |
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67 | |
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68 | most likely the starting time of the observation (file) |
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69 | |
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70 | * **AntennaName** - *String* |
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71 | |
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72 | the name of the antenna |
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73 | |
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74 | * **AntennaPosition** - *Double(3)* |
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75 | |
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76 | the WGS position of the antenna |
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77 | |
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78 | * **FluxUnit** - *String* |
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79 | |
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80 | the unit of the channel values i.e. *Jy* or *K* |
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81 | |
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82 | * **Observer** - *String* |
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83 | |
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84 | the name of the observer |
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85 | |
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86 | * **Obstype** - *String* |
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87 | |
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88 | probably telescope specific name/encoding |
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89 | |
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90 | * **Project** - *String* |
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91 | |
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92 | e.g. proposal project number, survey name |
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93 | |
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94 | * **Bandwidth** - *Double* |
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95 | |
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96 | self-explanatory |
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97 | |
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98 | * **nIF, nBeam, nPol, nChan** - *Int* |
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99 | |
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100 | the number of IF, Beam, Polarisation and Channel values in the scantable |
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101 | This is currently **FIXED** |
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102 | |
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103 | * **FreqRefFrame** - *String* |
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104 | |
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105 | redundant info - also in **FREQUENCIES** sub-table |
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106 | |
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107 | * **FreqRefVal** - *Double* |
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108 | |
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109 | no idea... |
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110 | |
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111 | Columns |
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112 | ------- |
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113 | |
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114 | * **SPECTRA** - *Float(nchannel)* |
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115 | |
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116 | the spectral channel data. |
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117 | |
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118 | * **FLAGTRA** - *uChar(nchannel)* |
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119 | |
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120 | the corresponding flags |
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121 | |
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122 | * **TSYS** - *Float(nchannel)* |
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123 | |
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124 | the channel-based system temperature values |
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125 | |
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126 | * **FLAGROW** - *uInt=-1* |
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127 | |
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128 | spectrum based flags |
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129 | |
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130 | * **TIME** - *MEpoch* |
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131 | |
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132 | The mjd time when the observation took place |
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133 | |
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134 | * **INTERVAL** - *Double* |
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135 | |
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136 | the integration time in seconds |
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137 | |
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138 | * **DIRECTION** - *MDirection* |
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139 | |
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140 | the associated direction on the sky |
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141 | |
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142 | * **AZIMUTH, ELEVATION** - *Float* |
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143 | |
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144 | the azimuth/elevation when the spectrum was collected |
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145 | |
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146 | * **SRCNAME** - *string* |
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147 | |
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148 | the name of the source observered |
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149 | |
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150 | * **SRCTYPE** - *Int=-1* |
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151 | |
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152 | the tyep of the source, i.e. indicating if it is an on source scan or |
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153 | off source. This will be used for calibration |
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154 | |
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155 | * **SCANNO, BEAMNO, POLNO, IFNO, CYCLENO** - *uInt* |
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156 | |
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157 | These columns index (0-based) the respective values. |
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158 | |
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159 | * SCANNO: the number odf the scan. A scan is usually multiple integrations |
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160 | (cycles) |
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161 | |
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162 | * CYCLENO: the integration number within a scan (sub-scan?) |
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163 | |
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164 | * IFNO: the index of the IF (spectral window) |
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165 | |
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166 | * BEAMNO: the index of the beam (in a multibeam system) |
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167 | |
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168 | * POLNO: the index of the polarisation, e.g. XX=0, YY=1, Real(XY)=2, |
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169 | Imag(XY)=3 |
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170 | |
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171 | * **REFBEAMNO** - *Int=-1* |
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172 | |
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173 | optional index of the reference beam in a multibeam obervation |
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174 | |
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175 | * **FREQ_ID, MOLECULE_ID, TCAL_ID, FOCUS_ID, WEATHER_ID, FIT_ID** - *Int* |
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176 | |
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177 | the reference indeces to the sub-tables |
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178 | |
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179 | * **FIELDNAME** - *string* |
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180 | |
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181 | the name of the field the source belongs to |
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182 | |
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183 | * **OPACITY** - *Float* |
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184 | |
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185 | maybe redundant - not used anywhere |
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186 | |
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187 | * **SRCPROPERMOTION, SRCVELOCITY, SRCDIRECTION** |
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188 | |
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189 | see livedata docs - not used in ASAP |
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190 | |
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191 | * **SCANRATE** |
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192 | |
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193 | see livedata docs - not used in ASAP |
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194 | |
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195 | ---------- |
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196 | Sub-Tables |
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197 | ---------- |
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198 | |
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199 | FREQUENCY |
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200 | --------- |
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201 | |
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202 | """""""" |
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203 | Keywords |
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204 | """""""" |
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205 | |
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206 | * **BASEFRAME** - *String=TOPO* |
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207 | * **FRAME** - *String=TOPO* |
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208 | * **EQUINOX** - *String=J2000* |
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209 | * **UNIT** - *String=""* |
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210 | * **DOPPLER** - *String=RADIO* |
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211 | |
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212 | """"""" |
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213 | Columns |
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214 | """"""" |
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215 | |
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216 | * **REFPIX** - *Double* |
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217 | |
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218 | * **REFVAL** - *Double* |
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219 | |
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220 | * **INCREMENT** - *Double* |
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221 | |
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222 | MOLECULES |
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223 | --------- |
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224 | |
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225 | """""""" |
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226 | Keywords |
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227 | """""""" |
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228 | |
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229 | * **UNIT** - *String=Hz* |
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230 | |
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231 | """"""" |
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232 | Columns |
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233 | """"""" |
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234 | |
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235 | * **RESTFREQUENCY** - *Double()* |
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236 | * **NAME** - *String()* |
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237 | * **FORMATTEDNAME** - *String()* |
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238 | |
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239 | FOCUS |
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240 | ----- |
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241 | |
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242 | """""""" |
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243 | Keywords |
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244 | """""""" |
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245 | |
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246 | * **PARALLACTIFY** - *Bool=False* |
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247 | |
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248 | indicates whether the observation was parallactified i.e. observed in such |
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249 | a way that the sum of the phases is zero. |
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250 | |
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251 | """"""" |
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252 | Columns |
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253 | """"""" |
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254 | |
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255 | * **PARANGLE** - *Float* |
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256 | |
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257 | the parallacticle angle of the observation |
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258 | |
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259 | * **ROTATION** - *Float* |
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260 | |
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261 | feed rotation |
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262 | |
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263 | * **AXIS** - *Float* |
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264 | |
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265 | ? ask Mark Calabretta |
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266 | |
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267 | * **TAN** - *Float* |
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268 | |
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269 | ? ask Mark Calabretta |
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270 | |
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271 | * **HAND** - *Float* |
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272 | |
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273 | the sign of the polarisation i.e. 1 for XY and '-1' for YX |
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274 | |
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275 | * **USERANGLE** - *Float* |
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276 | |
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277 | A user definable phase correction term |
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278 | |
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279 | * **MOUNT** - *Float* |
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280 | |
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281 | ? ask Jim Caswell |
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282 | |
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283 | * **XYOFFSET** - *Float* |
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284 | |
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285 | ? ask Jim Caswell |
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286 | |
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287 | * **XYPHASEOFFSET** - *Float* |
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288 | |
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289 | ? ask Jim Caswell |
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290 | |
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291 | .. todo:: add all sub-table descriptions |
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292 | |
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293 | ======================== |
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294 | Mapping to other formats |
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295 | ======================== |
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296 | |
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297 | MS |
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298 | == |
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299 | |
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300 | MeasurementSets are specifically designed for synthesis data. As such one of the |
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301 | main dfferences between the Scantable and a MS is how spectra are stored. |
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302 | An MS stores those in an n_pol x n_channel complex matrix (DATA column) and/or |
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303 | a n_pol x n_channel float matrix (FLOAT_DATA column) whereas the Scantable |
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304 | stores each polarisation seperately. In case of polarimetry data the (complex) |
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305 | cross-polarisation is serialised into real and imaginary parts, e.g. a total of |
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306 | four rows would describe one polarisation. |
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