{"id":166,"date":"2014-11-12T23:47:52","date_gmt":"2014-11-12T23:47:52","guid":{"rendered":"http:\/\/www.eaobservatory.org\/jcmt\/?page_id=166"},"modified":"2024-04-16T16:08:50","modified_gmt":"2024-04-17T02:08:50","slug":"observing-modes","status":"publish","type":"page","link":"https:\/\/www.eaobservatory.org\/jcmt\/instrumentation\/heterodyne\/observing-modes\/","title":{"rendered":"Heterodyne Observing modes"},"content":{"rendered":"<div id=\"toc_container\" class=\"no_bullets\"><p class=\"toc_title\">Contents<\/p><ul class=\"toc_list\"><li><a href=\"#Overview\"><span class=\"toc_number toc_depth_1\">1<\/span> Overview<\/a><ul><li><a href=\"#Quick_Guide\"><span class=\"toc_number toc_depth_2\">1.1<\/span> Quick Guide<\/a><\/li><\/ul><\/li><li><a href=\"#Sample_observations\"><span class=\"toc_number toc_depth_1\">2<\/span> Sample observations<\/a><ul><li><ul><li><a href=\"#Example_with_HARP\"><span class=\"toc_number toc_depth_3\">2.0.1<\/span> Example with HARP:<\/a><\/li><\/ul><\/li><\/ul><\/li><li><a href=\"#Jiggle-chop_and_Jiggle-pssw_observations\"><span class=\"toc_number toc_depth_1\">3<\/span> Jiggle-chop and Jiggle-pssw observations<\/a><ul><li><ul><li><a href=\"#Example_with_HARP-2\"><span class=\"toc_number toc_depth_3\">3.0.1<\/span> Example with HARP:<\/a><\/li><\/ul><\/li><\/ul><\/li><li><a href=\"#Scan_observations\"><span class=\"toc_number toc_depth_1\">4<\/span> Scan observations<\/a><\/li><li><a href=\"#The_details\"><span class=\"toc_number toc_depth_1\">5<\/span> \u00a0The details:<\/a><ul><li><a href=\"#Jiggle-Chop_observations\"><span class=\"toc_number toc_depth_2\">5.1<\/span> Jiggle-Chop observations<\/a><ul><li><a href=\"#General_description\"><span class=\"toc_number toc_depth_3\">5.1.1<\/span> General description:<\/a><\/li><li><a href=\"#Jiggle_patterns\"><span class=\"toc_number toc_depth_3\">5.1.2<\/span> Jiggle patterns:<\/a><\/li><li><a href=\"#Shared_vs_Separate_offs\"><span class=\"toc_number toc_depth_3\">5.1.3<\/span> Shared vs. Separate offs:<\/a><\/li><\/ul><\/li><li><a href=\"#Jiggle-Position_Switch_observations\"><span class=\"toc_number toc_depth_2\">5.2<\/span> Jiggle-Position Switch observations<\/a><ul><li><a href=\"#General_description-2\"><span class=\"toc_number toc_depth_3\">5.2.1<\/span> General description:<\/a><\/li><li><a href=\"#Jiggle_patterns-2\"><span class=\"toc_number toc_depth_3\">5.2.2<\/span> Jiggle patterns:<\/a><\/li><li><a href=\"#Shared_vs_Separate_offs-2\"><span class=\"toc_number toc_depth_3\">5.2.3<\/span> Shared vs. Separate offs:<\/a><\/li><\/ul><\/li><li><a href=\"#Grid_Position-Switch\"><span class=\"toc_number toc_depth_2\">5.3<\/span> Grid Position-Switch<\/a><ul><li><a href=\"#General_description-3\"><span class=\"toc_number toc_depth_3\">5.3.1<\/span> General description:<\/a><\/li><li><a href=\"#Grid_patterns\"><span class=\"toc_number toc_depth_3\">5.3.2<\/span> Grid patterns:<\/a><\/li><li><a href=\"#Shared_vs_Separate_offs-3\"><span class=\"toc_number toc_depth_3\">5.3.3<\/span> Shared vs. Separate offs:<\/a><\/li><\/ul><\/li><li><a href=\"#Frequency-Switch_observations\"><span class=\"toc_number toc_depth_2\">5.4<\/span> Frequency-Switch observations<\/a><ul><li><a href=\"#General_description-4\"><span class=\"toc_number toc_depth_3\">5.4.1<\/span> General description:<\/a><\/li><li><a href=\"#Baselines\"><span class=\"toc_number toc_depth_3\">5.4.2<\/span> Baselines:<\/a><\/li><li><a href=\"#Frequency_switch_amount\"><span class=\"toc_number toc_depth_3\">5.4.3<\/span> Frequency switch amount:<\/a><\/li><li><a href=\"#Noise_performance_and_map_size\"><span class=\"toc_number toc_depth_3\">5.4.4<\/span> Noise performance and map size:<\/a><\/li><\/ul><\/li><\/ul><\/li><li><a href=\"#Issues_from_Large_Position_Switches\"><span class=\"toc_number toc_depth_1\">6<\/span> Issues from Large Position Switches<\/a><\/li><\/ul><\/div>\n\n<h1><span id=\"Overview\">Overview<\/span><\/h1>\n<p style=\"text-align: justify\">The JCMT offers one &#8220;three-cartridge&#8221; heterodyne instrument, <a title=\"200 GHz band receiver \u2013 RxA\" href=\"https:\/\/www.eaobservatory.org\/jcmt\/instrumentation\/heterodyne\/namakanui\/\">N\u0101makanui<\/a> (230 GHz and 345 GHz undergoing commissioning, 86 GHz to be commissioned) and one 16-receptor receiver array, <a title=\"350 GHz band array receiver \u2013 HARP\" href=\"http:\/\/www.eaobservatory.org\/jcmt\/instrumentation\/heterodyne\/harp\/\">HARP<\/a>\u00a0(345 GHz).<\/p>\n<p style=\"text-align: justify\">The <a href=\"http:\/\/www.eaobservatory.org\/jcmt\/observing\/omp\/observing-tool\/\">Observation Tool (OT)<\/a> used to define an observation has three modes to map a source using a heterodyne receiver: stare, jiggle or scan mode. Stare mode is used to observe a single point or a set of points one by one by moving the telescope. A jiggle maps the source by moving either the secondary mirror or the telescope. Finally a scan maps the source by continuously scanning the sky. For heterodyne observing the scanning is done by raster scanning the telescope. The mapping method is combined with a switch mode &#8211; position, beam or frequency switching &#8211; not all switch modes can be combined with all mapping modes.<\/p>\n<p style=\"text-align: justify\">For large maps the scan (raster) mode is the most efficient observing modes for all receivers &#8211; it can only be used with position switching. For a small map a jiggle is most efficient and can be used with position or beam switching. For a single point or a set of single points use a stare observation with beam or position switching. With HARP it is also possible to use frequency switching with the stare and jiggle mapping method.<\/p>\n<p style=\"text-align: justify\">With beam switching, the secondary mirror is `chopped&#8217; (usually at 1 Hz, and in the azimuth direction, although chopping in any direction is possible) with the source in one beam. At a slower rate, typically every 10-30 seconds, the telescope is moved by an amount equal to the chop throw, to bring the source into the other beam (also called &#8216;double beam switching&#8217;). Thus a negative line signal is obtained when the source is in the reference beam, and a positive one when it is in the signal beam. Both signal and reference phases are necessary; if one attempts to omit the reference part of the cycle, standing waves are the likely result. Generally the technique is effective for sources of small angular size, and should produce better results whenever sky noise is a problem (most of the time). The maximum chop throw is 180 arcseconds. There is a <a href=\"https:\/\/www.eaobservatory.org\/jcmt\/instrumentation\/heterodyne\/calibration\/strong-continuum-het\/\">continuum mode<\/a> for beam switching, where the chop rate is much faster, which can be used if one is interested in the c<a href=\"https:\/\/www.eaobservatory.org\/jcmt\/instrumentation\/heterodyne\/calibration\/strong-continuum-het\/\">ontinuum level (e.g. for planets)<\/a>. However this mode is less efficient.<\/p>\n<p style=\"text-align: justify\">Position switching involves switching between the source and a specified reference position. It can be used with reference positions comparatively far away, but the switching is done rather slowly.<\/p>\n<p style=\"text-align: justify\">Beam switching produces much better baselines than position switching, which is important for sources with wide lines e.g. extragalactic sources.<\/p>\n<p style=\"text-align: justify\">Frequency switching involves switching between two target frequencies, rather than two positions on the sky, when observing spectral lines. This can be comparatively fast and efficient under some circumstances, but may not be well suited to certain types of sources (e.g. those exhibiting broad lines or a high density of spectral lines). Frequency switching is not possible with N\u0101makanui.<\/p>\n<p style=\"text-align: justify\">The particulars of how to construct each type of heterodyne observation are given in the\u00a0 <a title=\"Observing Tool\" href=\"http:\/\/www.eaobservatory.org\/jcmt\/observing\/omp\/observing-tool\/\">OT help pages<\/a>. There are <a href=\"https:\/\/www.eaobservatory.org\/jcmt\/instrumentation\/heterodyne\/calibration\/strong-continuum-het\/\">special considerations to be taken into account when observing sources with a strong continuum<\/a> such as the moon, the Sun, or planetary atmospheres.<\/p>\n<h2><span id=\"Quick_Guide\">Quick Guide<\/span><\/h2>\n<p>Source Structure\u00a0For Jiggles<\/p>\n<table>\n<tbody>\n<tr>\n<th>Map Size<\/th>\n<th><\/th>\n<\/tr>\n<tr>\n<td>Compact source<\/td>\n<td>Use a stare<\/td>\n<\/tr>\n<tr>\n<td>Moderate extent (less than 2 arcmins)<\/td>\n<td>Use a jiggle<\/td>\n<\/tr>\n<tr>\n<td>Large extent (greater than 2 arcmins)<\/td>\n<td>Use a scan<\/td>\n<\/tr>\n<tr>\n<td>Little to no emission close (&lt;180&#8243;) to target<\/td>\n<td>Use a beam-switched (BMSW) chop to the reference position<\/td>\n<\/tr>\n<tr>\n<td>Contaminating emission near source, or uncertain of region<\/td>\n<td>Use a position-switched (PSSW) observation to a reference further from the source (up to 3 degrees) or frequency switching<\/td>\n<\/tr>\n<tr>\n<td>Primarily after imaging or velocity\/frequency data<\/td>\n<td>Select &#8216;shared offs&#8217;<\/td>\n<\/tr>\n<tr>\n<td>Require accurate flux measurements over extended regions, or spatial smoothing of final map<\/td>\n<td>Select &#8216;separate offs&#8217;<\/td>\n<\/tr>\n<tr>\n<td>Highly compact core, with well-known position<\/td>\n<td>Select pixel-centered jiggle pattern<\/td>\n<\/tr>\n<tr>\n<td>More extended source, with actual position less known<\/td>\n<td>Select map-centered jiggle pattern<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<p>There are\u00a0<a href=\"https:\/\/www.eaobservatory.org\/jcmt\/instrumentation\/heterodyne\/calibration\/strong-continuum-het\/\">special considerations to be taken into account when observing sources with a strong continuum<\/a>\u00a0such as the moon, the Sun, or planetary atmospheres.<\/p>\n<h1><span id=\"Sample_observations\">Sample observations<\/span><\/h1>\n<p style=\"text-align: justify\">Stare (single-sample) or basic grid observations should be used when only a single spectrum (or a few spectral samples), is required on a source position (or several offset positions). If the source is compact, with emission-free positions relatively close (&lt; 180&#8243;), beam-switching to a reference position can be used (usually in Azimuth direction, unless one knows that a source is extended in a particular direction). This is called a sample-BMSW, or grid-BMSW. If the region is complex, or has foreground or background emission surrounding it, a position-switch mode, which allows selection of a reference up to three degrees away from the target, can be used instead, and is called a sample-PSSW.<\/p>\n<h3><span id=\"Example_with_HARP\">Example with HARP:<\/span><\/h3>\n<p style=\"text-align: justify\">HARP has\u00a016 detectors laid out on a 4 \u00d7 4 grid, with an on-sky projected beam separation of 30 arcsec. At 345 GHz the beam size is 14 arcsec, and the under-sampled field of view of HARP is 104 \u00d7 104 arcsec, as shown in Fig. 4. In a single-pointed observation, the map is under-sampled by a factor of 4.4 in area and by a factor of 17.6 with respect to the Nyquist frequency (\u03bb\/2D).<\/p>\n<p>&nbsp;<\/p>\n<p style=\"text-align: center\"><a href=\"http:\/\/www.eaobservatory.org\/jcmt\/wp-content\/uploads\/sites\/2\/2014\/11\/harp_array_spacing.gif\"><img loading=\"lazy\" class=\"alignnone size-full wp-image-968\" src=\"http:\/\/www.eaobservatory.org\/jcmt\/wp-content\/uploads\/sites\/2\/2014\/11\/harp_array_spacing.gif\" alt=\"harp_array_spacing\" width=\"360\" height=\"431\" \/><\/a><\/p>\n<p style=\"text-align: center\">HARP Detector layout (Reference: Buckle et al, MNRAS 2009)<\/p>\n<p style=\"text-align: justify\">An example of a stare observation, provided from the HARP publication paper, Buckle et al 2009, is shown here. At this time, two receptors were turned off in observations, hence the gaps in two corners.<\/p>\n<p style=\"text-align: center\"><a href=\"http:\/\/www.eaobservatory.org\/jcmt\/wp-content\/uploads\/sites\/2\/2014\/11\/harp_stare.gif\"><img loading=\"lazy\" class=\"alignnone size-full wp-image-966\" src=\"http:\/\/www.eaobservatory.org\/jcmt\/wp-content\/uploads\/sites\/2\/2014\/11\/harp_stare.gif\" alt=\"harp_stare\" width=\"360\" height=\"735\" \/><\/a><\/p>\n<p style=\"text-align: center\">Grid-position-switch stare observation, with receptors labelled. H00 and H03 have been turned off (top panel). The spectra from each position in the image (bottom panel) (Buckle, et al, MNRAS 2009).<\/p>\n<h1 id=\"jchp_grids\" style=\"text-align: left\"><span id=\"Jiggle-chop_and_Jiggle-pssw_observations\">Jiggle-chop and Jiggle-pssw observations<\/span><\/h1>\n<p style=\"text-align: justify\">If a source needs to be fully-sampled, and is less than 2 arcmins in extent, the Jiggle-chop or Jiggle-pssw mode is suggested. These observations rapidly step (&#8216;jiggle&#8217;) the secondary mirror to observe a grid-pattern of points until the area is fully sampled. A detailed description of the various options for this mode is given below. Again, depending on the surrounding emission in the region, either BMSW or PSSW to a reference position can be selected.<\/p>\n<p style=\"text-align: justify\">Whilst technically any grid-pattern can be selected for the jiggle mode (e.g. a 3 x 3 or 9 x 9 step map), two pre-designed modes are offered for the HARP array: HARP4, which is a 4 x 4, 16 point jiggle, producing 7.5&#8243; spacing, and is thus slightly under-sampled, and HARP5, a 5 x 5, 25 point jiggle, with 6&#8243; spacing, and slight over-sampling. The spacing for the jiggle mode can also be user-defined for the HARP and N\u0101makanui in the JCMT OT.<\/p>\n<h3><span id=\"Example_with_HARP-2\">Example with HARP:<\/span><\/h3>\n<p>An example for HARP is given here, again from Buckle et al 2009:<\/p>\n<p style=\"text-align: center\"><a href=\"http:\/\/www.eaobservatory.org\/jcmt\/wp-content\/uploads\/sites\/2\/2014\/11\/harp_jiggle_example.gif\"><img loading=\"lazy\" class=\"alignnone size-full wp-image-970\" src=\"http:\/\/www.eaobservatory.org\/jcmt\/wp-content\/uploads\/sites\/2\/2014\/11\/harp_jiggle_example.gif\" alt=\"harp_jiggle_example\" width=\"360\" height=\"325\" \/><\/a><\/p>\n<p style=\"text-align: center\">\u00a0Integrated intensity maps of jiggle position-switch observations in <sup>12<\/sup>CO 3 \u21922 towards a region in Serpens, utilizing the HARP4 pattern (left-hand panel) and HARP5 pattern (right-hand panel). Detectors H00 and H03 have been turned off. Spectra from the same spatial position near the peak are also shown, in a linestyle matching the arrow to the full map (bottom panel) (Buckle, et al 2009).<\/p>\n<p>&nbsp;<\/p>\n<h1><span id=\"Scan_observations\">Scan observations<\/span><\/h1>\n<p style=\"text-align: justify\">For regions larger than 2 arcminutes the scan mode is recommended, and is used most often for the array receiver HARP. Regions up to a degree or more in extent can be mapped in a single observation of under an hour when sampling at the fastest rate allowed by the spectrometer back-end, ACSIS. In this &#8216;raster&#8217; mode, the telescope is moved across a source and the data is integrated and continuously dumped to the back-end (max rate of 100ms). The array is rotated (see image below) to set the sample spacing perpendicular to the scan direction at 7.3 arcsec.<\/p>\n<p style=\"text-align: justify\">The off position is a single, specified reference position, observed in stare mode, which observes simultaneously an off position for each receptor. \u00a0The off-source integration time is a factor <em>f<\/em> greater than the length of the individual scanning sample time so that when combined with the \u2018on\u2019 observations, it gives optimum signal-to-noise ratio on the map, <span id=\"inline-formula-9\" class=\"inline-formula\"><img class=\"inline-graphic\" src=\"http:\/\/mnras.oxfordjournals.org\/content\/399\/2\/1026\/embed\/inline-graphic-9.gif\" alt=\"Graphic\" \/><\/span>, where <em>N<\/em> is the number of sample points per row, and <em>t<\/em> is the sample time. The noise in a single scan does vary due to the change in effective integration time when using an interpolated off-source observation.<\/p>\n<p id=\"p-94\" style=\"text-align: justify\">A basket-weave technique is normally used with scanning modes in order to minimize the effects of sky and system uncertainties in a map. This technique scans in different directions for each map, so that different receptors are used to sample the same sky point, thus reducing the effects of inaccurate flat-fielding and noisy or non-working receptors. The optimum change is a 90\u00b0 rotation of the scan direction, which can be set in the OT.<\/p>\n<p style=\"text-align: center\"><a href=\"http:\/\/www.eaobservatory.org\/jcmt\/wp-content\/uploads\/sites\/2\/2014\/11\/harp_raster.gif\"><img loading=\"lazy\" class=\"alignnone size-full wp-image-967\" src=\"http:\/\/www.eaobservatory.org\/jcmt\/wp-content\/uploads\/sites\/2\/2014\/11\/harp_raster.gif\" alt=\"harp_raster\" width=\"360\" height=\"417\" \/><\/a><\/p>\n<div id=\"F18\" class=\"fig pos-float odd\">\n<div class=\"fig-caption\">\n<p id=\"p-92\" class=\"first-child\" style=\"text-align: center\">The schematic diagram of a scan observation (Buckle et al 2009)<\/p>\n<p>See <a href=\"http:\/\/www.eaobservatory.org\/jcmt\/instrumentation\/heterodyne\/observing-modes\/harp-rasters\/\">here<\/a> for a comparison of rasters made with HARP using all different available Scan Spacings.<\/p>\n<\/div>\n<\/div>\n<p>&nbsp;<\/p>\n<div id=\"jigglechop\" class=\"section\">\n<h1><span id=\"The_details\">\u00a0The details:<\/span><\/h1>\n<h2><span id=\"Jiggle-Chop_observations\">Jiggle-Chop observations<\/span><\/h2>\n<h3><span id=\"General_description\">General description:<\/span><\/h3>\n<p style=\"text-align: justify\">This mode of observing is recommended for mapping \u2264 2 arcmin fields\u00a0where there are emission free positions relatively close (\u2264 180&#8243;)\u00a0to the target. Jiggle-Chop (JCHOP) observations utilize (i.e. &#8216;jiggle&#8217;)\u00a0the secondary mirror to observe a regular grid-pattern of points on the sky\u00a0as well as sky positions free of emission. To reduce systematics\u00a0arising from the secondary chopping to one direction on the sky, half\u00a0the observations are done at a sky position on one side of the target,\u00a0the other half at a position on the opposite side. This <i>nodding\u00a0<\/i>involves moving the whole telescope, but all other offsets require\u00a0moving only the secondary mirror. This method of observing the sky\u00a0reference is also referred to as <i>beam-switched<\/i>.<\/p>\n<p style=\"text-align: justify\">The advantage of JCHOP observations is that the patterns can be\u00a0executed quickly, with minimal telescope motions, and that the\u00a0frequent sky observations result in relatively flat baselines. JCHOP\u00a0observations are limited to targets that fit within the maximum\u00a0<i>throw<\/i> of the secondary mirror of 180&#8243;. Both the size of the\u00a0throw and the direction can be set by the observer in the MSB for the\u00a0observation, but a customary choice is to chop in AZ i.e. at a\u00a0constant airmass. If there are no sky positions free of emission\u00a0within 180&#8243; of the target, the observer will have to select a\u00a0position-switched observing mode instead.<\/p>\n<h3><span id=\"Jiggle_patterns\">Jiggle patterns:<\/span><\/h3>\n<p style=\"text-align: justify\">The choice of jiggle pattern requires some care and is not\u00a0straightforward. The detectors of HARP are referred to as\u00a0<i>receptors<\/i> and this terminology is used\u00a0for all heterodyne\u00a0receivers at the JCMT. The term <i>pixels<\/i> is used for positions in the output map (of course, all observations will produce a &#8216;cube&#8217; and the term &#8216;map&#8217;, as used here and below, refers to the spatial area mapped by the observation). A single-receptor receiver, such as N\u0101makanui, using a 5&#215;5 jiggle pattern will thus produce a 25-pixel map. HARP, with its 4&#215;4 array of receptors, using a 4&#215;4 pattern will produce a 16&#215;16 = 196 pixel map. Much of the complications associated with the jiggle patterns arise from the centering of the jiggle pattern and where the pointing target will end up in the resulting map:<\/p>\n<ul>\n<li>A <b><i>receptor-centered<\/i><\/b> jiggle pattern places the\u00a0target in the centre of the region mapped by one of the receptors and\u00a0it will also be on one of the pixels in the output map. All <i>NxN\u00a0<\/i>jiggle patterns such as 3&#215;3, 4&#215;4, or 5&#215;5, are receptor-centered and\u00a0primarily intended for use with the single-receptor receivers or<br \/>\ndual-pol receivers with two receptors on the same position on the sky.<\/li>\n<\/ul>\n<p style=\"text-align: justify\">HARP does not have a central receptor. When used with a\u00a0<i>receptor-centered<\/i> jiggle pattern it will align one of the inner\u00a04 receptors with the target instead. Hence, when a <i>NxN<\/i> jiggle\u00a0pattern is selected with HARP the target will be far off-center in the\u00a0resulting map, or, stated differently, the map will be very asymmetric\u00a0around the target position. The orientation of the map is determined\u00a0by position of the K-mirror, which can be at any of 4 angles,\u00a0differing by 90 degrees, depending on the elevation of observation. In\u00a0short, only a region of 1.5&#215;1.5 arcmin will be mapped predictably.\u00a0Combining observations from different elevations may result in a windmill\u00a0pattern around that region.<\/p>\n<p style=\"text-align: justify\">The four special patterns intended for use with HARP that fully\u00a0sample the footprint of the array are <i>harp4<\/i> and <i>harp5<\/i>,\u00a0coming in two variations each. The receptors in HARP are spaced 30&#8243;\u00a0apart and these patterns fill the 30&#8243;x30&#8243; postage-stamp of each receptor\u00a0with &#8216;4 by 4&#8217; or &#8216;5 by 5&#8217; points, spaced 7.5&#8243; or 6&#8243;\u00a0respectively. Compared to a Nyquist sampling of the nominal beam at\u00a0345 GHz, the harp4 patterns slightly under-sample, whereas the harp5\u00a0patterns over-sample. Harp4 is the recommended pattern for general map\u00a0making and the cube resulting from the 4&#215;4 receptors of HARP will have\u00a016&#215;16 pixels in e.g. RA, Dec and cover a region of 2&#215;2 arcmins. The\u00a0harp5 pattern will have 20&#215;20 pixels in RA, Dec covering the same\u00a0region.<\/p>\n<p style=\"text-align: justify\">Harp4 and Harp5 can be <i>pixel-centered<\/i> or <i>map-centered<\/i>.\u00a0To understand the distinction one needs to realize that the spatial\u00a0dimensions of a HARP cube will always have an even number of pixels, i.e. they do not have a central pixel. This presents two choices as to where\u00a0to position the target in the final map:<\/p>\n<ul>\n<li>A <b><i>pixel-centered<\/i><\/b> jiggle pattern will place the\u00a0target on one of the central 4 pixels of the output map, slightly\u00a0offset by half a pixel in each direction from the center of the\u00a0map. As is the case for the <i>receptor-centered<\/i> map, discussed\u00a0above, the map will thus be asymmetric around the target but only by<br \/>\none <em>map pixel<\/em>. Due to the possible orientations of the\u00a0K-mirror the edge of the map will this be uncertain by 7.5&#8243; or 6&#8243;,\u00a0respectively. The <i>pixel-centered<\/i> harp jiggle patterns are named\u00a0<i>harp4<\/i> and <i>harp5<\/i> in the JCMTOT.<\/li>\n<li>A <b><i>map-centered<\/i><\/b> jiggle pattern will place the target\u00a0exactly in the center of the output map, precisely <i>between the\u00a0central 4 pixels<\/i>. The resulting map will be symmetric around the\u00a0target position and independent of the K-mirror orientation. However,\u00a0none of the map pixels will align <em>exactly<\/em> with the target<br \/>\nposition, which should not be an issue for an extended source, but\u00a0might be a concern for a source with a point-like core. The\u00a0<i>map-centered<\/i> harp jiggle patterns are named <i>harp4_mc<\/i> and\u00a0<i>harp5_mc<\/i>. Map-centered patterns are also referred to as\u00a0<i>array-centered<\/i> since the target is placed on the center of the array\u00a0between the 4 receptors.<\/li>\n<\/ul>\n<p style=\"text-align: justify\">The four harp patterns are illustrated below. The cyan dots in each pattern\u00a0are the jiggle positions and the centers of the pixels in the output\u00a0map. In each figure the pointing target is indicated by the small\u00a0circle and either aligns with a pixel or the center of the map.<\/p>\n<p><a href=\"http:\/\/www.eaobservatory.org\/jcmt\/wp-content\/uploads\/sites\/2\/2014\/11\/HARP4.png\"><img loading=\"lazy\" class=\"alignnone size-full wp-image-366\" src=\"http:\/\/www.eaobservatory.org\/jcmt\/wp-content\/uploads\/sites\/2\/2014\/11\/HARP4.png\" alt=\"HARP4\" width=\"352\" height=\"488\" srcset=\"https:\/\/www.eaobservatory.org\/jcmt\/wp-content\/uploads\/sites\/2\/2014\/11\/HARP4.png 352w, https:\/\/www.eaobservatory.org\/jcmt\/wp-content\/uploads\/sites\/2\/2014\/11\/HARP4-216x300.png 216w, https:\/\/www.eaobservatory.org\/jcmt\/wp-content\/uploads\/sites\/2\/2014\/11\/HARP4-108x150.png 108w\" sizes=\"(max-width: 352px) 100vw, 352px\" \/><\/a><a href=\"http:\/\/www.eaobservatory.org\/jcmt\/wp-content\/uploads\/sites\/2\/2014\/11\/HARP4_mc.png\"><img loading=\"lazy\" class=\"alignnone size-full wp-image-367\" src=\"http:\/\/www.eaobservatory.org\/jcmt\/wp-content\/uploads\/sites\/2\/2014\/11\/HARP4_mc.png\" alt=\"HARP4_mc\" width=\"352\" height=\"488\" srcset=\"https:\/\/www.eaobservatory.org\/jcmt\/wp-content\/uploads\/sites\/2\/2014\/11\/HARP4_mc.png 352w, https:\/\/www.eaobservatory.org\/jcmt\/wp-content\/uploads\/sites\/2\/2014\/11\/HARP4_mc-216x300.png 216w, https:\/\/www.eaobservatory.org\/jcmt\/wp-content\/uploads\/sites\/2\/2014\/11\/HARP4_mc-108x150.png 108w\" sizes=\"(max-width: 352px) 100vw, 352px\" \/><\/a><a href=\"http:\/\/www.eaobservatory.org\/jcmt\/wp-content\/uploads\/sites\/2\/2014\/11\/HARP5.png\"><img loading=\"lazy\" class=\"alignnone size-full wp-image-368\" src=\"http:\/\/www.eaobservatory.org\/jcmt\/wp-content\/uploads\/sites\/2\/2014\/11\/HARP5.png\" alt=\"HARP5\" width=\"348\" height=\"492\" srcset=\"https:\/\/www.eaobservatory.org\/jcmt\/wp-content\/uploads\/sites\/2\/2014\/11\/HARP5.png 348w, https:\/\/www.eaobservatory.org\/jcmt\/wp-content\/uploads\/sites\/2\/2014\/11\/HARP5-212x300.png 212w, https:\/\/www.eaobservatory.org\/jcmt\/wp-content\/uploads\/sites\/2\/2014\/11\/HARP5-106x150.png 106w\" sizes=\"(max-width: 348px) 100vw, 348px\" \/><\/a><a href=\"http:\/\/www.eaobservatory.org\/jcmt\/wp-content\/uploads\/sites\/2\/2014\/11\/HARP5_mc.png\"><img loading=\"lazy\" class=\"alignnone size-full wp-image-369\" src=\"http:\/\/www.eaobservatory.org\/jcmt\/wp-content\/uploads\/sites\/2\/2014\/11\/HARP5_mc.png\" alt=\"HARP5_mc\" width=\"348\" height=\"492\" srcset=\"https:\/\/www.eaobservatory.org\/jcmt\/wp-content\/uploads\/sites\/2\/2014\/11\/HARP5_mc.png 348w, https:\/\/www.eaobservatory.org\/jcmt\/wp-content\/uploads\/sites\/2\/2014\/11\/HARP5_mc-212x300.png 212w, https:\/\/www.eaobservatory.org\/jcmt\/wp-content\/uploads\/sites\/2\/2014\/11\/HARP5_mc-106x150.png 106w\" sizes=\"(max-width: 348px) 100vw, 348px\" \/><\/a><\/p>\n<h3 id=\"jchp_shared\"><span id=\"Shared_vs_Separate_offs\">Shared vs. Separate <i>offs<\/i>:<\/span><\/h3>\n<p style=\"text-align: justify\">An important concept for JCHOP observations is one generally referred\u00a0to as <i>shared offs<\/i> (meaning <i>shared sky references<\/i>). This\u00a0concept originates from traditional scan-map observations in which the\u00a0telescope scans across an extended source before pointing to a sky\u00a0reference. By necessity, all positions observed along the scan will share the same sky reference (<i>off<\/i>). Since 50% of the noise\u00a0observed in a spectrum (i.e. <i>on-off<\/i>) results from the <i>off<\/i> this would lead to severe &#8216;striping&#8217; of successive rows\u00a0unless the integration time in the <i>off<\/i> position is sufficiently\u00a0increased so as to drop its noise significantly below the noise of the\u00a0<i>on<\/i> measurements. Calculations show that the integration time in\u00a0the <i>off<\/i> needs to be multiplied with <i>sqrt(np)<\/i>, where\u00a0<i>np<\/i> is the number of <i>on<\/i> positions in the row. Of course,\u00a0there is nothing special about scan-maps: the same applies for any\u00a0observations where a number of points share the sky reference, as is\u00a0the case for HARP jiggle-maps where the full 4&#215;4 or 5&#215;5 jiggle pattern\u00a0is observed before going to the <i>off<\/i>.<\/p>\n<p style=\"text-align: justify\">Observations benefit from <i>shared offs<\/i> in two ways, and more so with more points between the <i>offs<\/i> (the larger the\u00a0scan-map). To illustrate, consider 1s observations of 16 positions\u00a0(which would be a very small scan map). <i>Separate offs<\/i> would\u00a0require <b>32s<\/b> on the sky equally split between <i>ons<\/i> and\u00a0<i>offs<\/i>. Using <i>shared offs<\/i>, the observation needs only\u00a016+4=<b>20s<\/b>. But also <b>for each point individually the noise is\u00a0reduced<\/b> because of the longer observation of the\u00a0<i>off<\/i>. Expressed in a time required to reach a particular rms the\u00a0combined effect makes <i>shared off<\/i> observations <em>4 times\u00a0<\/em>faster than separate observations in the limit of many points. More\u00a0precisely, the factor is ~4\/(1+2\/sqrt(np)) or ~2.7 for the relatively\u00a0small HARP jiggle patterns and ignoring fixed overheads (which reduce\u00a0the factor to closer to ~2). Nevertheless, one night instead of two\u00a0nights is a huge gain, which is why <i>shared offs<\/i> are the default\u00a0for JCHOP observations.<\/p>\n<p style=\"text-align: justify\">Of course, <em>the above gain comes at a price<\/em>: all 16 or 25\u00a0jiggle-positions from every receptor are correlated because they have\u00a0the same sky reference (<i>off<\/i>). As long as the pixels are kept\u00a0separate this is not an issue because noise remains dominated by the\u00a0<i>on<\/i> measurement and the points can be regarded to be only weakly\u00a0correlated. However, any operation in which pixels are combined will\u00a0&#8216;suffer&#8217; from this correlation in the sense that it will combine\u00a0pixels that are not fully independent and the noise will hence not\u00a0drop with the customary sqrt(np) factor. Note that this both applies\u00a0to operations that use a <b><i>spatial convolution (smoothing)\u00a0<\/i><\/b>as well as <b><i>total flux measurements<\/i><\/b> that sum or average\u00a0over pixels such as <i>aperture photometry<\/i>. In the extreme, when\u00a0averaging data over the whole footprint of a harp4 observation, the\u00a0noise will be 1\/4th (16 independent receptors) instead of 1\/16th (196\u00a0pixels) of the noise per pixel. Finally: note that JCHOP <i>continuum\u00a0mode<\/i> observations always will have <i>separate offs<\/i>\u00a0because\u00a0there will be only a single jiggle position per sky reference.<\/p>\n<p style=\"text-align: justify\">In conclusion: <b>if imaging or velocity\/frequency data are the\u00a0primary aim of the observations, <i>shared offs<\/i> are hugely\u00a0beneficial and recommended; if accurate flux measurements over\u00a0extended regions are the primary aim or the final maps will be\u00a0spatially smoothed significantly, <i>separate offs<\/i> are\u00a0recommended instead.<\/b><\/p>\n<p>More details can be found <a href=\"http:\/\/www.eaobservatory.org\/jcmt\/heterodyne-observing-modes-more-details\/\">here<\/a>.<\/p>\n<p>&nbsp;<\/p>\n<h2><span id=\"Jiggle-Position_Switch_observations\">Jiggle-Position Switch observations<\/span><\/h2>\n<h3><span id=\"General_description-2\">General description:<\/span><\/h3>\n<p><em>(Since Jiggle-position switch observations share many\u00a0characteristics with the Jiggle-chops, please make sure to have read\u00a0the previous section).<\/em><\/p>\n<p style=\"text-align: justify\">This mode is recommended for mapping \u2264 2 arcmin fields where an\u00a0emission free position is relatively far away from the\u00a0target. Jiggle-PSSW (JPSSW) observations utilize the secondary to\u00a0observe a regular grid-pattern of points but, contrary to JCHOP, use a\u00a0position-switch of the whole telescope to observe sky positions free\u00a0of emission.<\/p>\n<p style=\"text-align: justify\">The advantage of JPSSW over JCHOP is that sky reference positions are\u00a0not restricted to a distance within 180&#8243; from the target and can be as\u00a0far as degrees away. The main disadvantages are the less accurate sky\u00a0subtraction due to the larger switch and the longer time between sky\u00a0references. Another disadvantage is the additional overhead resulting from moving the whole telescope to the reference position. The\u00a0position can be specified as either a relative position or a fixed\u00a0position. The switch will be to this single position only rather than\u00a0two positions on diametrically opposite sides of the target, as is the\u00a0case for JCHOP. (However, note that the reference position can be\u00a0defined in a coordinate system other than RA, Dec resulting in it\u00a0rotating from observation to observation in that frame). In order to\u00a0minimize the number of moves, the telescope executes an\u00a0OFF-ONs-ONs-OFF etc. pattern, with the switches pairs-wise associated.<\/p>\n<h3><span id=\"Jiggle_patterns-2\">Jiggle patterns:<\/span><\/h3>\n<p><a href=\"#jchp_grids\"><em>See Jiggle-Chop<\/em><\/a><\/p>\n<h3><span id=\"Shared_vs_Separate_offs-2\">Shared vs. Separate <i>offs<\/i>:<\/span><\/h3>\n<p><a href=\"#jchp_shared\"><em>See Jiggle-Chop<\/em><\/a><\/p>\n<p>More details can be found <a href=\"http:\/\/www.eaobservatory.org\/jcmt\/heterodyne-observing-modes-more-details\/\">here<\/a>.<\/p>\n<p>&nbsp;<\/p>\n<h2><span id=\"Grid_Position-Switch\">Grid Position-Switch<\/span><\/h2>\n<h3><span id=\"General_description-3\">General description:<\/span><\/h3>\n<p style=\"text-align: justify\">This mode of observing is recommended for sparse mapping or irregular\u00a0patterns. Grid-PSSW (GPSSW) observations don&#8217;t use motions of the\u00a0secondary mirror, but instead move the telescope between the target\u00a0grid-positions as well as sky positions free of emission. Likewise, it\u00a0is possible to do Grid-BMSW observations, but these will not be\u00a0further discussed here.<\/p>\n<p style=\"text-align: justify\">The advantage of Grid-PSSW observations is that the target positions\u00a0are under full control of the observer and that under-sampled maps or\u00a0non-regular patterns can be observed. Like JPSSW, the sky reference\u00a0can be far away from the target. A disadvantage is that moving the\u00a0telescope to execute the grid is relatively slow resulting in a larger overhead than with jiggle observations. In order to minimize the\u00a0number of moves, the telescope executes an OFF-ONs-ONs-OFF\u00a0etc. pattern, with the switches pairs-wise associated. The off-switch\u00a0will be to a single, specified, reference position. (However, note\u00a0that the reference position can be defined in a coordinate system\u00a0other than RA, Dec resulting in it rotating from observation to\u00a0observation in that frame).<\/p>\n<p style=\"text-align: justify\">[Note: it is possible to do a grid of jiggle-maps by putting a\u00a0&#8220;Jiggle-Eye&#8221; inside an &#8220;Offset&#8221; iterator. However, in this\u00a0case the jiggles will be executed as fully independent observations at\u00a0each grid point, resulting in independent cubes.]\n<h3><span id=\"Grid_patterns\">Grid patterns:<\/span><\/h3>\n<p style=\"text-align: justify\">Grid patterns can be set up using the <i>Offset<\/i> iterator in the\u00a0JCMTOT, either as a regular pattern or as a list of individual\u00a0offsets.<\/p>\n<h3><span id=\"Shared_vs_Separate_offs-3\">Shared vs. Separate <i>offs<\/i>:<\/span><\/h3>\n<p style=\"text-align: justify\"><i>Shared<\/i> and <i>separate<\/i> modes are available as for JCHOP\u00a0(see description there), but without being able to do a fast jiggle to\u00a0cover the pattern the distinction is rather irrelevant in\u00a0practice. With a minimum of 1s\/point and a maximum of 30s between\u00a0refs, multiple grid-points can only be observed for short integration\u00a0times. This means that relatively many telescope moves are required\u00a0for little integration time implying large overheads. Being more\u00a0efficient, <i>shared offs<\/i> mitigate the effects of the increased\u00a0overheads. On the other hand, short observations with <i>separate\u00a0offs<\/i> are particularly inefficient: because of restrictions imposed\u00a0by the data handling within the correlator these observations\u00a0<em>have<\/em> to switch to the reference after each grid-point, even\u00a0if the integration time per point is only as short as 1s.<\/p>\n<p style=\"text-align: justify\">Finally, as soon as the integration time per grid-point becomes such\u00a0that only a single grid-point can be observed before having to go the\u00a0sky reference (i.e. 15sec by default) there will be only one point per\u00a0reference and the distinction between <i>separate<\/i> and <i>shared\u00a0offs<\/i> disappears: the observations will use separate offs.<\/p>\n<p style=\"text-align: justify\">In conclusion: for any observations with integration times \u226515s\u00a0per grid-point the choice of <i>shared<\/i> or <i>separate offs<\/i> is irrelevant. For shorter integration times a choice for <i>separate offs\u00a0<\/i>comes with a heavy penalty in the form of excessive overheads.<\/p>\n<p style=\"text-align: justify\">More details can be found <a href=\"http:\/\/www.eaobservatory.org\/jcmt\/heterodyne-observing-modes-more-details\/\">here<\/a>.<\/p>\n<p>&nbsp;<\/p>\n<h2 style=\"text-align: left\"><span id=\"Frequency-Switch_observations\">Frequency-Switch observations<\/span><\/h2>\n<p style=\"text-align: justify\">Frequency-switch mode observing is possible for HARP observations. However it is an involved process that requires additional support and effort from the observer. Observers need to consider telluric lines (CO, Ozone that changes with time) and know that the baselines are in general sinusoidal. Reductions of such data are not currently handled well by ORAC_DR.<\/p>\n<\/div>\n<div id=\"jigglechop\" class=\"section\">\n<h3 style=\"text-align: left\"><span id=\"General_description-4\">General description:<\/span><\/h3>\n<p style=\"text-align: justify\">Frequency switching mode is suitable for sources with narrow lines (~&lt;\u00a010 MHz) and low line density. In such sources it has superior noise\u00a0performance for single point observations or small maps. Frequency\u00a0switching also has advantages in large maps when there is no nearby\u00a0line free region to use as a reference position.<\/p>\n<h3 style=\"text-align: left\"><span id=\"Baselines\">Baselines:<\/span><\/h3>\n<p style=\"text-align: justify\">Non-flat Baselines are common with frequency switching. There are several\u00a0reasons for this. The frequency change and the associated variation\u00a0in LO power changes the mixer impedance, coupling and conversion\u00a0efficiency creating baselines. For small frequency switches the\u00a0baselines are well behaved and can be removed by baselining. Another\u00a0cause is standing waves in the optics in front of the mixer. These\u00a0will move in the band pass when the frequency shifts and are not\u00a0cancelled. The standing wave between the cabin and secondary has a\u00a0frequency of ~ 16.2 MHz at the JCMT. Frequency switching by 16.2 or\u00a032.4 MHz reduces such standing waves. Thus frequency switching is most\u00a0suitable for observations of narrow lines not affected by\u00a0baselines. It should definitely not be used for continuum\u00a0observations. The JCMT uses an elaborate calibration scheme to reduce\u00a0the baselines which doubles the time integrating on calibration loads.<\/p>\n<h3 style=\"text-align: left\"><span id=\"Frequency_switch_amount\">Frequency switch amount:<\/span><\/h3>\n<p style=\"text-align: justify\">The baselines and the performance of the LO system limits the amount of the frequency shift. Currently switches up to 32.4 MHz have been tested with good results for both RxA and HARP. Since the line is present in both the signal and reference observation it will occur twice in the raw spectrum. After processing to average the two independent lines in the spectra there will be three features. This is not a problem for an isolated line but care has to be taken if there is more than one line in the band pass. Users should also be aware that lines from the earth&#8217;s atmosphere not are canceled. Terrestrial Ozone lines are routinely observed using frequency switching. Thus CO and Ozone can generate additional features in the spectra. The user needs to select center frequency (or velocity) and switch amount to avoid line blends.<\/p>\n<h3 style=\"text-align: left\"><span id=\"Noise_performance_and_map_size\">Noise performance and map size:<\/span><\/h3>\n<p style=\"text-align: justify\">Due to the fact the same line is observed both in the source and reference spectra the noise is improved by sqrt(2) after processing. However, observing modes that uses one reference observations (offs) for several on sources positions as jiggles and grids (with shared offs) and scan pssw becomes more efficient when the number of on source positions are large. <em>Neglecting overheads these methods are more efficient than frequency switching if more than 5 on source points share the same off<\/em>. The actual trade of point depends on overheads. Thus, in cases where there not is a good reference position in the vicinity of the source, frequency switch can maintain the advantages for larger maps. However, it is still recommended that the calibration is performed on a line free reference position. This requires that a reference position is defined in the OT target component. In cases with weak lines (line &lt;&lt; Tsys) and no continuum it is possible to calibrate on source.<\/p>\n<p>A discussion <a href=\"https:\/\/www.eaobservatory.org\/JCMT\/publications\/newsletter\/jcmt-n32.pdf#page=13\">Observing Using Frequency Switching<\/a> can be found in the <a href=\"https:\/\/www.eaobservatory.org\/JCMT\/publications\/newsletter\/jcmt-n32.pdf#page=13\">Spring 2010 JCMT Newsletter<\/a>.<\/p>\n<h1 style=\"text-align: left\"><span id=\"Issues_from_Large_Position_Switches\">Issues from Large Position Switches<\/span><\/h1>\n<p>Large Position switches are best avoided, as they can cause baseline ripples. In essence there are two types of ripples seen in the baseline of spectra. A a slow ripple that is typically caused by differences in reflection looking at the cold load. This creates uneven calibration within is easily seen when having a DC offset. For instance created by a large position switch in elevation.<\/p>\n<p>Additionally in some of the spectra a fast ripple typically for reflection between the mixer and sub-reflector is seen &#8211; although not neccessarily caused by large position switches.<\/p>\n<\/div>\n<p>&nbsp;<\/p>\n","protected":false},"excerpt":{"rendered":"<p>Overview The JCMT offers one &#8220;three-cartridge&#8221; heterodyne instrument, N\u0101makanui (230 GHz and 345 GHz undergoing commissioning, 86 GHz to be commissioned) and one 16-receptor receiver array, HARP\u00a0(345 GHz). The Observation Tool (OT) used to define an observation has three modes to map a source using a heterodyne receiver: stare, jiggle\u2026 <a class=\"continue-reading-link\" href=\"https:\/\/www.eaobservatory.org\/jcmt\/instrumentation\/heterodyne\/observing-modes\/\">Continue reading<\/a><\/p>\n","protected":false},"author":3,"featured_media":0,"parent":162,"menu_order":0,"comment_status":"closed","ping_status":"closed","template":"","meta":[],"_links":{"self":[{"href":"https:\/\/www.eaobservatory.org\/jcmt\/wp-json\/wp\/v2\/pages\/166"}],"collection":[{"href":"https:\/\/www.eaobservatory.org\/jcmt\/wp-json\/wp\/v2\/pages"}],"about":[{"href":"https:\/\/www.eaobservatory.org\/jcmt\/wp-json\/wp\/v2\/types\/page"}],"author":[{"embeddable":true,"href":"https:\/\/www.eaobservatory.org\/jcmt\/wp-json\/wp\/v2\/users\/3"}],"replies":[{"embeddable":true,"href":"https:\/\/www.eaobservatory.org\/jcmt\/wp-json\/wp\/v2\/comments?post=166"}],"version-history":[{"count":145,"href":"https:\/\/www.eaobservatory.org\/jcmt\/wp-json\/wp\/v2\/pages\/166\/revisions"}],"predecessor-version":[{"id":13058,"href":"https:\/\/www.eaobservatory.org\/jcmt\/wp-json\/wp\/v2\/pages\/166\/revisions\/13058"}],"up":[{"embeddable":true,"href":"https:\/\/www.eaobservatory.org\/jcmt\/wp-json\/wp\/v2\/pages\/162"}],"wp:attachment":[{"href":"https:\/\/www.eaobservatory.org\/jcmt\/wp-json\/wp\/v2\/media?parent=166"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}