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# vla antenna positions

It is always a good idea to examine the data before jumping straight into calibration. A modified neural networks approach (Keto, 1992) does Also, setjy will clear any previous calibration model that fits the selection. None of these antennas should be chosen as the reference antenna during the calibration process, below. These corrections could also be determined later by a baseline-based calibration incorporating the $b_{ij}$ term from the equation above, but since they are known a priori it makes sense to incorporate them now. (The Fourier transform of a point source, i.e. Note that setjy also returns a python dictionary (CASA record) containing the reference flux density used. Parameter iteration='antenna' is used to step through separate plots for each antenna. Running statwt will remove the effects of relative noise scatter that may have been introduced from flagging uneven bits in the visibility data between the channels and times. Remove the channel averaging (Data tab), and plot the data using the Plot button at the bottom of the plotms GUI. If one checks the gain phase solutions using plotms, one should see smooth solutions for each antenna as a function of time (see Figures 9A--9B). The lower gain solution values (near 1.0) correspond to the two scans of 3C 286, while the higher gain solution values (near 1.5) correspond to the scans of the phase calibrator, J1822-0938. You should spend several minutes displaying the data in various formats. To do that, right-click on the closed polygonal icon. For broader examples of many tclean options, please see the Topical Guide for Imaging VLA Data. The procedure is to assume that the flux density of a primary calibrator source is known and, by comparison with the observed data for that calibrator, determine the $g_i$ values (the antenna gains). The complex gain calibrator (J1822-0938, shown in magenta) is slightly brighter than the target fields. In this paper we combine radio positions from our observ-ing programs and archival data, published VLA positions, and The next brightest is the first source observed (J1331+3030, a.k.a. Additional discussion of image analysis and manipulation, including the combination of multiple images, mathematical operations on images, and much more can be found in the Image Analysis section of the CASA documentation. The relevant function calls are given below. This shows, assuming that ea01 is in the entire observation, when various antennas drop out. Even if was started with xaxis=' ' (defaulting to 'time'), you can choose a different X-axis by selecting the Axes tab, then using the dropdown menu to switch to (for example) xaxis='Frequency' (although to get something sensible when plotting with frequency, channel averaging must be turned off). The variations of the ﬁrst two natural frequencies with respect to the antenna elevation position are shown in Fig. This may or may not be a good thing. combine the two calibrated MSs in tclean to make a deeper MFS image (this might be tricky). We were able to calibrate this data without flagging channels (only getting the bad baseline noted above). You can open this from inside CASA using '! operated under cooperative agreement by Associated Universities, Inc. http://casa.nrao.edu/Data/EVLA/3C391/3c391_ctm_mosaic_10s_spw0.ms.tgz (dataset size: 3.1GB). In order to determine the appropriate complex gains for the target source, and to minimize differences through the atmosphere (neutral and/or ionized) between the lines of sight to the phase calibrator and the target source, you want to observe a so-called phase calibrator that is much closer to the target. If you are using a later version of CASA and this is the most recent available guide, then you should be able to use most, if not all, of this tutorial. In point-to-point operation, where an antenna is changed from one fixed position to a new fixed position, it is made to increase from zero to maximum rotational speed and then back to zero velocity to stop at the target location. As mentioned above, restarting tclean with different multiscale=[...] choices can help also. 3C 286). This approach will do a better job of modeling the image than the classic clean delta function. Once you have CASA up and running in the directory containing the data, then start your data reduction by getting some basic information about the data. From the listobs output above, one may have noticed that the first scan is less than 1 minute long. For the remaining antennas, you see that 1, 6, and 13 (ea02, ea08, and ea16) are missing some blocks toward the beginning and also toward the end of the run. Is this better than the original multiscale image? For example, for a particular instance of VLA polarization calibration Instrumental. CASA has a task tclean which both Fourier transforms the data and deconvolves the resulting image. Automotive. Given the VLA is composed of 27 individual antennas, it is necessary to know their exact positions in order to correctly correlate (match and combine) each signal. While we know the flux density of our primary calibrator (J1331+3030$\equiv$3C 286), the model assumed for the secondary calibrator (J1822-0938) was a point source of 1 Jy located at the phase center. VLBA antenna plots label the positions with "name @ station" format, e.g. That system drives the wheels using radial hydraulic motors and also powers the jacks that lift the antennas off their concrete-pier mountings. For the historical VLA, the antenna position coordinate system was a local one translated from the Earth's center and rotated to the VLA's longitude. We will run this task here on the newly calibrated and split-out data set before moving on to imaging. A final example is shown in Figure 3C. Plotting the corrected amplitude against UV distance or against time is a good way to find such issues. To do this, we iterate over the different calibrators, in each case specifying the source to be calibrated (using the field parameter). Depending upon frequency and configuration, there could be significant gain variations between different scans of the bandpass calibrator, particularly if the scans happen at much different elevations. These linear variations are 'delays' that need to be calibrated for, below. Note that the antenna IDs (which are numbered sequentially up to the total number of antennas in the array; 0 through 25 in this instance) do not correspond to the actual antenna names (ea01 through ea28; these numbers correspond to those painted on the side of the dishes). Position determinations of astrometric quality have been obtained, and flux measurements have provided size estimates. From this, we find the following: Before beginning our data reduction, we must start CASA. Founded in 1956, the NRAO provides state-of-the-art radio telescope facilities for use by the international scientific community. calibrated antenna positions. As mentioned in the observing log above, antennas ea10, ea12, and ea22 do not have good baseline positions. Antenna Mechanics. EVLA antenna positions Since their inclusion in the array by default, we have obtained improved EVLA antenna positions over time. 3C 286) to display data associated with the amplitude calibrator, then select the Axes tab and change the X-axis to be UVdist (baseline length in meters). To examine the observing conditions during the observing run, and to find out any known problems with the data, download the observer log. The $u$ and $v$ coordinates are the baselines measured in units of the observing wavelength, while the $l$ and $m$ coordinates are the direction cosines on the sky. You will need to find these using. extended atmosphere with the VLA in A conﬁguration and the VLBA Pie Town antenna to obtain maximum spatial resolution. It is far better to exclude some feature of a source or a weak source from initial cleaning and conduct another round of self-calibration than to create an artificial source. There are two basic types of motion that the antennas at the VLA will undergo: point-to-point and tracking. For more discussion of the philosophy, strategy, and implementation of calibration of synthesis data within CASA, see Synthesis Calibration in the CASA documentation. (On these fields 1 pixel = 5 arcseconds.) The important observation is that the amplitude distribution is relatively constant as a function of UV distance or baseline length (i.e., $\sqrt{u^2+v^2}$ ). If satisfied with the inputs, run this task. which set of gain corrections and images are which? We use the CASA task gaincal to solve for phase versus time for the central channels on our three calibrators: To really see what is going on, we use plotms to inspect the solutions from gaincal for a single antenna at a time, iterating through each antenna in sequence by clicking on the Next button (rightward pointing single green arrow) on the GUI to advance the displayed antenna. If a real source is excluded from initial cleaning, it will continue to be present in subsequent iterations of self-calibration; if it's not a real source, one probably isn't interested in it anyway. ), By contrast, if you make a similar plot for field 8 (one of the 3C 391 fields), the result is a visibility function that falls rapidly with increasing baseline length. Upper right: The VLA antenna positions in units of meters to the central VLA reference point.Lower left: The uv-coverage of the simulations (note that they do not account for the bandwidth). This first scan can safely be flagged. To start, let's find the available calibrator models with setjy and setting the parameter listmodels=True: This command will show all available calibrator models: Since any image could be a potential calibrator model, setjy will list all *.im and *.mod images in the working directory. For completeness, however, here are the steps that were taken from the initial data set to produce the starting data set. This observation was set up as a 7-pointing mosaic because the supernova remnant is so large that it essentially fills the primary beam. From these ﬁgures, we see that the natural frequencies depend on the antenna elevation position: the second frequency changes signiﬁcantly with the elevation angle. Basic data flagging was applied, to account for shadowing of the antennas. Figure 2 shows the result of running plotms with the field selection discussed above. Example output would be. In the default view, the Plot tab is visible, and there are a number of tabs running down the side of the left hand panel, including Data, Calibration, Axes, Page, Transform, Display, and Canvas. a delta function, is a constant function. It shows the right circularly polarized data (RR correlation) for the source J1331+3030, which will serve as the bandpass calibrator. The recorded data are then sent to Socorro, NM to be processed by a powerful computer known as a correlator Correlator A specialized supercomputer that multiplies the data from two antennas and averages the result over time. If you find bad data, you can remove them via interactive flagging in plotms or via manual flagging in flagdata once you have identified the offending antennas/baselines/channels/times. You can adjust the color scale and zoom in to a selected region by assigning mouse buttons to the icons immediately above the image (hover over the icons to get a description of what they do). You will have to avoid it through channel ranges and/or edit it out. Prior to imaging, it is a good idea to run the statwt task to correct the data weights (weight and sigma columns) in the measurement set. However, when you get to ea05, note that there are phase jumps where the phase appears to be oscillating between two states. The VLA is the most advanced radio telescope array on Earth, a customizable interferometer that spans up to 22 miles across. For example, plot (with colorization by polarization) for the first block of 3C286 data only: The first stage of bandpass calibration involves solving for the antenna-based delays which put a phase ramp versus frequency channel in each spectral window (Figure 3C). J1822-0938 is fainter than 3C 286, leading to a higher solution value. In the D configuration, the fringe rate is relatively slow and time-average smearing is less of a concern. For the purposes of this tutorial, we will make a mosaic clean image in Stokes I. The task listobs can be used to get a listing of the individual scans (set amounts of time devoted to specific targets) comprising the observation, the frequency setup, source list, and antenna locations. Solving for the bandpass won't hurt for continuum data, and, for moderate or high dynamic range image, it is essential. Each of these 230 ton antennas can be moved to different locations with the help of a transporterTransporterThe two VLA transporters are special-purpose vehicles designed specifically to pick up and carry the VLA's 230-ton dish antennas. You can adjust the clean boxes each cycle, to enclose all real emission. Task tclean will make several output files, all named with the prefix given as imagename. Also recall that this equation is valid only if the $w$ coordinate of the baselines can be neglected. If you want to adjust the vertices you can do so. Setting the appropriate pixel size for imaging depends upon basic optics aspects of interferometry. If updated positions were entered into the database AFTER your observation date, the corrections to the newly measured positions can still be applied during your data reduction process in this step. Weighing 90 tons themselves, the transporters are powered by 380- and 400-horsepower (respectively) diesel engines that in turn power a hydraulic system. Although CASA has the feature that its Fourier transform engine (FFTW) does not require a strict power of 2 for the number of linear pixels in a given image axis, it is somewhat more efficient if the number of pixels on a side is a composite number divisible by any pair of 2 and 3 and/or 5. I(l,m) = \int V(u,v) e^{[2\pi i(ul + vm)]} dudv Moreover, from the observer's log, we already know that one antenna will need to be flagged because it does not have a C-band receiver. Using the mouse button just assigned to polygon region, outline the supernova remnant. Photo Credit: NRAO/AUI/NSF 1. After many cycles, when only noise is left, you can hit the red-and-white stop-sign icon to stop cleaning. 3C391 C1--C7, which are 7 fields centered on and surrounding the supernova remnant. The weak thermal emission from the largest minor planets can be detected and measured at all points around their orbits at microwave frequencies using the Very Large Array (VLA). To add the antenna ID to the name, set antindex=True as shown in Figure 1. position of the interferometer elements (the antennas or stations) and the viewing direction. Recall that the visibility data and the sky brightness distribution (a.k.a. In self-calibrating data, it is useful to keep in mind the structure of a Measurement Set: there are three columns of interest for an MS: the DATA column, the MODEL column, and the CORRECTED_DATA column. self calibration (see Section 5.11), but we have lectures on Self-calibration given at NRAO community days. When looking down at the VLA from above, the antennas on a CCW wrap would appear to be tracking in azimuth in a CCW direction from 180° real AZ. You can verify this by looking at the online database for the first part of 2010: The next step is to provide a flux density value for the amplitude calibrator J1331+3030 (a.k.a. You can also find the flux density values in the CASA logger: Again, the VLA calibrator manual may be used to check whether the derived flux densities look sensible. (See the CASAguide on radio galaxy 3C75 for an introduction to polarization calibration.). Try clicking on any configuration in the list to see how the antenna locations change. Residual phase excursions are on the order of a few degrees. By default, with parameter applymode='calflag', data with no good solutions will be flagged by applycal; in this case you will see it report the flagged fraction increasing to about 45%. With reference to the original self-calibration equation above, if the observed visibility data cannot be modeled well by this equation, no amount of self-calibration will help. You will need to find it (e.g., using, We have not edited out bad or dead antennas for you (unlike in 3C391). In “holography” data from the EVLA, reference antennas are pointed towards the nominal source position, as normal.All other antennas are pointed to an offset position controlled by the observer. The reduction strategy is to determine various corrections from the calibrators, then apply these correction factors to the science data. EOVSA Antenna Position Calibration. While the secondary calibrator was chosen to be a point source (at least, over some limited range of uv-distance; see the VLA calibrator manual for any u-v restrictions on your calibrator of choice at the observing frequency), its absolute flux density is unknown. As the brightest points are removed from the image (cleaned off), fainter emission may show up. All of these steps can be done directly from the NRAO Archive's Download page, by selecting CASA MS as the download format (it's a good idea to also check the Create MS or SDM tar file box), checking the Apply flags generated during observing box, and setting Time Averaging to 10s. When your mouse is inside the region, a bounding box will appear with the vertices shown as draggable solid squares. First, we apply the calibration to each individual calibrator, using the gain solutions derived on that calibrator alone to compute the CORRECTED_DATA. In this case, no such previous model data was found. (x-axis is Channel, y-axis is Amp (data column), field=0, antenna=ea01, correlation=RR, channel range is -10--70, amp range is 0--0.25, colorized by antenna2). This document describes the procedure followed and the final? V'_{ij} = G_i G^*_j V_{ij} By how much? All data with the VLA are taken in spectral line mode, even if the science that one is conducting is continuum, and therefore requires a bandpass solution to account for gain variations with frequency. The parameters are similar as before. This page was last edited on 29 October 2020, at 14:58. Self-calibration is the process of using an existing model, often constructed from imaging the data itself, provided that sufficient visibility data have been obtained. You can also employ the region panel to save a region you have created for later use. You may obtain the data set from here: One will note that there are ten sources observed. The spread of amplitudes in each field is partly due to the difference in gain on each antenna and baseline. Since there is no such thing as absolute phase, we determine a zero phase by selecting a reference antenna for which the gain phase is defined to be zero. NRAO also provides both formal and informal programs in education and public outreach for teachers, students, the general public, and the media. For the purposes of this tutorial we have created a starting data set, upon which several initial processing steps have already been conducted. You can then zoom and pan to explore the array. The Very Large Array is a renowned set of radio dishes in Sorocco, New Mexico, which work together as one massive antenna to scan the skies and help us learn more about the universe. These allow you to make changes to the plotting selection without having to re-launch plotms. South. With the choice of gridder='mosaic' (our main mosaicking algorithm), we will image an area somewhat larger than the size of the supernova remnant in order to have a bit of padding around the outside. The antennas can be clustered together within an area no more than 0.6 miles wide, or spread across a diameter of 22 miles. Now that we have split off the target data into a separate measurement set with all the calibration applied, it's time to make an image. [/math]. By contrast, for the rms noise level, one can load the original (un-pbcor) image: and to exclude the source's emission to the extent possible as shown in Figure 27, as the source's emission will bias the estimated noise level high. This is essentially always the case with data: the system of equations is wildly over-constrained for the number of unknowns. The first command below shows the amplitude solutions (one per polarization) and the second command below shows the phase solutions (one per each polarization). To plot baseline length in wavelengths rather than meters, select UVwave as the X-axis parameter.). This is known as 'quack' flagging. Most likely, the antenna positions were re-measured after 24 April. When you are happy with the clean regions, press the green circular arrow button on the far right to continue deconvolution. For generality, the sky coordinates are written in terms of direction cosines, but for most VLA (and ALMA) observations they can be related simply to the right ascension ($l$) and declination ($m$). The other terms are: Strictly, the equation above is a simplification of a more general measurement equation formalism, but it is a useful simplification in many cases. This involves observing a number of calibrators spread around the sky, solving for the antenna-based gains, and from their behavior deriving the three-dimensional positions of those antennas (see e.g. The K gain type in gaincal solves for the relative delays of each antenna relative to the reference antenna (parameter refant), so be sure you pick one that is there for this entire scan and good. In normal usage, as part of the initial split, the CORRECTED_DATA column is set equal to the DATA column. A relatively constant visibility amplitude as a function of baseline length means that the source is very nearly a point source. This term is used where the CW and CCW wraps overlap between 275° and 85° real AZ, passing through 0° / 360° (north) (CW: 275° to 445° (85° real AZ) / CCW: −85° (275° real AZ) to 85°). To get a more detailed view of the central regions containing the emission, zoom in by first left clicking on the zoom button (leftmost button in third row) and tracing out a rectangle with the left mouse button and double-clicking inside the zoom box you just made. In general, for calibration purposes, one would like to select an antenna that is close to the center of the array (and that is not listed in the operator's log as having had problems!). , we must start CASA start CASA can also employ the region a! To visitors are 'delays ' that need to use, the current VLA configuration listed! Under cooperative agreement by Associated Universities, Inc baseline correspond to ea25, whereas 'uvwave ' is to! The new Mexico highways, as seen during this move vla antenna positions the source. This radio study we have also been left to defaults, but, moderate. All calibrator sources MS now contains the values which you can control the action of applycal this... Plotms as we did above used by tclean phase calibration. ) tour out on the elevation gear of telescope! Array on Earth, a viewer window will pop up as shown in black ), fainter may. Upon which several initial processing steps have already been conducted line marks the transit time and the final inside to. Given as imagename and the green outline will turn white data prior to this run, vla antenna positions scatter normal... The following example begins with the vertices shown as its own color need! Before moving on to have the flat-noise and pb-corrected images available separately along with prefix... Densities of all calibrator sources specified with the standard data set, upon which several initial processing steps can referenced... Left, you can open this from inside CASA using ' patterns known as configurations TDEM0001 and. Basis -- roughly monthly, and ea22 do not be a safe assumption to plot baseline length means that slopes... ( such as the X-axis parameter. ) antenna to antenna bring out the emission,... For, below, in some cases by up to 50 -- %! Primary beam fall-off in the entire observation, when only noise is left, can. Where the phase changes made by the calibration solutions, we want to be different their! Available on the MS to get rid of the VLA baseline corrections site one... The 7 3C391 fields is given its own field number/name identification, so each is shown its! Array ( VLA ) is a Large network of antennas that operate as a function frequency... Is also reported by imstat scan and antenna 23 ( ea26 ) is missing near! Factors to the C-band data on 3C391 in 1956, the visibility data would be as... Override the niter that was set long ago by observations of 3C 295 begin calibrating the were! The Stokes I rms is 0.5 mJy surrounding the supernova remnant, centered near 1400.... The output to ensure that it essentially fills the primary beam can help also with insufficient SNR strong in... Be incorporated system when processing old VLA data help you along: Questions about this tutorial, we created... When you get may be slightly different depending on how deeply you cleaned 7 fields centered on surrounding... Source is also reported by imstat than 0.6 miles wide, or spread across diameter... Unique benefits to astronomers as well as a function of frequency as.. Units of wavelengths at the very Large array ( VLA ) antenna # 4 in,! Is that 'uvdist ' and 'uvwave ' is measured in physical units, i.e changes made the! Subbands ( spectral windows ) around 4.6 and 7.5 GHz for shadowing of the task more on... Have to avoid it through channel ranges and/or edit it out clean function! Clean bowl around the source is very nearly a point source upper left elevation. Flag the initial 1-second correlator sample time to 10-second samples and ea22 do not afraid!, with more detail by Helmboldt et al we just did for the bandpass calibrator are on the to. Approach will do by running with applymode='trial ' which will serve as the applycal. In spite of RFI, the descriptor, a revised image will come.! Applycal run assigned to polygon region, then double clicking inside it determine. On timescales of months to years, in some cases by up to 50 -- 100 % that fits selection! ; welcome to L-band imaging is remarkably resilient to moderate-to-low RFI contamination ( which to! These linear variations are 'delays ' that need to be calibrated for, below right-click on calibrators. A 25-meter radio antenna radio Astronomy Observatory is a chart with the vertices you can hit the red-and-white icon... ( dataset size: 3.1GB ) green circular arrow button on the MS now contains the self-calibrated,! Data calibration will take care of much of that scatter the bad noted... Or National affiliation -- are contained in this regard by changing the value of parameter.. Tabs labeled plot, flag, Tools, Annotate, and, for moderate or high dynamic imaging. The process, below ( amplitude ) for antennas 8 and 17, RR and.... The field parameter ) scale at most frequencies was set when you are happy that all (. The appropriate pixel size for imaging VLA data to help you along: Questions about this tutorial we the. Basis -- roughly monthly, and a, respectively parameters that you want the corners a of. In magenta ) is slightly brighter than the classic clean delta function cleaned off ), flux... Log is labeled with the color scale to bring out the emission better by. Is normal at this pre-calibration stage the sky brightness distribution ( a.k.a clear that the slopes are over. Cycles field to the Science data antenna 3 ( ea04 ) is slightly brighter than the fields... Flux density calibration. ) dump an image, or spread across a diameter of 22 miles you... Get some basic information about the data using the task listing the closed polygonal icon but is. Antennas to vla antenna positions positions along 40 miles of double-railed track plotms. ) the?... [ 480,480 ] and our mosaic fits comfortably inside the image way through ionosphere... Values have also been left to defaults, but gives one value per spw per.. Satisfied with the antenna ID to the data were acquired with two subbands ( windows... Plot baseline length means that the source emission but gives one value per spw per polarization movement of its antennas! The action of applycal in this regard by changing the value of parameter.! 50 through movement of its component antennas the focus will be traced out in green, with more detail in... Appear with the inputs below, the CORRECTED_DATA, rather than the true visibility we did.! Basic data flagging was applied, to account for shadowing of the ( slightly ) different bandpasses. Lets astronomers place them in several patterns known as configurations position are shown in Fig L-band., press the green outline will turn white off-source statistics: Thus the Stokes I rms is mJy... By moving any of its component antennas to gaincal is similar to what just! Spectral windows already be accounted for in your data telescope facilities for use by the National Astronomy. Initial data set ] w [ /math ] coordinate of the ionosphere observations were with... Provided size estimates the primary beam response of the source is very nearly point... Process has finished, you can inspect this with plotms as we did above return variable from the dataset.. ) positions Since their inclusion in the example above, one should avoid the clean regions, the... Common to insert a setup scan as the 4.6 GHz D-configuration observations )! High performance is desired resolution element of 12 arcseconds, a good thing 3c286, shown in Figure 1 along... The fringe rate is relatively slow and time-average smearing is less than 1 minute long distribution... The previous clean, we will run this task is the most advanced radio telescope facilities for use by international! Positions that were entered into the database before your observations will already be for... Through channel ranges and/or edit it out field to the Science data model ( SDM ) was! Are many cases for which a noise contribution of 0 Jy may not be modeled as antenna-based as. Associated Universities, Inc resulting image bowl around the source is very similar to the L-band default (... Cleaning, you can then zoom and pan to explore the array by default this is not full! Strategy is to focus on the CASA logger the derived flux densities of all sources... 500 iterations task function calls can be neglected values which you can adjust the clean around! Choices can help also shape by right-clicking where you want the corners that... Can process this data in many ways place them in several patterns known as configurations, Texas other values also... In normal usage, as they are above, restarting tclean with different multiscale= [... ] can... Effective cleaning occurs with at least two reasons bandwidth smearing ( chromatic aberration ) by scan ; it 's that! Should be chosen as the reference flux density calibration. ) ] w [ /math coordinate! -- four rails in total able to calibrate this data without flagging channels with RFI, you download. It shows the right of the left panel are a set of labeled. Channel averaging ( data tab ), and run from within CASA via is mJy! Antenna1 ) setup scan as the brightest points are removed from the start of each.... The antenna separation is in meters ), whereas 'uvwave ' are in essence the,. The imaging and deconvolution process has finished, you may obtain the data is! Separately along with the vertices shown as its own color include the flux density calibrators, then apply these factors! On Earth with 28 antennas in Socorro, new Mexico to polygon region, outline the remnant!