INNAME Input UV file name (name) INCLASS Input UV file name (class) INSEQ 0.0 9999.0 Input UV file name (seq. #) INDISK 0.0 9.0 Input UV file disk unit # BCHAN 0.0 2048.0 Lowest channel number 0=>all ECHAN 0.0 2048.0 Highest channel number BIF 0.0 100.0 Lowest IF number 0=>all EIF 0.0 100.0 Highest IF number 0=>all STOKES Stokes type to fit. SELBAND Bandwidth to select (kHz) SELFREQ Frequency to select (MHz) FREQID Freq. ID to select. SOURCES Source name (1 only) ANTENNAS Antennas to include. 0=all SUBARRAY 0.0 1000.0 Subarray, 0 => 1. TIMERANG Time range to use. DOCALIB -1.0 101.0 > 0 calibrate data & weights > 99 do NOT calibrate weights GAINUSE CL/SN table to apply. DOPOL -1.0 10.0 If >0 correct polarization. PDVER PD table to apply (DOPOL>0) BLVER BL table to apply. FLAGVER Flag table version DOBAND -1.0 10.0 If >0 apply bandpass cal. Method used depends on value of DOBAND (see HELP file). BPVER Bandpass table version SMOOTH Smoothing function. See HELP SMOOTH for details. UVRANGE Range of baseline lengths in kilo wavelengths. SOLMODE Soln. mode: 'A&P','A','RE', ' ' OPCODE 'GAUS' or 'SPHE' NGAUSS 0.0 4.0 Number of components Guess of model parameters GMAX Peak of component (JY) GPOS (X,Y) position (asec) GWIDTH (BMAJ, BMIN, PA) of comp. (asec, asec, deg) 0->Use small value. GAINERR Antenna gains 0=>1 NITER 0.0 10000.0 Maximum # of iterations EDROP 0.0 Tolerance level Solve for model parameters? DOMAX -1.0 1.0 Solve for GMAX? DOPOS -1.0 1.0 Solve for GPOS? DOWIDTH -2.1 1.0 Solve for GWIDTH? INLIST List of sources up to 9999 IMSIZE 0.0 Min, max allowed component size (asec.) IMPORTANT PRTLEV 0.0 2.0 Print level,0=>none, 1,2 some DOCAT -1.0 3.0 0=>NO CC file. 1=>write CC. INVER Output CC file version no. 0=>create new FITOUT Not ' ' => write results to this text file

UVGIT Task: This task is a version of UVFIT with a different fitting method (the one used in IMFIT). It seems to converge better in some circumstances (where UVFIT simply returns the initial guess). This task will fit a model consisting of elliptical Gaussians or optically thin, uniform spheres to a uv data file. Output can optionally be written into a CC (components) table associated with the input uv data base. Multiple channels or IFs can be averaged before fitting. A circular Gaussian can be used by using an appropriate value for DOWIDTH. The current version will handle up to 2500000 visibilities. If there is more data in the file it should be averaged before running UVGIT. There is also a limit of 390 parameters to be solved for (60 components, 30 antennas). Errors are the formal errors from the fitting process and can be unrealistic. Adverbs: INNAME.....Input UV file name (name). Standard defaults. INCLASS....Input UV file name (class). Standard defaults. INSEQ......Input UV file name (seq. #). 0 => highest. INDISK.....Disk drive # of input UV file. 0 => any. BCHAN......First channel to use. Data from BCHAN to ECHAN are averaged, 0=>all. ECHAN......Highest channel to use. 0=>all higher than BCHAN BIF........First IF to use. Data from IF BIF to EIF are averaged 0=>all. EIF........Highest IF to use. 0=>all higher than BIF STOKES.....The desired Stokes type of the data: 'I','V','Q','U','RR','LL','RL','LR' SELBAND....Bandwidth of data to be selected. If more than one IF is present SELBAND is the width of the first IF required. Units = kHz, 0=> all SELFREQ....Frequency of data to be selected. If more than one IF is present SELFREQ is the frequency of the first IF required. Units = MHz, 0=> all FREQID.....Frequency identifier to select (you may determine which is applicable from the OPTYPE='SCAN' listing produced by LISTR. If either SELBAND or SELFREQ are set their values overide that of FREQID, however setting SELBAND and SELFREQ may occasionally result in an ambiguity, in which case the task will request that you use FREQID. SOURCES....Selected source from multi source file, only 1 allowed. ANTENNAS...A list of the antennas to include in the solutions If any number is negative then all antennas listed are NOT to be used to determine solutions and all others are. All 0 => use all. SUBARRAY...Subarray number to use. 0 => 1. TIMERANG...Time range of the data to be used. In order: Start day, hour, min. sec, end day, hour, min. sec. Days relative to ref. date. DOCALIB....If true (>0), calibrate the data using information in the specified Cal (CL) table for multi-source or SN table for single-source data. Also calibrate the weights unless DOCALIB > 99 (use this for old non-physical weights). GAINUSE....(multisource) version number of the CL table to apply to the data. SN version GAINUSE will be applied for a single source file. 0 => highest. DOPOL......If > 0 then correct data for instrumental polarization as represented in the AN or PD table. This correction is only useful if PCAL has been run or feed polarization parameters have been otherwise obtained. See HELP DOPOL for available correction modes: 1 is normal, 2 and 3 are for VLBI. 1-3 use a PD table if available; 6, 7, 8 are the same but use the AN (continuum solution) even if a PD table is present. PDVER......PD table to apply if PCAL was run with SPECTRAL true and 0 < DOPOL < 6. <= 0 => highest. BLVER......Version number of the baseline based calibration (BL) table to appply. <0 => apply no BL table, 0 => highest. FLAGVER....Specifies the version of the flagging table to be applied. 0 => highest numbered table. <0 => no flagging to be applied. DOBAND.....If true (>0) then correct the data for the shape of the antenna bandpasses using the BP table specified by BPVER. The correction has five modes: (a) if DOBAND=1 all entries for an antenna in the table are averaged together before correcting the data. (b) if DOBAND=2 the entry nearest in time (including solution weights) is used to correct the data. (c) if DOBAND=3 the table entries are interpolated in time (using solution weights) and the data are then corrected. (d) if DOBAND=4 the entry nearest in time (ignoring solution weights) is used to correct the data. (e) if DOBAND=5 the table entries are interpolated in time (ignoring solution weights) and the data are then corrected. BPVER......Specifies the version of the BP table to be applied. <0 => no bandpass correction done. SMOOTH.....Specifies the type of spectral smoothing to be applied to a uv database . The default is not to apply any smoothing. The elements of SMOOTH are as follows: SMOOTH(1) = type of smoothing to apply: 0 => no smoothing To smooth before applying bandpass calibration 1 => Hanning, 2 => Gaussian, 3 => Boxcar, 4 => Sinc To smooth after applying bandpass calibration 5 => Hanning, 6 => Gaussian, 7 => Boxcar, 8 => Sinc SMOOTH(2) = the "diameter" of the function, i.e. width between first nulls of Hanning triangle and sinc function, FWHM of Gaussian, width of Boxcar. Defaults (if < 0.1) are 4, 2, 2 and 3 channels for SMOOTH(1) = 1 - 4 and 5 - 8, resp. SMOOTH(3) = the diameter over which the convolving function has value - in channels. Defaults: 1,3,1,4 times SMOOTH(2) used when input SMOOTH(3) < net SMOOTH(2). UVRANGE....Range of allowed baseline lengths in kilowavelengths. SOLMODE....Solution mode: 'A&P ' => amplitude and phase. 'A ' => Amplitude only 'RE ' => Real part only ' ' => 'A&P ' OPCODE.....The model type. 'GAUS' = gaussian, 'SPHE'=> Optically thin homogenous sphere. NGAUSS......The number of components to use in the fitting. 0->1. Maximum number is four using adverbs. Ignored if INLIST is not blank. GMAX........The peak value for each component. MUST be given. Ignored if INLIST is not blank. GPOS........The position (X,Y) for each component. The values are in arcsec. No default. Ignored if INLIST is not blank. GWIDTH......The major axis, minor axis and position of major axis for each component. The values are in asec with no default. For uniform spheres only the first width parameter is used which is the radius in asec, the other values should be zeroed. Ignored if INLIST is not blank. GAINERR.....The antenna gains to use for each antenna. a positive value means fit that parameter with the given initial guess. Zero means use 1.0 and don't fit. Negative values mean use the absolute value as the antenna gain but don't fit. NITER.......The maximum number of iterations to use in the fitting. 0-> NGAUSS * 100. EDROP.......A tolerance level for cutoff. 0->10**(-12) A value >1 relaxes the convergence criterum, A value <1 (but >0) increases. One or more factors of ten may be appropriate. Flags for holding any of the parameters constant 1 -> let parameter vary. (> 0 -> 1) -1 -> keep parameter fixed. (<= 0 -> -1) DOMAX.......Flags for GMAX Ignored if INLIST is not blank. DOPOS.......Flags for GPOS. If SOLMODE='A' is used then the position of at least one component MUST be fixed. Ignored if INLIST is not blank. DOWIDTH.....Flags for GWIDTH, if DOWIDTH(2,n) = -2 then use a circular gaussian of width GWIDTH(1,n). For uniform spheres ONLY DOWIDTH(1,n) should be set true (+1). Ignored if INLIST is not blank. INLIST......Set this blank unless you want an input components list. Text file containing one line per source, giving channel, gmax(i), gpos(1,i), gpos(2,i), gwidth(1,i), gwidth(2,i), gwidth(3,i), domax(i), dopos(1,i), dopos(2,i), dowidth(1,i), dowidth(2,i), dowidth(3,i) blank separated free format and trailing zeros may be omitted. A comment field may follow dowidth(3,i) which will be appended to the output line in the FITOUT file. Input lines in INLIST are limited to 512 characters and the comment field may be no longer than 256 characters. If the INLIST adverb is not blank, this file is used to set the NGAUSS, initial guess parameters, and whether they are fit or held fixed. There is a limit of 60 components for any one channel and lines in the file beginning with # are comments and those with channel not matching BCHAN are ignored. IMSIZE......Minumum and maximum size allowed for components in arc seconds. MUST be given. PRTLEV......Print level for messages from fitting routine: 0 => none, 1 => some, 2 => lots. (mostly inscrutable stuff) DOCAT.......0-> Don't write a CC extension file with the results. 1-> Write the derived gaussian components to a CC file. The source name is added as a table keyword; use PRTAB to examine. INVER.......Output CC file version number desired. 0=>Create a new extension file. FITOUT......If not blank, the name of a text file to receive a summary of the fit results. A one line title plus one line per component are printed. The text is appended to pre-existing files. The text includes a line giving column titles and one line per component giving in order: channel, component number, RA position and error, Dec position and error, Peak flux and error, major axis and error, minor axis and error, position angle and error, postfit RMS, Chi-squared, start time and end time of the interval. Units for time are days, for flux and RMS Jy, for position offsets and sizes arc-seconds or milli-arcseconds, and for position angles degrees.

UVGIT: Task to fit a model brightness distribution to uv data. DOCUMENTOR: Chris Flatters, NRAO PURPOSE UVGIT fits a model brightness distribution comprising up to four discrete components to uv data by adjusting the component parameters to minimize the chi-squared statistic for the ideal visibilities derived from the model and the observed data. The model components may be elliptical Gaussian brightness distributions or optically-thin spheres according to the setting of the OPCODE adverb; UVGIT can not handle a mixture of Gaussian and spherical components. The optimized model may be saved as a clean component (CC) file by setting DOCAT and INVERS. You may also use UVGIT to derive constant telescope gain corrections. This is useful for calibrating VLBI data where there may be significant differences in the a priori flux scaling at each antenna. DATA SELECTION UVGIT will operate on both single-source and multi-source data files. If multi-source data is used then a single source must be selected using the SOURCES adverb. Multichannel and multi-IF data may be used as input. All selected channels and IFs will be averaged together. VLBI data should therefore be fringe-fitted before running UVGIT and any bandpass corrections that are available for spectral- line data should be applied. UVGIT will not use more than 2500000 visibilities. If the selection criteria include more than this number of visibilities then UVGIT will use the first 2500000 visibilities that meet the criteria. If there are more than 2500000 selected visibilities, it is advisable to average the data so that uv coverage is not lost. A priori amplitude calibration should, of course, be applied to the data. The STOKES adverb should normally be restricted to 'I', 'RR' or 'LL'. FITTING A MODEL BRIGHTNESS DISTRIBUTION UVGIT works by minimizing the chi-squared statistic (the sum of the squared deviations of the data from the ideal visibilities). As is usual case for minimization techniques, UVGIT will find a local minimum of chi-squared with respect to all of the model parameters that are allowed to vary. In order for this to be a global minimum, UVGIT must be given an initial guess for the model parameters that is close to the location of the global minimum. You supply the initial guess by giving the number of parameters that you want to use and their parameters. Gaussian components are specified by giving a peak brightness (GMAX), a position in arcseconds relative to the phase center (GPOS), a major axis size in arcseconds, a minor axis size and a position angle for the major axis in degrees (GWIDTH) for each component. If the major axis or minor axis size is given as zero a small number will be used instead. Note that component widths are constrained to be between IMSIZE(1) and IMSIZE(2) during the fit so you should make sure that IMSIZE(1) is less than your smallest GWIDTH and that IMSIZE(2) is greater than larger than your biggest GWIDTH. Always check IMSIZE before running UVGIT since its usage in other programs tends to favour IMSIZE(1) being equal to IMSIZE(2) which will force UVGIT to use circular Gaussian components all of the same size. The same parameters are used for spherical components with the exception that only the major axis size is specified using GWIDTH and the other two components of GWIDTH are ignored. You should then choose whether you wish to fit to amplitude and phase or amplitude only. If you have VLBI data that has not yet been self-calibrated in phase them you should use amplitudes only (SOLMODE='A') otherwise you should normally use both amplitudes and phases (SOLMODE='A&P'). The next step is to choose which parameters you will allow to vary during the fit. Do this by setting the appropriate flags in DOMAX, DOPOS and DOWIDTH to +1 for the parameters that will vary and -1 for those that will be held constant. If you are using Gaussian components you can force any of the to be circular by setting DOWIDTH(2, i) to 2 (to constrain the i-th component). Note that if you are fitting to amplitudes only then the position of at least one component may be fixed. You may then start UVGIT to find the optimized parameters. I recommend that you run UVGIT with PRTLEV set to 1. This will give a small amount of information about every iteration. If the iteration number goes over about thirty or if you don't see the chi-squared statistic (labelled F in the iteration summary) go down significantly in the first few iterations then UVGIT is going nowhere and you might as well abort it and start again with a different model (small numbers of iterations can also be a problem as discussed below). PRTLEV 0 doesn't give any information about the iterations and PRTLEV 2 gives much more information than is needed. EVALUATING THE RESULTS UVGIT will report the post-fit chi-squared statistic and rms. Ideally, the rms should be close to the average thermal noise for a data point after averaging over the selected channels and IFs. If the rms is much higher than this you obviously have a poor fit to the data. To use the post-fit chi-square you need to determine the number of degrees of freedom, d = k * v - p where k = 1 if you are fitting amplitudes only or k = 2 if you are fitting amplitude and phase, v is the number of visibilities used and p is the number of parameters allowed to vary. If the reduced chi-squared statistic (obtained by dividing the post-fit chi-squared by d) is about 1 or less then you have a good fit; it it is much greater than one then you have junk. More quantitatively, you can use the chi-squared statistic to get the probability of obtaining the observed data given the model in question (assuming errors have a Gaussian distribution). You will probably need a statistics program to calculate this since d is usually much larger than the number of degrees of freedom for which the chi-square distribution is tabulated in reference books. This may not seem particularly useful on its own but given two models, A and B, with no reason to prefer one over the other and given the probabilities of obtaining the observed data given model A, P(d|A) and that of obtaining the observed data given model B, P(d|B), the probabilities of either model being correct given the observed data are in the ratio P(A|d) / P(B|d) = P(d|A) / P(d|B) (this is a trivial application of Bayes' theorem). More generally, the higher the probability of obtaining the data given a particular model, the more you should believe the model. You should note, however, that the calculation of the chi-squared statistic assumes that the data are weighted by the inverse of the noise variance; if this is not true then the value of the chi-squared statistic is useless (although lower chi-square numbers are still preferred over larger ones). NOTES ON THE ERROR BOUNDS UVGIT presents nominal error bounds on all of the parameters that were allowed to vary during the fit. These errors are actually the square roots of the diagonal elements of the covariance matrix (the matrix obtained by inverting the Hessian matrix -- the matrix where element ij is the partial derivative of chi-squared with respect to the i-th and j-th model parameters) for the final model parameters. Provided that the data weights reflect the expected noise in the data these elements give the formal 1-sigma error for each parameter with all other components at their fitted value. This means that they will underestimate the true errors since the error ranges of the parameters are all interconnected; it is, however difficult to derive any better estimates. A further complication arises from the algorithm used by UVGIT. UVGIT uses a member of the quasi-Newton family of minimization algorithms. Quasi-Newton algorithms build up the Hessian matrix through successive approximations. This has the consequence that the error estimates will not be accurate unless UVGIT goes through several iterations. If UVGIT converges in less than about 5 iterations you may want to change one or more of the parameters (to force it to do more iterations) and run it again to get better error estimates. FITTING ANTENNA GAINS You may also introduce antenna gain factors as parameters to be fit. If GAINERR(i) is zero the gain factor for antenna i is fixed at unity (the default); you may fix the gain factor at another value, C, by setting GAINERR(i) to -C or you may allow UVGIT to fit the gain factor by setting GAINERR(i) to a positive value (which is taken as the starting guess). You can use the gain factors output by UVGIT to correct antenna gains using CLCOR (set CLCORPRM(1) to the factor given by UVGIT for the antenna in question, set the remaining elements of CLCORPRM to zero and set OPCODE to 'GAIN'). This is useful for VLBI data where different antennae may have significant systematic errors in gain measurements. Unless you know the flux density of the source, in which case the model fluxes should be fixed by setting all elements of DOMAX to -1, the gain of at least one antenna in the array should be fixed otherwise the flux scale will drift.