-----DATA specification------ INNAME Input UV file name (name) INCLASS Input UV file name (class) INSEQ Input UV file name (seq. #) INDISK Input UV file disk unit # SRCNAME Source Name (1 only) SUBARRAY Subarray TIMERANG Time: start day,hr,min,sec stop day,hr,min,sec ANTENNAS Antennas to include UVRANGE Range of projected spacings (thousands of wavelengths) STOKES 'I', 'HALF', 'RR', and 'LL' default = 'I' FREQID Default FQ table entry (-1) BCHAN First chan ( 0 => 1 ) ECHAN Last chan ( 0 => BCHAN!!!) BIF First IF ( 0 => 1 ) EIF Last IF ( 0 => EIF !!!) DOCALIB -1.0 101.0 > 0 calibrate data & weights > 99 do NOT calibrate weights GAINUSE CL (or 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. ---- Input MODEL file ----- INFILE DOFIT shortcut for model specs --- Output MODEL file ----- OUTFILE -------SCREEN output------- PRTLEV recommended value = 3 +100 => explain noise +10 => show intermed. values ---- Output DATA file ----- OUTPRINT -- Output AIPS DISK file -- OUTNAME (name) OUTCLASS (class) OUTSEQ (seq. #) OUTDISK (disk #) OUTVERS (content type - see HELP 0: D 1: D-GS 2: S 3: GS 4: D/G 5: D/S 6: D/(GS) 7: D/|D| NOISE (1) a priori noise [Jy] (not already in weights) (2) outlier threshold [sigmas] (3) low-amp cutoff threshold [sigmas] APARM Task Enrichment Parameters Early Stopping Criteria: ( set <0 to turn off ) (1) percent change in Chi-squared 0 => 1 percent ( <0 to disable ) (2) greatest fractional meaningful change in parameters: 0 => 0.01 Misc. controls: (3) LMM initial factor (4) print covariance matrix in OUTFILE (1=yes,2=no) default=yes for a single component model or if matrix is small [<16x16] NITER max # iterations ( <1 => 40 ) ------Self-Calibration------- SOLMODE self-calibration mode ['' => none requested] traditional modes: 'A' => ampl. self-cal 'P' => phase self-cal 'M' => per comp self-cal 'T' => force closure 'B' => ignore closure !! SOLINT and DPARM used only if self-cal not requested !!! SOLINT solution interval [ sec! ] (=< 0 --> 100 years) DPARM Specifies which parameters to solve for in each iteration (1,2) start/stop solving for self-cal phases (3,4) start/stop solving for self-cal amplitudes (5,6) start/stop solving for non- self-cal (source) parameters [default = always solve at at all iterations] WEIGHTIT 0.0 3.0 Modify data weights function BADDISK Disks not to use for scratch

OMFIT Task: OMFIT fits a source model to the UV data directly. Optionally, this task can simultaneously self-calibrate the data using the model fit. There is no pre-compiled limit to the number of input visibilities. THIS IS DEVELOPMENT CODE written by Ketan Desai who no longer works for NRAO. Its support is questionable. Adverbs:[out-of-date, see inputs list above instead] 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. SRCNAME....Source name to be gridded. Must specify if input is a multi-source data set, otherwise all sources are included. 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. ANTENNAS...List of antennas to use in selecting data. Only baselines between antennas appearing on this list are used. If the list is empty, all antennas are allowed. UVRANGE....Range of allowed baseline lengths in kilowavelengths. STOKES.....The desired Stokes type of the data: 'I','RR','LL', and 'HALF' are allowed where ' ' defaults to 'I' and 'HALF' processes 'LL' and 'RR' separately FREQID.....Frequency identifier to select (you may determine which is applicable from the OPTYPE='SCAN' listing produced by LISTR). BCHAN......Channel range to select. ECHAN BIF........IF range to select. Only one or all IFs EIF can be selected. 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....version number of the CL table to apply to multisource files or the SN table for single source files. 0 => highest. DOPOL......If > 0.5 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). INFILE.....KEYIN format file containing additional control parameters for the model-fit (see EXPLAIN OMFIT for simple and detailed examples). DOFIT......This is a new feature - and currently poorly documented. Basically, this feature allows you to enter a model at the POPS prompt instead of specifying an INFILE. OUTFILE....KEYIN format file to which to write final fitted model. This file can be used as an INFILE for another invocation of OMFIT. If the last character in the OUTFILE string is a '+' (plus) character, the model will be appended to the output file (or create a new one) all without the trailing plus sign. Without the plus, the OUTFILE will be written only if it is a new file. PRTLEV.....Output print level 0 => just the facts please - gimme the answer! 1 => basic info about the data (un)used in the fit 2 => some intermediary information 3 => more intermediary information - recommended level. 4 => too much intermediary information 10 => WAY TOO MUCH information [good for debugging - intermediary parameters are displayed in internal units ] OUTPRINT...Text file containing model-fit residuals OUTNAME.... OUTCLASS... OUTSEQ..... OUTVERS....Content to write to output AIPS data set 0: data 1: data - [gain model * source model] 2: source model 3: gain model * source model 4: data / gain model 5: data / source model 6: data / [ gain model * source model ] NOISE(1)...Pre-fit amplitude (a priori) noise in Jy. 0 -> 1 Note - this enters into CHI-squared displayed at the end and should be the additional noise not reflected in the weights. Thus if the weights are actually 1/sigma^2 in Jy already, set NOISE(1) = 1 (the NEW default). If the wights should be 100 times bigger to be properly calibrated, set NOISE(1)=0,1, Estimated thermal noise per requested Stokes visibility. If you set STOKES='I','RR','LL',or ' ', give here the thermal noise per I visibility. If set 'HALF', give here the thermal noise per 'RR' or 'LL' [assumed the same]. NOISE(2)...Threshold for editing data in sigmas (0 -> 10). Flag data when residual exceeds NOISE(2)*NOISE(2)*Chisq. Data editing parameter. Data that differ from the model by more than APARM(2)*(model-fit residual estimated for current model) will be discarded at each iteration. NOISE(3)...Low threshold for editing data in sigmas (0 -> 0). Flag data when amplitude less than NOISE(3) * SQRT(Chisq). Data editing parameter. Data with amplitudes below APARM(3)*(model-fit residual estimated for current model) will be discarded for each iteration. Early stopping criteria: APARM(1)...Chi-squared must change less than this amount (in percent) to stop. 0 -> 1 (set < 0 to turn this off) APARM(2)...Greatest fraction change in any parameter must be less than APARM(2) to stop. 0 -> 0.01. (set < 0 to turn this off) APARM(3)...LMM initial factor - determines how fast OMFIT moves, bigger values mean slower movement. (-60 < APARM(3) < 60) APARM(4)...Covariance matrix information will be written to OUTFILE at each iteration if OUTFILE is set and APARM(4) not = 2. Set = 1 to see these data when the number of global parameters exceeds 16. ****** The following APARMs are no longer used ******* APARM(5,6).Defaults = 0.01,0.05. APARM(5) is a threshold value to which the fractional change in Chi-Squared over the previous iteration is compared. APARM(6) is a threshold value to which the ratio of each parameter's change to its estimated error bar is compared. If the changes to Chi-Squared and all the parameter change/error-bar ratios fall below these thresholds, the iteration will terminate early because the model is not changing. Set either APARM(5) or APARM(6)<0 to force the full number of iterations to be performed. APARM(7)...Default = 0. [allowed range = -60 ... 60] 2**APARM(6) is the initial value to use for the Levenberg- Marquardt Scaling parameter: GAMMA. Large Positive GAMMA initially ignores covariances between parameters and proceeds as a Steepest Descent algorithm. Large Negative GAMMA initially accepts all covariances between parameters and proceeds as an Inverse Hessian algorithm. In ENGLISH: GAMMA controls how much attention the model-fitting procedure initially pays to the covariance between the fitted parameters. If GAMMA is large, the model-fitter starts by varying each parameter independentl of all others. If the parameters are highly covariant, i.e. if varying one has much the same effect as varying another, this is not a good idea. A small GAMMA tells the model-fitter to pay attention to these covariances from the beginning. The program increases or decreases GAMMA at each iteration depending on how successful the last step was in reducing Chi-Square; APARM(6) just sets the initial value. APARM(8)...Minimum SNR used to edit self-calibration solutions. If the SNR of a self-calibration solution falls below APARM(7), the weight of the SN table entry is set negative -- it is NOT removed. Default is not to reject any self-calibration solutions. NB: currently, OMFIT's SNR calculation does not conform to the rest of AIPS. APARM(8) is currently disabled. APARM(9)...APARM(9) controls one final criterion for baseline selection and model refinement control. If self-cal is selected: APARM(9)=0 forces baselines not part of a triangle to be dropped while APARM(9)<>0 passes all baselines. This behaviour is reversed when self-cal is not selected: APARM(9)=0 passes all baselines, APARM(9)<>0 forces baselines to be a part of closing triangles. SELF-CALIBRATION INPUTS: NITER......Default = 40. Number of iterations to perform SOLMODE....Self-Calibration mode selection string. Default = ' ' (no self-calibration) Allowed values: 'A' amplitude only self-cal 'P' phase only self-cal 'AP' full self-cal - amplitude and phase. 'MP' multiple phase-only self-cal 'MA' multiple amplitude-only self-cal 'MAP' multiple full - self-cal "multiple" self-cal ==> independent self-cal for each model component. Additionally: 'T' explicitly requests that closure be forced 'B' explicitly requests that closure be ignored default: closure is forced only if self-cal is selected SOLINT.....Default = 36500*86400 [seconds]. Solution interval to use if self-calibration is requested. DPARM(1-2).specify at which iterations self-cal phases are solved for. if self-cal phases not requested, phases are never solved for. default = 1,NITER IF phase self-cal requested via SOLMOD DPARM(3-4).specify at which iterations self-cal amps are solved for. if self-cal amps not requested, amps are never solved for. default = 1,NITER IF amp self-cal requested via SOLMOD DPARM(5-6).specify at which iterations non-self-cal parameters are solved for. default = 1,NITER WEIGHTIT...If > 0, change the data weights by a function of the weights just before doing the solution. Choices are: 0 - no change weighting by 1/sigma**2 1 - sqrt (wt) weighting by 1/sigma may be more stable 2 - (wt)**0.25 3 - change all weights to 1.0

OMFIT: This task simultaneously self-calibrates and model-fits UV data. The actual (non-linear) model-fitting is done using the Levenberg-Marquardt method of least-squares fitting (see, e.g., the _Numerical Recipes_ chapter on modeling data, also Nash, Compact Numerical Methods for Computers). The matrix inversions are performed using LAPACK SVD code. [Thank you LAPACK and NETLIB!!!!]. DOCUMENTOR: Michael Rupen, NRAO/Socorro. PROGRAMMER: K. M. Desai, NRAO/SOC, UCSB, NRL/DC, NRAO/CV RELATED TASKS: UVFIT, CALIB, [IMFIT, JMFIT, IMSTAT]? PURPOSE SOURCE MODELS INITIAL GUESES FOR THE ANGULAR SIZE WHY USE OMFIT SIMPLE INFILE EXAMPLE FULL DESCRIPTION OF INFILE FORMAT COMPLICATED INFILE EXAMPLE TERMINOLOGY and COMMENTS FEATURES 'UNDER DEVELOPMENT' PURPOSE ======= OMFIT is a model-fitting program that fits the UV-data directly. OMFIT can fit for different modeltypes simultaneously. Individual models may span different ranges of frequency space with [possible] overlap. Different Stokes parameters may be selected for fitting; a few models can be fit to multiple STOKES parameters simultaneously. The following types of models are currently implemented: The Source Models ================= The following models are currently supported: Model Type #parameters parameter description parameter unit *1* 'DOT' Point 3 Flux density of point source [Jy] source Offset east of phase center [mas] Offset north of phase center [mas] 'POL' Point 4 Flux density of point source [Jy] source Stokes polarization [AIPS stokes code] Offset east of phase center [mas] Offset north of phase center [mas] 'GAU' Gaussian 6 Flux density of Gaussian [Jy] Offset East of phase center [mas] Offset North of phase center [mas] Major axis angular size [mas] Axial ratio (minor/major) [dimensionless] Position Angle of major axis [deg. East of North on the sky] 'DISK' Disk 7 Flux density of Disk [Jy] Offset East of phase center [mas] Offset North of phase center [mas] Major axis [mas] Minor axis [mas] Position Angle of major axis [deg. East of North on the sky] *2* Order of the bessel function [dimensionless] 'SPHR' Disk 5 Flux density of spherical shell [Jy] Offset East of phase center [mas] Offset North of phase center [mas] *4* Average radius [mas] *4* Shell f [dimensionless] 'MAS' Maser 10 Flux density of Gaussian [Jy] Offset East of phase center [mas] Offset North of phase center [mas] Major axis [mas] Axial ratio (minor/major) [dimensionless] Position Angle of major axis [deg. East of North on the sky] Center of Gaussian spectral line profile [channels] FWHM of Gaussian spectral line profile [channels] Drift Eastwards per channel of position from center of spectral line profile [mas/channel] Drift Northwards per channel of position from center of spectral line profile [mas/channel] 'GAINS' Gain #IF*#POL Antenna number [as in AN table] station gains [dimensionless] ordered as either IF1 IF2 ... IFn or IF1RR IF1LL IF2RR ... IFnLL *1* Any modeltype specified using a 'Y' prefix, eg 'YDOT', will have all its angular size parameters (major axis, position offsets) interpreted in arcseconds instead of milliarcseconds (mas). *2* Suitable choices for this parameter allow the use of a ring or a thin sphere model [see below]. *3* See full infile below for an implementation of GAINS. A triplet consisting of antenna number, mean gain, and vary flag may be specified for each antenna in the data set. **NB** when amplitude self-calibration is specified, although specified gain values are read in and applied, they are not solved for. *4* The shell has inner radius r_in and outer radius r_out. The fitted radius, r_geo, is the geometrical mean of r_in and r_out (r_geo = sqrt(r_in*r_out)). Shell-f works this way: r_out = r * (1 + shell-f/2.) r_in = r / (1 + shell-f/2.) shell-f is close to but not equal to (r_out - r_in)/r_geo Ie. if r_out = 1.25 and r_in = 1.0, then r_geo is 1.118 and shell-f = 0.23607

Initial guesses: angular size: ============================== If the angular size is specified in arcseconds and a fiducial baseline length is determined using UVPLT, the product should equal a scale-factor which is typical of each modeltype: Modeltype Angular size def Fiducial baseline length Scale factor GAU FWHM Vis fcn half power pt 91.1 DISK Uniform disk radius First null of vis fcn 240.0 DISK Ring radius 150.0 DISK Thin sphere radius 280.0 The DISK model generates a thin sphere when n = 1. The DISK model generates a uniform ring when n = -2. The DISK model generates a uniform disk when n = 0. As may be apparent from the above list, different types of models are being added and requests will be accepted.

WHY USE OMFIT? ============== OMFIT has the following advantages over UVFIT: +) OMFIT does not have any built-in limit on the number of visibilities to be fit. The data do not have to be pre-averaged to fit within a fixed maximum number of visibilities; nor is any time-averaging done within the task itself. +) OMFIT can simultaneously solve for different model types. This means you can request that, e.g., an Elliptical and a Circular Gaussian be fit simultaneously to the data. +) OMFIT can self-calibrate the UV data while model-fitting. +) OMFIT can also perform multiple self-calibration, i.e., solve for different self-calibration solutions in different directions on the sky. This may be useful for low-frequency work, or when using the phased-VLA for VLBI work. +) OMFIT can solve for antenna-based IF and polarization dependent time-independent gain factors. This can be useful for transferring a final amplitude scale from a calibrator to a target source. OMFIT's most significant disadvantages are: -) OMFIT runs more slowly than UVFIT. This is because... - OMFIT does not load the visibilities into memory - self-calibration involves lots of overhead - multiple self-calibration has even more overhead Fortunately, self-cal and multiple self-cal are not usually necessary, and turning them off recovers much of the speed of UVFIT. OMFIT now uses dynamic memory allocation which should also help recover some of the overhead. Gains do not cost much in terms of speed and should be used instead of amplitude self-cal whenever possible.

SIMPLE INFILE EXAMPLE ===================== The INFILE can be quite complicated, so this description begins with the simplest possible file: ! ## MINIMAL INFILE !------------------------------------------------------------------------ ! This is a minimal INFILE ! ! Type ID Bchan Echan FlTot vary? Epos vary? Npos vary? Maj vary? ! Min/Maj vary? PA vary? 'GAU' 1 7 7 45.0 'T' 5.0000 'T' 7.0000 'T' 8.5000 'T' 0.90 'T' 9.0 'T' / / ! DO NOT REMOVE !------------------------------------------------------------------------ ! ## END OF MINIMAL INFILE This infile requests fitting one component, labelled #1 (Icomp) in I/O, in channel 7 (Bchan, Echan). The initial total flux is 45 Jy (FlTot); the source is located (5, 7) mas (East, North) of the phase-center (Epos, Npos). The model is to be an elliptical Gaussian (Type) with initial parameters: major axis= 8.5mas (Maj), minor/major axis axial ratio= 0.9 (Min/Maj), position angle for the major axis= 9 degrees East of North (PA). All parameters are allowed to vary during the fitting process. **** The comment line entitled "DO NOT REMOVE" is required for proper operation of the KEYIN input subroutine used by OMFIT. Since generation of a suitable INFILE is often frustrated by KEYIN's fussiness, it is recommended that this line not be left out, not even accidentally [thats not funny...] **** A more complicated sample INFILE follows the full description of the INFILE format. FULL DESCRIPTION OF INFILE FORMAT ================================= INFILE is used to provide basic control parameters and is divided into logical lines. Logical lines are delimited by the '/' character. The '!' character serves as a comment character. Comment lines CANNOT appear _within_ a single logical line, i.e., 1 10 101 'DOT' ! oops 5 'T' 1.0 'T' 2.0 'T' / is NOT legal, while 1 10 101 'DOT' 5 'T' 1.0 'T' 2.0 'T' / ! whew is OK. An INFILE may have multiple logical lines; it is terminated by a single otherwise blank logical line. Description of logical line format (model specification): Each logical line corresponds to one component to be fit, and consists of a number of entries separated by blanks (or carriage returns): 1st entry: MODELTYPE component type, eg 'DOT', see list below 2nd entry: ID source number for multi-source files; used as an "identification number" for both single and multi-source files. 3rd entry: BEG-CHAN first channel in which this component exists 4th entry: END-CHAN last channel in which this component exists Sub. entries: Pairs of parameters for the model. Each pair consists of a parameter value (initial guess), followed by 'T' or 'F': 'T' -> find the best fit to that parameter 'F' -> keep that parameter fixed Last entry (/): character indicating end of logical line ============================================================================== COMPLICATED INFILE EXAMPLE ========================== ! ## BEGIN COMPLICATED SAMPLE 'INFILE' !------------------------------------------------------------------------ ! This is a ridiculously complicated infile ! Mtype ID Bch Ech Flux flag EastPos flag NorthPos flag ... 'YGAU' 469 1 1 3.25208 'T' 82.31136 'T' -76.53779 'T' .69031 'T' .96562 'T' 39.51926 'T' / 'YGAU' 468 1 1 2.33739 'T' 81.62495 'T' -83.76611 'T' 1.02294 'T' .95342 'F' 86.40079 'T' / 'YDOT' 467 2 2 .11282 'T' 65.22981 'T' -76.88685 'T' / 'GAU' 465 3 3 .11964 'T' -89554.05 'T' 80003.69 'T' 1001.43 'T' .99732 'T' -12.58175 'T' / 'YDOT' 462 3 3 1.04904 'T' -66.15225 'T' 85.19714 'T' / 'GAINS' 2 2.0 'T' / 'GAINS' 0 1.1 'T' / 'GAINS' 4 1.5 'F' / / ! end inputs ! Required Blank Line - leave alone ! ! ## END COMPLICATED SAMPLE 'INFILE' Note the following characteristics about this example: - This file specifies a model for three channels. - Each channel is modelled independently. [there are no features that span more than one channel.] - Channel 1 has two gaussian components, the axial ratio and position angle component 468 will not be solved for. - Channel 2 has only a single point source component. - Channel 3 has both a point and a gaussian component. - The size and position parameters of the gaussian component in channel 3 are specified in milliarcseconds. - The default gain information overrides all previously encountered gain information. *** - All antennas will start with gains of 1.1 except for antenna 4 which will start with 1.5. - The mean gain factors specified here will multiply the model visibility. Eg, initially the model visibility on baseline 2-4 will be multiplied by 1.1*1.5 = 1.65 . - mean gain factors will be determined for all stations except station 4. *** except *** if amplitude self-calibration was requested through SOLMODE = 'A', 'AP', or 'MAP'; in that case, mean gains will not be determined for any station. - The order of the logical lines is arbitrary [except for the default gain specification as described above]. ============================================================================ TERMINOLOGY and COMMENTS ======================== [3apr98 - outdated section (kmd) ] global parameters - these are all model parameters that are not self-cal parameters, eg source flux, major axis size, minor axis size, position angle, ra offset, dec offset, etc. local parameters - these are all model parameters that are self-cal related and come in two flavours, local phases which are self-cal phases, and local amplitudes which are self-cal amplitudes. DoF - The number of observed visibilities minus the number of model parameters. "Discarded ?? global and ??? local params" - the singular value decomposition method used to invert the normal matrices may choose to discard some singular values [effectively discarding linear combinations of model parameters] based upon the input value of CUTOF. This message simply warns the user that some linear combinations of his/her parameters are being discarded in the model fitting procedure. 1-sigma error calculation method - The principal problem with calculating error bars in AIPS is that the weights, while possibly well known in a relative sense, are unknown by an overall scale factor. If this were not so, SWSR would be the Chi-Square statistic, RSWSR would be the reduced Chi-Square, and the goodness-of-fit of the model could be evaluated. OMFIT reports two estimates of the 1-sigma error. The 'formal' 1-sigma error is shown in the OUTFILE output and uses the user estimated thermal noise per visibility [default value = 1 Jy]. The 'suggested' 1-sigma error is printed to screen when OMFIT runs and also is sent to the OUTFILE. This error assumes that the model is a perfect representation of the data so that the post-model fit residual is also an estimate of the thermal noise per visibility. This error bar is probably what you want since presumably you do think that the model fits the data well. OMFIT is perfectly happy to allow you to solve for self-calibration parameters and source position offsets. It is important to realize that these two types of parameters are somewhat degenerate - especially when there is only one source component in the model. ============================================================================ FEATURES 'UNDER DEVELOPMENT' ============================ See the section immediately following the local includes in $QPGOOP/OMFIT.FOR . You may request additional features, or report bugs via email to kdesai@nrao.edu . ============================================================================ Comments from an M. Bietenholz, an experienced user of OMFIT It will probably never be particularly easy to use. Its a bit clunky because in typical usage, you have to have your input text file open in an editor outside of AIPS to make changes between runs of OMFIT, but there is probably no reasonable way of moving all the possible model parameters to adverbs. It is also sensitive to the input file format. I always use it on uv-files which have already had the vis. averaged together across both freq. and IF, I can't remember whether this is just to reduce number of points to fit or if OMFIT doesn't work for multiple IFs. A related but larger issue is that very often, the dominant errors in the visibilities are in fact calibration errors not noise, which are highly correlated from 1 vis to the next in the typical case of vis.recs. being a few secs. Using such small vis records means that OMFIT uses a huge number of degrees of freedom, and gets uncertainties which are ridiculously small. I usually UVAVG to scans (for typical phase-ref VLBI scan lengths of ~1-2 min) before feeding to OMFIT. However, the issue of the real number of degrees of freedom and getting usable uncertainties does not have an easy solution, so probably some intelligence and diligence on the part of users will be unavoidable. I also think OMFIT does not recognize degenerate cases, such as amplitude self-cal *and* fitting your source flux density. I don't remember what happens when you do this, I think it happily converges somewhere or other but doesn't necessarily let you know that the uncertainty on your fitted flux density is actually infinite. There is some code to try an prune degenerate parameters from the fit, but I think it only works for single parameters that have no effect on the fit, e.g., p.a. of a *circular* Gaussian. I don't know whether it would be easy to check fo the most common cases of degenerate combinations of parameters - ie. you need at least one source flux-density or gain-amplitude parameter NOT to be free. The other issue that will be difficult to avoid is that you need a reasonable starting model. Its not uncommon for there to be local minima in the Chi2 hypersurface. Getting the starting position of your model wrong by more than a beamsize or so is likely to converge to some local minimum in the Chi2 space that has nothing to do with the best fit. My 2-dim picture is that the Chi2 surface has some concerted dip near the best fit, where you have some hope of finding the global min by crawling downslope, but far away from the best-fit its probably just some random hilly surface, so crawling downslope will get you to one of an infinite number of local minima. I don't think there is much than can be done about this. The parameter space is probably generally too big to allow a brute-force grid search amongst different starting points (a la difmap), so probably the only way to get somewhat reliable results out of OMFIT is to a) have a pretty good idea of your starting points, especially positions and b) to run it several times. My recipe is usually to give it starting parameter vector [x1], let it converge (hopefully) to [y], and then restart it with a starting point on the opposite side of [y], ie. something like [2*y - x1]. With luck it will again converge to [y] and you're good. If not, a few more runs are in order. And that is just the sourc-model parameters, I've been mostly taking it on faith that starting the selfcal parameters at amp=1,phase=0 is okay. ---- notes from another time --------------- You probably want to set APARM(1) to something quite small, I usually use ~1/(5*N_Vis), 1 percent is not good for lots of vis. To make my life easier, I usually run OMFIT to get a reasonable fit first to figure out the noise level (rms per vis or something like in OMFIT output), and then set NOISE(1) to that level. This doesn't change anything in the fit, it just scales the Chi2 values that get printed out, so that they wind up close to Degrees_of_Freedom and its easier to judge the significance of changes to the fit. A gotcha here is that I think Ketan did some wierd scaling of Chi2 internally. As long as you don't change the vis data set you are okay (ie. Chi2 is arbitrarily scaled but comparisons of different Chi2 values for different fits are useful). If, however, you change the vis data, then it's dangerous to compare Chi2 values. Another one of my hacks is to print out the unscaled Chi2 so you can estimate the significance of e.g., throwing out an antenna. Once you've got a fit, it is best is to re-run OMFIT with a few different starting values, usually not too far from the fitted values, but try and start it with starting values on on different sides of the fit point to make sure you are finding a global, not a local minimum. Ie, run it with a starting value of major=1.0mas; converges to 1.532 mas; so rerun it with a starting value of 0.5mas - do you still get 1.532 mas? Repeat w/ starting value of 3mas. Total flux density is usually pretty robust against this, but for things like disk size (if vis-fn has nulls in) this can be a gotcha. If you are fitting antenna gains as well, convergence can be an issue, I've resorted to multiple runs of OMFIT with a fixed e.g., disk radius to build up manually a disk radius vs. chi2 curve. It uses log format for e.g., disk radii, so best to use a non-zero but small compared to resolution value. Check also that it *has* converged; I use PRTLEV 3; in some cases even with e.g., NITER=500 it will not formally converge. I think it is usually close to the an actual min in Chi2 in these cases so the final fitted values are probably okay, but - hey, you're in the world of far too many free parameters.