| fitpsf | noao.digiphot.apphot | fitpsf |
fitpsf -- model the point spread function with an analytic function
fitpsf image box
FITPSF models the stellar brightness distribution of objects in the IRAF image image using non-linear least squares techniques and writes the list of model parameters and associated errors to the file output . Initial coordinates for the objects are read from the image cursor or the text file coords . Pixels in a subraster of width box * scale are extracted and used in the fit.
The coordinates read from coords are assumed to be in coordinate system defined by wcsin . The options are "logical", "tv", "physical", and "world" and the transformation from the input coordinate system to the internal "logical" system is defined by the image coordinate system. The simplest default is the "logical" pixel system. Users working on with image sections but importing pixel coordinate lists generated from the parent image must use use the "tv" or "physical" input coordinate systems. Users importing coordinate lists in world coordinates, e.g. ra and dec, must use the "world" coordinate system and may need to convert their equatorial coordinate units from hours and degrees to degrees and degrees first.
The coordinates written to output are in the coordinate system defined by wcsout . The options are "logical", "tv", and "physical". The simplest default is the "logical" system. Users wishing to correlate the output coordinates of objects measured in image sections or mosaic mosaic pieces with coordinates in the parent image must use the "tv" or "physical" coordinate systems.
If cache is yes and the host machine physical memory and working set size are large enough, the input image pixels are cached in memory. If cacheing is enabled and FITPSF is run interactively the first measurement will appear to take a long time as the entire image must be read in before the measurement is actually made. All subsequent measurements will be very fast because FITPSF is accessing memory not disk. The point of cacheing is to speed up random image access by making the internal image i/o buffers the same size as the image itself. However if the input object lists are sorted in row order and sparse cacheing may actually worsen not improve the execution time. Also at present there is no point in enabling cacheing for images that are less than or equal to 524288 bytes, i.e. the size of the test image dev$ypix, as the default image i/o buffer is exactly that size. However if the size of dev$ypix is doubled by converting it to a real image with the chpixtype task then the effect of cacheing in interactive is can be quite noticeable if measurements of objects in the top and bottom halfs of the image are alternated.
FITPSF can be run either interactively or in batch mode by setting the parameter interactive . In interactive mode starting x and y positions can either be read directly from the image cursor or read from the text file specified by coords . In batch mode the estimated positions can be read from the text file coords or the image cursor parameter icommands can be redirected to a text file containing a list of cursor commands.
The currently available cursor commands are listed below.
Interactive Keystroke Commands
? Print help
: Colon commands
v Verify the critical parameters
w Save the current parameters
d Plot radial profile of current star
i Interactively set parameters using current star
f Fit current star
spbar Fit current star, output results
m Move to next star in coordinate list
n Fit next star in coordinate list, output results
l Fit remaining stars in coordinate list, output results
e Print error messages
r Rewind the coordinate list
q Exit task
Colon Commands
:show [data/fit] List the parameters
:m [n] Move to next [nth] star in coordinate list
:n [n] Fit next [nth] star in coordinate list, output results
Colon Parameter Editing Commands
# Image and file name parameters
:image [string] Image name
:coords [string] Coordinate file name
:output [string] Output file name
# Data dependent parameters
:scale [value] Image scale (units per pixel)
:fwhmpsf [value] Scale factor (scale units)
:emission [y/n] Emission feature (y), absorption (n)
:sigma [value] Standard deviation of sky (counts)
:datamin [value] Minimum good data value (counts)
:datamax [value] Maximum good data value (counts)
# Noise description parameters
:noise [string] Noise model (constant|poisson)
:gain [string] Gain image header keyword
:ccdread [string] Readout noise image header keyword
:epadu [value] Gain (electrons per adu)
:readnoise [value] Readnoise (electrons)
# Observation parameters
:exposure [string] Exposure time image header keyword
:airmass [string] Airmass image header keyword
:filter [string] Filter image header keyword
:obstime [string] Time of observation image header keyword
:itime [value] Exposure time (time units)
:xairmass [value] Airmass value (number)
:ifilter [string] Filter id string
:otime [string] Time of observation (time units)
# Fitting parameters
:function [string] PSF model (radgauss|elgauss|moments)
:box [value] Width of the fitting box (scale units)
:maxiter [value] Maximum number of iterations
:nreject [value] Maximum number of rejection cycles
:kreject [value] Rejection limit (sigma)
# Plotting and marking functions
:mkbox [y/n] Mark the fitting box on the display
The following command are available from within the interactive setup menu.
Interactive Fitpsf Setup Menu
v Mark and verify the critical fitpsf parameters (f,s,b)
f Mark and verify the full-width half-maximum of the psf
s Mark and verify the standard deviation of the background
l Mark and verify the minimum good data value
u Mark and verify the maximum good data value
b Mark and verify the half-width of the fitting box
The fitting parameters are function , the functional form of the model to be fit, maxiter , the maximum number of iterations per fit, kreject , the K-sigma rejection limit and nreject , the maximum number of rejection cycles. The currently available functions are a 2D moments analysis "moments", a 2D radial Gaussian "radgauss", and a 2D elliptical Gaussian "elgauss".
The weighting of the fit is determined by the parameter noise in the datapars file. The two options are constant , in which all the weights are set to 1 and poisson in which the weights are equal to the inverse of the counts divided by the image gain read from the datapars gain or epadu parameters plus the square of the readout noise determined from the datapars parameters ccdread or readnoise . If function is either "radgauss" or "ellgauss" then the datapars parameter fwhmpsf is used to determine the initial guess for the Gaussian sigma. The datapars parameter threshold determines the intensity threshold above which the moment analysis is performed.
In interactive mode the following quantities are printed on the terminal as shown below, for the radial Gaussian, elliptical Gaussian and moments functions respectively.
image xcenter ycenter rsigma amplitude sky err
image xcenter ycenter xsigma ysigma rot amplitude sky err
image xcenter ycenter rgyrat ellip pa amplitude sky err
In both interactive and batch mode the full output is written to the text file output . At the beginning of each file is a header listing the values of the parameters when the first stellar record was written. These parameters can be subsequently altered. For each star measured the following record is written for the radial Gaussian, elliptical Gaussian, and moments functions respectively.
image xinit yinit id coords lid
xcenter ycenter rsigma amplitude sky
excenter eycenter ersigma eamplitude esky ier error
image xinit yinit id coords lid
xcenter ycenter xsigma ysigma rot amplitude sky
excenter eycenter exsigma eysigma erot eamplitude esky ier\
error
image xinit yinit id coords lid
xcenter ycenter rgyrat ellip pa amplitude sky
excenter eycenter ergyrat eellip epa eamplitude esky ier\
error
Image and coords are the name of the image and coordinate files respectively. Id and lid are the sequence numbers of stars in the output and coordinate files respectively and xinit and yinit are the initial positions. Xcenter and ycenter are the computed x and y positions of the object. Rsigma, xsigma and ysigma are the distance from the center of the Gaussian at which the Gaussian is equal to exp (-0.5) of its central value. Xsigma and ysigma refer to those values along the major and minor axes of the ellipse respectively. The amplitude and sky refer to the amplitude of the Gaussian function and a constant background value respectively. If function = "moments" amplitude and sky refer to the total intensity above threshold and sky is the threshold value. Rot and pa are position angles of the major axis measured counter-clockwise with respect to the x axis. Rgyrat is the radius of gyration of the object and ellip its ellipticity. Quantities prefixed by an e represent the errors in the corresponding fitted parameters.
If all went well in the fitting process the error code stored in the ier field described above is 0. Non-zero values of ier flag the following error conditions.
0 # No error
401 # The fitting box is off the image
402 # The fitting box is partially off the image
403 # There are too few points to fit the function
404 # The fit is singular
405 # The fit did not converge
1. Compute the radial Gaussian function parameters for a few stars in dev$ypix using the display and the image cursor. Setup the task parameters using the interactive setup menu defined by the i key command. Use uniform weighting.
ap> display dev$ypix 1 fi+ ... display the image ap> fitpsf dev$ypix 11 noise=constant ... type ? to see the help screen ... move the image cursor to a star ... type i to enter the interactive setup menu ... enter maximum radius in pixels of the radial profile or type CR to accept the default value ... set the fitting box width, fwhmpsf, and sigma using the graphics cursor and the stellar radial profile plot ... typing <CR> leaves everything at the default value ... type q to quit the setup menu ... type the v key to verify the parameters ... type the w key to save the parameters in the parameter files ... move the image cursor to the stars of interest and tap the space bar ... a one line summary of the fitted parameters will appear on the standard output for each star measured ... type q to quit and another q to confirm the quit ... the full output will appear in ypix.psf.1
2. Compute the radial Gaussian function parameters for a few stars in dev$ypix using the contour plot and the graphics cursor. Setup the task parameters using the interactive setup menu defined by the i key command. Use uniform weighting.
ap> show stdimcur ... save the current value of stdimcur ap> set stdimcur = stdgraph ... define the image cursor to be the graphics cursor ap> contour dev$ypix >G ypix.plot1 ... store the contour plot of dev$ypix in the file ypix.plot1 ap> fitpsf dev$ypix 11.0 noise=constant display=stdgraph ... type ? to get a short help page on the screen ... move the graphics cursor to a star ... type i to enter the interactive setup menu ... enter the maximum radius in pixels of the radial profile or type CR to accept the default value ... set the fitting box width, fwhmpsf, and sigma using the graphics cursor and the stellar radial profile plot ... typing <CR> leaves everything at the default value ... type q to quit the setup menu ... type the v key to verify critical parameters ... type the w key to save the parameters in the parameter files ... retype :.read ypix.plot1 to reload the contour plot ... move the graphics cursor to the stars of interest and tap the space bar ... a one line summary of the fitted parameters will appear on the standard output for each star measured ... type q to quit and q again to confirm the quit ... full output will appear in the text file ypix.psf.2
3. Setup and run FITPSF interactively on a list of objects temporarily overriding the fwhmpsf and sigma parameters determined in examples 1 or 2. Use uniform weighting.
ap> daofind dev$ypix fwhmpsf=2.6 sigma=25.0 verify-
... make a coordinate list
... the output will appear in the text file ypix.coo.1
ap> fitpsf dev$ypix 11.0 fwhmpsf=2.6 noise=constant coords=ypix.coo.1
... type ? for optional help
... move the graphics cursor to the stars and tap space bar
or
... select stars from the input coordinate list with m / :m #
and measure with spbar
... measure stars selected from the input coordinate list
with n / n #
... a one line summary of results will appear on the standard output
for each star measured
... type q to quit and q again to confirm the quit
... the output will appear in ypix.psf.3 ...
4. Display and fit some stars in an image section and write the output coordinates in the coordinate system of the parent image. Use uniform weighting.
ap> display dev$ypix[150:450,150:450] 1
... display the image section
ap> fitpsf dev$ypix[150:450,150:450] 11.0 noise=constant wcsout=tv
... move cursor to stars and type spbar
... type q to quit and q again to confirm quit
... output will appear in ypix.psf.4
ap> pdump ypix.psf.4 xc,yc yes | tvmark 1 STDIN col=204
5. Run FITPSF in batch mode using the coordinate file and the previously saved parameters. Use uniform weighting. Verify the critical parameters.
ap> fitpsf dev$ypix 11.0 coords=ypix.coo.1 noise=constant verify+ \
inter-
... output will appear in ypix.psf.5 ...
6. Repeat example 5 but assume that the input coordinate are ra and dec in degrees and degrees, turn off verification, and submit the task to to the background. Use uniform weighting.
ap> display dev$ypix 1
ap> rimcursor wcs=world > radec.coo
... move to selected stars and type any key
... type ^Z to quit
ap> fitpsf dev$ypix 11.0 coords=radec.coo noise=constant \
wcsin=world verify- inter- &
... output will appear in ypix.psf.6
ap> pdump ypix.psf.6 xc,yc yes | tvmark 1 STDIN col=204
... mark the stars on the display
7. Run FITPSF interactively without using the image display.
ap> show stdimcur
... record the default value of stdimcur
ap> set stdimcur = text
... set the image cursor to the standard input
ap> fitpsf dev$ypix 11.0 coords=ypix.coo.1 noise=constant
... type ? for optional help
... type :m 3 to set the initial coordinates to those of the
third star in the list
... type i to enter the interactive setup menu
... enter the maximum radius in pixels for the radial profile or
accept the default with a CR
... type v to enter the default menu
... set the fwhmpsf, sigma, and fitting box size using the
graphics cursor and the stellar radial profile plot
... typing <CR> after the prompt leaves the parameter at its default
value
... type q to quit the setup menu
... type r to rewind the coordinate list
... type l to measure all the stars in the coordinate list
... a one line summary of the answers will appear on the standard
output for each star measured
... type q to quit followed by q to confirm the quit
... full output will appear in the text file ypix.psf.7
ap> set stdimcur = <default>
... reset the value of stdimcur
8. Use an image cursor command file to drive the FITPSF task. The cursor command file shown below sets the fwhmpsf, sigma, and noise, computes the model fit parameter values for 3 stars, updates the parameter files, and quits the task.
ap> type cmdfile
: fwhmpsf 2.6
: sigma 5.0
: noise constant
442 410 101 \040
349 188 101 \040
225 131 101 \040
w
q
ap> fitpsf dev$ypix 11.0 icommands=cmdfile verify-
... full output will appear in ypix.psf.8
In interactive mode the user should not change the type function to be fit after the first record is written to the output file. In this case the file header and record structure will not match.
It is currently the responsibility of the user to make sure that the image displayed in the frame is the same as that specified by the image parameter.
Commands which draw to the image display are disabled by default. To enable graphics overlay on the image display, set the display parameter to "imdr", "imdg", "imdb", or "imdy" to get red, green, blue or yellow overlays and set the mkbox switch to"yes". It may be necessary to run gflush and to redisplay the image to get the overlays position correctly.