New Version of the DAOPHOT Crowded-Field Photometry Package Released

The major algorithm changes are listed below.

• The analytic component of the psf model is no longer restricted to a Gaussian function. The user may choose between 6 alternatives, including a Gaussian function, two Moffat functions, a Lorentzian function, and two Gaussian + Lorentzian functions, or allow the psf modeling code to choose the most appropriate analytic model based on a goodness of fit criteria.
• Constant psf models may be purely analytic (analytic function + 0 look-up tables) as well as empirical (analytic function + 1 look-up table).
• Variable psf models may vary quadratically with position in the image (analytic function + 6 look-up tables) as well as linearly with position in the image (analytic component + 3 look-up tables).
• The analytic component of the psf model is no longer computed by fitting the model to the first psf star. Instead the data from all the psf stars, appropriately weighted by signal-to-noise, is used to compute the analytic component of the model.
• The ability to down-weight bad pixels, always present in the psf fitting code, was added to the psf modeling code.
• The ability to combine data in the cores of relatively faint stars with data in the wings of saturated bright stars in order to compute high signal-to-noise look-up tables was added to the psf modeling code.
• The ability to refit the local sky brightness during psf fitting was added to the psf fitting tasks.

Six new tasks were added to daophot.

• daoedit is an interactive task which uses the IRAF lpar/epar/unlearn tasks, the image display, and where appropriate radial profile plots of selected stars to review and / or edit the algorithm parameter sets.
• findpars is a parameter set task which sets the values of the object detection algorithm parameters required by the daofind task.
• pcalc is a task which performs simple arithmetic operations on a numerical field in a daophot photometry file, e.g., shifting the coordinates of an object.
• pfmerge is a task which creates a new photometry file suitable for input to the daophot psf fitting tasks peak, nstar, and allstar, by combining existing photometry files that have been written by different tasks, e.g., allstar and phot.
• pstselect is a task which creates a candidate psf star photometry file suitable for input to the psf task by selecting bright relatively uncrowded stars from an existing photometry file.
• setimpars is a task which saves and restores the daophot algorithm parameters for a specific image.

• The datapars parameter ``threshold'' has been moved to the findpars parameter set and defined in units of the datapars parameter ``sigma'' rather than in units of counts. Together ``threshold'' and ``sigma'' set the brightness of the faintest star that can be detected by the daofind task.
• The daofind object detection parameters ``nsigma'', ``ratio'', ``theta'', ``sharplo'', ``sharphi'', ``roundlo'', ``roundhi'', and ``mkdetections'' have been moved to the new findpars parameter set described above.
• The new parameter ``function'' has been added to, and the existing parameter ``varpsf'' has been replaced with ``varorder'' in, the daopars parameter set. ``function'' and ``varorder'' set the form of the analytic component and the number of look-up tables in the psf model computed by the psf task.
• The new parameters ``nclean'' and ``saturated'' have been added to the daopars parameter set. ``nclean'' sets the number of iterations the psf task must make when computing the psf model look-up tables. ``saturated'' specifies whether or not saturated stars will be provisionally accepted as psf stars by the psf task.
• The new parameters ``fitsky'', ``groupsky'', ``sannulus'', and ``wsannulus'' have been added to the daopars parameter set. These parameters are used by the peak, nstar, and allstar tasks to determine if and how the group sky value will be recomputed during the psf fitting process. If ``fitsky'' is "yes" the group sky value is recomputed; if ``fitsky'' is "no" (the default) the values computed by the phot task are averaged to compute the group sky value. If ``groupsky'' is "yes" (the default) the sky value for each pixel contributing to the fit is set to the average of the current sky values of all the stars in the group; if ``fitsky'' is "no" the sky value for each pixel is set to the average of the sky values of stars in the group within ``fitrad'' of that pixel. In the case of peak and nstar the pixels used to recompute the sky are within one ``fitrad'' of the star; in the case of the allstar task the sky pixels used to recompute the sky are defined by ``sannulus'' and ``wsannulus'' parameters.
• The parameters ``flaterr'' and ``proferr'' have been added to the daopars parameter set. ``flaterr'' and ``proferr'' are used by the psf fitting code in the peak, nstar, and allstar tasks to define the flat-fielding and interpolation components of the error model, respectively. In previous versions of daophot these values were fixed internally.

• The daofind task parameter ``convolution'' specifying the name of the output density enhancement image has been renamed ``starmap'', and a new parameter specifying the name of the output background density image ``skymap'' was added. The sum of these two images should model the input image except at the image edges and in regions containing bad data.
• The daofind task has been modified to read the datapars parameters, ``datamin'' and ``datamax''. These parameters are used by daofind to reject bad data from the computed density enhancement and background density images.
• The psf task has been modified to accept lists of input images and photometry files and write lists of output psf images and photometry files. Previous versions of psf accepted only a single input image and photometry file, and wrote only a single output psf image and photometry file.
• The psf task has been modified to accept an input list of candidate psf stars (new parameter ``pstfile'') and output a list of the psf stars (new parameter ``opstfile'') actually used to compute the psf model. The new parameter ``matchbyid'' specifies whether the psf stars are matched to stars in the input photometry file by id number (the default) or by positional coincidence.
• A new parameter ``interactive'' was added to the psf task. If ``interactive'' is "yes", the psf task accepts commands from the user via the image display cursor; if ``interactive'' is "no", psf can either accept and execute commands from a cursor command file or compute the psf model using stars in the input psf star list without intervention by the user.
• The residuals plots drawn by the new psf task are displayed after the psf model is fit instead of as the psf stars are selected, and have a different meaning than they did in the old psf task. The new residuals plots show the residuals from the best fit to the entire psf model, not from the best fit analytic component of the model. Ideal residuals plots are flat with per pixel rms noise values appropriate to the detector and the data, not the doughnuts users are used to seeing.
• The psf task plotting code is now capable of drawing radial profile plots in addition to surface and contour plots.
• The peak and nstar tasks have been modified to read the daopars parameter ``recenter''. The ``recenter'' parameter determines whether the psf fitting code will compute new x and y coordinates or use the input values. The allstar task has always been able to read the ``recenter'' parameter.
• The peak and nstar tasks have been modified to read the daopars parameters ``clipexp'' and ``cliprange''. In previous versions of daophot these values were fixed at 8.0 and 2.5 internally, and only the allstar task could read these parameters.
• A new parameter ``rejfile'' with a default value of "default", has been added to the peak, nstar, and allstar tasks. If ``rejfile'' is not equal to "" then stars which fail to be fit for one reason or another are written to a separate rejections file instead of to the output photometry file.
• Error code and message columns have been added to the output photometry files written by the peak, nstar, and allstar tasks so the user can determine precisely why the code failed to fit a particular star.
• The substar task has been modified to accept an input list of stars to be excluded from the subtraction process (new parameter ``exfile''). Stars in the "to be excluded list", e.g., psf stars, whose ids match those in the input photometry file are not subtracted from the input image.
• Minor changes have been made to the pappend, pdump, and prenumber tasks. These are described in the accompanying article on the ptools package.

Users who have not yet upgraded to IRAF 2.10.3BETA, or who are running IRAF on platforms for which IRAF 2.10.3BETA is not yet available, can acquire the updated version of daophot by retrieving and installing the external package digiphotx from the IRAF network archive.

Lindsey Davis

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