Index: trunk/doc/psphot/psphot.tex =================================================================== --- trunk/doc/psphot/psphot.tex (revision 4896) +++ trunk/doc/psphot/psphot.tex (revision 4900) @@ -51,5 +51,5 @@ data rate. The prototype telescope alone is expected to produce typically $\sim 700$ GB per night of imaging data. These images will -not be limited to high galactic lattitudes, so large numbers of +not be limited to high galactic latitudes, so large numbers of measurable stars can be expected in much of the data. The combination of the high precision goals of the astrometric and photometric @@ -83,5 +83,5 @@ \item Sextractor : pure aperture measurement with rudimentary object subtraction. pro: fast, widely used, easy to automate. con: - poor object separation in crowded regions, psf-modelling is only + poor object separation in crowded regions, PSF-modeling is only beta (psfex), what models are available? @@ -89,5 +89,5 @@ con: IRAF-based, aperture photometry. -\item galfit : detailed galaxy modelling. not a multi-object PSF +\item galfit : detailed galaxy modeling. not a multi-object PSF analysis tool. con: does not provide a PSF model, not easily automated. very detailed results in very slow processing. only a @@ -112,13 +112,18 @@ Perl (or potentially Python). -\note{Discussion of the lessons learned from experience with previous - analysis programs. 1) Flexible PSF model: functional form should be - easily modified. 2) PSF variation is fundamental : PSF - representation should incorporate 2-D variations. 3) Speed fitting - with accurate parameter guesses. 3) Make good use of moment - information to speed analysis. 4) careful definition of PSF - validity tests. 5) careful analysis of aperture corrections. 6) - flexible non-PSF models. 7) Good code abstraction to simplify - modification. } +\note{Add discussion of the lessons learned from experience with previous + analysis programs} + +\begin{itemize} +\item Flexible PSF model: functional form should be + easily modified. +\item PSF variation is fundamental : PSF representation should incorporate 2-D variations. +\item Speed fitting with accurate parameter guesses. +\item Make good use of moment information to speed analysis. +\item Careful definition of PSF validity tests. +\item Careful analysis of aperture corrections. +\item Flexible non-PSF models. +\item Good code abstraction to simplify modification. +\end{itemize} \section{Description of the PSPhot analysis steps} @@ -189,15 +194,15 @@ The noise image, if not supplied is constructed by default from the -flux image using the configuration supplied values of GAIN and -READ_NOISE to calculate the appropriate Poisson statistics for each -pixel. In this case, the image is assumed to represent the readout -from a single detector, with well-defined gain and read noise +flux image using the configuration supplied values of \code{GAIN} and +\code{READ_NOISE} to calculate the appropriate Poisson statistics for +each pixel. In this case, the image is assumed to represent the +readout from a single detector, with well-defined gain and read noise characteristics. In some obvious cases, this assumption will not be valid. For example, if the input flux image is the result of an image stack with significantly variable number of input measurements per pixel, it will necessary to supply a noise image which accurately -represents the noise as a function of position in the image. - -\subsubsection{Intial Object Detection} +represents the noise as a function of position in the image. + +\subsubsection{Initial Object Detection} The objects are initially detected by finding the location of local @@ -231,5 +236,5 @@ Once a collection of peaks have been identified, basic properties of the objects are measured. First, the local sky flux is measured -(using Median? user-specifie method?) within a square annulus with +(using Median? user-specific method?) within a square annulus with user-defined dimensions (\code{INNER_RADIUS} and \code{OUTER_RADIUS}). \note{rejection of some peaks based on the local sky measurement?}. @@ -250,5 +255,5 @@ PSPhot uses an analytical model to represent the shape and flux of an object. An important concept within the PSPhot code is the -distinction bewteen a model which describes an object on an image and +distinction between a model which describes an object on an image and a model with describes the point-spread-function across an image. @@ -258,5 +263,5 @@ ($x_o, y_o$), the elliptical shape parameters ($\sigma_x, \sigma_y, \sigma_{xy}$), the model normalization ($I_o$) and the local value of -the background ($S$). A specific object will have a partiular set of +the background ($S$). A specific object will have a particular set of values for these different parameters. @@ -278,7 +283,7 @@ each a function of the object centroid coordinates: \begin{eqnarray} -\sigma_x & = & f1(x,y) \\ -\sigma_y & = & f2(x,y) \\ -\sigma_{xy} & = & f3(x,y) \\ +\sigma_x & = & f_1(x,y) \\ +\sigma_y & = & f_2(x,y) \\ +\sigma_{xy} & = & f_3(x,y) \\ \end{eqnarray} @@ -314,23 +319,24 @@ these types of circumstances are abstracted, and a method is provided to return the necessary function to the higher-level software. For -example, each model type has its own function to define an intial +example, each model type has its own function to define an initial guess for the model, or a function to determine the radius for a given flux level. These are then registered as part of the model function code. Another function is then used to return the appropriate function for a specific model type. For example, the -psModelLookup_GetFunction will return the psModelLookup function for a -given model type. This mechanism makes it very easy to add new model -functions into the PSPhot code base. To add a new model function, the -programmer simply defines a new model name (a string), the set of all -necessary model lookup functions, and places the reference to the -model code at the appropriate location in the psModelInit.c routine. -It is not necessary to specify the PSF model functions independently -or the object model functions. Nor is it necessary to identify the -intended use of a given object model function (ie, PSF-like object, -galaxy, comet, etc). Any model can be used for the PSF model. The -code currently uses a fixed translation between the object model -parameters and the PSF model parameters. It also defines a specific -order for the 4 independent parameters. \note{it may also require -that two of the PSF-like parameters represent the shape in some way}. +\code{psModelLookup_GetFunction} will return the \code{psModelLookup} +function for a given model type. This mechanism makes it very easy to +add new model functions into the PSPhot code base. To add a new model +function, the programmer simply defines a new model name (a string), +the set of all necessary model lookup functions, and places the +reference to the model code at the appropriate location in the +psModelInit.c routine. It is not necessary to specify the PSF model +functions independently or the object model functions. Nor is it +necessary to identify the intended use of a given object model +function (ie, PSF-like object, galaxy, comet, etc). Any model can be +used for the PSF model. The code currently uses a fixed translation +between the object model parameters and the PSF model parameters. It +also defines a specific order for the 4 independent parameters. +\note{it may also require that two of the PSF-like parameters +represent the shape in some way}. \subsubsection{PSF Object Candidate Selection} @@ -339,6 +345,6 @@ identify a collection of objects in the image which are {\em likely} to be PSF-like. PSPhot uses the object moments to make the initial -guess at a colletion of PSF-like objects. At this point, the program -has measured the second order moments for all objects identified thier +guess at a collection of PSF-like objects. At this point, the program +has measured the second order moments for all objects identified their peaks, as well as an approximate signal-to-noise ratio. All objects with a S/N ratio greater than a user-defined parameter are selected by @@ -353,9 +359,9 @@ the bin size is approximately 0.1 image pixels. The binned $\sigma_x, \sigma_y$ plane is then examined to find a peak which has a -significance greater than XXX. Unless the image is extremely sparce, +significance greater than XXX. Unless the image is extremely sparse, such a peak will be well-defined and should represent the objects which are all very similar in shape. Other objects in the image will tend to land in very different locations, failing to produce a single -peak. To avoid detecting a peak from the unresoled cosmic rays, +peak. To avoid detecting a peak from the unresolved cosmic rays, objects which have second-moments very close to 0 are ignored. The only danger is if the PSF is very small and too many of these objects @@ -381,5 +387,5 @@ iterative, and rejects the $3-\sigma$ outliers in each of three passes. This fitting technique results in a robust measurement of the -variation of the PSF model parametesr as a function of position +variation of the PSF model parameters as a function of position without being excessively biased by individual objects which fail drastically. Objects whose model parameters are rejected by this @@ -400,5 +406,5 @@ the PSF objects. The difference between the aperture and fit magnitudes ({\em ApResid}) is a critical parameter for any PSF -modelling software which uses an analytical model to represent the +modeling software which uses an analytical model to represent the flux distribution of the objects in an image. @@ -408,5 +414,5 @@ between two images. Whether the goal is calibration of a science image taken at one location to a standard star image at another -location, or the goal is simply the repetative photometry of the same +location, or the goal is simply the repetitive photometry of the same star at the same location in the image, it is always necessary to compare the photometry between two images. If this measurement is to @@ -450,5 +456,5 @@ Consider a typical bright object with a flux of (say) 40,000 counts in an image of background 1000 counts per pixel, with FWHM of 4 pixels. -In principle, the flux of this object should be measureable with an +In principle, the flux of this object should be measurable with an accuracy of roughly 0.57\% ($\frac{\sqrt{40000 + 1000 \times 12}}{40000}$). However, the measurement of the sky is limited at some @@ -572,14 +578,14 @@ PSPhot will use the user-selected galaxy model to attempt the galaxy -model fits. In the configuration system, the KEYWORD GAL_MODEL is set -to the model of interest. All suspected extended objects are fitted -with the model, allowing all of the parameters to float. The initial -parameter guesses are critical here to achieving convergence on the -model fits in a reasonable time. The moments and the pixel flux -distribution are used to make the initial parameter guess. Many of -the object parameters can be accurately guessed from the first and +model fits. In the configuration system, the keyword \code{GAL_MODEL} +is set to the model of interest. All suspected extended objects are +fitted with the model, allowing all of the parameters to float. The +initial parameter guesses are critical here to achieving convergence +on the model fits in a reasonable time. The moments and the pixel +flux distribution are used to make the initial parameter guess. Many +of the object parameters can be accurately guessed from the first and second moments. The power-law slope can be guessed by measuring the isophotal level at two elliptical radii and comparing the ratio to -that expected. +that expected. For each of the galaxy models (in fact for all object models), a @@ -642,5 +648,5 @@ Finally, PSPhot can simply ignore the fitting process and instead glean information about the fainter sources on the basis of the peak -charateristics. In this option, the image is smoothed with the PSF +characteristics. In this option, the image is smoothed with the PSF model, and the peak for each object is measured. The peak flux and the local peak curvature theoretically give sufficient information to @@ -675,5 +681,5 @@ will automatically result in inconsistent interpretation of the noise. -For a difference image, both positive and negetive objects will be +For a difference image, both positive and negative objects will be present. The basic peak detection algorithm will only trigger for the positive sources. One solution is to simply apply PSPhot to both the @@ -703,5 +709,5 @@ a 30 second exposure. Even a main belt asteroid at roughly 1 AU would have reflect motion of approximately 1 degree per day, equivalent to -1.25 arcsec in a 30 second exposure, and could be noticably smeared +1.25 arcsec in a 30 second exposure, and could be noticeably smeared and non-PSF-like. A trailed-star model can be used to characterize these types of objects. 2) Small offset stars, either due to