astropy:docs

- Index
- Modules

### Navigation

- next »
- « previous |
- imexam v0.5 »
- Imexam User Documentation »
- Introduction

# Introduction

imexam is meant as a replacement for the IRAF imexamine task. You should be able to perform all of the important functions that imexamine provided in IRAF, but you also gain the power and flexibility of python.

It currently provides display support with a DS9 window, either one you already have open, or one that imexam explicitly opens. It communicates with the DS9 window through the XPA messaging system. A Ginga widget using the matplotlib backend is also available for viewing images and arrays. The package is designed so that it may accept other display devices in the future.

Note

For DS9, it is important to know if you have XPANS installed on your machine and available through your PATH if you plan to use the nameserver functionality. XPANS is the XPA Name Server, it keeps track of all the open socket connections for DS9 and provides a reference for their names. If you DO NOT have XPANS installed, then imexam will still work, but you should either start the DS9 window after importing imexam through the imexam.connect() interface, OR after you start DS9 from the shell, make note of the XPA_METHOD address in the File->Information dialog and use that in your call to connect: window=imexam.connect(XPA_METHOD) so that communication with the correct socket is established. As a convenience, a full installation of the XPA software is packaged with this module. You may choose whether or not to make use of this. There are lines in the setup.py file which are commented out, removing these will compile the XPA software during the full imexam installation and place a copy of the xpans executable in your scripts directory.

In order to use the Ginga widget display you must have Ginga installed. More information about Ginga can be found in the package documentation: http://ginga.readthedocs.org/en/latest/

You can access this help file on your locally installed copy of the package by using the imexam.display_help() call after import. This will display the help in your web browser.

Note

All information returned from this module should be considered an estimate of an actual refined result, more precise analysis of the data should be performed for verification before publication.

## Installation

These are some tips on installing the package, or tracking down problems you might be having during or after installation.

imexam can be installed from the source code in the normal python fashion:

    python setup.py install

If you want to have access to the photometry features of the imexam analysis, download and install photutils - one of the astropy associated packages. It can be found here: https://github.com/astropy

If photutils is not installed, imexam should issue a nice statement saying that the photometry options are not available upon import, and any time an analysis key is pressed during the imexam() function loop which requires photutils to render a result.

imexam displays plots using matplotlib, if you find that no windows are popping up after installation it’s probably the backend that was loaded. One quick way to get things started is to start ipython with the –pylab option, this will make sure the proper display backend loads when matplotlib is imported:

    ipython --pylab
    >import imexam

If you install Ginga you will have access to another display tool for your images and data. You can find the source code on GitHub, but you can also install it with pip or conda.

## Usage

imexam is a class based library. The user creates an object which is tied to a specific image viewing window, such as a DS9 window. In order to interact with multiple windows the user must create multiple objects. Each object stores all the relevent information about the window and data with which it is associated.

For example, in order to open a new DS9 window and use the object “viewer” to control it, you would issue the commmand:

    viewer=imexam.connect()

“viewer” now has associated methods to view, manipulate and analyze data in the DS9 session. When you start the connection, you also have the option of specifying a currently open DS9 window using the target keyword. This keyword can contain the name, the actual text name that you gave the window, or the address of the window. The address of the window can be found in the File->XPA->Information menu item, is stored as “XPA_METHOD”, and is of the form “82a7e75f:58576” for inet sockets, and a filepath for local sockets. The following is an exaple of connecting to an already active DS9 window which was started outside of imexam:

    viewer=imexam.connect("82a7e75f:58576")

    or

    viewer=imexam.connect("my_window_title")

When imexam starts up a DS9 window itself, it will create a local socket by default, even though the default socket type for DS9 is INET. However, imexam will first check to see if XPA_METHOD was set in your environment and default to that option. If you are experiencing problems, or you don’t have an internet connection (the two might be related because the XPA structures INET sockets with an ip address), you can set your environment variable XPA_METHOD to ‘local’ or ‘localhost’. This will cause imexam to start a local(unix) socket which will show an “XPA_METHOD” that is a filename on your computer. imexam defaults to a local socket connection to allow for users who do not have the XPA installed on their machine or available on their PATH. The full XPA source code is available in the imexam package. If you don’t have the XPA on your path, simply point it to that location, or copy the xpans executable to the location of your choice, and make sure you update your PATH. Any time DS9 is started it will start up the xpa nameserver automatically. Then all the xpans query options will be available through imexam (such as imexam.list_active_ds9()). imexam itself uses Cython wrappers around the get and set methods from the XPA for it’s communication which is why the fully installed XPA is not necessary.

If you wish to open multiple DS9 windows outside of imexam, then it’s recommended that you give each a unique name. If you’ve forgotten which window had which name, you can look in the same XPA info menu and use the “XPA_NAME” specified there. If you haven’t given them a unique name, you can list the available windows using imexam.list_active_ds9() (as long as xpans is running) and specify their unique address.

imexam will attempt to find the current location of the DS9 executable by default, but you may also supply the path to the DS9 executable of your choice using the path keyword when you call connect. The fully optional calling sequence is:

    imexam.connect(target="",path=None,viewer="ds9",wait_time=10)

    Where target is the name of the ds9 window that is already running, path is the location of the ds9 executable, viewer is the name of the viewer to use (ds9 is the only one which is currently activated), and wait_time is the time to wait to establish a connection to the socket before exiting the process.

In order to return a list of the current DS9 windows that are running, issue the command:

    imexam.list_active_ds9()

If you are using the Ginga widget, the interaction with the imexam code stays the same, you simply specify that you would like to use Ginga in the call to connect:

    viewer=imexam.connect(viewer='ginga_mp')

“ginga_mp” tells imexam that you’d like to use the Ginga widget with the matplotlib background. Future support for the Ginga QT widget will be added at a later date.

In order to turn logging to a file on, issue the command: window.setlog(). The log will be saved to the default filename imexam_session.log in the current directory unless you give it another filename to use. Here’s an example of how that might work:

    import imexam
    window=imexam.connect('ds9')
    window.setlog() <-- turns on logging with default filename
    window.imexam() <-- all output will be logged to the file and displayed on the screen
    window.setlog(on=False) <-- turns off logging to file
    window.setlog(filename='my_other_log.txt') <-- turns on logging and sets the save filename

The log will look something like this, you can see it contains a mention of the command used along with the results

    gauss_center
    xc=812.984250   yc=706.562612

    aper_phot
    x       y       radius  flux    mag(zpt=25.00)  sky     fwhm
    812.98  706.56  5       1288669.29      9.72    11414.53        4.83

    show_xy_coords
    813.5 706.625

    gauss_center
    xc=812.984250   yc=706.562612

    gauss_center
    xc=239.856464   yc=233.444783

    aper_phot
    x       y       radius  flux    mag(zpt=25.00)  sky     fwhm
    239.86  233.44  5       126601.26       12.24   11574.32        -12.67

    show_xy_coords
    253.0 234.75

    gauss_center
    xc=239.856464   yc=233.444783

More detailed examples can be found in the examples section of this documentation.

Note

More information on DS9 can be found at: http://ds9.si.edu/site/Home.html

## Common Problems

You’re getting the following error statement when you try to connect() to a DS9 window, or display an image:

    XpaException: Unknown XPA Error : XPAGet returned 0!

You can first try using local unix sockets by setting your environment variable XPA_METHOD to local:

    setenv XPA_METHOD local

That will create local unix file sockets for communication with ds9. If that doesn’t solve the problem, see if your path includes the location of xpans, the XPA name server. If you have it installed, but it’s not on your path, put it there. You can also compile and install the XPA software included with imexam by editing the setup.py file:

> - Uncomment the section towards the bottom of the file which instructs you to uncomment to compile the full XPA
> - Also uncomment the last line with CMDCLASS and add that line inside the parenthesis for the setup command just above.
> - Now you can reinstall imexam the normal way and it will also build the XPA executables and store them in cextern/
> - Make sure the path to the executables is on your system PATH

Now you can start an ipython window, import imexam and try starting a new DS9 connection. If this still doesn’t solve your problem, send email to the developers or open an issue in github.

If you are using Ginga and the plotting window seems to block, check to see if you’ve specified the QT backend in any of your matplotlib defaults and try turning it off:

    matplotlib.use('Qt4Agg')  <-- remove this and see if it helps