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Wise Observatory
Fast Photometry - Detailed description

For the benefit of the WET community we give here a few details about the Wise Observatory standard ccd based fast photometry system. At the heart of the system there is a photometrics 1 inch square 1024x1024 pixel tektronix ccd. The software running this system is the pmis package. It can be run either under windows 3.1 or under windows 95. For fast photometry applications windows 95 is a more comfortable environment to work in. The main reason for this is the ease with which a windows 95 machine can be networked to a Linux machine running a Samba server. The Linux machine runs an astronomical software package such as IRAF and can reduce and display the data of the last file written to the networked drive on the Linux machine by the ccd machine. Given that the user has a suitable interface and software there is no reason why the ccd should not be directly controlled by the Linux machine. However, if the system is to be run this way the user should be aware that the ccd application must have top real-time priority. The Pmis software supports a versatile macro language which enables complete control of the ccd. There follows a description of setting up and executing a fast photometry run. Links are provided to the macros which may be consulted by WET users who are attempting to implement a similar Fast Photometry system with a different ccd and software.

The PMIS end

  • Set the telescope to the WET program star coordinates.
  • Take a full field binned image to identify the field. (iacquire)
  • Having located the program star select one or more nearby stars for comparison purposes.
  • From the various stars to be included in the fast photometry field determine the size of the rectangular box which will amply surround the set of objects. Some 30 pixels should be allowed with our system between the center of any of the stars and the edge of the defining rectangle. This places each star within a 0.7 arc minute window.
  • It is convenient to center the field close to the central point chosen in 4 above. (moveit)
  • Take an unbinned image of say a 5 arc minute area about the field center. (iareab followed by mtake)
  • Select and define the specific part of the image which is to be used for fast photometry. (photbox)
  • Take and display an image to be sure the correct image has been defined for fast phometry. (obs)
  • Start the fast photometry sequence. (photseq)

    iacquire takes a full field 4x4 binned image.
    input: Time of exposure in seconds.
    output: Display of field suitably scaled.
    note: Because the image is binned a 10 second exposure is equivalent to a 3 minute unbinned exposure regarding stellar magnitude.

    moveit allows the observer to move a selected point in the displayed image to the field center.
    input: (1) rotation angle of the instrument mounted on the telescope.
    (2) a cross positioned by the mouse anywhere in the image.
    output: Telescope setting motions in RA and DEC which are immediately implemented by a program shelled to by the macro.

    iareab selects a square region on the chip for the next exposure.
    input: (1) number of pixels to define the height and side of the area.
    (2) binning parameter, 1 unbinned or 2.
    output: image display suitably squeezed.

    mtake sets up an exposure with a name a time and filter selection. It performs the exposure. It displays the image suitably scaled. It prompts the user to save the image if desired.

    photbox lets the observer select a suitable sub-image area for the fast photometry sequence.
    input: (1) sub-image width in pixels.
    (2) sub-image height in pixels.

    obs is a basic pmis command. It takes an image and displays it. Generally the image must be rescaled to show the detail it contains. It serves to check that the defined area contains the required objects for the fast photometry sequence.
    input: (1) exposure time in milliseconds.

    photseq performs a fast photometry sequence. Every exposure in the sequence is started as the seconds roll through zero. The exposure is made for a defined time. The area defined in photbox is read out from the ccd and immediately written to a network drive. There is no displayed image. An exposure of 7 seconds is accompanied by a dead time of 3 seconds.
    input: (1) object name.
    (2) exposure time in seconds.
    (3) filter name.

    The computer clock

    The computer clock can be reset to UT either automatically every hour or manually between runs. (link to a PASCAL programs source, for setting the clock).

    The IRAF end

    A simple IRAF script was written for online/offline reduction of the images. In the first image the user should choose the variable star and a reference star (in future versions, more then one reference star is allowed). For each image an aperture photometry is conducted and relative magnitude is calculated, and displayed on screen.

    Download the IRAF scripts fast_phot.cl script - for online aperture photometry.

    For other IRAF scripts, including offline automatic daophot photometry.

    Contact : Peter Ibbetson

    Constructed: November 1998, Eran Ofek, written by: Peter Ibbetson.