Canada-France-Hawaii Telescope

A User's Manual For the CCD Data Acquisition User Interface

Generic tools


The following generic tools are found in every observing session involving CCDs:


A CCD exposure is initiated by clicking the Expose button in the menubar, filling out the form and pressing Accept.

The user can enter the following:

Note that the observer and object name fields are ``sticky''. The exposure type defaults to ``object'' each time when opening the form.

The Median option provides an online capability for a fast and accurate combine algorithm for image sequences (Iterations > 1). When you select ``normal'' median data option, a background process will start (the process is currently handled by ``wiki'' the summit Sun SPARCstation). After each image is acquired, a copy will be automatically sent to wiki, which will then convert the FITS file to IRAF file format. After completion of the acquisition of the sequence of images, a ``combine'' task will be invoked. It is based on the well tested and widely used CCDRED/COMBINE task in the NOAO/IRAF data processing environment. After extensive testing, we have decided to use the ``average sigma clip'' option offered by this task; it provides for the final image a mean identical to a ``median'' option, but with a standard deviation better by 10%-15%. The average sigma clip algorithm works as followings: a mean and a standard deviation value are computed for each image pixel from the set of images, after rejecting the minimum and maximum values, then pixels which values are more than X (X~3) times the standard deviation are rejected, and a final mean value is computed. The current implementation of this option is extremely reliable and has been extensively tested. We strongly recommend that you use it for bias/flat/dark sequences. Many observers are not storing the individual images of a sequence anymore, which saves both storage space and time. Currently, we are saving all the images of a sequence to the optical disc, but we are planning to save only the final combined image in the second semester 1991. The current processing time for a sequence of ten 1024x1024 images is 10 minutes; this will be reduced to 90 seconds for ten 2048x2048 images by the end of 1991, with the installation of more powerful computers.

Once the fields have been editted/selected, take the appropriate action with one of the four Control buttons.

Start a CCD exposure by pressing Accept. A CCD status icon will then appear. It contains the following information:

Moreover, a set of 5 buttons appear on the bottom of the icon. They offer the following actions:

At the end of a CCD exposure, the data is transferred from the CCD controller to the Data Acquisition Computer disc, and stored under the FITS file name that was automatically assigned. Also, a copy of the file is sent through the CFHT network down to Waimea, where it will be automatically saved to an archive system. In case you ``lose'' or delete a file by mistake on the summit disc, don't panic, make a note of the file number, and have someone retrieve the file from the optical disc for you.


This form controls the area of interest and the binning factor to use when reading out the CCD. For CCD exposures controlled with Expose, Raster applies, for focus exposures controlled with Focus, Fraster applies.

The Raster and Fraster forms allow the user to modify the following parameters:


The Focus form allows the user to define and start a (manual) focussing sequence.

The user can select:

When activated with Accept, Focus uses the raster parameters defined in Fraster.


This form allows the user to control the preflashing of the CCD. Preflash defines the duration of the preflash. This must be set depending on the individual CCD requirements. A preflash time of 0.0 must be entered for CCDs with no preflash.


The Modes form allows the user to select automatic or manual image display after an exposure and automatic or manual graphic display after an exposure.

Image: Image display

The Image form

The Image form allows the user to display the image of his choice. After selecting/editing the fields, clicking accept will start a new process call SAOimage or CFHTimage (the local version of SAOimage.

The SAOimage/CFHTimage image display


The SAOimage display package was originally created by Mike VanHilst at the Smithsonian Astrophysical Observatory. It is a stand-alone image display with associated built in functions. The CFHT version, CFHTimage, has been modified to incorporate a few custom features.


Upon initial examination of the SAOimage window (after activating Image on the image of your choice) you will notice several different areas:

Modifying the image scaling and look up table

We describe here two examples of image scaling and look up table modification.

Faint intensity levels: for this you need to select scale in the top menubar, then histeq in the bottom menubar. This will apply an histogram equalization to the image, with emphasis on the mode of the histogram. Most of the time, the mode of the histogram is close to the median sky level, so that the scaling will be perfectly adequate to reveal faint objects. The selection of color in the top menubar allows the user to modify the image contrast and thresholds by pressing the left mouse button, and dragging the cursor in the image while still pressing on the left button. As you do this, you will see the image contrast and thresholds change. Note that in the color menu, you can select invert to flip the look up table with white becoming black and vice et versa.

High intensity levels: To be able to see the highest intensity pixels in an image, from SAOimage commands (see the ``Image'' form for an alternative way), select scale, then sqrt, this will compress the intensities and the look up table will allow a good look at the highest values.

Pan and Zoom

Pan and Zoom is very easy within SAOimage. For this you have first to select Pan in the top menubar.

Zoom: in the bottom menubar, you can now see a set of zoom options including 2, 4, 1/2 and 1/4. Clicking on any of these buttons will zoom the image by the corresponding factor. The alternative is to click with the middle mouse button in the image display: you can zoom by as much as you want (!) by a series of clicks. To unzoom click on ``zoom 1''.

Pan: position the cursor on the image location that you would like to see in the middle of the display, then click with the left mouse button and the selected image area will move to the center of the display. One can also use the full image display in the small top left area: a similar cursor action will move the (green) box center to the desired image location, and the large image display will be updated.

Other useful options

Graph: Graphic display

The Graph form

The Graph form allows you to view cuts in an image. The options are as follows:

Grapher usage

Grapher is an integrated interactive plotting program. The Grapher window opens after your selection of Accept in the Graph form. It provides the following possibilities:

IQE: Image quality evaluation

The IQE form allows for the computation of various statistics from a FITS file. These include simple minimum and maximum values, as well as more complex values such as mean, standard deviation, full-width-at-half-maximum, and centroids.

At the top of the IQE form, you will notice a large field for the FITS file name. This will be the file on which statistics are performed, and all locations, both entered and reported, will be relative to this file and in image pixel units.

The following options can be modified:

IQE output:

After the Accept button is pressed, the output of these statistics is presented in a popup window, as text. After you are finished viewing this data, press the ``Quit'' button that appears across the bottom of the result popup.

Files: Files disc/tape management

The Files form allows four types of action to be applied to FITS files:

These FITS files can be selected in several ways (by clicking in the appropriate box):

If you have selected All files, Already on tape, or Not on tape, you can narrow your file selection further by selecting one or both of the following:

After selecting the action desired, and the set of FITS files to apply this action to, pressing the Accept button will result in the desired action being performed. As each file is processed, it will be listed in a popup window. When the last file has been handled, a message will be displayed in this window to that effect, the popup window should then be ``closed'' with the ``Quit'' button. As files are listed in this popup window, those that have been saved to tape will be surrounded by parentheses, while those that have not been saved to tape will appear simply as the file name. It follows that selecting only files that have been saved to tape will result in all the file names being enclosed in parentheses.

It is suggested that you use the list-files action to verify that the files you have selected are indeed the appropriate files, and then bring the Files form up again to perform your action.


To end the session, click on Aloha. This will bring up another form which asks you if you want to really quit (then press Accept) or not (press Cancel).

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