Canada-France-Hawaii Telescope

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

Generic tools

Summary

• Expose: form used to initiate exposures.
• Raster: form used to set up CCD raster sizes.
• Focus: form used to initiate manual focus procedure.
• Fraster: form used to set up CCD raster for focussing
• Chip: form to set up preflash parameters
• Modes: form used to set up automatic display and TCS enabling
• Image: form used to display and image with the SAOimage/CFHTimage tool.
• Graph: form used to display image cuts.
• IQE: form used to compute image quality.
• Files: form used to manage image files on disc and copy to tape.
• Suggestions: form used to send suggestions for improvements or fixes to the system manager.
• News: delivers the latest news on the Data Acquisition User Interface and the instrument.
• Aloha: form used to exit the observing session.

Expose

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:

• Exposure type: object, bias, flat, dark or comparison.
• Exposure time in seconds.
• Number of iterations (not applicable for ``object'' type).
• Median processing: on / off
• Object name: will be written to the FITS header.
• Observer: will be written to the FITS header.
• Comment: will be written to the FITS header.

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:

• The file name that has been automatically linked to the CCD data being acquired. It has the form of a number, representing the exposure number in the CFHT archive system, followed by a letter: o for object, f for flat, d for dark, b for bias, c for comparison. The extension .fits is added to indicate that the data on disc is stored in FITS format. The file name is a crucial parameter to allow you to locate your data, either on the summit discs while observing, or on the CFHT archive system .
• The sequence number of the exposure (if you are running a sequence).
• The status of the CCD controller: expose when the CCD is integrating, readout when the data is being read out.
• A count down timer: it indicates the remaining exposure time while the CCD is integrating.

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

• Stop will stop the exposure. This will read-out the CCD and create a file of the appropriate name. You can do this at any time during an integration. This will of course shorten the exposure time.
• Abort will abort the exposure. No CCD read out is provided and consequently the data is lost.
• Hold will stop the exposure, but will not read out the CCD. You can later use Restart to resume the exposure. Although the total integration time is not modified, the CCD dark current will keep accumulating while the exposure is on hold. The total dark time is therefore the exposure time plus the hold time (the field DARKTIME in the FITS header will indicate the exact dark current time).
• Break will break a sequence. The exposure currently being taken will be lost, and no further image will be taken.

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.

Raster/Fraster

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:

• Xc: the center of the raster along the CCD rows.
• Yc: the center of the raster along the CCD columns.
• Xs: the size of the raster along the CCD rows.
• Ys: the size of the raster along the CCD columns.
• Xbin: the binning factor along the CCD rows.
• Ybin: the binning factor along the CCD columns.

Focus

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

The user can select:

• Video focus mode. This mode reads the CCD out as fast as possible, and displays the image only on the video system. No files are generated. The following fields affect video focus:
  • The exposure time in seconds.
• Analytical focus mode. This mode takes an exposure, writes it to disc, and then runs IQE on it to provide a quantitative evaluation of the focus. The following fields affect analytical focus:
  • The exposure time in seconds.
  • The border of the frame to ignore in the background computation for the image quality computation (edges often have saturated pixels from the CCD control electronics in this mode).
  • The location of the background computation.

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

Chip

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.

Modes

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.

• File name: image to be displayed. current.fits refers to the most recent image acquired. Also previous.fits refers to the image acquired previous to the most recent one.
• Contrast: ``Linear'' or ``Equalized Histogram''. ``Linear'' will display the image scaled linearly between a minimum and a maximum value computed automatically or entered manually (see ``Level setting''). ``Equalized Histogram'' will compute the image histogram, and select its scaling based on the histogram mode: it is usually quite well adapted to seeing the faintest object in an image.
• Level setting: ``Automatic'' will compute the min and max values in the image and scale the greyscale to map the full range of intensities. ``Manual'' will take the min and max values entered by the user to map the greyscale.

The SAOimage/CFHTimage image display

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

• A large image display area. This is where the image is displayed with the best resolution.
• Two small display areas on the upper right corner:
  • The left one represents the full image in the memory buffer. The box (green if you can see green) indicates which part of the image is being seen on the large display area (under pan/zoom).
  • The right one is a zoomed portion of the image around the cursor, when it is moved in the large image display.
• A menubar: the top row corresponds to sets of possible actions, while the bottom row corresponds to direct actions. Clicking with the left mouse button on a button in the top row will bring a new set of buttons, single actions, on the bottom row.
• A greyscale at the bottom of the window. The greyscale coding will change according to any modification of the look up table.
• A coordinate display in the upper left window area. When SAOimage is opened from ``Image'', the coordinate display (i.e. X,Y position and intensity value) will update as you move the cursor.

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

• new: display a new image.
• track: small zoom window is updated as the cursor is moved in the large display window. This is set automatically when SAOimage is opened from Image.
• coord: coordinate display is updated as the cursor is moved in the large display window. This is set automatically when SAOimage is opened from Image.
• print: send image display to PostScript printer. This can take some time: 10 minutes for a 1024x1024 image on a QMS printer, 1 minute on a Sparc printer.
• raise: raise the full SAOimage window (including all sub-areas) above all other windows.
• QUIT: quit and kill the SAOimage process.

Graph: Graphic display

The Graph form

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

• Cutting along: X axis to cut along a CCD row, Y axis to cut along a CCD column.
• Cut # (1-2-3): up to three different cuts can be displayed simultaneously on the grapher display. Clicking on the box below ``cut #'' enables or disables the corresponding cut.
• For each cut you have the following options:
  • File name: the name of the file to be cut.
  • Position of cut: the row or column number to which the cut has to be applied.
  • Width to average: the number of rows or columns to be averaged before display.
  • Subtraction position: by clicking in the box, the grapher will subtract the columns/rows identified by the Subtraction position/width to average fields from the main cut defined above. This is for instance quite useful for a quick sky subtraction from a raw spectrum.

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:

• Plots: pop-up menu (press on the left mouse button, and keep pressing while you browse through the menu) allows to reconfigure each plot (file name, cut location, subtraction).
• Options: pop-up menu allows to modify the line type, compute statistics on the graph, or add labels.
• Print: pop-up menu allows to print the displayed graph to the summit or waimea printers.
• Zoom: pop-up menu allows to select a zoom factor from 1/4 to 4.
• Show plot: by clicking on the appropriate boxes, you can enable or disable one of the 4 plots allowed simultaneously.
• Scaling adjustment: you can tell the program to automatically adjust the X and Y scales by clicking in the ``X scale'' and ``Y scale'' boxes.
• Pan: by either clicking or dragging the black horizontal bar at the bottom of the window, you can pan across the plotted line or column.

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:

• Subraster options: The subraster options are necessary to narrow down the area to perform the statistic on. The use of the entire FITS file as the area from which to perform statistics is not recommended, due to the nature of the calculations. For example, the centroid values reported from a FITS file (or subraster!) containing more than one star will be between the stars! The user is responsible for the proper selection of the subraster so as to include only those object(s) upon which the statistics are wanted. To accomplish this, first select the radio button labeled ``As Follows:'', and then fill the desired values into the ``Xc'', ``Yc'', ``Xs'', and ``Ys'' edit fields. Use the raster image in the lower right of the form as a guide to the meanings of these variables. It is expected that you will use an image display tool (SAOimage) to determine your values. If the FITS file only contains the items you are interested in, perhaps because of the use of raster options when obtaining the data, you can select the ``Entire data area, ignoring edges'' radio button. The edge of the data is always ignored, due to various edge effects found in CFHT CCD's.
• Values computed: This area of the form allows the selecting of the desired values, using a standard list of checkboxes.
• Background from: The background for the FWHM values can be generated as the mean of the entire edge of the selected area, which is normal for use on images. For spectroscopic data, the use of the first and last X (or Y) values may give a better approximation of the actual background. Selection is via radio button.
• Other option: The FWHM values are usually computed on the maximum pixel's X and Y coordinate vectors. If the entire area selected is useful for the calculations, the ``Collapse for Fwhm/Pwhm'' may be selected. This may be useful for spectroscopic data.

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:

• List FITS files: list the file names (default action).
• List FITS Headers: display the selected files' FITS headers.
• Write FITS files to tape: write the selected FITS files to magnetic tape.
• Remove FITS files: remove the selected FITS files from disc.

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

• One file: perform the selected action on only one file, you can enter to file name on the field reserved for it.
• All files (default): performs the selected action on all the files in the directory.
• Already on tape: performs the selected action only on files which have been saved to tape.
• Not on tape: performs the selected actions only on files which have not yet been saved to tape.

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:

• File Type Override: performs the selected action only on the files with the selected file type: o for object, f for flat, b for bias, d for dark, and c for comparison.
• Range Override: performs the selected action only for the files with exposure numbers (``odometer'') falling into a given range which may be entered in the two dedicated fields.

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.

Aloha

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).