8.4: Technical Requirements for DEIMOS Image Display

A typical DEIMOS frame which includes some prescan and overscan information from all 8 CCDs will consist of about 8500 columns and 8200 rows. As 16-bit integers this requires nearly 140 Megabytes of storage, and when converted to IEEE floating point this storage doubles.

8.4.1: Requirements for the Data Acquisition System

The demands of the image display exceed those of many existing systems. For the sake of speed and for other considerations this places certain extra requirements on the CCD data acquisition system (DAS) (described in Chapter 6). These requirements are reiterated here beccause they are driven by the needs of the image display.

8.4.1.1: Mosaicking Needs

For typical imaging cases the pixels will be read from 4 CCDs, and in spectral mode pixels will be read from 8 CCDs. For quick look and for the sake of pre-existing image display tools these image sections will be mosaicked as they are stored in memory.

The software which receives the streams of pixel values from the CCDs should be able to do the following:

When the images are written to FITS files on disk the user should have the option of storing them as:

8.4.1.2: Slitmask Alignment Exposures

The slitmasks require that the telescope be precisely pointed such that the light from each object passes through its slit. Each slitmask will have a number of rectangular apertures through which it will be possible to observe alignment stars (see Chapter 3). In order to avoid sacrificing large regions of slitmask the alignment stars will typically be chosen as close together as possible and clustered along lines parallel to the dispersion direction. These constraints permit the alignment images to be handled by the same software used for normal CCD readouts. Only a small fraction of the CCDs will be usefully illuminated so the DAS need not readout the entire imaging area. The slitmask alignment exposures do not require calibration by prescan or overscan pixels; they should not be stored or displayed. In order to accelerate the readout the DAS must be able to do the following:

8.4.1.3: Gathering of Image Statistics

Several astronomers have expressed interest in seeing some kind of quick statistical analysis of the image data as it is first displayed. This interest was prompted the fact that the displayed images may be binned by a factor of 4 to 8. With such binning it will be difficult for astronomers to perform a manual scan of the images for defects.

As the sections of the image data arrive from the CCD controllers the data acquisition system should accumulate some simple statistics. Some of these will assist the astronomers to make data quality assessments; others will serve to accelerate the operation of the image display software.

Subject to the constraint that the calculations do not significantly delay the readout and storage of the CCD image, the following statistics should be measured for each separate CCD amplifier:

8.4.2: Requirements for the Image Display Graphical User Interface

In previous Keck detectors (i.e., HIRES and LRIS) the detectors have been single CCDs read through multiple amplifiers. The pre/overscan data have been located outside the bounds of the image data. This permitted the image display algorithms to consider only the central block of pixels when reducing the image from 16-bit integers to an 8-bit (or less) grey-scale. In DEIMOS not only will there be overscan data in the midst of the image, but in both spectral and imaging frames there will be regions of CCDs which are not illuminated.

8.4.2.1: Overview of Hardware and Protocol requirements

Single monitors that can display 8k x 8k images do not exist. Monitors that can display 2k x 2k pixels do exist, but they are quite expensive and will typically not be available at remote sites. ``Monitor walls'' can be built by stacking arbitrary numbers of commonplace monitors, but again these will not be available at remote sites.

We choose to work within the bounds of typical existing displays which have about 1k x 1k pixels and 8-bit color depth. If circumstances provide a larger or deeper display then the display should be able to take advantage of them.

8.4.2.2: Display of World Coordinate System Information

The GUI will utilize WCS information in the FITS headers. As the mouse tracks over the image it will provide a real-time update of the coordinate position on the sky (for image frames) or the slitlet/object and observed wavelength information (for spectral frames).

The precise WCS mappings for DEIMOS will be highly nonlinear and may exceed the capabilities of the simple cases described in the FITS WCS draft. The real-time tracking must make use of the simple components of the WCS mapping as described in the main body of the WCS draft. If the fully accurate and nonlinear WCS mapping can be computed fast enough it may supercede the simple transformation; otherwise upon user request the GUI should perform the mapping to full accuracy.

For any kind of frame the user should have the option of displaying WCS information in any one of the following coordinate systems:

When the cursor is positioned over pixels that contain calibration (prescan or overscan) data the position display should revert to indicate the X and Y pixel data. It will simultaneously indicate the type of the calibration pixels.

For imaging frames the user should have the option of displaying WCS information in any of the following coordinate systems:

For spectral frames the coordinate system will be known to high precision because of the requirements for aligning the slit masks. The user should have the option of displaying WCS information that tells the following:

Steve Allen <sla@ucolick.org>
$Date: 1996/03/19 06:29:41 $