Image Display and Control
|The Image Display and
Image Control Windows permit the user to view the
guide camera image, optimize the display, make measurements of the image,
and control certain critical guider functions such as size and placement of the guide
reticle. A good understanding of display and control is essential to making the most
of the guider. Many users of Lick facility instruments will find these windows familiar,
having used the very similar Lick data-taking
When using the actual Image Control Window in the guider, a click of the center mousebutton on most of the widgets displays the authoritative help file for the corresponding function. The descriptions of image control functions in this manual are meant to complement those original help files and to address issues specific to the guider's implementation of the software.
Image Display Window|
|The image display window refreshes automatically each time an image arrives from the camera. For typical guiding, the refresh rate will be from several times per second to thirty seconds or more. The appearance of the image is controlled from the Image Control Window, illustrated below. Apart from the image itself, the display may contain any or all of the following guider features: compass, sky box, guide reticle, and markers. The user may also make row and column plots of the image, and draw boxes for use with other guider options.|
Image Control Window
|The Image Control Window provides numerous ways to view and measure the image, and to control the appearance of the display. Careful control of color mapping and contrast can reveal much detail that would otherwise be lost.|
|The compass indicates the sky directions on the image. It will display the correct
directions provided the version of the guider software appropriate to telescope, focus, and
instrument has been run. The compass accounts for field roatation (3-m coude and CAT) and TUB
rotation (3-m cassegrain), and for flips in orientation due to diagonal mirror position
(3-m and 1-m cassegrain).
The compass may be dragged anywhere on--or parked just off--the image using the mouse. It may be hidden or shown using the Show/Hide option in the guider control window.
The compass can be a useful diagnostic. If there appears to be a guiding problem, check that the compass directions are correct by moving the telescope. If not, you may have the wrong version of the guider software running, or signs in the parameters section of the guider control window.
|The total flux under the sky box is measured. An average sky brightness is calculated and
subtracted from the guide reticle with each exposure. The guider also uses the
sky brightness meaurement to monitor the guide star by comparing the flux in the guide
reticle to the sky flux. By this means, the guider determines whether the star is in the reticle
and is sufficiently bright. The user-settable parameter
Sigma Threshold sets the level for relative flux below which the guider assumes the star is lost and
The sky box may be repositioned and resized using the mouse: left click and hold anywhere inside the box and drag to reposition; click and hold on the small white handle on the upper left corner of the box and drag to resize. The skybox should always be positioned on blank sky, preferrably not too far from the guide reticle, and should be large enough to contain a good sample of sky. It may be shown or hidden using the Show/Hide option in the guider control window.
|The guide reticle consists of two pairs of two oppposed boxes. The guide star or other guide object is
ususally (though not always) positioned at the center of the reticle. Flux is measured under each box with each
exposure. The flux data are used to sense and correct deviations in tracking in one of several ways, depending
on the algorithm chosen in the guide choices part of the camera
The pairs of boxes may be resized, their separation changed, and the entire reticle repositioned using the mouse. To resize the top/bottom or left/right pair (opposing boxes are always the same size and will resize in a single motion) grab and drag the small white square in the upper left corner of the top or left box, respectively. To change the separation between pairs of boxes, click and hold inside one of the boxes in the pair (but not where it overlaps another box), and drag. To reposition the entire reticle, click and hold on the small red handle at its center, and drag.
The reticle may be shown or hidden using the Show/Hide option in the guider control window.
|Markers are crosshairs that can be placed on the image to mark locations. Up to a dozen markers can be displayed. They are numbered 0-11 and are color coded in three groups of four (note that there is a dark red, middle green, and light blue, so that you can choose the best contrast with image background). Markers are displayed using the markers menu in the guider control window. A marker can be dragged anywhere on the image using the mouse. The last location for each marker is remembered, even if guider software is stopped and restarted.|
|Row and column plots may be drawn by placing the cursor on the display and typing "r" or "c". A plot window will appear showing a plot of a single row or column at the position of the cursor. The plot may then be scaled or zoomed from within the plot window.|
|Boxes may be drawn on the image display by typing "b" with the cursor on the display, then clicking to define the first corner, dragging out the box, and again clicking to define the opposite corner. Boxes are important for windowing the CCD as described in the Camera Control section of this manual, and for defining regions of the CCD to use for histogram equalization.|
|POSITION & INTENSITY|
|When the cursor is on the image, this box displays the image coordinates of the its current position, and the value (intensity) of the pixel at those coordinates. Remember that in the display, the origin is at the upper lefthand corner.|
|ZOOM & PAN|
|When he cursor is on the image, the circle serves as a magnifier window, displaying a 5X magnified view of the area immediately around the cursor. The magnifier may be used by left-clicking in the magnifier circle and then moving the cursor around in the display. Alternatively, left-clicking in the display will produce an image in the magnifier, centered on the cursor.|
|This box controls panning and shows the current relationship of the image to the x-window in which it is displayed. The green box represents the full CCD image, the black box represents the x-window. To pan, click the cursor on the spot within the green box (representing the image) on which you want to recenter.|
|These buttons zoom or unzoom the image by the factor displayed on the button.
Note that with the cursor on the image itself, the right mouse button will redisplay the image, centered on the pixel under the cursor. This is useful in combination with the zoom buttons for zooming in on a particular spot: first put the cursor in the image on the spot you wish to zoom in on, click to recenter, then select the appropriate zoom factor.
|CONTROLLING THE COLORMAP|
ADJUSTING CONTRAST & COLOR: THE BIG BLUE BAR
This portion of the image control window permits the user to control how the colormap displays pixel values (intensities). Mastery of this widget is particularly important for getting the most out of guider images.
The solid blue bar always represents the full range of the colormap (0 to 255). The straight, horizontal line below is a range of pixel values--the full range of the CCD (0 to 65535) unless "fine" is selected (see below). Between the two a coarse histogram of the pixel values for the whole image is plotted, with the horizontal line as its X-axis.
The length and placement of the blue bar determines how the colormap displays pixels of different values. The bar may be lengthened or shortened to change the range of pixel values over which the colormap is spread, effectively changing image contrast.
To adjust contrast, shorten or lengthen the bar by clicking and dragging from either end. Shortening the bar will reveal low contrast features, but will cause more of the values to fall outside of the colormap's range, resulting in some wrapping or clipping of the colormap (see "clip" below). Lengthening the bar will include more intensity levels in the range of the colormap, but will reduce contrast and tend to hide faint features. Optimum contrast is found by experimenting, but the histogram plot is a good clue to finding the best length for the blue bar. In general, the bar should be about the same length as the width of the histogram's prominent peaks.
The bar may also be slid horizontally, without altering its length, to change the pixel values corresponding to the beginning and end of the colormap. Grab in the middle and slide it left or right. Placing the bar over the histogram peak usually gives the best result.
N.B.: Lengthening and shortening the bar can be a bit tricky, especially when it's short to begin with. The trick is to make sure that the cursor is placed near enough to the end of the bar so that tilts to the left (as illustrated) or right. When the cursor is tilted, left clicking the mouse will grab the end of the bar. To slide the whole bar, the cursor must be pointed straight up. When the cursor is not near enough to manipulate the bar, it assumes the shape of a mini Gumbi.
The upper set of numbers shows the scale of the histogram's X-axis and represents the full range of the CCD, or a subsection of that range when "fine" is selected (see below). These numbers will not change unless "fine" is selected or unselected. The lower set of numbers shows the range of pixel values over which the colormap (represented by the blue bar) is currently stretched. These will change as the bar is shortened, lengthened, or slid.
With "clip" unselected (button is gray) the colormap will wrap for pixel intensities lying outside the range of the blue bar. With "clip" selected (button turns yellow) wrapping is prevented: all pixels above the range of the bar will have the color corresponding to the highest value in the range, those below to the lowest.
Selecting "fine" causes the X-axis to assume the blue bar's range of values at the time "fine" is selected. This in turn causes the blue bar to expand along the full length of the line, facilitating fine manual adjustments to its length.
Selecting "lock" prevents autoscaling and is recommended for most guider imaging.
The vertical color slider at far right in the image control window slides the colors along the numbers of the colormap, changing the color of the display but not affecting how the colormap's numerical values are mapped to pixel values.
|Transformation and Colormap Choices|
|The user can choose to display a linear rendering of the pixel values or one of several arithmetic translations.
Transformations are performed on the pixel values prior to display. Pixel values in recorded images are not
affected in the saved image.
Logarithmic and square root transformations simply take the log and square root of the raw image and display the resulting pixel values. Histogram equalization gives greater stretch to pixel values that appear often in the image and compresses pixel values that appear less often. This transformation enhances low-level features when most of the field is dark be very useful in revealing faint sources.
The histogram equalization box option allows the histogram to be computed from a subsection of the image. Define a box as described above and select the "H. Eq. Box" option. The entire image will be scaled according to the histogram computed within the box.
|The user can choose from several predefined colormaps. Choosing "Other..." displays a menu with a variety maps. "Invert" and "B & W" are self-explanatory. "Contour" draws a contour plot on the image.|
|Other options include various ways to measure the image (Itv and Digitals), and a function for displaying images from disk (New Image). These features are well described in the writeup on the Lick data-taking display.|