Image Display

Oasis can display any standard astronomical FITS images that contain the header information necessary to determine sky coordinates (e.g. projection, scale, rotation). FITS files which have missing or improper coordinate/projection information will be displayed but cannot be overlayed with source tables, grids, etc.

In addition, the user has completed control over the following:

Each these has their own control panels and can be instantiated via buttons (or menu items) on the main Oasis window.


Image Resampling

The image resampling control window allows the user to choose which subset of an image file to extract for viewing. This can involve either cropping out the region of interest or subsampling the data (i.e. every nth pixel/row) or both. Images which are too large to fit in memory are automatically subsampled for initial display.

Operations such as contour generation and area statistics can be run on sampled images, though a warning is displayed since the information generated will be based on just the pixels in memory and not the original file.

The region sampled is shown on a thumbnail representation of the original image (in the upper left corner of the pop-up). The currently sampled portion of the image file display is outlined here as a box, with another box representing the area which will be sampled next (which the user presses the "Resample" button).


Data Input Transforms

When reading a FITS astronomical image file, the pixel values must be converted to 8-bit integers for display purposes. To achieve this, the user must decide what attributes of the data are most important (full dynamic range, details of a specific set of values) and whether a non-linear scaling should be used. For instance, a common tactic is to transform the data onto a logarithmic scale since the brightest sources are often orders of magnitude brighter than the low-level variations in the sky background. On a log scale both the background variations and the relative brightness of the sources can still be seen.

Another common technique is to skip the very brightest and dimmest pixels (essentially truncating the brightness distribution histogram). This usually concentrates the variation in the image colors into data regions where important physical phenomena are occurring.

The Oasis data transform panel (the top half of the Color Manipulation pop-up) provides controls for this transformation. The user is free to set the data minimum and maximum (i.e. the cutoffs mentioned above) either is file data units or as a percentile of the data values for the whole image. A good approach is to start with a percentile approach, then switch to small adjustments to the absolute values once the display is close to what the user wants.

The user can choose between a linear transform, the log transform described above, or a "log-log" tranform (i.e. log(log(x))) for images that have a more extreme data range (not uncommon in astronomy where images often contain very bright stars on a low-level background).

A static data histogram of the original image pixel values is displayed for reference and there is a reset button to return to the default value (a data range of 1% to 99%).


Color Manipulation

Data read from FITS image files passes through two transforms before being displayed. First, it is converted from its original data units (which are usually floating-point values, often 64-bit precision) to 8-bit integers for display purposes. This is described in the section on Data Input Transforms.

Given an 8-bit image, the user must then decide how this should be mapped into a set of colors. Oasis provides a set of color tables, ranging from a simple gray-scale (low 8-bit values go to black, high to white with a smooth gradient in between) to various psuedo-color color tables (e.g. "thermal", where the colors cycle through a spectrum from black to blue to green to red to white).

For fine-tuning purposes, any color table can also be stretched linearly through an interactive set of sliders (controlling the slope and center of the linear transformation). This overlaps with the data range tranforms referred to above but has the advantage of being interactive (whereas the data transform has to reread the file). If the user finds they want to stretch the color table by a large amount, they should probably adjust the data transform instead.

Oasis displays a histogram of the 8-bit values in conjuction with a ramp image. This allows the user to see how many pixels in the image will have a particular color. This histogram is similar to that displayed for the original file in the data range section of the panel, but represents the values after the data cutoffs and any non-linear transforms have been applied.

A color reset button (for stretch parameters, not color table) is also provided.


Zooming and Panning

The zoom/pan control window allows the user to set a zoom factor either in integer increments, powers of two, or directly by entering a value manually. Changing this factor causes Oasis to zoom in/out around the currently displayed center. This zoom factor can be fractional.

Panning is accomplished through a thumbnail representation of the whole image (in the upper left corner of the pop-up). The currently displayed (zoomed up) portion of the image being shown in the main display is outlined here by a green box, which the user can grab and move as desired.

The current location of the zoom center is displayed on this panel for reference.


FITS Header Display

Oasis will display a summary of the information in any FITS file (including extensions). This tool will be eventually be used to allow access to extension data and planes in data cubes, though this capability is not yet available.


Cursor Feedback

The Oasis main display has a selectable cursor feedback state. The current state can be set explicitly from the menu or via certain buttons (e.g. the Examine Area button) or implicitly (e.g. when a focal plane overlay is instantiated).

Specific cursor feedback modes are as follows:


At the bottom of the main Oasis window is an area for controlling and viewing the current coordinates of the cursor when it is on top of the image. The user is free to set the coordinate system (e.g. Equatorial Julian, Galactic), the epoch (e.g. 1950, 2000, 1983.5), and the display format (e.g. sexagesimal, decimal degrees). For each coordinate system there is a default format defined (e.g. sexagesimal hours/degrees for Equatorial).

To the right of this is a feedback area for the current cursor coordinates. Any necessary precession or coordinate conversion (including Besselian <-> Julian) is performed automatically.

Note that the coordinate feedback is on at all times and the other modes are in addition to it.


Source Information

Oasis is capable of displaying data from an arbitrary number of source tables as overlays on the base image. It can also display those tables themselves. When locations are selected on the image, any sources nearby are marked, both on the image and in the tables.

However, displaying all these tables by default can rapidly clutter up the screen to the point where it is difficult to maneuver so Oasis does not make them visible by default (they can be made visible using the layer control window). Instead, selected sources are displayed in a "source info" window. This window shows the contents of a single record from a single table at a time and provides a mechanism for rapidly scrolling through the list of all selected sources.


Pixel/Area Definition

If the user desires to examine an area (usually by pushing the Area Definition button), the cursor behaves as follows:

  • Clicking on the image defines the initial center of an area. This location will be marked and outlined appropriately for the desired area shape (pixel, circle, box).
  • For a box or a circle, the outline has initial size which can be adjusted by grabbing the control points on the outline. The center point is also a control that will move the region as a whole.
  • Clicking well away from the control points will end this area adjustment phase and fire an area change event to all interested parties (see the information on the displayed layers for details.
When the area examination is in single pixel mode, an area change event is sent immediately with each mouse click.

Moveable Overlays

Most map overlays represent a set of specific locations on the sky and are displayed statically. One notable exception are telescope focal plane outlines and similar observing information. Oasis provides a mechanism for displaying such layers and a cursor feedback mode for controlling them.

Currently, the only instance of this that is included is an outline of the SIRTF focal plane, though others could easily be added. When a focal plane layer is instantiated, a separate control comes up for setting the focal plane rotation angle and the cursor feedback mode is set to control the focal plan location.

In this mode, clicking on a location moves the center of the focal plane there. Dragging a location on the focal plane to a place on sky move the focal plane accordingly. This is particularly useful for lining up instruments with sky objects.