Keywords: gcx

Subsections

- FITS Header fields
- Viewing Image Files
- Saving and Exporting Images

# Image Files

The basic format used by GCX for image files is FITS. Internally, images are represented using 32-bit (single-precision) floating-point values. The images are read and saved as 16-bit integer files. It it relativey easy to modify the program to read outher FITS formats (like 8-bit or floating point), should the need arise.

GCX will read and write fits files compressed in the gnuzip format (ending in `.fits.gz`) transparently. A zipped file name can be used whenever a regular fits file name is required.3.1

While the way image data is represented in the FITS files is well specified, additional information from the fits header can vary between different programs. Since the data reduction functions of GCX make use of quite a few fits header values, it is important to understand how they are interpreted.

Some camera control programs generate broken 16-bit FITS files that have `BZERO` set to 0 and all values stored as unsigned numbers. When loaded, values larger than 32768 appear as negative numbers. To accomodate these files, GCX provides the File and Device Options/Force unsigned FITS option, which will make the program interpret all values in frames with `BZERO`=0 as unsigned. Note that files are always saved in the standard format.

# FITS Header fields

The default names of the fits header fields have been set to what the author considers the most common. It is however possible to change any of the field names by editing the options or the `~/.gcxrc` file.

After loading a fits file, you can examine it's header by selecting File/Show Fits Header.3.2When new frames are created by GCX from images captured by a CCD camera, the frame is annotated from one of the following sources:

- The General Observation Setup Data options;
- Object information from the currently selected target object;
- Frame information from the camera.

The table below details the most important fits header fields set and read by GCX :

| Name | Type | Meaning | Comments
| `CRPIX1/2` | Real | Coordinates of reference pixel | Set to the center of the frame; Used to set the initial WCS.
| `CDELT1/2` | Real | Scale of image in degrees per pixel | Set initially from the observation setup focal length, pixel size and binning; updated after WCS fitting.
| `CROTA1` | Real | Rotation of image in degrees | Set initially to 0; updated after WCS fitting.
| `CRVAL1/2` | Real | World coordinates of the reference pixel | Set from the target object coordinates; update after WCS fitting.
| `FILTER` | String | Name of filter used | Set from the current observation data or filter wheel status; used for photometric reductions.
| `ELADU` | Real | Image electrons per AD unit | Set from camera parameters; used by the photometry routines in the error model.
| `RDNOISE` | Real | camera read noise in AD units | Set from camera parameters; used by the photometry routines in the error model.
| `EXPTIME` | Real | integration time in seconds | Set from camera paramters; used by the scintillation noise evaluation routine
| `JDATE` | Real | Julian date of the start of integration | Used, among other things to calculate the frame airmass
| `MJD` | Real | Modified Julian Date of the start of integration | An alternative to JDATE
| `DATE-OBS` | String | The date/time of start of exposure in the format specified by the fits standard | An alternative to JDATE
| `TIME-OBS` | String | The time of day portion of the date/time, used when the `DATE-OBS` field only sets the date | An alternative to JDATE
| `APERT` | Real | Telescope aperture in cm | Set from general observation options; Used to evaluate scintillation noise
| `LAT-OBS` | String | Latitude of observation site in DMS format | Set from general observation options; Used to calculate airmass
| `LONG-OBS` | String | Latitude of observation site in DMS format; Eastern latitudes are negative | Set from general observation options; Used to calculate airmass
| `AIRMASS` | Real | Frame airmass | If present, disables airmass calculation and sets the airmass value
| `OBJECT` | String | Target object name | Used to set the initial WCS if other information is not available
| `OBJCTRA` | String | Target right ascension in HMS format | Used to set the initial WCS if other information is not available
| `RA` | String | Target right ascension in HMS format | Used to set the initial WCS if other information is not available
| `OBJCTDEC` | String | Target declination in DMS format | Used to set the initial WCS if other information is not available
| `DEC` | String | Target declination in DMS format | Used to set the initial WCS if other information is not available
| `SECPIX` | Real | Image scale in arcseconds per pixel | Used to set the initial WCS if other information is not available

# Viewing Image Files

To pan around the image, either use the scrollbars, or place the cursor over the point that you want in the center of the image and press the spacebar or the center mouse button.3.3You can pan back to the center of the image using ctrl-L or select Image/Pan Center from the menu.

To zoom in, place the cursor over the point you want to zoom in around, and press the = key (same key that has the `'+'` symbol). To zoom out, press -. The Image menu also has Zoom In and Zoom Out options.

When loading a frame, the image cuts are automatically selected for a convenient display of astronomical frames. The background is set at a somewhat dark level, and the dynamic range is set to span 22 times the standard deviation of the intensity across the frame. You can always return to these cuts by pressing 0 or selecting Image/Auto Cuts.

Pressing 1 - 8 will select various predefined contrast levels. 1 is the most contrasty: the image spans 4 sigmas, while 8 spans 90 sigmas. 9 will scale the image so that the full input range is represented (the cuts are set to the min/max values of the frame). Selecting Image/Set Contrast/... from the menu will accomplish the same effect.

To vary the brightness of the background, use B (Image/Brighter) and D (Image/Darker).

Another (sometimes more convenient) way of making contrast/brightness adjustments is to drag the pointer over the image. Dragging horisontally will change the brightness, while dragging vertically will adjust the contrast.

It is important to know that all the ajustments described only apply to the display. The internal representation of the frame (and of course the disc file) is never changed in any way.

Further adjustments to the way the image is displayed can be made by bringing up the Image/Curves&Histogram dialog. This dialog shows a portion of the frame's histogram, overlapped by the currently set intensity transfer function. The image cuts are placed at the red vertical bars in the histogram window.

The shape of the transfer function can be altered by changing the Gamma and Toe parameters. Gamma controls the overall shape, while the Toe controls what happens in the leftmost portion of the curve. Increasing the toe will prevent the transfer function from having a very high slope near zero, as would be implied by the gamma setting.

# Saving and Exporting Images

The image currently displayed can be saved in the FITS format by selecting File/Save FITS As. When frames are saved as FITS, their values aren't affected by the display settings. Another option is to export the file to a different format (presently, only the 8-bit pnm format is supported). When exporting, the intensity mapping used for displaying the image is used to convert from the original frame to the 8-bit output image.