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7. Loading images

To blink images or do astrometry you must first load them into memory. The `fitsblink` program accepts 16-bit two-dimensional FITS images. The program also reads the header and extracts the following information:

- WCS values from the CRVAL, CRPIX, CDELT and CROT keywords. If these keywords are present, the program will also display celestial coordinates at the cursor position.
- Time and date of the mid-exposure. It seems that there is no standard on which keywords define the time of mid-exposure. Different CCD camera manufacturers use different keywords in programs which aquire data from cameras and write a FITS file. `Fitsblink` tries the following things to calculate the correct time of mid-exposure:
  1.
    Checks for UT-CENT keyword. If this keyword is present it takes its value as the time of mid-exposure.
  2.
    If UT-CENT is not present, it calculates time of mid-exposure from values of UT-START or TIME-BEG keywords and EXPOSURE keyword.
  3.
    If there is no EXPOSURE keyword, time is calculated as the mean time of UT-START or TIME-BEG and UT-END or TIME-END values.
  4.
    EXPOSURE and EXPTIME keywords are considered to be synonims.
  5.
    Date expressed in either dd/mm/yy, dd/mm/yyyy, dd-mm-yyy or yyyy-mm-dd formats is read from the value of the DATE-OBS keyword. It is understood that the date is valid for the start of exposure and the program makes necessary correction if the mid-exposure falls on the next day.
- Object name from the OBJECT keyword. This is later used in the astrometry report.
- Telescope description from the TELESCOP keyword.
- Instrument (CCD camera) name from the INSTRUME keyword.
- Observer name(s) from the OBSERVER keyword.

To load an image, press the "Load" button. A file selector will appear and you can navigate through directories to find the wanted file. When you find it, double-click on it or select it and press the Ready button. It is also possible to type in the file name. `Fitsblink` loads either FITS images or gzipped fits images, thanks to the CFITSIO library. After loading it tries to make an automatic level adjustment so that stars exhibit good contrast relatively to the background. This procedure may produce not so good results for images with very bright objects or if some non-image data is stored in the image area. When the image is loaded, it is scaled and transfered to an X Windows structure called Pixmap for faster blinking operation. The original image data remains in the memory. `Fitsblink` can in principle read unlimited number of images. You are limited by the available memory of your computer and X terminal, of course.

There is an alternative method of image loading, which is particularly suitable when the images are first processed by some program which analyses the images and finds how are they shifted and identifies moving objects. You need to prepare a file with the following structure:

    filename1 x_offset1 y_offset1
    filename2 x_offset2 y_offset2
             .
             .
             .

If you name this file eg. `test.inp`, `fitsblink` will load files named in the first column and shift the images by the offsets given in the second and third columns. The purpose of this is to have properly aligned images at the moment they are loaded into memory.

Additionally, following every line with the file name it is possible to append a list of marker coordinates. For every marker you need to specify also its size and a word describing the object it represents. You can also specify marker color and marker shape. There must be four, five or six white-space separated items in such a line, for example

    filename1 x_offset1 y_offset1
    45 100 5 asteroid red box
    67 29 5 asteroid blue
    87 216 5 nova
    filename2 x_offset2 y_offset2
             .
             .
             .

The default color is `red` and the default shape is `circle`. More about markers in section 10. Do not forget to press return in the last line of batch file. For a practical example, see the Quick tutorial section.

Next: 8. Erasing an image Up: Fitsblink 2.1 user manual Previous: 6. Getting help