Creating a preliminary transformation

This stage of the recipe uses the five reference stars identified in the previous stage as `fiducial marks' to define a preliminary astrometric calibration. However, before starting, a superficial glance at the DSS image (for example in Figure ) and the JKT image (Figure ) reveals that they are rotated with respect to each other by 180$^{\circ}$. Sometimes uncalibrated images show such gross rotations with respect to the standard orientation, sometimes they do not (the `standard orientation' has north at the top, east to the left and Right Ascension increasing from right to left, that is `the wrong way round'). If they do then it is best to rotate them before attempting the astrometric calibration.

This recipe assumes that GAIA is still running and that the catalogue of reference stars created in the previous stage is still available. If not, then start GAIA and load either the local catalogue of reference stars that you created in the previous stage, or file ngc1275usno.tab, which is the equivalent example file.

1. Load image ngc1275jkt.sdf into GAIA.

2. Adjust the colour table and magnification:
   1. click the Auto Cut: button for 98% (in the bottom right of the control panel in the centre top of the window),

   2. set the magnification: click the Scale: button (in the bottom left of the control panel in the centre top of the window) and set it to 1/2x,

   3. set the colour table: click the Color Map: button (in the lower right of the control panel in the centre top of the window) and set it to heat. Also set the Intensity Map: button (directly below the Color Map: button) to default.

3. Rotate the image to the standard orientation. Click on the button marked with two horizontal arrows and then the one marked with two vertical arrows (these buttons are located on the bottom row of the control panel towards the top of the main window).

   It should now be straightforward to identify objects in the JKT image with the corresponding objects in your print-out of the DSS image (created in the previous stage) or in Figure .

4. Click on the Image-Analysis button on the menu-bar along the top of the main window. Choose the Astrometry calibration item and then Fit to star positions from the next menu. The Fit astrometry reference positions dialogue box shown in Figure  should appear.

   Figure: The main GAIA dialogue box used to create an astrometric calibration

   

5. Indicate the celestial coordinate system in which your star coordinates are expressed. You do so by specifying suitable values using the coordinate system:, Equinox: and Epoch: buttons. The coordinates will usually be FK5, equinox J2000 or for older catalogues FK4, equinox B1950. The epoch defaults to J2000 for FK5 coordinates and B1950 for FK4 coordinates. For the present recipe the defaults of FK5, equinox J2000 are correct.

6. To import the catalogue of selected reference stars click on the Grab button.

   A selection box allowing you to specify the catalogue required will appear. The selected reference stars will usually be the first catalogue in the list.

   Once the catalogue has been imported the reference stars should be listed under `Reference positions' in the Fit astrometry reference positions dialogue box (Figure ).

7. You now need to measure the positions (in pixels) of these stars in the JKT image. Click on the first reference star in the list, make a mental note of its star name and click on the Edit button. The dialogue box shown in Figure  should appear.

   Figure: The GAIA dialogue box holding details for a reference star

   

8. Now click the Pick object... button. The dialogue box shown in Figure  should appear. The black box in the upper portion of the window displays a section of the main image centred on the position of the cursor. Referring back to your paper copy of the DSS image or Figure , position the cursor over the corresponding first reference star (the star names should match).

   If there are any other significant features visible in the box, you should reduce the size of the box using the `zoom buttons' (`Z' and `z') immediately below the image box.

   Once you are happy with the sample size, re-position the cursor over the star and press the left mouse button. The pixel coordinates at the centre of the feature are displayed in the Image X: and Image Y: fields within the object picker dialogue box (Figure ).

   Figure: The GAIA object picker dialogue box

   

9. Raise the object picker dialogue box (Figure ). You should find that the pixel positions of the star have been copied into the X and Y fields.

   The star is now fully specified, so press the Enter button. After confirmation, this will amend the details of the star in the list of reference stars in the Fit astrometry reference positions dialogue box (Figure ).

10. Repeat the procedure for the remaining reference stars. All the dialogue boxes remain open, so it is relatively simple to cycle through, measuring the positions of the remaining stars. Hint: double-clicking on a star in the reference star list is equivalent to clicking on it and then clicking the Edit button.

11. Once you have entered all your reference positions, close the dialogue boxes shown in Figures  and by pressing the Close button in each one.

12. Click on the Marker menu on the menu-bar along the top of the Fit astrometry reference positions dialogue box (Figure ), choose the Size option and set it to 21. This option makes the markers a convenient size when plotted on the JKT image. You may also need to set the Outline colour option to white (depending on which colour table you have set).

    Now click the Fit/Test button in the same dialogue box. GAIA uses the known celestial coordinates and measured positions of the reference stars to define an astrometric calibration. It then uses this calibration to work out the pixel positions corresponding to each of your reference coordinates, and displays markers in the main image at these pixel positions. You should find that a marker is drawn more-or-less on top of each of your reference stars.

    If the markers are not properly aligned then you have probably measured the wrong star or entered an incorrect Right Ascension or Declination value. You should correct the reference positions, and then press the Fit/Test button again. To correct the reference positions: Edit the details of the offending star, as above, then press Fit/Test again to re-calculate the astrometric calibration.

    Cheat: you can load a file containing the celestial coordinates and measured positions for the five stars marked in Figure . Click on the File menu in the menu-bar along the top of the Fit astrometry reference positions dialogue box (Figure ) and choose the Read positions from a file... option. A file-picker appears. Use it to load file refstars.prelim. Then click the Fit/Test button as before.

13. Once you are happy with the calibration, press the Accept button.

14. Finally, save the astrometric calibration by clicking the File menu button in the main window, and then the Save as... menu item. Use the resulting dialogue box to save the image (with the astrometric calibration) in a new file, perhaps called ngc1275jktpre.sdf. This is most simply done by entering the new file name in the Selection box, and pressing OK. Do not worry if a message is displayed saying that the WCS could only be saved as an AST native representation.