Finding astrometric reference stars

The purpose of this stage is to find a small number of astrometric reference stars, imaged on the CCD frame, which can be used to define the preliminary astrometric calibration. Traditionally finding reference stars is a long-winded task involving consulting printed atlases and catalogues. However, the on-line resources available to GAIA allow the process to be automated and simplified. Proceed as follows.

1. You need to extract a region of the Digitised Sky Survey (DSS) roughly corresponding to the region imaged in the CCD frame. The first part of the recipe in Section  gives exactly the procedure required. Either: repeat this procedure, load the image that you created when previously working through Section  into GAIA, or load file ngc1275dss.sdf into GAIA (the last is the required region, already extracted from the DSS).

2. Adjust the colour table until the image appears as in Figure . Click on the View menu and select the Colors... option. A panel will appear. Set the colour scale algorithm to Linear, the colormap to ramp and the intensity to neg. Then click on the Close button.

   Set the magnification by clicking on the Scale: button (in the bottom left of the control panel in the centre top of the window) and setting it to 2x.

3. Overlay the DSS image with objects selected from the USNO at ESO catalogue, following the second part of the recipe in Section .

4. The next step is to choose five stars in the image to act as reference stars. These stars should be:
   • easily identified,

   • reasonably bright,

   • stars rather than galaxies (because star images have more precisely defined centres),

   • isolated from other stars and galaxies (to avoid images which blend together),

   • spread reasonably uniformly over the image,

   • not very close to the edge of the image (later you will need to to identify the corresponding stars in the JKT image, and the two areas of sky are not exactly the same).

   Figure  shows five suitable stars in the DSS example image. To select the stars: hold down the shift key and click on each star in turn (without releasing the shift key). As you do so the selected star is highlighted in both the image and the catalogue windows. You can add as many stars as you like, but five is adequate.

   Figure: DSS image with reference stars marked

   

5. Copy the selected stars to a new table dialogue box by clicking on the Options menu in the catalogue dialogue box and choosing the Extract selected item. A new catalogue dialogue box listing just the selected objects will appear. Henceforth you will work with this catalogue dialogue box.

6. To double-check which objects you have selected: click on the Graphics menu in the main window and choose Clear. All the catalogue object markers will disappear.

   Re-plot the selected objects by clicking on the Plot button towards the bottom of the new catalogue dialogue box. Label the chosen objects by clicking on the Options menu in the catalogue dialogue box menu-bar and choosing Label all objects.

7. It is useful to print out a copy of the image with the reference stars marked, for use in the next stage of the calibration. If you are using the colour table described above you will need to adjust it so that the star identifications are legible, typically by inverting the image to make the stars white against a dark background (as in Figure ), rather than vice versa.

8. Print out a paper copy of the image by clicking on the File option (the rightmost option in the menu-bar along the top of the main window) and choose the Print... option, followed by Image.... Use the ensuing dialogue box to save the image as a postscript file, which you can then print.

9. Save the catalogue of selected objects. Click on the File menu in the USNO at ESO (1) dialogue box (the leftmost item in its menu-bar) and choose the Save as... option. You should specify a file-type of .lis so that the catalogue is saved in the `ASCII Header' format (see Section ).

You now have a list of suitable reference stars. You can quit GAIA at this point, but it is better to proceed directly to the next stage of the recipe.