Tweak Astrometry Calibration Toolbox

This window is designed to allow you to tweak your astrometric coordinate system by adding in a linear transformation that corrects the positions on your image. To use it you need to go through several stages:

  1. Obtain reference positions (e.g.) from a catalogue.
  2. Position them over the corresponding image features.
  3. Perform a fit or assign a transformation.
  4. Test the coordinate system.
  5. Accept it if you're happy.
See below for more about these stages.

Obtaining reference positions

You can get reference position information from many sources (consulting your coworkers about how they get such information is often a good start), but the most easily available in GAIA is from the Guide Star Catalogue (GSC) and US Naval Catalogue (USNO) which you can query using the "Catalogues..." item under the "Data-Servers" menu on the main window.

Copying positions from a catalogue window

After you've made a query to a catalogue for some reference positions, you can copy these to this window using the "Grab" button. This creates a dialog window from which you need to select the catalogue you're interested in.

Modifying a reference position

All the reference positions are listed in the reference table. To modify one of these values you either:

  1. Select the required line in the reference positions table and now press the "Edit" button. This opens a dialog window in which you can edit the positions (take care to press the "Enter" button to update the entry).
  2. Double click mouse button 1 over the corresponding image graphics marker. This will result in the same dialog window being revealed.

Adding a reference position

To do this just press the "New" button and fill in the dialog window. Remember to give a valid "id" (which can be any unique string or number) and press "Enter" to create the new object. You can enter more than one object at a time by editing new values and pressing "Enter" again.

Deleting a reference position

Either select the row in the reference table and press the "Delete" button, or press <Control-Mouse-Button-1> over the marker. Note you can delete many rows at a time by selecting all their entries in the reference table.

Positioning reference points on your image

Each reference position is associated with a graphics marker that is displayed over the image (the default is a small circle). The position of this marker identifies the point on the image that corresponds to the correct position on the sky.

Moving a graphics marker

You do this by:
  1. Dragging the marker around over the image (you do this by pressing, and holding down, mouse button 1 and then moving the mouse, when your cursor is over the marker).
  2. By selecting the required line in the reference positions table and then pressing the "Edit" button. This opens a dialog window in which you can edit the positions (take care to press the "Enter" button to update the entry).
  3. Double clicking mouse button 1 over the marker will also create a dialog window in which you can edit the values.

Moving graphics markers as a whole

If you want to move all the markers as if they where coupled together (so that for instance they all move by the same amount, or rotate/scale about the same point). Then you can achieve this in two ways:

  1. Use the sliders and/or entry fields in the "Simple transformations" section. To enter a value by hand just type the new value into the appropriate entry field and then press <Return>. The markers will now be updated (note that you are not restricted to the range of values allowed by the sliders).
  2. Deselect the "Move markers individually:" option. Now when you move one marker they will all move together. You'll also notice a cross marker appearing on the image, this identifies the rotation/scale centre and can be moved just like any other marker. To rotate or scale all markers, press mouse button 2 over one and drag the marker to its new position.

Seeing which markers correspond to which reference positions

Just place your cursor over the marker you're interested in and the appropriate row in the reference table will be highlighted. Double clicking the marker also shows the values associated with the object.

Pressing mouse button 2 over a selected row in the table attempts to centre the image about the associated marker.

Accurately centering all the markers on stars

If your reference positions are all stars (or non-confused peaked objects), then you can accurately position the markers by centroiding. Just press the "Centroid" button.

Trying out a fit

After you've got sufficient reference positions created and you've positioned their graphics markers to your satisfaction, you can attempt a tweak by pressing the "Fit/Test" button. The results of the fit are displayed in a scrollable window just below the "Test" button.

The fitting process uses a linear transformation between the projected X and Y positions (i.e. those your current calibration provide) and the new positions that you've determined. You can control the level of refinement by selecting the "Options" menu and choosing a different type of linear fit used to make the refinement (this is useful when you know certain properties of the coordinate system). The results that you see are those of this fit, not of the sky coordinate system to the image.

Note: that if you've applied a simple transformation to the markers (using the sliders and entry fields) then you should press the "Assign" button rather than the "Fit/Test" button (actually this makes little difference when you have many reference positions, as it is almost identical to using "Fit/Test" with a solid body fit, but if you want to apply a simple transformation to the coordinates without reference positions, then it is essential).

Checking the fit

After doing a tweak you can assess it visually by checking the new marker positions, which are updated. You can also read off positions using the main window facilities, check the axes orientations using a grid overlay (see "Overlay axes grid..." under "Image-Analysis") and replot any catalogues positions from query windows.

Saving the new astrometry

When you're happy with the quality of your new calibration press the "Accept" button to close the window. This updates the image with the new astrometric system, but to retain this information you need to save the image to disk (see the "Save as..." item in the "File" menu).

Miscellaneous features

Changing the appearance of the object markers

Open the "Graphics" menu. Under this are many options for setting the size, shape and colour of the markers.

Clearing the markers from the image

You can do this by closing the window, or by using the "Clear" item under the "Graphics" menu.

Redrawing the markers

Open the "Graphics" menu and select the "Redraw" item.

Removing markers which are not on the image

Press the "Clip" button.

Saving the reference positions

You can write a plain text file copy of the reference positions using the "Write positions to a file..." item in the "File" menu. These can be read back later using the "Read positions from a file...".

Reading reference positions from places other than catalogue windows

You can read in reference positions from plain text files using the "Read positions from a file..." item in the "File" menu. The format of these files should be either:

  1. id ra dec x y
  2. id ra dec
  3. ra dec
Each field should be space separated, and ra's and dec's should be written as either "hh/" or "hh/dd mm" strings or dd.dd values. Note that this format is different to that used by "Local Catalogues", which use a tab-separated design that also allows parameters, column naming etc. It is designed to be simple to create using text editors and other programs. You can manipulate and save local catalogues, but you will need to "Grab" the values on each occasion.