In this tutorial you will learn how to use AIP to make precise positional measurements for objects in a CCD image. AIP can extract reference stars from any of the following catalogs of precise stellar coordinates...
In order to obtain an astrometric solution one of these databases must be available. Alternatively, a subset of one of these databases must be available as a file for stars which appear in the CCD image to be analyzed. An approximate right ascension and declination must be known for the center of the CCD image.
AIP will assist the user to identify reference stars in the image by matching them with stars in one of the catalogs of precise stellar coordinates. The stellar positions will be overlayed on the image. The positions for all reference stars will be measured and an astrometric solution (plate constant solution) will be determined which will transform x,y positions in the image to right ascension and declination. Target objects may be selected interactively and the right ascension and declination will be determined. A report file containing the results can be generated.
Pick out a CCD image for which you know the approximate right ascension and declination for the center of the image and follow along with the steps outlined below.
Step 1: Load the image
It is advisable to begin without any open images or tool windows. As appropriate, use the following selections...
Window | Close All ImagesWindow | Close all open Tool Windows
Open an image. Then use...
Measure | Astrometry
Step 2: Create an Overlay
To create a reference star overlay you will need to specify at least an approximate value for the focal length of the optical system that was used to obtain the CCD image. Enter the focal length in mm in the Focal Length text box on the Astrometry Tool. This will scale the generated overlay so it matches the scale of the CCD image.
For some images you may also need to use the Mirror X and/or Mirror Y buttons to change the orientation of your image. Your image should be oriented so that north is toward the top and east is toward the left (roughly). This is the orientation that is used for the overlay.
Click on the Load Ref Data button and the Select Overlay window will appear. At the top you will see a set of five radio buttons. Each radio button corresponds to one of the database types that AIP can read. The Reference File button refers to a file that AIP can generate after reference stars have been read from one of the large databases. Select the appropriate choice for your application.
If you select the Reference File option, you will be prompted to navigate to the reference file you desire. If you select one of the CD-ROM databases, you must enter the right ascension and declination for the center of the field you intend to analyze along with the angular size of your image. You can also specify the Magnitude Limit for reference stars to consider, or accept the default limit. You will be prompted to insert the appropriate CD. You will also be prompted to see if you wish to save the stars that are extracted from the database to a Reference File. If you select this option you will be able to continue with the analysis of your image at a later time without needing one of the CD-ROM databases.
After reference star data has been read you should click Toggle Overlay. The overlay will then appear as a collection of red circles, with a blue cross marking the center of the field.
Step 3: Align the Overlay
In general, the first time you begin to analyze an image, the overlay and the image will not be alligned. The overlay is always presented with north toward the top and east toward the left of the display. As necessary, you must adjust the display of your image to match this convention. You may need to use the Mirror X and/or Mirror Y buttons. For example, if the recommended setup for SBIG cameras is followed, it is likely that the image will be displayed with south at the top. If this is the case, it will be necessary to click the Mirror Y button. Should it also be necessary to click the Mirror X button?
You will need to identify a pattern of stars that you can see in the image and that appears in the overlay. Try looking for a bright triangle or trapezoid. Once the orientation is correct, you will need to shift and rotate the overlay until the stars match up with positions shown on the overlay.
In the Shift Overlay frame you will see four arrowed buttons grouped around a center button marked X1. Click the X1 button and it will change to X10. This is the rate at which the overlay will shift when the arrowed buttons are clicked. Try shifting the overlay until at least one star near the center of the image is alligned with its overlay position. In general the overlay will need to be rotated somewhat to match the star field. Try entering angles in the Parallactic Angle textbox, or use the associated slider. You should ultimately be able to match nearly all the stars on the overlay with stars in the image.
If too many stars are shown in the overlay, you may want to adjust the magnitude limit by using the Mag Limit text window and slider on the upper right of the Astrometry Tool window. If too few stars are visible you may wish to set a fainter magnitude limit. For an acceptable plate constant solution it is desireable to have two dozen (or so) reference stars identified in your image.
Step 4: Select a set of Reference Stars
Select the circled star in the center of the image and then click the Select as Reference button. A green circle will now surround this star and the label R1 should appear. This star will be used as the computational tangent point, or the reference for all subsequent measurements. Always try to select a clean star image that will give a good centroid determination. Avoid stars embedded in nebulosity or stars that exhibit blooming.
Select two more reference stars. For these second two stars try to select objects that are not near the center of the image. Select stars that form an open triangle with the three stars not near to being on a straight line. Again, you want clean star images, and you can only select stars that appear on the overlay.
As soon as three stars have been selected, the Recalculate Focal Length and PA button will be enabled. If you click this button (a minimum of three reference stars are required) the overlay will be realligned to sub-pixel accuracy and the focal length will be computed by using all stars to solve for the pixel scale. Click this button as soon as you have selected three stars. You should see the parallactic angle and the focal length change slightly. It turns out that this a useful way of making an empirical determination of the focal length for your optical system. If the focal length that is computed seems significantly different from what you know to be the correct value, it may be that AIP is using an inappropriate pixel size. If necessary, adjust the pixel size being used by AIP by clicking the Set Pixel Size button in the Display Control window.
Select a few more reference stars. Try to have your reference stars uniformily distributed across the image. In particular, try to make sure you select reference stars that surround any object or objects for which you wish to determine precise coordinates. As soon as you have selected a minimum of four reference stars a formal plate constant solution which includes residuals and error estimates becomes possible. Once this is available the RA and DEC is updated at the bottom of the Astrometry Tool window. This coordinate display will be updated whenever the left mouse button is depressed while the cursor is in the image. Of course, you may also use Recalculate Focal Length and PA at any time.
Step 5: Select the Object to be Measured
Use the cursor to select an object in the image for which you wish to determine precise coordinates. Then click the Select as Target button. The Enter Target Name window will appear. Enter a name for the target and click OK. A yellow circle will appear around the target with a label T1. If you make a mistake, or do not get the object you need, click the Clear Target(s) button and try again. If necessary, you may need to reduce the search radius for the centroid finder by using the Search Radius slider.
You may select as many targets as you wish.
Step 6: Generate a Report
Click the Measure button. The Astrometric Residuals window will appear. This window will show the residuals for each reference object computed using the derived plate constant model. If it is not already visible, open the Data Log window. In this window you will see the calculated RA and DEC for each target object along with their formal statistical uncertainities. A magnitude estimate for each target object is also provided.
To generate a report, click the Report button. The Enter Astrometry Report Data window will appear. The Verbose report contains... everything. The Single-line report is the format required for sending data to the Minor Planet Center (MPC). For either option you will be required to enter an object (or project) name, the time, and the date. You will then be prompted for a location to save the report file. Click Save to save the file. These files are simple text files which can be viewed or edited using any text editor or word processor.
You can also view these files from within AIP. To view these report files or the log files proceed as follows...
File | Open Log (text) File...
This will open a file select dialog. Select the file you wish to view and it will be displayed in a small text window.
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