Photometrica - Instant photometry of remote telescope CCD images

Step 4: Identifying Stars

After checking the image in general, the next step is typically to identify your target and comparison stars. If you are not familiar with the field you have imaged you can

1) Manually compare the image with a map
2) Import data from file
3) Import data from the AAVSO comparison star database

To demonstrate we will use both method 1) and 3), starting with 1). Click the target name link in the File List page to open the image:

Figure 1: Open the image

As the mouse moves across the image the R.A/Dec. is displayed next to the mouse pointer. This makes it easier to locate stars.
By comparing with a star map we locate V Cnc, and when clicked the FITS file is processed on the server. Starting at the coordinates of the click, Photometrica searches a circular region with radius as specified by the aperture radius in the toolbar above the image. It snaps to the brightest star in that region. TIP: If you are working in a crowded field try narrowing the search area by using a smaller aperture.

When a centroid is found a dialog like this pops up:

Figure 2: The information window

This information window lets you change the name of the star, what type it is (Target, Comp or Check), the gap between the aperture and the sky annulus, and the sky annulus width. It also displays essential information such as SNR and FWHM. Note that the sky annulus settings can vary from star to star, but the aperture radius applies to all measurements, and can thus only be changed for all stars simultaneously. This is done from the toolbar above the image.

The star name is displayed in the reports so we change the default name from 'Star 1' to 'V Cnc'. By default the star is a target star. Click the update link in the lower left corner to save the changes and close the dialog window.

You can open the window again by moving the mouse over the aperture (innermost circle). It closes without saving when the mouse moves outside the dialog box again.

Now we need to add at least one comp star. After identifying a suitable comp star from the chart, we click on it in the image and the dialog for this measurement pops up:

Figure 3: Adding a comparison star

In addition to changing the name we check the 'Comp' radio button and enter the V magnitude. This procedure will have to be repeated for every comp star.

One might also want to use a check star. In that case change the type to 'Check' and click 'Update'. If you enter a magnitude for the check star it will be displayed in the report which makes it easy to compare known and estimated magnitude.

Next: Importing Comp Stars