Difference Image Photometry - problems in wide field cameras


Wide field cameras frequently come with their own inherent difficulties with this technique since it is not always possible to optically correct the image to be in perfect focus and have a constant PSF across the camera. Here's an example of a wide field image taken with a 2048x2048 pixel CCD camera (again of M31). The actual PSF matching between images is accomplished with a Fourier technique described in Phillips and Davis (1995) The left side panels shows a 128x128 pixel subimage close to the center of the original frame (upper left panel) with the mean galaxy background subtracted from the image, and its difference image (lower left panel). A suitable star close to this region has been used to empirically determine the PSF matching convolution kernel to match the image pair being differenced. All structure in the unsubtracted data has been effectively removed; the residuals around the brightest star are an unavoidable artifact due to it being saturated on the original CCD image.

However, applying the same convolution kernel to a region located 500 pixels away (upper right panel) shows large systematic residuals in its difference frame on the scale of the PSF (lower right panel). This indicates a poor match of the PSF in this region and shows that there is no unique solution to the matching convolution kernel applicable to the entire frame. Effective subtraction of the full image can only be done by modeling the spatially varying PSF kernel using the limited number of appropriate PSF matching stars on the frame.

Once the images are PSF-matched with a spatially varying convolution kernel the quality of the subtraction for the entire frame becomes comparable to the lower left panel. The inset image in the lower right corner is the new difference image located in the region of the box in the upper right panel. A clear detection of a point source is now made, which was almost completely hidden in the systematic residuals in the first attempt at matching the PSF. All differences are displayed in the same intensity range; the intensity range of the upper panels is five times larger.