APERTURE PHOTOMETRY AND SKY DETERMINATION
Before we can perform the final nonlinear least-squares profile fits, we must first obtain starting guesses for the remaining unknown parameters: the brightness of the star and the local brightness of the sky (we already have decent guesses at the x, y coordinates of the centroid, from FIND). In principle, we could derive a starting guess at the magnitude from the central -value of each star, from FIND. However, we certainly wouldn't want to use the -value that we could have gotten from FIND as the sky value, because this estimate is based on very few pixels, and those few pixels are already known to contain some significant object.
DAOPHOT's PHOTOMETRY routine gets improved estimates of the brightnesses of star and sky by synthetic aperture photometry. In effect, the software draws a simple, roughly circular, aperture around the estimated position of the star, and the total amount of "light" within the aperture is determined by simply adding up the data numbers. At the same time, the software draws a circular annulus around the position of the star, and builds up the histogram of brightness values found in the pixels in that annulus. PHOTOMETRY presently uses an algorithm originally developed for the "mountain software" package at Kitt Peak to derive a robust estimate of the average sky brightness from this histogram: in an iterative procedure, the mean and standard deviation of the histogram are computed, the tails beyond a certain number of standard deviations from the mean are chopped off, the mean and standard deviation are recomputed, and the process continues until the mean and standard deviation stop changing. The local sky brightness is defined as the estimated mode of this truncated histogram. However, because of Poisson errors the peak of histogram will be ratty, so the literal mode of the distribution, as defined by that brightness value which occurs most frequently, will be rather poorly defined. Furthermore, if your image data are integers, then the literal mode can also only be an integer. In general, this may not be quite as accurate an estimate of the sky value as you want. Therefore the modal value which is actually used is estimated from
(see Fig. 4-3) which - I believe - is strictly true for a skewed Gaussian. As I said, I deserve absolutely zero credit for developing this algorithm, but I have found it works well and quickly enough that so far I have felt little urge to try to improve on it. However, it may be getting to be time to try for an improved sky estimator. I suspect that some really clever person could work up a good, robust, precise mode-finder using something like the (a, b) weight-fudging scheme which I outlined in the second half of Lecture 3, and the expected sky which can be computed from first principles on the basis of the read-noise and gain of the detector (as distinguished from that which is estimated empirically from the observed histogram).
Figure 4-3. |
Why do we want to use the mode of the sky-brightness histogram, rather than the mean or the median. Well, think about it. The mode, the peak of the distribution, is that location where most of the histogram's volume is contained in the shortest distance. If you adopt the mode of the histogram as your estimate of the sky brightness per pixel, you suffer the smallest possible probability of making a large error in the sky brightness of any given pixel. You don't really want to know what the sky brightness would be if there were no stars in the frame, you want to know what the typical sky brightness is in a pixel with typical starlight contamination. Then, when this brightness is subtracted from each pixel in the small, circular star aperture, the amount of flux remaining is the most likely estimate for the net flux due to that star alone, exclusive of the typical contamination due to the sky and the other stars in the frame.
Obviously the stellar brightness measurement which is obtained here is still only a crude approximation of the actual stellar brightness. First of all, the light of some other stars may or may not be leaking into the aperture - there will always be some positive or negative difference between the "typical" contamination by starlight in that part of the frame, and the contamination which actually occurs in that particular aperture. To minimize the random errors engendered by this uncertainty, the aperture cannot be made very large. Since the aperture is not very large, some of the light of the star you want to measure will fall outside the aperture; as a result, you wind up underestimating the star's total flux. However, as long as you use the same small aperture for every star, all stars will be underestimated by the same fractional amount, and their relative magnitudes - which is all you need to get the profile-fitting started - will be OK. How you recover the flux which fell outside that small aperture, so that you can place the magnitudes from this CCD frame on an absolute photometric system, will be part of my last lecture.