On Mon, Jan 23, 2012 at 3:51 PM, Mike S <[email protected]> wrote: > On 1/23/2012 3:02 PM, Jim Lux wrote: >> >> How well could you do with something like the camera in the iPhone4 >> facing up. The front camera is VGA resolution > > > A lower bound can be estimated. > > A cell phone (iPhone 4 rear camera) camera sensor has a resolution of what? > ~2600 pixels wide with a 45 degree field of view - that's ~ 60 arc seconds > per pixel, which is about 4 seconds of time. The Dawes limit is about 1 > second (17 arc-seconds) for a perfect .25" lens. Obviously worse with a VGA > resolution camera.
The goal is not to create an image. A blur is actually better and I've read of people intentionally using de-focus. What you do in compute a best fit of the system point spead function (PSF). Or with many blobs in the field you do a convolution of the image with the system PSF. The end product is not an image but a table of X,Y coordinates of each detected star. You don't need to detect every star. Then you search a star catalog and find thebest fit transformation matric that takes you from X,Y to the catalog. The matric is your real "product". Typically you should expect about 1/10 of a pixel resolution at the end. And then you take hundreds of images every night and average them and you continue maybe for years. If you were designing a camera for this purpose you make it so that a typical star would cover maybe five pixels across so that the 5 by 5 pixel subimage would look like a Gaussian function. The centroid of the function is your X,Y for the star. So you see that even with 5 pixel blurs you can likely find X,Y to much better than one pixel width. This helps with noise too, noise would be a poor fit to a 2D Gaussian function. (and also there would be no catalog star for a noise hit) Chris Albertson Redondo Beach, California _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
