Exploring the PSFPosted by Chris Flynn Dec 29, 2011 12:54PM
As a follow up to the knife edge imaging of the moon with the Canon 35
mm camera, I took images of a laserpointer shining into the lens.
Laserpointer was set on with tape and shone into the camera lens from a
distance of about 5 meters. Camera was on a good tripod. The laser
pointer was aligned to be pointing into the lens by watching to see it
emerge from the back of the camera out the viewfinder and onto a wall
behind the camera. Eyes were kept well away from everything!
Exposure times of a few hundredths to a few thousandths of a second gave good halos.
Laser was pretty well centered. There is a weak secondary image off to lower right. The main image is saturated in the core out to about 10 pixels radius.
The laser has a strong core surrounded by a quite uniform intensity "platform", after which the light falls away like a power law.
The radial profile of the laser (blue) compared to the moon (green) is shown below.
The laser pointer's profile falls off faster than the moon -- but not very much faster.
If we assume that the laser is a good point source, then there is scattering in the lens/CCD combination in the camera which is seen as a power law at large R.
Slope of the lunar halo is about -2.0, whereas the slope of the laser pointer halo is about -2.6. Getting close to the diffraction limit of -3!
The lunar profile is then interpreted as a combination of both scattering in the camera and scattering in the atmosphere. Its slope is shallower -- so atmospheric scattering totally dominates at large R (scale both powerlaw falloffs to the same flux at log(R)=2 or 100 pixels to see this -- atmospheric scatter would then dominate internal lens scatter by about an order of magnitude out at R=1000). Not sure what's happening close in, as the laser pointer has a strong uniform intensity "platform" of light around the core -- easily seen by eye just by pointing it at a piece of paper. A point like source of light, like a street lamp seen at a large distance might be a way around this problem.
I also imaged the laser pointer projected onto a white wall -- long exposure times of many seconds -- and got the same result -- similar core and platform and a halo with the same slope.
All very interesting !
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