iansmith
Newbie
I have a hobby setup here that is giving me troubles. Long winded stuff follows...
I have an f4.5 18" parabolic mirror with an 80" focal length set up and a point light source at twice the focal length and offset to the side just a little bit.
If you put a white card to the side of the light, a perfect image is formed, as all the light is converging at that spot. Move the card back and the image grows. If you put your eye right up to the convergance point, the effect is that the tiny pinpoint of light completely fills the mirror.
A few inches after that spot I would like to make all the reflected light paralell so a camera can photogtaph it. As is, it spreads out so wide so fast that most camera lenses only can get the center beams. Anything on the edge is clipped, usually by the cameras iris apature, even when opened fully.
I am assuming that with the light bent into paralell rays, it will all get into the camera ok. No idea if I can focus on it, but anyway, here is the question...
What variables do I need to deal with? Do I need a lens that matches the mirrors f number of 4.5? Do I need a parabolic lens, if they are even made? Do I need to get a specific size/shape based on how far the lens is from the convergance point, or will one work at any distance?
I wish I had a degree in optics, but I don't.. so.. help?
Thanks!
I have an f4.5 18" parabolic mirror with an 80" focal length set up and a point light source at twice the focal length and offset to the side just a little bit.
If you put a white card to the side of the light, a perfect image is formed, as all the light is converging at that spot. Move the card back and the image grows. If you put your eye right up to the convergance point, the effect is that the tiny pinpoint of light completely fills the mirror.
A few inches after that spot I would like to make all the reflected light paralell so a camera can photogtaph it. As is, it spreads out so wide so fast that most camera lenses only can get the center beams. Anything on the edge is clipped, usually by the cameras iris apature, even when opened fully.
I am assuming that with the light bent into paralell rays, it will all get into the camera ok. No idea if I can focus on it, but anyway, here is the question...
What variables do I need to deal with? Do I need a lens that matches the mirrors f number of 4.5? Do I need a parabolic lens, if they are even made? Do I need to get a specific size/shape based on how far the lens is from the convergance point, or will one work at any distance?
I wish I had a degree in optics, but I don't.. so.. help?
Thanks!
Disaster_Area
Gadget Monger
what about just positioning a ground glass focusing screen where you want the image to be photographed... If I get what you're doing, the same process is basically going on inside a LF camera and hitting the ground glass at the back.
antiquark
Derek Ross
If you remove any lens from your camera and put the mirror 80" away from the film plane, then distant things will be in focus. Then you can move the mirror back and forth slightly to focus it. If you have an SLR, you should also be able to see an image through the viewfinder.
However you'll have lots of trouble aiming it, 80" is a pretty long lens.
However you'll have lots of trouble aiming it, 80" is a pretty long lens.
Disaster_Area
Gadget Monger
just replace the white card with the focusing screen and take a picture of that.
Al Kaplan
Veteran
It might be easier to help you if we knew what you are trying to do with the set-up.