Re: ATM Lenses

["Asaf S.-T." <asafs@ibm.net>]

>Hi
> I don't know of any particular web site.
>The main thing to remember it the equations
>for focusing images:
>
>1/f = 1/a + 1/b
>
>for positive lenses
>and:
>
>1/f = -1/a + 1/b
>
>for negative lenses.
>
>It seems that you have at least several relay achomats from
>what you and others have discribed. With these you can make
>a collimated beam using one of these and a point source
>light. You could use a mini mag-lite bulb since these are
>quite bright and almost point source. Adjust the lense
>such that the exiting beam is as parallel as you can get
>it ( light cast on white paper doesn't get larger or smaller
>as you move away from lens ). The distance to the light is
>the focal length of this lens, we'll call this the collimating
>lens. Not to be confused with collimating your telescope. 
>Place any positive lens into this beam and the
>length to a focused spot is the focal length for
>any of your positive lenses. For the negative lenses, you'll
>need a converging beam. This will be a little trickier
>because you need a distance between the beams focus and
>the collimators lens that is at least as great as the
>focal distance of the negative lens that your measuring.
>Since you don't know this, you may have to readjust some.
>Start by readjusting your collimating lens to provide a
>spot about 12 inches from the lens. Put the negative lens
>into the beam and adjust such that you get the original
>parallel beam projected as you had with the original collimating
>beam. Measure the distance between the lens and the point
>that the beam was focused without the negative lens and that
>is the focus distance for that lens.
>Anyhow, that should get you started.

Thanks for another great lesson, Dwight.

Thanks, Asaf.

----------------------------------------
Asaf Shtull-Trauring, Amateur Astronomer

http://www.maxnm.com/asaf/