Re: [APML] Petzval hot spot?

[Chris1011@aol.com]

In a message dated 9/18/2002 11:11:08 AM Central Daylight Time, aa6g@aa6g.org writes:


> In my mind the notion of "correcting for light falloff" mis-identifies
> what is really being done


In effect what you guys are willing to put up with is an effectively smaller aperture over part of the film plane. So why not allow the manufacturer to deliver to you a smaller front objective, which, when mated with a large rear Petzval lens, will give the same (although reduced) illumination over the whole film plane?

It is interesting that in the old days of wide field photography there existed a 3 element design (Taylor triplet) that does indeed produce full illumination and true flat field over a large format. The way it is done is to use the middle element as the field stop. In one 4"F5 design I'm familiar with, the 4" clear aperture lens used a 5" front and 4.8" rear element. 

If you have the time to think about it, the same problem exists in Newtonians. If the secondary mirror is made just the right to catch only the on-axis rays, then only the center of the image is fully illuminated. How many people realize this and never notice the smooth light drop-off to the edge of the field? How to get full illumination? You can either increase the size of the secondary, or increase the size of the primary mirror (doing a little of both will result in uneven illumination again). In neither case will the on-axis aperture be increased, but it will result in an equal off-axis aperture. Increasing the size of the primary mirror is the way Schmidt cameras produce even illumination. In that case, the defining field stop and aperture is the front corrector diameter. 

Roland Christen