Re: [APML] Tak FSQ-106

[Chris1011@aol.com]

In a message dated 5/2/2002 7:41:50 PM Central Daylight Time, tonyhallas@foothill.net writes:


> Roland dragged me into this.... the scope definitely has a hot spot in the center... but with all the techniques that we have now it should be possible to effectively neutralize it. The AP 4" f/6 does not have this much of a hot spot... Both scopes are excellent scopes... Roland can comment on the individual designs...


I'm sorry to have dragged you. I know you hate to get into controversial debates. 

In a telescope where the main lenses are up front, the only thing that governs field size is the opening near the film plane, i.e. the focuser and field flattener elements. The extreme example of this is an 8x10 view camera where the lens can be pinhole size, and guess what??? the coverage is 8x10 (yes, there is theoretically a small light falloff that is governed by cos of the angle between lens and edge of field). In this case, the fully illuminated field is fixed by the largest opening near the film plane. 

In the case of a Petzval, the incoming light beam from the relatively long focus front set is bent inward by the second set to form the final fast focal ratio. The light from this second set is convergent and will have the largest effective aperture right in the middle of the film plane. The effective aperture will then taper off slowly to zero at the edge of the secondary lens diameter. 

Petzval lenses are well understood by lens designers - no magic about how the light goes from here to there. Similar misunderstanding exists about Cassegrain scopes. If you look at typical light paths that are drawn of SCTs for example, you always see only the very central on-axis rays. Never do you see them draw the off-axis bundles because you would then immediately see that the baffle tube cuts off the outer rays - in some cases fully half the light is missing. 

Roland Christen