Posted 14 August 2012 - 06:43 PM
At WSP, quite a few years ago, Mike Zammit of Star Structure along with Steve Kennedy, a fine fast mirror maker in California and user of our Denk II Binos with both 1.4X and 1.2X OCS* had let me know that the super fast scopes they were using had been shown through ray tracing to have some noticable fall off of illumination with the lower power 1.2X optical lens. Not real bad, but observable.
Basically, to transfer the image plane through the light path of the Denks (this would hold true with any binoviewer), the optical lens must be located rather deeply into the focal plane of the scope, often entering the upper cage a bit. Here's where it gets a bit tricky when deciding whether to use the 1.2X or the 1.4X lenses. The 1.2X requires .75" deeper location than the 1.4X. however, most of you who set out to have a very large and fast dob not only want a short "stepstool-ready" scope, but you want a wide true field of view (TFOV) too. Mike Z. did some on site ray tracing at WSP and determined that going with the 1.4X would be a better option due to the very steep cone in what was an F3.75 mirror, if I remember correctly. remember that it needs to go into the focal plane .75" LESS.
The clear aperture of both the 1.2X and 1.4X OCS lenses were and still are 37mm. The new optic as mentioned is 45mm clear aperture and has the same lower power 1.2X factor.
I was asked if I could make a low power 1.2X with as much clear aperture as possible to negate this clipping. I felt that in a sense, it was not a good option to make 100 optics, especially if the typical blanks that we were already using were 40mm and new tooling, New Cells, retainers, raw material etc. would be required in large numbers. For how many fast dobs? 10? Yikes!
So I sort of declined and had been telling customers with fast dobs that opting for the 37mm 1.2X or 1.4X was a decision they would have to make, weighing the tradeoffs between low power and some falloff/clipping, or less of it with the higher power 1.4X lens.
Well, things being what they are, I am a huge dob fan. I use my own 20" F/f5 with a John Hall mirror (Pegasus) for many years now. But I felt it was finally time to go ahead with this optic since more and more super fast mirrors are showing up and Steve Kennedy felt this would be a very good thing.
My opinion is that ANY dob F/5 or faster should choose this upgrade. F/5 is fast in the telescope world. Typically, large Dobs are expensive and well worth it! So, why not have the largest OCS optic possible in a 2" focuser doing the work? I need a good night to really cross test the 37mm vs the 45mm in my own F/5 but I can say I'd use the 45mm in any case to be sure there is an easily observable gain. I used the 1.2X 37mm in my scope and always had. Now of course, I will use the 45mm! It's here if people want it. Its an honest effort to make an OCS with largest possible clear aperture for a 2" focuser available for those who want one. If I'm trying to grab some photons, I want as many as possible.
*OCS=Optical Corrector System, a term that has been sort of hijacked and used by some others with slight changes. Note that the values of 1.2X and 1.4X are derived via the introduction of the low power optic in the Denk Bino's power switch, used in combination with the large OCS optic. Used alone, these OCS cells would render appx. 2.3X (lower power)vs 2.5X (higher power)in the Denk Binoviewers.