Re: The last "Final Iteration" of 2.5" F-4 finder scope
Dale asked: "How did you determine the diagonal was not quite large enough? Was it by viewing or some other method?"
The measurements taken from the design were plugged into the software and the geometry values "adjusted" until I was able to remove all the red flags. If I barely squeeked under the threshold with the 1" diagonal mirror, the Amici prism, which is smaller would certainly have been worse.
The conical section on such a small, fast mirror has a very short aspect ratio. It appeared likely that if I barely have enough secondary mirror without a reversing roof prism diagonal, that I would be in trouble when I added it.
I speculated as much early on in a post with Tony a week or more ago. He acknowleded the potential problem.
Sure enough, when I tested the scope without the diagonal, on Jupiter, I ran out of in-focus.
I had to incrimentally shorten the tube, thus bringing the primary closer to the secondary. Light is already spilling off the sides, without the reversing diagonal.
The roof prism diagonal arrived and when installed I once again found I have a shortage of in-focus by the 2 inches it adds.
If the section of the light cone is already too big for the existing secondary, moving the primary closer is only going to make it worse. It also threatens to vignet inside the 1.25" focuser.
I will not go with a larger secondary. It would block off more of the primary. I'll simply shorten the tube length and live with the consequences. It is, after all, just a finder scope and I'll still have 75% of the rays.
That's good enough
--- In firstname.lastname@example.org, "atmpob" <atmpob@...> wrote:
> How did you determine the diagonal was not quite large enough? Was it by viewing or some other method?
> Dale Eason
> --- In email@example.com, "art.bianconi" artbianconi@ wrote:
> > In the process of completing the scope as a generic Newtonian I found
> > that the 1" secondary mirror is not quite large enough.