Re: ATM Eight inch F8 design

[Brad Davy <bdavy@bigdog.fred.net>]

At 02:04 PM 6/18/97 -0700, Mel Bartels wrote:
>Ah ha, Mike, I think we are getting hung up on terms!

You can say that again!

>Use diagonal.exe to
>find the diagonal size that gives you at least 70% illumination at the
>radius of the field of view you will be working at.
This is where I'm having problems.  How do I determine the field of view I
will be working at? Jay Anderson and I have been discussing this off-line,
and I sent the following:

Everyone talks about the 100% and 75% illumination, which is to my way of
thinking, additional light that is actually slightly off axis.  Obviously,
the axis can only point at one object at a time, so any object that is
slightly off the axis will not have rays parallel to the axis, and those
rays will not be reflected to the focal point.  Is that what this is
referring to - i.e. an object that is x degrees off axis will have 75% of
the light reflected by the secondary mirror, and its image will be y inches
(radially) away from the eyepiece center, with the "illuminated field"
being 2y in diameter?

Now if this is true, then the "illuminated field" should be equal to the
diameter of the first lense in the eyepiece in order to make full use of
the eyepiece.  Obviously, for very wide eye-pieces (40mm in my case), it
might be necessary to sacrifice some of the "illuminated field" to avoid
diffraction from the secondary.  If this "illuminated field" can be
obtained with a smaller secondary, all the better.

If you're still with me, my conclusion is that I should physically measure
the radius of the first lense of my lowest power eyepiece (since the focal
plane is very close to that lense) and then size my secondary to achieve at
least 70-75% illumination at the outer radius of that lense.
Brad
bdavy@fred.net               http://www.fred.net/bdavy/home.html
bdavy@nox.cs.du.edu