> Another Crayford question. I've started designing a focuser and have
> looked at the drawings of the JMI NGF 2". One goal of the focuser design
> is to minimize its height so that a small diagonal may be used. It seems
> as though JMI achieves this by allowing the focusing tube to extend about
> 1.4" into the tube when it is racked in.
>
> Why is this acceptable? If I have used Mel Bartels' newt program
It is hard to make a focuser that is low, sturdy, and has a lot of travel. If you give up an inch of focuser travel, or add an inch of height, you can reduce the intrusion into the main tube.
Frequently the inside diameter of the Newtonian's tube is made 2" or more bigger than what might be considered "necessary" if one only considered light paths. This additional space allows tube air currents to flow near the edges of the tube without wiggling the image. I feel that this is usually a good idea for a high performance scope, although there is a tradeoff with bringing the eyepiece closer so you can use a smaller diagonal mirror. An unintended side effect of this is it allows the focuser to poke into the tube some without blocking any light, although the focuser intruding into the tube probably messes up the tube currents.
Personally, I don't like the focuser protruding into the main tube for most designs.
>
> Second question: If the light cone at the bottom of the focuser with it
> fully racked in is less than 1.25" does this imply there is no value in
> using a 2" eyepiece? Thanks for the help.
>
I'm not sure what you mean by "light cone" here. You might mean the light cone of just the on-axis rays or the light code of 100% illumination given a particular size diagonal mirror and aperture stops. But neither of these is what is usually thought of as the "light cone" at the end of the focuser. How did you calculate the size of the light cone?
In the usual way I think of the light cone (in this case defined as all the light contributing to an image) the size varies depending on the field of view (and thus depends on the size of the field stop of the eyepiece). An eyepiece with a 1.25" field stop will always have a light cone bigger than 1.25" at the end of the focuser tube. The light cone is really a truncted cone, with the minumum width at the focal plane.
A two inch eyepiece with a field stop larger than 1.25" will always give a wider field of view than any 1.25" eyepiece in the same telescope. This wider field of view may not be fully illuminated, but I would say that it is probably still of considerable value, except in really extreme cases.
Chuck