Howdy! If you have much experience with Newtonian telescopes there is no doubt you know the difference between high and low profile focusers. High profile focusers are good for refracting telescopes. Low profile focusers are typically used on Newtonian telescopes. The reason for this is because the lower the eyepiece, the smaller the secondary mirror. The smaller mirror obstructs less light, causes less diffraction, and is easier to mount sturdily. But how do you maximize this principle?
Did you ever hear of a negative profile focuser? Now you have!
Sometime before WWII my uncle Jerry Raths built a Richest Field Telescope. In those days there was some debate over what telescope type would show you the most stars in one field of view. Some said a small telescope with a very wide true field of view. You point it at the Milky Way, and you can't count the stars. Some prefered the large telescope with a relatively narrow field of view. You point it at M-13, and you can't count the stars. Now, in either case, you need an eyepiece with a very wide apparent field. And you need a secondary mirror which provides significant illumination at the edge of the field.
At the time uncle Jerry was a very well-known amateur telescope maker in the Phoenix area. He spent 70-some years teaching people to grind their own mirrors, first at the Detroit Astronomical Society. He was known as a maker of precision, high-resolution mirrors. His best student went on to become Star-Liner Instruments of Tucson. My uncle Jerry Raths is the very person who taught John Dobson how and why to parabolize a mirror. This is after Jerry's 6" stole the whole show away from Dobson's 22". Jerry also built the World's Largest Camera Obscura with an 8" lens he ground. You went inside it. That wasn't good enough for him, so he ground and made a 12" camera obscura. I have the 12" lens and two of the three flats he made for it.
Uncle Jerry made the small version RFT, the kind you hold in your hands as you sit in a chair. He used a Sonotube. He made his own mirror mounts. He made an F/5 4.75" glass mirror, and a secondary out of obsidian. They didn't have wide-field Naglers and such back then. Jerry used an Erfle with the tube and field stop removed. This eyepiece was too wide to fit into a 2" focuser. The entrance aperture itself was nearly 2". He achieved an apparent field somewhere around 150+ degrees. Pretty much unheard-of back then.
In 1999 Jerry gave the scope to me. I saw his original three-vane spider had been replaced with a 360-degree circular spider. He had already minimized the diffraction spikes, but then he removed them. But that was wobbly. I used his materials and made them into a 180-degree curved-vane spider. Now it is much stiffer and holds collimation better. I also made removable altitude bearings, and an alt-az mount on a pier. The whole thing disassembles and fits into a box I made, which doubles as the pier.
So if we diagram the 4.75" F/5 scope, we see the secondary mirror is very large when the eyepiece is in a low-profile focuser. To get the edge illumination we need a 3.5" mirror. But that obstructs almost all of the light going in. What to do? Jerry removed the tube from the eyepiece, and mounted it flush with the tube. This brought it so low he got by with a 1.7" secondary.
Well, his Jaeger eyepiece lasted a long time. But the decades wore on, and eventually the lenses in the eyepiece began to separate. It wasn't too bad at first, but then it became like looking through a kaliedoscope. Not much fun, and so the scope got little use. I decided to give the eyepiece away and use a modern eyepiece. But the focal plane is inside the telescope!
I had three choices. At first I disassembled a 2" 31mm Erfle eyepiece and mounted it like he did. But his had a built-in helical focuser, and I had nothing like that. A standard low-profile focuser would require me to move the primary mirror up into the tube farther so I can reach focus. But then the secondary would become too small for the intended purpose. The 100%-illuminated part of the field becomes very small.
So I made a negative-profile focuser. It actually lowers the eyepiece into the tube, till it reaches the current focal plane. The travel is 1.7". If I need to, I can lower an eyepiece into the light path. Using modern 2" eyepieces means the clarity is improved. And instead of being stuck with a 32mm focal length, I can now use higher magnification eyepieces.
My design is really just a variation of the currently-fashionable Crayford focuser. I used whatever materials I scrounged up. I also figured out how to do it with simple tools. None of the precision and expense normally required by a Crayford. I suppose you could call it a vertically-mounted sled focuser, if you wish. Unlike a low-profile Crayford, my design has a very large focusing knob. It is easy to use in the dark with gloves on. And it provides a finely-adjustable focus motion without slop, no need for a 10x reduction gear.
One day I got a new eyepiece. It has the stoopid indentation in the focuser part. I really hate that dent. It is supposed to capture the setscrew, so the eyepiece does not fall out if the screw comes loose. But really, how often did that ever happen? Some moron thought it would be a good idea to make his product this way, and all the others followed suit. All he did is make us mad when we loosen the screw, and the eyepiece does not come out till we fight with it some more.
But what is really the worst, they always put this long indentation, and the edge of it is under the screw. With this eyepiece, every time I would tighten the setscrew it hit the edge of the indentation, the eyepiece would pop up, and tilt over. When I used the eyepiece again and again, finally the damned thing broke! The 1.25-inch barrel part stayed in the focuser, and the expensive part fell off! A damaged eyepiece is exactly what the original moron said he was preventing. Fortunately for me, it fell in my hand, instead of the pavement. But now I have a stripped out thing I am afraid to use.
That is why I found a way to address the issue. Sure, my focuser is not ideal, but it does away with the problem. The ideal thing is to build eyepieces without the obnoxious "feature", like they used to do. This eyepiece has a lens inside that 1.25-inch barrel, so I can't just replace it with an old non-indented one. My focuser gets a better grip on the eyepiece. It provides a more even pressure around the barrel, instead of pinching two opposite points. The eyepiece is straighter in the focuser.
And so now the telescope is fun again, and gets used.