Louis D

2 minutes ago, F15Rules said:

Your 32mm plossls will likely vignette a little (show a darker edge to the field of view) but should still give a nice view.

Oh, they'll vignette more than a little.  The outer 25% drops off to blackness before the fieldstop in my Arcturus binos.  I still like the views of nebulae in them because it really brightens them up by compressing all the light into a smaller image than with higher powered eyepieces.  That, and the eye relief with glasses is super comfortable.  Enjoy using them.  They're a lot of fun once you get a good setup.

11 hours ago, Stu said:

The speed of light is constant in a vacuum but it does change (slow) in other media such as glass. I think astronomers only normally consider the speed in a vacuum because that is what it is for the vast majority of its journey to us from the stars.

The shorter light path on a prism however has nothing to do with the speed of light, it is simply due to the geometry of the prism vs the mirror i.e. The prism has a physically shorter light path. I'm not totally sure why, but putting my Baader Zeiss T2 prism next to the BBHS T2 mirror, the prism is clearly a smaller package.

Well, EdZ on CN stated the following back in 2012:

The optical path of a prism of same size is shortened (by physics) by the slowing of light traveling thru glass. The convergence of the rays is slowed and we can determine the optical path length by taking the physical length divided by the refractive index. So prisms (of same size) will ALWAYS have a much shorter optical path than mirrors.

So I'm going to go with EdZ because he always seems to have a pretty good grasp of optical physics.

7 hours ago, Knighty2112 said:

Don't quite get how the light path would be shorter than less expensive options here?

Prism diagonals have a shorter light path than mirror ones because the speed of light traveling through glass is slower than through air.  At least, that's what I've heard.  I'm no physicist or optician.

5 hours ago, Knighty2112 said:

Got a Altair 2" dielectric mirror diagonal.  Got 80mm focus on my dual sped focuser, so hoping that should be enough also to bring the binoviewer into focus OK.

I couldn't reach focus with my OPT 2" dielectric diagonal my AT-72ED, so I ended up getting a 1.25" WO CF dielectric.  It works great, no flexing, no induced aberrations, and it weighs less.

Before giving up on the scope's altitude bearings, try locating a sheet of pebbly, gloss formica.  In the past, Wilsonart Ebony Star Formica was the go to choice, but I think it's discontinued, so you'll have to locate something similar.  Ask around on the ATM forums what's best these days.  Cut the sheet into strips and glue them to the curved bearing surfaces attached to the mirror box.  Next, locate some blocks of Teflon (PTFE) and countersink screw them to the rocker box, one toward the front and one toward the back of the bearing surface.  Check that Obsession's bearings, I think you'll find a setup similar to this to guide you.  Nylon can be substituted for Teflon if the motion has too little sticktion for your liking.

4 hours ago, Peter Drew said:

It seems better to use stainless steel ballrace bearings and stainless steel strips on the side bearings and a stainless steel disc running on bearings for the azimuth axis.

I seem to recall seeing a goto truss design similar to this.  The bearings reduce friction to a manageable level for the slewing/tracking motors.  Still, there's no grease outside of the bearing races.

4 hours ago, FLO said:

Mert noticed it should be greased

I've owned several truss dobs, and not one uses grease on the side bearings.  Virgin Teflon blocks riding on Ebony Star Formica (or equivalent) should need no additional lubrication.  Nylon blocks can be substituted for Teflon to add a little more sticktion.

You might try mounting fans to blow across the front of the mirror.  I've heard good things about this approach removing the boundary layer of air just above the mirror.  Kind of counter-intuitive.

If the bearings had been teflon on pebbly Formica, no lubrication should have been needed at all.  In fact, the motion can be too smooth with too little sticktion to hold heavier eyepieces.

Reminds me of the Stabilite mirrors that Zambuto abandoned over a decade ago because it was too expensive to deal with print-through issues during figuring.  Has Sky Watcher found a way to deal with this?  Both were/are radial rib designs.