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Shortening the trusses

Started by tomhole, 12/07/2003 12:09PM
Posted 12/07/2003 12:09PM Opening Post
Based on input from a little birdie, I emailed Mike Harvey to ask him about his experience with shortening his truss poles for binoviewing. I don't think I'll be trying this anytime soon. With his permission, here is what he sent back:

"ISSUES? You want ISSUES? I'll give you ISSUES! smile

Forget it...that way lies madness. I went through this years ago (I've been using binocular viewers since the late 80's).
The first problem you run into is that, once you've cut the poles, you've moved the secondary ~5 inches closer to the primary. The immediate result is that your 1.83" diagonal is smaller than the light cone at that point. The result is a loss of effective aperture. My 18" Obsession, for example, was suddenly performing like a 14" (maybe less).
Let me add that collimation is also a real bitch when you can't see the whole primary.
OK, so in this case, I replaced the 3.1" diagonal with a 4". Seem simple? Nah!
The larger diagonal would not fit under the focuser with the spyder in its original position. I had to get Dave Kriege to make some truss pole extensions that connected to the upper UTA ring and projected OUTWARD 4". This was where the spyder had to be mounted to place the secondary under the focuser.
At this point the scope was impossible to balance and I had to add ~15 lbs. of counterweights to the back of the mirror box.
After all this I was dismayed to find that the larger secondary caused a significant reduction in contrast that was especially noticeable on just the objects I hoped to observe...faint nebulae and galaxies!
I finally gave up and accepted the need for a Barlow, but was ever hopeful that we'd one day have something like the Denkmeier OCS. Thankfully, that day finally arrived.

The only solution I can see to operating optimally with no additional image amplification at all is to build a scope from scratch that would allow more in-travel without introducing a larger diagonal. An off-axis design immediately comes to mind. The object would be to design it in such a way that the secondary is precisely placed to project the focal plane into the bino-viewer.

Hope this helps...