Boris: adventures at 12 feet altitude
CONTENT FOR NOVAC Members:
Below are the two images of the rocker box and mirror box from "Boris" that I am giving away.
The Saga of "Boris" the long focal length 'Big Dob'
This is a summary discussion of my extensive experiences "liberating" a 20.4" f/6.7 mirror from a sub-optimal structure back to the original optical performance that won it Second Place in the Optics competition at Stellafane in 1991. This was one of my most frustrating ATM experiences, and a challenge that I persevered at until I'd solved it. The scope needed a 10' ladder to reach the eyepiece at zenith (the focal length is 130"+), and was just too large for my yard, and too much of a hassle to transport to dark skies. The optics and hardware now reside with a new owner with dark skies and asperations to build the structure this fine and storied mirror deserves.
The mirror was ground by Tom Massey, a longtime member of
the club in N.Y. that helps put on NEAF. Massey spent ~2 years polishing
and figuring the mirror in 1990-91. He ultimately made three Dobs of > 20",
and sold all three and a travel trailer as a package deal when he retired in
~2002 and moved to Florida. The scope was rebuilt from what had by all
reports been a nicely performing if large and unwieldy scope and into a
modern looking (K&B style) underperformer. It wouldn't render tight star
images at much over 100X when I got it; when I sold its optics, I had it
performing nicely at ~600X (which was all the seeing conditions would
tolerate when I tested it).
When I bought the scope I suspect it was a "problem child" that the previous
owner didn't know how to fix. Among the things I've tweaked since owning
the scope, I:
 |
remounted the primary mirror cell (it was canted in the
base, so moving any one |
|
shortened the primary |
|
made the collimation tool-free for both primary and secondary (each had required an allen wrench), |
|
made an ad hoc series of bracing to stiffen the undersized
truss tubes (my mod is more like a classic Serrier truss, I think). |
I purchased a "Tallman" 10 foot aluminum orchard ladder(http://www.tallmanladders.com/astronomer.html) factory modified for astronomy by making the steps closer together. This three-legged ladder is infinitely more stable than a convention four-legged ladder. I used to demonstrate this by sitting on the top step and leaning vigorously from side to side. The only downside of the Tallman is its narrow (2 3/4") rungs. These steps are so narrow that my feet would begin to ache after a couple of hours of observing, and I'd find myself tired enough to want to stop observing. (This coming from someone who can typically observe 8-10 hours after a full work day!) My solution was to buy a rubber honeycomb patterned "anti-fatigue mat" from Costco and cut strips off that I duct taped to each step. That imparted enough "give" to the narrow steps that they no longer caused my feet to ache. Of course, they did double the weight of the ladder... <g>
Me with the scope as purchased
Boris pointed straight up
With the 10 foot "Tallman" tripod ladder and camping cell foam baffles in place
The braces used to stiffen the overly thin truss tubes and reduce their
"whippiness". Each pair of truss tubes has a spreader put
in ~40% of the way from the bottom, and a compression strap binds all four
braced pair into a single unit. Unsightly, but it worked well! I
later added bungie cord ties to ensure that a brace could not come lose
and fall onto the primary.
Boris's original mirror cell with the portion I cut off to allow for side mounted fans to blow onto mirror face
Boris after I fixed skewed orientation of mirror cell and made wooden knobs to allow tool-free collimation adjustment