|Solar filter Stabilizing Ring
(elegant simplicity to ensure a no-sag fit)
|Perhaps I'm overly persnickety at times, unable to resist the urge to tamper with... er, "improve" my equipment...
especially when the mix involves astro-toys and long, dreary New England winter nights. One such nit that I had not
yet picked, was the coupling of my rather hefty Coronado SM90/T-Max H-a solar filter to the retractable dewshield
of my scope (an Astro-Physics Traveler).
The dewshield seemed most stable when fully extended, but did sag a tad under the weight of the filter assembly.
Retracting the dewshield worsened the situation, as it was then free to rock back and forth. My usual mounting
procedure entailed firmly seating the Coronado adapter ring upon the retracted dewshield, allowing me to ensure it
was fully and thus squarely positioned. Next, I'd tighten the thumb screws and tug on the adapter while extending
the dewshield, rocking it slightly to get it all the way forward (where the least tipping was encountered). I would
orientate the T-max tuning wheel either at the very top or dead-bottom, so as to use its tilting mechanism to cancel
out any sag.
This worked, but the aforementioned "can't leave well enough alone" syndrome kept nipping at me... a challenge if
you would. But the system worked well enough, and so was not a big issue.... that is before I decided to add
another SM90/T-max to the shebang, doubling the weight and increasing the moment arm. Hmmmm... what to do,
what to do?!?
In a very fortunate turn of events, former ASNH club president and friend Dave Johnson generously offered his
time and expertise to help to come up with a solution (Dave has extensive fabrication experience, and has opened
up access to the Bethany Observatory machine shop to club members).
And so we mulled over potential solutions and possible pitfalls they involved. I wanted the filter assemblies to be
located as close to the objective as possible, mainly to be more easily reached for adjustments while at the
eyepiece, but also since it seemed to offer the best hope for stabilty. Early on I discovered that theT-Max threads
were slightly different than those in the scope's lens cell (rats!), but this proved a boon rather than a bane, as it
prodded us to look elsewhere for a solution. Oh, we briefly considered altering the threads on the T-max, but
among other things it would have then limited its use to this particular scope.
Thoughts then turned to machining a coupling ring which would accept the threads of the T-max on one end, and
the scope's cell threads on the other. Concerns though included the potential of causing the threaded lens
retainer to loosen or tighten (damaging the lens or pinching the optics), metal filings falling upon the lens after
repeated install/remove cycles, placing considerable levered load upon the retainer (again, risking damage or
pinching as well as filter sag), and also the possibility of scratching the lens with the outer edge of the assembly,
as it would be precariously close to the objective (youch!)
So we moved our thoughts to securing a retracted dewshield. Months ago I had considered shimming the base of
the retracted dewshield with foam, but that seemed like such a schlock approach. After noodling around a bit, we
came up with the ideal solution... simple, elegant, no alterations or potential harm to the equipment, and oh yeah...
it works like a charm... we'd fabricate a stabilizing ring at the base of the dewshield! Or more correctly, I should say
Dave fabricated it (thanks a million, Dave!)
To ensure proper fit and to aid installation, the entire focuser end of the OTA was removed, allowing the dewshield
and tube to be accurately measured. The OD of the ring was designed for a snug fit within the felt-lined dewshield,
and the ID was oversized enough to allow a felt lining to be secured to it, protecting the finish of the scope. The
outside front edge of the ring had the radius eased, so as to provide self-centering and easier seating of the
dewshield as it makes contact.
The felt was attached to the ring with spray adhesive, close-trimmed, and inserted on the OTA after the dewshield
had been reinstalled. The below animation shows the completed assembly in the extended position (the way I
formerly used it) and in the fully retracted and now super-secure position. Note the aluminum ring at the left of the
tube... that's the baby!
|Now all I need is sunshine, some time, that second filter, and we'll be rolling!