Vixen's Porta mount replaces a Tele Vue Tele-Pod for grab-and-go
observing with the TV-85.
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First
Impressions:
The mount head unit appears to be very well manufactured. Aluminum tripod
and all metal fittings are solid, however the center spreader arms (right)
are plastic and don't instill much confidence that they will stand up
to rigorous use.
The fit and finish is above average, white paint flaking on some of the painted tripod parts being the worst of the cosmetic blemishes.
Five pages of printed instructions are provided and sufficiently explain the basic operation of the mount, the attachment of the dovetail adapter to a telescope, and a description of the few adjustments that can be made.
Setup and TV-85 Installation:
The Porta is delivered with the alt/az mount head attached to it's tripod;
there is no indication in the printed instructions or anywhere else
that the head is intended to be removed from the tripod as a matter
of routine.
The fine adjustment knobs simply snap onto the altitude and azimuth adjustment shafts and fit snugly.
Attaching the AVT-1011 adapter to a Tele Vue clamshell-type ring system is a simple matter of removing the mounting studs from the clamshell and attaching the dovetail adapter in their place. There are additional holes the bottom of the plate to allow attachment of the scope to a Tele Pod cradle.
As the clamshell must now be rotated 90° to move the mounting point to the starboard side of the OTA, repositioning of my Starbeam pointer from left side to right was required, as was the reversal of the clamshell to allow unobstructed access to the thumb screw (now located at the bottom of the OTA when mounted on the Porta).
Mounting of the scope to the mount head is a snap - just position the scope so the dovetail bar is flush with the block, tighten the main and safety bolts, and adjust the clamshell position fore or aft to balance the scope.
The
altitude and azimuth tension is adjustable with a hex set screw on each
axis, but there seemed to be no difference in tension regardless of whether
the screws are fully loosened or tightened, or anywhere in between; perhaps
the stiffness will work itself out over time.
A black rubber cover over the azimuth axis can be removed to access two hex wrenches (left) which are used for tension adjustments and removal of the dovetail block from the mount. The wrenches are held in place by two magnets - a nice touch.
The tripod legs can be adjusted to place height of the altitude axis between 34" (with legs fully retracted) and 54.5" (with legs fully extended). For storage and travel the entire package can be compacted to a little under 39" in overall length.
Operation:
The biggest flaw in the Tele Pod design was the mount handle, which was
a great aid to accurate positioning but also limited the maximum elevation
well short of the zenith - the arm hits against the stationary (in
alt) fork arm assembly.
With my TV-85, the Porta will reach the zenith easily as long as the scope is positioned between the tripod legs, otherwise the scope's diagonal will touch the tripod leg before the scope reaches 90° (right).
There is a tendency on my part to grab the alt/az adjustment handles to point the scope (no doubt attributable to mental conditioning from using the Tele Pod for a year with the pointing handle). The Porta's operation guide warns that doing so risks damaging the adjustment shaft mechanisms. Coarse pointing must be done with hands on the optical tube or fork arm only.
Future Plans:
In order to attach the Tele Vue dovetail adapter plate it was necessary to remove
the two threaded studs from the clamshell mount. This
now requires me to rethink how I must attach the TV-85 to the custom rail
on my Celestron 8". As designed, the rail is drilled to accept the two clamshell
mounting studs at various positions, with just enough clearance at the underside
of the rail to thread a couple of hand nuts to secure the TV-85 to the rail.
(The full skinny on the rail can be found here.)
Having
the AVT-1011 adapter installed obviously throws
a wrench in the elegant design of the rail. The solution I have
in mind is to mount the same dovetail block that's on the Porta to the
C8 rail. I positioned the block loosely on the rail (right)
just to eyeball whether it would work with the existing holes; it won't
- some additional holes will definitely be needed on the rail. (Two of
the holes actually do line up, but one is the threaded hole for the camera
mount and the other is non-threaded for the TV clamshell stud.)
A couple of additional holes on the rail will be necessary to mount the dovetail block when removed from the Porta fork arm. Depending on the cost and availability, I may purchase a spare dovetail block to keep permanently attached to the Celestron.
I also intend to explore the possibility of mating the Porta head to a nice wood tripod such as the Tele Vue Gibraltar. This would probably require another custom piece of hardware. The aluminum tripod can be kept as a travel unit.
Additional Photos:
