Arguably the most significant development in amateur astronomy in the last
50 years has been the Dobsonian telescope. Go to any star party, and perhaps
half of the scopes on the field will be Dobs. Never before have so many people
been able to see so much.
Unfortunately, to see deeper than ever before the Dob owner has sacrificed
equatorial tracking. In this respect, the APO crowd has it right. The nudge-wait-look
cycle of Dob operation is not conducive to observation of fine detail at high
magnification such as double-star or planetary work. The “look” part
of the cycle is so small that it rarely occurs during that moment of perfect
seeing. In deep sky observing one can also benefit by having a long uninterrupted
gaze to realize faint detail in galaxies, nebula, and clusters.
These thoughts were weighing on my mind as I began the planning for a 10”
f/5 Dob. It appeared that the lightest suitable German equatorial (GEM) would
be a Losmandy G11. This is a fine mount, but is best described as “transportable”,
not “portable”. The cost was approximately $1,900. At that time cheap
Chinese GEMs had not yet available. While one can obtain an appropriate Chinese
mount for about half the cost of a G11, the weight and bulk issues remain. An
additional consideration would be the need to construct a solid-tube scope for
use with GEM, which created issues with fitting the scope in my Honda Accord.
After some research I decided to order a single-axis Tom Osypowski platform
for about $900. (Dual axis models are available for higher costs for those interested
in imaging.) This mount promised to offer equatorial tracking in a package that
weighed less than 20 pounds and was about the same size my eyepiece case.
By nature equatorial platforms are latitude specific. That is, at the time
of order you specify the latitude at which you plan to use it. By shimming the
mount it can be made to work at other latitudes, but there is a limit to this.
In practice, actual latitude can vary plus or minus five degrees from what the
platform is built for.
The platform was delivered on-time given the 5 month estimate at time of order.
As you can see from the accompanying photos, it is constructed from premium
plywood (which appears to be ApplePly) and finished in a honey-maple stain.
Construction is rugged, finish and workmanship are excellent. The drive is powered
by a 9 volt battery. My observing sessions tend to be short and scattered around
my schedule, so I have not tracked battery usage but it appears to be low.
There are three main parts to the platform - the roller drive, the bottom board,
and the top board. The drive has three controls: On/Off, a hemisphere switch,
and a knob for speed adjustment. I have never had to adjust the speed on my
platform. Note that the top and bottom boards are not physically connected.
For moving or transporting the unit there is a bolt to hold the pieces together.
You can also see from the photos that the platform as handles. The larger one
is for carrying purposes, the smaller one for resetting the platform (described
below).
To use the platform, you place it on the ground with the motor side facing
north and remove the transport bolt. Check the bubble level, and you’re
ready to go. In practice, it hardly takes longer than reading this paragraph
- a far cry from a typical GEM.
The platform does not replace the ground board of your Dob. You merely place
the entire telescope on the platform. This will add about six inches to the
overall eyepiece height. Once on the platform, activate the stepper motor and
observe. A red LED indicates the platform is tracking. (Motor noise is present
but not loud.) The Dob still moves in normal alt-az fashion. The only difference
is when you stop moving the scope, the target stays centered. In terms of stability,
I can detect no difference in vibration damping time between my scope being
on the ground or on the platform.
Alignment is not critical for visual observing. If you notice drift, the platform
is not pointing to the north pole. Merely drag the south end of the platform
to the right or left as needed. This can be done with the scope still on the
platform (well, at least for my scope). I can leave the platform running for
the length of its travel and the object will be centered in a low power field
when I return.
Equatorial platforms can track for about one hour before they reach the end
of their travel arc. When this happens you turn off the stepper motor and pull
up on the reset handle. This will tip the platform to the east and it is now
ready for another hour of tracking. This is a very simple and fast operation,
but you will lose whatever happened to be in the eyepiece at that time. While
this might sound like a nuisance, I have not found it to be so yet. Platform
resetting does result in the platform tipping about 7 degrees off horizontal.
If your Dob has a very loose azimuth bearing this could present an issue, for
normal loadings and friction it will not.
Pros
- Delivers tracking at a fraction of the weight, bulk, and cost of a traditional
GEM.
- Extremely portable, easy to store.
- Sets up in less than a minute - every time.
- Self contained, with no extra counterweights or power cords.
- Does not require special plates or rings for mounting the OTA, truss tube
friendly.
- Allows fine optics to work at top magnification.
Cons
- Does not offer true N-S-E-W movements for star-hopping like a traditional
EQ mount. Still the same orientation issues as a regular alt-az mount.
- Moving your observing site to a substantially different latitude will require
a new platform. You won’t be taking this one to the Winter Star Party.
Would I recommend this product? It delivers on all advertised claims. Materials
and workmanship are exceptional. High-power work becomes a joy, not a tedious
tracking task. The platform would also be well suited to public observing sessions.
If you do any observing above 200x, you’re crazy not to own one. It is
one of the essential accessories for a Dob owner.
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