Hi DJDD,
Thanks for the post.
Gary Kopff here from Wildcard Innovations, manufacturers of the
Argo Navis Digital Telescope Computer.
Short exposure astrophotography is increasingly popular on
Dobsonian mounts.
One can achieve results both with undriven mounts such as can be
seen in Mike Salway's article at
http://www.iceinspace.com.au/index.p...63,201,0,0,1,0 and
mounts that have a tracking capability such as these stunning
examples of deep space objects on Johannes Brachtendorf's web
site using an Argo Navis and ServoCAT -
http://homepages.uni-tuebingen.de/jo....brachtendorf/
When one looks through the eyepiece of an Alt/Az mount, the
apparent field of view (FOV) will appear to rotate over time.
To overcome this 'field rotation phenomena' when taking
astrophotographs, there are several alternate things one might
consider doing.
The first consideration is to not provide any tracking at
all and to keep the imaging time so short that tracking effects
and field rotation effects don't become pronounced. By
capturing multiple images and using software processing, one can
'stack' the images and in fact some packages can provide for
limited amounts of software field de-rotation.
The next alternative consideration may be to tilt the mount's Az
axis over to align it with the refracted equatorial pole of the
sky. In other words, turn the mount into an equatorial mount.
This has the advantage that the mount need only track in a
single RA axis and there will be no field rotation at the
eyepiece for the camera. This is often achieved by Dob owners by
using an equatorial table. This device not only tilts the scope
over at an angle required for your observing latitude on Earth,
but also has a simple tracking motor that allows the mount to
move in the RA axis.
The next alternative consideration is to retain the Alt/Az
configuration and have tracking in both axes and to fit a field
de-rotator to the eye-piece adapter. This then rotates the
camera in the same direction as the field rotation. This
approach is the most common on professional observatories, the
largest of which tend to use Alt/Az configurations. However,
hardware field de-rotators are not quite so common in the
amateur world, even though they are available.
The next alternative consideration is to retain the Alt/Az
configuration and contine to track in both axes but keep the
exposure times short again. This is the approach used by
Johannes Brachtendorf as cited above.
As has been posted by other respondents, the Intelliscope system
is a PUSHTO system rather than a GOTO system. It falls into a
class of device known as a Digital Setting Circle.
A little historical background. Mechanical setting circles are
nearly as old as the invention of the telescope itself. One
problem they have, however, is that if the mount has what are
known as geometric or flexure "fabrication errors", then their
ability to assist the operator to locate or report a specific
co-ordinate in the sky is compromised.
One example of a geometric fabrication error is when the Az
axis is not at right angles to the Alt (or Dec) axis.
One example of a flexure error is where the optical tube
assembly flexes under gravity.
In the mid-1980's, digital replacements for mechanical setting
circles started to appear for amateur telescopes. However, just
like their mechanical counterparts, so-called "Digital Setting
Circles" (DSC's) could not compensate for the fabrication errors
within the mount.
Also like their mechanical counterparts, they made no provision
to compensate for effects such as atmospheric refraction.
From a technological implementation standpoint, DSC's also used
early generation, low performance 8-bit CPU's, with limited
memory. Due to these hardware limitations, their software was
also very compromised. For example, their ability to compute the
positions of the planets was compromised, resulting in low
precision in locating these types of objects.
Fast forward to the 21st Century and the introduction of the
Argo Navis Digital Telescope Computer (DTC). Unlike mechanical
setting circles or digital setting circles, the DTC falls into a
different class of device.
The Argo Navis DTC has a dual CPU architecture including a high
performance 32-bit CPU with high speed memory and caches. This
platform then allows for highly advanced software and features.
A DTC can take into account effects such as atmospheric
refraction, precession and nutation. It can store a much large
number of objects with much higher positional precision. It has
the computational grunt to not only compute the positions of
planets, comets, asteroids and man-made satellites, but to do so
in 'real-time'. What's more, it can assist the operator in
analysing and potentially compensating for range of common
systematic geometric, flexure and eccentric bearing errors
within the mount/OTA.
One could just purchase an Argo Navis, along with an optical
encoder kit. We provide kits for a range of commercial mounts
including many of the popular generic Dobsonians sold here in
Australia. The kit is very easy to install. With this
combination, one then has the best-in-class PUSHTO solution with
all the benefits of the Argo Navis DTC mentioned above.
To bring motorised tracking to the mount, there are a variety of
options. The most advanced system on the market is undoubtedly
the ServoCAT servo motor controller. If one also purchases a
ServoCAT and a pair of servo motors one then retains the Argo
Navis and in fact interfaces it to the ServoCAT via a
communications cable. Now rather than PUSHO, one has full GOTO
support. The Argo Navis actually performs all of the object
offset and tracking rate calculations on behalf of the ServoCAT.
Just dial up an object and push a button and the mount will slew
and track with all the benefits of having a DTC installed. The
ServoCAT has the smarts to know how to speed up and slow down
the motors by just the right amounts. A bit like getting into a
high quality elevator in a high-rise building. It is very smooth
and quiet.
The ServcoCAT has a price/performance point best matched to
telescopes in the 14" to 48"-plus aperture class.
There are also systems such as the Sidereal Technology
motor controller (Sitech) which again Argo Navis is designed to
control. The Sitech controller is a commercial
embodiment/refinement/spin-off from the Mel Bartel's system one
respondent mentioned. The advantage Argo Navis system has when
used in conjunction with the ServoCAT or Sitech controllers is
that you don't need to lug around a PC or laptop.
The Intelliscope system has no provision for controlling a slew
and track GOTO system and falls into the DSC class of devices.
Even if you are a visual user rather than an astrophotographer,
an Argo Navis DTC PUSHTO solution will assist you in locating and
identifying objects. It is designed and manufactured here in
Australia and is exported the world over.
Argo Navis is used on everything from tiny refractors and binocular mounts
to some very large aperture telescopes.
Argo Navis is even used in some professional observatories, such
as on the largest optical professional telescope in Canada as well as at the
Lowell Observatory in Flagstaff, Arizona. The good news is that
you can readily move it from one scope to another when it comes
time to upgrading your scope.
You can read more about the Argo Navis DTC on our web site -
http://www.wildcard-innovations.com.au/ including find a copy of
the User Manual here -
http://www.wildcard-innovations.com....mentation.html
You can read more about the ServoCAT on StellarCAT's web site
here -
http://www.stellarcat.biz/
Your plan to look through multiple scopes is a good one. On a
personal note, aperture always wins. Recommend you consider the
largest aperture that you are comfortable with as far as cost
and portability. Certainly I would recommend 10" as a starting
point.
If you have any questions, please feel free to contact me by email or phone
if you so desire.
Thanks again for the post.
Best Regards
Gary Kopff
Managing Director
Wildcard Innovations Pty. Ltd.
20 Kilmory Place, Mount Kuring-Gai
NSW. 2080. Australia
Phone +61-2-9457-9049
Fax +61-2-9457-9593
sales@www.wildcard-innovations.com.au
http://www.wildcard-innovations.com.au
is a good one.
and so will keep my reply brief. your provided in the post a wealth of info, which will probably cover most future questions I have. 
Some of the interesting points are the reasons for tracking problems- flexure and geometric errors.
