Quote:
Originally Posted by mark3d
how sensitive is the system? does it guide your telescope exactly to the target? or do you still need to then find your target e.g. with a wide angle eyepiece? i just dont think of the dob mount as being a particularly precision mounting  |
Hi Mark,
Thanks for the post.
The 10,000 step encoders provide an effective resolution of around 2.16 arc minutes
per step on each axis.
All things being equal, if the mount were 'perfect' you could ideally guide
down to close to the resolution of the encoders.
However, as you mention, various fabrication errors within the mount/OTA
can increase your pointing error residuals. For example, there are typically various
systematic geometric, gravitational flexure and eccentric bearing errors that
contribute to degrading the raw pointing performance of the mount.
The good news is that Argo Navis has a built-in feature called the Telescope
Pointing Analysis System (TPAS). It is a very sophisticated piece of software
that can analyse and potentially compensate for many of these classical
systematic fabrication errors.
What's more, it reports the results to you in real statistical terms. For example,
it will provide you with the Root Mean Square (RMS) pointing performance of
the mount both before and after corrections have been applied.
Anecdotally, on mount such as your LightBridge 10", you might achieve
a corrected pointing performance of typically somewhere between
3 arc minutes to 6 arc minutes RMS, whole sky.
3 arc minutes RMS whole sky would mean that approx. 68% of objects would
fall within a diameter of twice that, namely 6 arc minutes. Nearly all objects
would fall within a radius of three times the RMS, or a diameter of 18 arc minutes.
Your mileage may be different.
Last week, a customer in Europe who has an home-built 12" Dob
was initially experiencing large pointing error residuals. He then used
his Argo Navis to sample some star positions and to perform a TPAS
analysis. He then applied a TPAS model, after which his pointing
performance improved dramatically. He wrote -
Quote:
|
... the fitted RMS in REVIEW DATA (with the novel five stars) was 3.2’, which I guess is pretty good? I went on to search some small open clusters such as NGC 654 and 659 in Cas with the 8 mm Ethos (150x) and the pointing accuracy was just ASTOUNDING!! Just all objects across the entire sky came really pretty close in the center of the eyepiece. What more do I want?
|
Given they are made of wood, what might surprise you is that the raw
pointing performance, that is the pointing performance before any TPAS
corrections are applied, of some Dobs is sometimes actually significantly better
than some of the expensive CNC machined equatorial mounts.
Why would this be so? The reason is that a common weak link is the operator
when they don't mount the OTA squarely on a GEM, either because the OTA
is not 'square' in the rings or because the dovetail has not be attached
'squarely'. Fortunately, such an error, referred to as Collimation in Hour Angle,
or CH for short, is easily identified and corrected by TPAS.
On mounts such as machined equatorials, once TPAS corrections are applied,
it is not uncommon to see RMS pointing performance effectively become
encoder resolution limited.
If a mount has a non-systematic error, such as a sudden mirror flop, then
no system can correct it. Such problems are best cured at the source.
There are a large number of customers using Argo Navis on 8", 10" & 12"
generic Dobs, including the LightBridge range and we have encoder installation
kits available for them.
Thanks again for the question.
Best Regards
Gary Kopff
Managing Director
Wildcard Innovations Pty. Ltd.
20 Kilmory Place, Mount Kuring-Gai
NSW. 2080. Australia
Phone +61-2-9457-9049
Phone +61-2-9457-9593
sales@wildcard-innovations.com.au
http://www.wildcard-innovations.com.au