Argo Navis
The 12, 16 and 20-inch telescopes are driven by Argo Navis and ServoCAT . What are my experiences with these systems? In short, very good and I’ve become quite dependent on them. Although there are quite some digital setting circle systems around, the Argo Navis is probably the most advanced and powerful system. One strength of the Argo Navis system is its very intuitively operation and user friendliness. The other, and perhaps strongest point lies in a system called TPAS, which stands for Telescope Pointing Analysis System. What’s that? The superb manual explains it all very well, so just some short notes here.

Any mount has its systematic faults. For instance, the mechanical Azimuth axis may not fall exactly together with the optical axis. This will result in a less than optimal pointing accuracy. And with 'optimal' pointing accuracy I mean the ability to select a deep sky object and find it with the Argo Navis system, while using a ~200x-magnifying eyepiece and get the target close to the center of this eyepiece. And then repeat this for another target that is located at a very different portion of the sky. Actually, when that works out properly, it’s really a lot of fun!

For the Argo Navis to work properly, you have to align on two stars, which is just as the usual procedure of many go-to systems. The TPAS, however, uses an algorithm that analyzes a larger number of 'alignment' points in the sky that are collected during a longer session. So, instead of the two stars, you now align the telescope on five to ten stars. The TPAS algorithm analyses these points and assigns a statistical factor, that defines and describes the error, based on the collection of aligned stars. The next thing you do now is to define several models. For instance, the TPAS can compute the systematic error of the telescope in Azimuth. Or, in another model, TPAS can compute the systematic error of the telescope in Altitude. Upon accepting the resulting statistical factor the computer again calculates the distribution of the aligned stars. So suppose there is indeed a systematic error in the Azimuth mounting. After computing and applying the Azimuth factor, the pointing accuracy of the telescope will improve dramatically. And this will also be the case with a computed Altitude factor after using a different, Altitude related model. Playing a bit with these different models very quickly reduces the pointing error of the telescope to encoder resolution and that means within a few bow minutes! After having installed the Argo Navis on a number of telescopes I built, I now know more or less what the critical steps are (see below) and I've been able to obtain very low systematic errors in the mounts. All the same, at the beginning of each observing session, I always perform a short (5 to 7 stars) alignment, run TPAS and thereby achieve quickly a high level of pointing accuracy.

Installation tricks
So, after explaining what I think are strong points of the system, are there any weaknesses? Sure, one "drawback" is that you need to craft the telescope very carefully to achieve a good pointing accuracy. Of course this has little to do with the Argo Navis system, and everything with your construction abilities. An example. As said, the optical axes have to precisely coincide with the mechanical axes. But how precise is that? Is a 3 mm deviation on a total length of 2000 mm acceptable? Particularly with wood it doesn’t get any better I guess. But just look at the two pictures. Even after painstaking carpentry the laser light falls outside the hole in which the Argo Navis encoder sinks (below, middle). So, the optical axis (as illustrated by the laser light hitting the centre of the mirror, below, left) is at that point ~4 mm off the mechanical axis (below, middle), which coincides with the hole in the pivot bold. It needed a couple of adjustments in the mount to have the axes fall together, not completely (below, right), but pretty close.

But what if this mounting error persists or is not noticed at all? It will probably result in less good pointing accuracy. And even then it will probably show up in a systematic error when TPAS is used and the computer can compensate for that.

Another example of a mounting issue. It can happen that upon moving the telescope you observe no changing encoderstep counts on the Argo Navis computer. The manual states that this is almost always due to encoder slippage. What’s that? And what's the usual cause? Most often there is some torsion on the tangent arms that are attached to the encoders. That in turn causes tension on the encoders themselves and they stop counting. Personally I find the easiest solution to simply not fix the tangent arms too tight with the delivered screws. Leaving the possibility to just move up and down a tiny bit is sufficient to release the tension and avoid encoder slippage.

One exception may be the Az encoder system. Look at the picture of the Azimuth pivot bold of the Argo Navis (below, right). The bold fits into a nut that is welded to a metal plate (below, right). Now it’s probably impossible to weld a nut really square--square to a metal plate and this indeed appears to be happening. When the nut is slightly not rectangular with respect to the groundplate, the bold is neither and nor is the encoder axis. So, when the scope moves around, the tangent arm will go up (below, left) and down (below, middle) a bit, when not fixed to the bottom. But when fixed to the bottom, there is inevitably some torsion on the tangent arm, which causes encoder slippage. When I found this out, I made sort of push-pull system (below, right) and got the pivot bold square in relation to the ground board. That worked perfectly well!

So the bottom line is, to get as much out of the Argo Navis system as it deserves, careful construction of the telescope is an absolute must!

Finally, the contacts with Gary Knopff (of Wildcard, the company that makes the Argo Navis) are always a pleasure. He’s always very helpful and responsive.

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