Dob driver and Roboscope

After years observing with an equatorial mounting, I could not get use to the feeling of hand guiding. For me, tracking is not only a significant comfort but also an effective means to perceive fine details thanks to the concentration on the object observed and not on its tracking. Two possibilities to solve the problem are an equatorial table or a motorization in alt/azimuth. Of course, the two solutions can be homemade or bought.

To buy an equatorial platform, one can go to Equatorialplatforms or get a custom unit built to your latitude by John Lightholder. If one wishes to make an EP, one can go to see the realization of Jean Marc Becker who is a model of ingeniousness, as all his creation. The motorization in alt/azimuth can be made with some knowledge in electronics and then controlled by a notebook computer. On Mel's website, it's possible to find all the necessary elements. Olivier Lobet, Thierry Restout and Cyrille Tessier currently have developed a project of motorization and an automatic pointing system. I choose the System Tech 2000 that offers an evolutionary and compact design advantage once installed. The purchase in the USA involves a 7% of custom charge and 19.6% of VAT, it should not be forgotten that these taxes also apply to the shipping costs.

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The system is delivered with a documentation of its use and a documentation of the installation, which seems complete, but some delicate points are not described at all. There are two versions of the DDII, the basic Dob Driver and the more elaborate system, which requires a portable computer, the Roboscope. Dob driver is composed of a hand control to drive the motors. It's simple to use, just follow the object wanted by activating the buttons of the hand paddle and during a few moments the integrated calculator sends the orders of follow-up to the motors. This basic system can be enough for people who already have digital setting circles for pointing or for who wish to continue using the starhop method while having the scope track the object.

The Roboscope is sold by Tech 2000 to drive the alt/Azimuth motors. It allows for the calibration of the steps of the motors and the " backlash " or in other words the play coming from the motors as well as the telescope mounting. An initialization on just two stars is all that is needed to use the function " goto " using catalogues (Messier, NGC, IC, King, Caldwell, User) or while entering an objects co-ordinates. The system uses DOS software, which does not require a powerful machine to run (a 386sx is enough), the connection is made using the parallel port using a long cable. I personally use a Dell320N+, with a 386 whose lighting of the matrix was dead so Thierry installed a green LED, which gives an ideal light at night. These types of portables can be found at a small price in second hand stores.

An English document describes some problems, which can decrease the accuracy of the software for pointing and tracking. The pointing accuracy of the software is supposed to be close to the hundredth of degree! Of course, that does not take account of the mechanical inaccuracies. The use of the software is very simple for any person with some English notions. We asked the editor to have sources of the software or at least for the text files to be able to translate it in French, but alas the answer of the editor was negative.

I worked out a file of stars to use as reference marks. It consists of brilliant stars distributed uniformly over the sky. That allows a precise centering if the need is felt and feel free to write me and I can send it to you! (6Ko) or download it here.

First of all, it is necessary to set the frequency and the repetition delay of the keyboard. If the frequency is not high enough or too high, the motors will have a nonlinear operation at the time of initialization on the two stars. Tech 2000 indicates that it does not influence the quality of pointing and initialization, but our experiment showed the opposite. If one deals with a computer in which the BIOS does not make it possible to regulate speeds and times, it is necessary to integrate the following line in the AUTOEXEC.BAT: MODE CON: RATE=32 DELAY=4.

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The first experiments were failures because the documentation does not approach the purely mechanical part of the installation. First of all, the system is delivered with steel casters without bearings; they are completely unusable as much in azimuth as in altitude. Several months of development and tests enabled us to discuss all the critical items and to have a very good result, i.e. a good tracking and a nearly perfect pointing using the goto function. The hardest part relates to the azimuth. It is necessary to have a flat and rigid base to be used for tracking.

After some testing of various materials, we chose an aluminum plate of 1.5mm thickness glued with adhesive neoprene on the base. We reinforced the base with a strong plywood for Thierry’s scope (54mm) and with aluminum angles between the feet for mine. The significant pressure caused by the motors and the casters tend to bend the base when the pressure is between the feet of the base; it is crucial to reduce the flexion as much as possible.

A base of composite polystyrene foam and plywood can give the same result but it is necessary to use a strong thickness of polystyrene. The pressure of the tracking roller marks the aluminum’s surface but it does not destroy it (like it does with Formica).

Thierry made casters of quality for us with roller bearings. The needle bearings with double cage allow a rotation without to much friction, even on the short rotation of the azimuth base. The casters are inserted with a pressure in a grove made with a router. The cutting accuracy during the installation is important in order to have the base and the fork parallel. Needless to say, ordinary tracking does not required all these things nor these casters. Simple Teflon pads fixed on the base are perfect.

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On the photo Ci above, one can note one of the four circular Teflon pads fixed inside the fork to prevent any possible drift of the mirror box.

The pivot is also a crucial point that is not approached in the documentation of Tech2000; it is absolutely necessary to reduce the play as much as possible and to avoid any friction, which might prevent the good counting of the driving steps. First of all we used an aluminum stage with a stainless pivot but the best results were brought by a stage made out of Teflon and the stainless pivot with Teflon grease lubrication.

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It's important to take care and maintain good parallelism of the mirror box with the fork so the optical tube runs vertically, the software can compensate for an error with a function of centering which enables it to make the average but, it is clear that a good orthogonal will give a much better precision of pointing.

The altitude motor gave us less problems; we used a granulose material for the bearing and traditional Teflon pads, following the rule of one kilogram per cm2 of Teflon pads. Thereafter Thierry employed roller bearings to replace the Teflon and that removed any effect of stickiness and gave an instantaneous response to the requests of the motors. On the other hand the laminating tends to be destroyed under the action of the. We thought of replacing it with an aluminum band but the difficulties of implementation (rectilinear cut of the aluminum, bending, joining always tough with aluminum) led us to seek another material. According to a friend we chose ertalon (or polyamide or PA6, to see list of the plastics). We found it in the form of plate of 1m X 2m and 1mm thickness for a price of 330FF. It is a flexible material but extremely resistant to wrenching and compression. The contact with the wheel of drive made out of crenellated stainless is perfect. Careful however, there is a certain adherence with the Teflon which does not make it a good material for a manual operation, on the 406mm of Thierry who has roller bearings in altitude that does not give any problem, on my 508mm with the Teflon pads I have to use polyamide only on the motor side, on the other I kept the laminated. To cut polyamide blade we used a large cutter guided by a rule. First surprised, after the cut of the first blade, the blade of the cutter was dead; the polyamide has an exceptional resistance. Joining with neoprene is easy and reliable, a good choice is the neoprene gel which allows an accurate positioning of the blade; it is also easier to put the glue on the blade before the adhesive obliges with a joining " contact ", when it is still "soft" positioning is simpler. The tests are really good; the motors react instantaneously with a very reduced inertia compared to the laminated. During a bad handling, I pushed the tube whereas the motor was not disconnected; I was very surprised to note that the polyamide was not even striped by the stainless wheel! We also discovered an unexpected effect with this material, the soundproofing. Until now the noise of the motor was propagated in the pivot and came to develop in the scope, the polyamide prevents this propagation of the sound and makes the motors quiet. We found only good points while using this material, so we will test it on the base should not improve the tracking but the noise could be better compared to aluminum (the Tech 2000 is not very noisy but it is even better if we have a minimal noise to celestial wonders).

Once all these mechanical improvements are made, the automatic pointing accuracy becomes very good. The objects are almost always in the center of the field, regardless of where the telescope is pointed in the sky. Only when pointing very lowon the horizon is the error significant. This residual error has its cause in refraction and also can be of a mechanical nature. In the same way, one should not forget to center an object before setting out again towards another object, or else these errors will compound. Slewing becomes very smooth, high magnification can be used without the steps of the motors being perceived. However, take care to set the maximum speed of the motor with the dob driver before effectively using the roboscope, thispoint is crucial !!

To allow an easy use of the portable computer Thierry manufactured a shelter on the telescope. It follows the rotation of the instrument and avoids having any wires on the ground. The risks of breakage are much more significant when wires are on the ground. A numerical keypad extension enables one to make centering by using the directional arrows, that avoids having to transport the portable to the top of the ladder.

The notebook is well protected from moisture and the cold under a plastic sheet. All is removed easily in the same way as a standard truss is attached to the diagonal cage.