During the spring of 2008 I was looking for a new equatorial mount. Because I’m mainly involved in astro photography I was

looking for a very stable mount with a good tracking accuracy. In addition, the mount must be able to carry several telescopes with

a large aperture (25 cm or more). Finally I decided to order the virtually unknown "Mesu-Mount at Lucas Mesu of Mesu-Optics1.

The mount was delivered in December 2008 by Lucas Mesu personally. Setting up the complete mount, including telescopes, lasted

less than 10 minutes.

The Mount

The starting point for the development of this equatorial mount was that it should be a mobile mount without meridian flip, for the

advanced amateur astronomer. Furthermore the mount had to carry large size telescopes, preferably more than one, and be able to

track very accurate. This resulted in a divisible mount, the "tripod" including the “RA” drive and the declination house including

“DEC” drive. Both components weigh about 25kg each and are easy to merge.

Further specifications of the mount:

Drive : friction wheels

RA / Dec friction wheels : 30cm diameter

Reduction RA-axis : 3000 : 1

Reduction Dec-axis : 3000 : 1

Capacity : 2 x 65kg

Meridian flip : no

Backlash : no

RA and Dec encoders : 60000 steps through a 1:6 reduction (theoretical resolution = 21,6”/step)

Pointing accuracy : < 30” (provided that the mount is polar aligned)

Periodic error : ± 2”

GOTO speed : software adjustable to ± 8 °/sec

Auto guiding : ST4, guide speed software adjustable

Propulsion : servo motors

Control : GOTO (with hand controller or through PC planetarium program)

Power consumption : average 400mA (guiding 90% of the time)

The mount also has a provision to keep dust and dirt out of it. This is done through a small fan which is slowly rotating and creates

a slight overpressure in the mount. Therefore, the friction wheels remain free of dust and dirt which have a negative impact on a

precise operation of the mount.

Coincidence or not but in the year that the

telescope was invented 400 years ago there

appeared a new mount on the market, made

in the Netherlands. It is a mobile mount for

the demanding amateur both for observing or

imaging with large telescopes from his

backyard as well as from dark remote places.

In this article I would like to discuss my first

experience with this mount.

Rob Kantelberg

www.astro-imaging.com

The Mesu-Mount, a high-end mobile mount

The Mesu-Mount dismantled

The friction drive The axes of the Mesu-Mount are driven through friction2. This drive is chosen for its high efficiency, its smooth course and its high

stiffness. Furthermore, the cost compared to a worm wheel drive is significantly less.

The friction drive for the RA axis consists of a shaft with a diameter of two millimeters which drives, through an ingenious system

of friction wheels, a large friction wheel. The thin shaft itself is driven by a servo motor. The final transfer ratio is 3000:1. This

means that the angular displacement of the telescope, with one revolution of the motor shaft, 432” is. Therefore the telescope can

be positioned very accurate. An encoder is mounted on the servomotor that generates 2000 pulses per revolution. This encoder is

used to control the motor speed through pulse width modulation. This technique results in a very good speed control system which

also consumes much less power than alternatives with stepping motors. This low power consumption is important for mobile use. A

small battery of 7.2 Ah can produce 400mA for 18 hours which is often enough for a whole weekend use without recharging the

battery. Moreover, there was a battery and charger included in the delivery. In use, it’s very pleasant that the system is backlash

free. I previously used systems which had some backlash but these systems have the disadvantage that if the telescope is not

perfectly balanced, the backlash compensation overshoots due to the imbalance. A backlash free system is very pleasant when the

motors have to change direction even in cases of slight imbalance.

Schematic overview of the friction drive

Due to these advantages the ServoCat servo system3 is chosen for the propulsion. The used servo motors of the ServoCat system

have such a large range that speeds of more than 8°/sec can be achieved, this couldn’t be achieved with the stepper motor systems I

looked at. Another nice feature that I use is the spiral search function of the ServoCat. With this function it’s possible to scan large

sky areas in four different ways. To summarize, four main advantages of this servo control:

- Low power consumption, small battery needed.

- Uniform velocity resulting in a very precise guide speed.

- Even without a PC fully functional for use in the field.

- A huge dynamic range in terms of speed (slewing speed is over 2000 times faster then guiding speed).

GOTO

The ServoCat system comes in combination with an Argo Navis GOTO controller4 with more than

29,000 items pre-programmed (Messier, NGC, IC, planets etc). Furthermore, the user can also

program another 1100 items. The ServoCat system can also be connected to the serial bus of a PC in

order to operate the mount through a planetarium program, click on an object press enter and the

mount navigates to the object and places it in the field of view of an eyepiece or CCD camera.

However, in combination with the Argo Navis, the ServoCat is a complete telescope controller

without the need of a PC. The Argo Navis is in its class the most advanced digital setting circle and in

combination with the ServoCat a very suitable controller for Mesu-Mount. The best of all is that the

Argo Navis is easy to use in the dark due to the big selection wheel and two push buttons. Besides a

large number of functions that I have not used yet, I would like to mention the following very useful

features:

- Compensation for atmospheric refraction.

- Compensation for deflection of the telescope tube and other components.

- Useful function in order to align the mount to the celestial pole.

- Entering orbital elements for tracking satellites and comets.

- A very useful feature to identify objects.

Many more institutions are standard in the Argo Navis menu where other telescope control systems need a PC and additional

software. In the context of this article it goes to deep to describe both systems in detail but more information can be found on the

websites of the suppliers. Furthermore, the whole system can be demonstrated by Mesu-Optics.

Periodic error

By applying very carefully produced friction wheels for the RA-axis the mount has a very good intrinsic periodic error of ± 2" (see

graph). This periodic error has a period of approximately 600 seconds which means that auto guiding easily can reduce this to

values below 1". Harmful to a friction drive is dirt on the running surfaces of the friction wheels. To prevent this, the mount is

equipped with a small fan, with an air filter, which creates a light over pressure in the mount ensuring that dirt and dust remains

outside the mount.

Periodic error of the Mesu-Mount

My setup

Some results

With the above setup the following image were made.

M35 en NGC2158 (Imaged made with an 15cm F/6.3 Newton and Canon 10D)

My current telescopes arsenal consists of a 25cm Meade Schmidt-

Cassegrain, a 25cm Newton, a 15cm Newton and a William Optics

72mm Megrez APO. On the photo you see the mount equipped with

the 25cm Meade SCT, 15cm the Newton and the 72mm Megrez (not

visible in this picture) which are, together with various cameras and

other accessories, easily handled by the Mesu-Mount. Because the

Mesu-Mount is transportable and very easy to disassemble / assemble

it is interesting to go to dark places with reasonably large telescopes in

order to observe or to make astro images. You must have a car with

proper space in order to be able to transport everything. With a little

practice it would be able to setup and align the mount on the celestial

pole within half a hour.

M101 (Imaged made with an 25cm F/5 Newton and Canon 10D)

M51 (Imaged made with an 15cm F/6.3 Newton and Canon 10D)

Conclusion The Mesu-Mount fulfilled my expectations completely. Due to its special construction it is much stiffer than a conventional

German mount and it can carry two big telescopes, rather than one big telescope and useless counter-weights. Furthermore, you

suffer no longer from meridian flip at the time that objects are best visible. The servo control is very quiet and uniform. As shown

in the periodic error graph this mount can be qualified as a top mount. Further improvement with respect of periodic error seems to

be pointless because of the seeing which is normally 3 arc-seconds or more. Due to the chosen drive, without backlash, it’s possible

to guide very precise (on seeing level). This might be interesting for the use of large high resolution telescopes but this is not yet

tested. Due to the simple and reliable construction the mount can be setup and aligned in half an hour. In terms of volume the

mount is slightly larger than a conventional mount. In terms of weight the mount is not too heavy looking to the load of 2x65 kg it

can handle. In a not too small car, the mount, telescopes and all other equipment can be transported to dark places.

1 The supplier of the Mesu-Mount is Mesu-Optics : www.mesu-optics.nl

2 A comprehensive comparison of different drive is made by Dr. Frank Melsheimer of DFM Engineering.

The article can be found at www.dfmengineering.com/news_telescope_gearing.html 3 The ServoCat system is made by StellarCat in America, see http://www.stellarcat.com

4 The Argo Navis is made by Wildcard Innovations in Australia, see http://www.wildcard-innovations.com.au