Review: A heavy, easy to move mount: the Gemini-41

Figure 1: The author's Celestron 11 on a Gemini G41 mount. Approximately 16 kg of weight is needed to balance the C11.

For some time now I have been working on a automated supernovae search program. A key issue with this type of research is to observe as many galaxies possible in a limited time frame. This means you need a robotic mount that performs well on both slewing and tracking. The CCD-chip covers about 15 min of arc and every slew will have to place the object on chip. In a typical night at least 300-500 slews will be done. The images are not guided as searching for a guide star would cause a lot of overhead. This means the mount will have to track very precise when working at a resolution of 1"/pixel. A low and smooth periodic error is needed.

Everything will be installed in a observatory but each year the mount will be used on an astronomy holiday in the Southern part of France. The mount should be portable or at least semi-portable.

My main target was, to buy a mount capable of:

    * -fair price-quality ratio
    * -capable of robotic operation
    * -able to handle 20 kg of instrument load
    * -periodic error of +- 5”
    * -portable
    * -good customer-service
    * -ASCOM-compliant

Scanning the market

A quick survey rapidly showed that little mounts are in relevant when such demands are required: Paramount, Astrophysics 1200 and 900, Losmandy Titan, Parrallax HDG150, Millenium Mount, Gemini G41, 10 Micron, Celestron CGE, Lichtenknecker M100, E. Alt, ...

After carefully consideration I decided to buy a Gemini G41 Observatory +, a mount made in Hungary with a fair price (+- 7000 EUR complete with accessories and tripod). All other mounts are, when bought in Europe at least, far more expensive, especially when you add all the needed accessories. On top of that certain mounts are hard to get, with long waiting lists as a result.(Astrophysics)

The Paramount, Astrophysics, Titan and Parallax are heavy observatory mounts that cannot be easily transported and that cost more then 12 500 EUR. The 10 Micron runs at about 10 000 EUR and was too expensive. The Losmandy G11 has a mediocre reputation concerning tracking and cannot handle a Celestron 11 or 14. The Celestron CGE has with a periodic error way over 20" not enough accuracy to track well enough for unguided exposures. I am also reluctant concerning the FS2 controller because the functionality is rather limited.

The mount was purchased directly with Gemini in Hungary. The manufacturer is Andras Dan and customer service is nonsense and quite well. Other dealers are Ecliptica in the Netherlands and the duo Telescope-Service and APM-telescopes in Germany. There are also dealers based in Italy and Portugal.
When ordering the mount there is a delivery time of about three months. This seems alot but Astrophysics have a delivery time measured in years, Paramount is about 6 months. About a week before shipment you are asked to pay the invoice in full. Because Hungary now is a EU-member, this can be done with a simple and easy bank-transaction (with IBAN and BIC-code). There are no further costs involved. The VAT-rate in Hungary is 25%.
The mount is delivered without a power supply, written manual and software. The manuals and software can be downloaded on Gemini's website (for free). A good (laptop) power-supply that can deliver 24V and 5A stabilized current can be bought at a computer store for about 60 EUR.

Field or Observatory +?

The G-41 can be ordered in two versions: Field and Observatory. The field-version is lighter (15kg) compared with the Observatory+-version (25kg). This only has consequences in terms of payload that can be carried. The PE accuracy is the same : +- 5"

Figure 2: The field-version of the G41. Have a look at the illuminated polar finder.

According to the manufacturer the Observatory + is capable of carrying a C14 or 30cm Newton, the Field edition would hold a C11 or 25 cm Newton. I am sure this is true visually, but I find a C14 too large for CCD-photography. For a while I had a very long 25 cm Newton installed on the mount that weighed about 27kg. The mount had no problem with the weight, but because of the tube length (1.6m) there were problems using this setup in combination with a CCD. Visually there was no problem at all.

My mount currently carries a Celestron 11 SCT and a 8 cm Orion ED refractor. The mount carries these telescopes very easily. The setup is extremely stable. There is about 20kg of counterweight needed to balance everything.
Bits and bolds

The mount is made out of black anodized aluminum and looks very nice. The two gears (RA and DEC) have a diameter of 217 mm and are made out of bronze. The counterweight bar is made of a 30 mm thick INOX bar and is 50 cm long. The counterweights are made of the same material and weigh 8 kg a piece. Inside them there is a ring to lock them into place without damaging the counterweight bar.

The polar finder is illuminated and has a objective diameter of 30 mm. This is a very nifty tool to quickly align the mount to the pole. The large diameter and large magnification ensures it is easy to see dim stars and align the mount very easy. The polar finder is quite accurate. Drift alignment after alignment with the polar finder shows there is no drift for over 5 minutes at 200x power. This is more then adequate to make guided exposures. If you want a really precise polar alignment you will have to drift align thoroughly.

The stepper motors have 200 steps per revolution. On top of that they are microstepped (8 bit). The motor is connected directly to the drive shaft with a reduction gearbox. In theory each microstep will move the mount about 0.23 ". The mount tracks very smooth, without sudden jumps, what can be clearly seen in the PE-curb. A worm-revolution takes 200 seconds. Do make sure the motors get enough power and have enough torque, otherwise microsteps will be skipped. If the motors do not get enough power you will clearly see the steps at about 600x.

Figure 3: The chart shows about 30 minutes of data. The data was filtered so it would not show RA-drift. The periodic error is about 7 to 8". Large irregularities are caused by wind gusts or a train moving along my house.

No backlash of was observed and the mount reacts well to corrections. The issue with a previous model that a correction in RA would lead to a correction in DEC is clearly solved. The total PE measured with PEMPRO is 8.3 arc seconds, peak to peak.

The Field-version can be ordered with the 'No-Dec Cable', the Observatory+ as this option standard installed. The ‘No Dec Cable’ means that only two cables run from the mount to the controller: the RA-cable and the PE-encoder for the RA-axis. The Dec-cable is within the mount. This is a nice feature for robotic work as this is one cable less that can get stuck and cause damage.

A minor issue in the construction of the mount is the way the motor is connected to the mount. Very, VERY (I think they threw it out of the airplane) rough handling from the parcell service made the DEC-motor dangle loose in stead of being firmly attached when the mount arrived. The manufacturer immediately shipped replacement parts and ensured to deal with the problem in future mounts.

Another problem I observed was retrograde motion in Declination. When you instruct the mount to move North it would first move South a bit. Basically this was caused because the worm was pushed upwards before it would start to rotate. Again the manufacturer quickly dealt with the problem. Some other users also had this problem and some others did not. After the fix the mount shows no retrograde motion or backlash in declination (and RA of course). All current mount models have a new assembly that should deal with both the problems. (http://www.astronomy.hu/update12_2005.htm/)

A tripod to put the mount on is available. The tripod is very firm and stable. There is no bubble level, not on the mount or on the tripod. I use a carpenter bubble to first align the tripod.
The mount is shipped in a very solid and re-usable PE-case. The transport case can hold the mount, counterweight axis and the controller. The mount is hung into the case on a solid bar. This ensures no damage with normal transportation.

The mount has a warranty for 5 years, the electronics 1 year!

Volts and Pulses

Three different controllers are available for this mount: Pulsar, FS2 and Dynostar. FS2 and Dynostar are used for a number of mounts, like the Milenium Mount and the mounts of Eckhard Alt. My choice was the Pulsar that is offered by Gemini itself. This controller has more functionalities then the FS2 controller. For example it can do intelligent pole crossings, swap tubes, home position, auto stops, ... furthermore this controller will retain PEC information in the memory, even if the unit is switched off.

The controller is supplied without a power supply. I found buying a laptop power supply far out the cheapest. The power supply can deliver stabilized 5A 24V power. I paid about 60 EUR which is a lot cheaper then buying a transvertor for a 12 V power supply. The controller is made of two parts: the hand-controller and the drive unit. All cables to the mount and hand controller are leading away from the drive unit. The hand controller is equipped with a small optical joystick. In real life this is a nice feature to surf the moon, especially because the motor speed can be set on progressive acceleration. There are a number of shortcut keys on the hand controller. These allow to do much used functions in a user friendly way. The menus are user friendly and easy in use. A minus is that you have to scroll your way through the object library. This isn't important to me, but I can suspect a number of users will find this irritating. You can also connect a GPS to the system ( I didn't). Autoguiding can be done through RS232 or a Meade autoguider port. With a optional adapter, SBIG users can also use this port.

Pulsar Features

This controller offers a lot of nice features. Next to GOTO and adapted tracking rates (sidereal, King's, Lunar, Solar, user) this controller has some intelligence as well. You can program a home position very easily. This is a parking position of the telescope when not in use. Once power is switched back on, no further initialization is necessary, the mount remembered it's exact location.
Pole crossing will ensure that every slew is done in the most economic way. When possible, slewing will be done over the pole in stead of going round it thus saving a lot of time.
To prevent the tube crashing into the mount an autostop can be programmed. In that case a no-go area is defined. This stop will make sure that GOTO's will not go into this area. There is also a local autostop for sleepy astrophotographers. This function will stop tracking at a certain point.
The swap tube functionality will take the tube to the other side of the mount and point it exactly where is was before. This is really nice when photographing an object for several hours near the meridian. Remind that to get this feature to work you will need a perfectly aligned OTA versus DEC-axis and perfect polar alignment. This takes some time to achieve, but the result is well worth the effort.

PE can be programmed both manual and from the PC. The PC Pulsar software enables to download the PE-table so enhancements can be made. Pempro can also be used to program PE.

Standard only the Messier-database in included and about 40 reference stars. There is room to put the entire NGC-catalogue and 2000 user-defined objects. They can be uploaded with a PC to the controller and are available at the Gemini website.

The manual could be more elaborate, because some features are not clear straight away. The controller firmware is updatable and regulary new versions and bug fixes are released.

ASCOM ?

ASCOM is an acronym of Astronomy Common Object Model and has been for some years a standard in controlling mounts and focussers. This package will hide the technical complexity of a particular mount for a programmer. The same command will then have the same effect on all mounts. The old ways would require a different set of commands for every specific mount. This enables everybody with basic programming skills to write a simple but effective program to control a wide range of mounts.
It's a pity CCD-camera's are not yet supported by ASCOM!

The G41 will support ASCOM, though not directly. The G41 does accept most basic commands of the LX200. An added command set is available to do what an LX200 is not capable off. Because the LX200 is ASCOM-compliant you can use every planetarium program like Cartes du Ciel or The Sky. I personally use the shareware package Cartes du Ciel.

In conclusion

The Gemini 41 is a middle heavy mount with excellent features and great precision. The periodic error is sufficiently small to allow for unguided photography and can handle heavy Newton and SCT-telescope with great ease. There are some minor issues with the software and hardware, but problems are addressed quickly and thoroughly by the manufacturer. I feel this is one of the few mounts that are currently available within this price range that can perform like this and can easily be moved to dark sites. I have been using this mount for more then 3 years now and I am still very happy with it ! Currently the mount is holding a heavy 10" Newton and doing a great job on variable star photography.