Jump to content

  •  

- - - - -

Gemini G41


Discuss this article in our forums
For some time now I have been working on an 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 20' 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 2"/pixel. A low and smooth periodic error is needed.

Everything will be installed in an 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

Figure 1: The 25 cm Newton and 8cm Orion ED are mounted on the Gemini G41 by means of heavy rings made by Parallax. Approximately 27 kg of weight is needed to balance the mount.

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

  • fair price-quality ratio
  • capable of robotic operation
  • able to handle 30 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, Parallax HDG150, Millennium Mount, Gemini G41, 10 Micron, Celestron CGE, Lichtenknecker M100, E. Alt, ...

After careful 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. 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 that is provided with some mounts as PEC is lost when the power is interrupted.

Figure 2: The mount without telescope. The grey weight weighs 11 kg.

The mount was purchased directly with Gemini in Hungary. The manufacturer is Andras Dan and customer service is 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 back-order of about three months. This seems a lot but Astrophysics have a back-order 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 also no more problems with customs. 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).

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 has consequences in terms of payload that can be carried. The PE accuracy is the same: +- 5"

Figure 3: 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.

My mount currently carries a 25cm Newton in a Pertinax-tube (16kg) and a 8 cm Orion ED refractor. Everything is firmly held together with a set of very strong Parallax tube rings, supported by a INOX-plate. The tube is 1.6 meters of length, that's already quite a lot. Everything added I need 27 kg of counterweight to balance the rig. Everything is rock solid,

even with strong winds.
Item Field Observatory+
Own weight 16 kg 25 kg
Loading capacity 20 kg 30 kg
Periodic Error +- 5”
Gear size 217 mm bronze
Counterweight Axis 30 mm
Controller Pulsar, FS2, Dynostar
PEC encoder Option Standard
Without DEC-cable Option Standard

Motors stepping motors with 200 steps/revolution

Polar finder 12 x 30 illuminated
Power Supply 12-24V min 5A not included
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.

The stepper motors have 200 steps per revolution. On top of that they are micro stepped (8 bit). The motor is connected directly to the drive shaft with a reduction gearbox. In theory each micro step will move the mount about 1". The seems a lot, but due to mechanical slowness, none of this can be observed. The mount tracks very smooth, without sudden jumps, what can be clearly seen in the PE-curb. A worm-revolution takes 200 seconds.

Figure 4: The curb shows 2 worm revolutions. The periodic error is about 7 to 8". In the middle a fluctuation can be observed. This was probably caused by a wind gust or other vibration and is non periodic. A small amount of RA-drift can also be seen. This was caused because the mount was not properly polar aligned.

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 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 parcel service

made the DEC-motor dangle loose instead of being firmly attached when the mount arrived. The manufacturer immediately shipped replacement parts and ensured to deal with the problem in future mounts.

A tripod to put the mount on is available. I did not order it and so I cannot tell much about it. From what I am hearing from other users it would be a very firm and stable 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. Truly a wonderful case and definitely worth the money.

The mount is under 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 Millennium 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). Auto guiding can be done through RS232 or a Meade auto guider 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 auto stop 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.

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.

ASCOM?

ASCOM is an acronym of Astronomy Common Object Model and has been for some years a standard in controlling mounts and focusers. 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 Carts due Ceil or The Sky. I personally use the shareware package Carts du Ciel. The controller firmware is updatable and regularly new versions and bug fixes are released.

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. 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.




0 Comments



Cloudy Nights LLC
Cloudy Nights Sponsor: Astronomics