GTOCP5 Control System
We have developed a new control system which combines the precision performance of Renishaw Absolute Encoders with the whisper-quiet motion of brushless micro-step servo motors to produce a very accurate observatory-grade mount that can also be set up in just a few minutes in the field. The mount can be operated manually via the clutches or electronically with the encoder loop providing the pointing and tracking functions.
- Use of Absolute Encoders means that the mount always knows where the axes are pointing and don't need to establish home on power-up.
- The mount can be operated manually via the clutches or in full GoTo mode via keypad or external software.
- Absolute Encoders are dead accurate in slewing, pointing, tracking, guiding. Periodic error and backlash, reversal delays in Dec are eliminated.
- Rapid feed-forward servo loop stiffens the axes against outside disturbances such as cable drag and wind loading.
- Both axes can be run at custom tracking rates from sub-arc second motions per hour to 1000x sidereal.
- The GTOCP5 brushless servo system can be run from 12 volts to 24 volts with max slewing from 1200X (5 deg/sec) to 1800X (7.5 deg/sec).
- The power to the motors is self-limiting and cannot cause any kind of damage to the windings or the electronics, even when in a stalled state.
GTO Keypad (optional purchase)
The Keypad is a self-contained computer that allows you to control all essential telescope functions and includes an extensive database of objects. Many people do not want to bother with a computer in the field and like the handiness of the Keypad and its ability to operate in very cold temperatures. The Keypad is an ideal choice for these users.
We have designed the interface and firmware to be as intuitive and easy to use as possible, while filling it with terrific features to make your observing session productive and pleasurable.
Optimized for Astronomy
Astro-Physics mounts and the Astro-Physics GTO Servo Control System are designed from the outset as tools for celestial observing and imaging. These mounts do not have any military or defense applications. Applications like missile, airplane, or very low earth orbit satellite tracking require faster tracking rates and more rapid changes to those tracking rates than our mounts allow. Note that the ISS is generally within the mount’s tracking rate capabilities.
Mach2GTO and Renishaw Absolute Encoders
Mach2 Tracking Performance with Absolute Encoders - PE = 0.21 arc seconds
All of our 1100GTO and 1600GTO mounts with Absolute Encoders have always used the Renishaw Resolute Absolute Encoders for precise positioning and tracking. Now our newest mount, the Mach2 has Renishaw encoders built into every mount. What are the main differences between the Renishaw and other less expensive relative encoders?
Lab Certified Accuracy!
The Renishaw encoder accuracy is verified against a laboratory standard. Each ring comes certified with its own calibration measurement. The maximum guaranteed error on these rings translates to less than 1/2 arc second per hour tracking error.
Highest Resolution for Unparalleled Tracking Accuracy!
The Renishaw Resolute read-head interpolates a special barcode that is engraved on the matching stainless steel ring into 67 million individual addresses. Every address is unique and fully reproducible down to sub-arc second levels. This is not possible with any relative encoder system.
Ripple and Resonances – NOT with Astro-Physics!!
Relative encoders use a simple ring that has engraved marks with separations on the order of 60 to 100 arc seconds between ticks. In order to achieve sub-arc second resolution, the gaps between these marks are filled in electronically by a method known as interpolation. Even the best interpolation methods have errors on the order of 5% (3 to 5 arc seconds). When used in a telescope drive system these relative encoders introduce a fast moving ripple in the RA tracking rate of 3 to 5 arc seconds, although the average or RMS value of this error can be quite low.
These peak excursions cause stars to move slightly back and forth in the RA direction during sidereal tracking. This tracking ripple can remain hidden when imaging with short wide-field scopes, but will manifest itself when using long focus instruments for high resolution work. It is generally a fast moving error, so it cannot be guided out.
No Sub-divisional Error!
Because of the ever present interpolation error (SDE) with low cost relative encoders, we decided long ago that we would not use them. Renishaw absolute encoders use a different type of interpolation system that smooths out the SDE to a level that is not detectable. There is no mechanism for a relative encoder to remove the SDE.
Never Needs Homing. Simply Polar Align; Power Up; and GO!!
An absolute encoder never needs to be homed. It is always home the minute power is applied. It always knows where it is and transmits the exact shaft position to the control box at all times.
Dependability to Astro-Physics Standards!
We have used Renishaw encoders for many years on our larger mounts, both in portable setups and in observatory remote installations. They have been totally reliable. Renishaw encoders are also considered to be the gold standard in the machine tool and robotics industries where precision is required. Yes, they are much more expensive than a simple relative encoder, but for the type of accuracy required in a telescope mount, there is nothing out there that comes close. We know, we have been evaluating options for years.
Benefits of Absolute Encoders
The utility of absolute encoders varies from brand to brand. Some manufacturers make very limited use of encoders and use relative (not absolute) encoders to provide a potential benefit to tracking and nothing else. In regards to premium mounts, assuming dual on-axis absolute encoders, the benefits may be:
- The mount always knows where it is pointed regardless of power loss, movement of the axes when the power is off, bumping the mount or wind movement.
- Homing positions can be set to whatever orientation a user wants, without limitation.
- No rehoming is required for remote locations should there be a power loss.
- Periodic error is fully corrected. No need for occasional PE measurement curves.
- Zero backlash in both axes for precision guiding.
- Dual axis absolute encoders allow for variable tracking that automatically adjusts for refraction parameters.
- Very accurate pointing makes it easy to find very faint objects.
- Very precise tracking such that unguided imaging may be accomplished with an appropriate optical/imaging setup.
- Very precise guiding, if needed, with instant response in both axes. Guiding is only needed to correct for optical/imaging train issues, not mount issues.
- Corrects for polar misalignment and repeatable mechanical flexures.
- Very precise dual-axis tracking is very important for high precision tracking on objects like comets, asteroids, artificial satellites, where the mounts use a "self-guiding" function based on orbital elements.