1100GTO Absolute Encoders -
Installed on 1100GTO-AE and 1100GTO-AEL

Absolute Encoders

In order to meet the needs of the most demanding astro-imagers and researchers, we have integrated the optional Renishaw RESOLUTE Absolute Encoders into the 1100GTO-AE and 1100GTO-AEL mounts. These encoders and readheads comprise a true absolute non-contact optical encoder system, which offers the latest in technological advances to ensure accurate and reliable performance in the field. What will this mean for you? Near perfect periodic error correction and your mount will never be lost, which will result in greater productivity and hassle-free imaging sessions. Your mount will be invisible, as it should be.

Consider these features when the absolute encoders are installed in both axes:

  • Absolute encoder knows the exact position of the gear angle of the mount when the power is applied.
  • No separate homing sensor is needed. Current software sets a home position with the CW down and the scope pointing at the pole - equivalent to the Astro-Physics Park 3 position.
  • The R.A. encoder is used to set the exact sidereal rate to eliminate periodic error of the worm gear. When used in conjunction with our Astro-Physics Command Center (APCC) Pro version software (purchased separately), the sidereal rate can be adjusted in small increments and set to any custom rate that is necessary to counter R.A. and Dec. drift due to flexure, atmospheric refraction, polar misalignment, etc.
  • Virtual elimination of Dec. backlash symptoms when auto-guiding. Absolute encoders will close the loop in both RA and Dec axes for very accurate guiding. The response, especially in Dec will be instant and will eliminate all backlash delays during reversal of the motors. This is important when using a long focal length scope.
  • High immunity to dirt, scratches and light oils to withstand real-world observatory conditions.
  • Sealed readhead for high reliability in harsh environments.
  • Integral set-up LED on readhead for quick set-up and instant “health check" any time.
  • Built-in position-checking algorithm constantly monitors calculations for ultimate safety and reliability.
  • Low temperature version available.

RESOLUTE uses a unique single optical absolute track (a world first) combined with sophisticated optics. The readhead acts like an ultra fast miniature digital camera, taking photos of the coded scale of the encoder ring. Photos are analyzed by a high-speed DSP to determine absolute position. The built-in position check algorithm actively checks every reading. Advanced optics and position determination algorithms are designed to provide low noise and low sub-divisional error. The result is unparalleled performance.


  • One-piece 4" diameter stainless steel ring with a single track, true absolute scale marked directly onto the periphery. High accuracy graduations ensure outstanding metrology and reliability.
  • Resolution: 67 million pulses per rotation, 0.019 arc second per pulse.
  • Accuracy is +- 3 arc seconds over the entire 24 hour, 360 degree rotation. +- 0.2 arc sec per hour typical accuracy.
  • RESOLUTE Absolute Encoder:
    Operating: 0 degrees C to +80 degrees C (32 F to +176 F)
    Storage: -20 degrees C to +80 degrees C (-4 F to +176 F)
  • RESOLUTE ETR (Extended Temperature Range) Encoder:
    Operating: -40 degrees C to +80 degrees C (-40 F to +176 F)<
    Storage: -40 degrees C to +80 degrees C (-40 F to +176 F)
  • Operation down to -40 degrees C (-40 degrees F) in non-condensing environments is guaranteed by Renishaw. However, we have not tested the extreme ends of the range ourselves. The encoder is also tough enough to survive the physical punishment of harsh environments with solid steel ring scales. NOTE: If you plan to operate the mount in severe cold temperature environments, please contact Astro-Physics regarding special lubrication requirements for the 1100GTO that may be needed.

The images below were taken using a 1600GTO mount with the Absolute Encoders installed. As mentioned earlier, the 1100GTO will use the identical encoders, readheads and software. Your results should be similar using either the 1600GTO or 1100GTO.

Unguided exposure: 10 minute (600 seconds)M106

This test exposure was taken with the 175EDF refractor at F8.3 at 1452mm FL, 9 micron pixel size. Click on the thumbnail to view the full image including the enlarged section, which shows the pinpoint star image.

Unguided exposure: 5 minute (300 seconds)

This image is the result of six separate 5 minute unguided test exposures taken with the 175 StarFire EDF. Unguided imaging was necessary because the H-alpha filter basically wiped out all star signals needed for the internal guide chip. One of many reasons that unguided imaging is a great advantage. Click on the thumbnail to view the enlarged image.

Unguided exposure: 10 minute (600 seconds)

This image consists of four ten minute exposures that were done at 1200 mm focal length with an SBIG STL-1100. It was done in Luminence filtration. No separate guiding was done. Click on the thumbnail to view the enlarged image.


We feel that even longer exposures with refractors are feasible since, typically, they only have a tiny amount of flexure compared to mirror scopes. This is particularly true if the object is straight up. However, since many optical systems have inherent flex issues and possible mirror movements and tracking rates vary from one part of the sky to another, APCC (Astro-Physics Command Center Pro version software, purchased separately) provides additional corrections to make unguided imaging a reality for a wide variety of optical systems, assuming that the error is repeatable and can be modeled. Tests at the University of Hawaii with our 3600GTOPE equipped with an R.A. Precision Encoder and using the APCC Pro version with the APPM pointing model (beta version) already have shown that unguided exposures are possible with large Ritchey-Cretien imaging scopes. This will greatly enhance their research program.

Software Controls for the Absolute Encoders

The commands related to the Absolute Encoders can be issued through the easy-to-use interface of the  Astro-Physics Absolute Encoder Utility (Windows-based) provided on a CD-ROM. This program utilizes the Astro-Physics ASCOM V2 driver.

Note: The Astro-Physics Command Center (APCC) software is not included as part of this package. It is available as an optional purchase.

Order Options

1100GTO-AE and 1100GTO-AEL: If you plan to use Absolute Encoders, we encourage you to order your mount with the R.A. and Dec. encoders fully installed and tested prior to shipment.

Absolute Encoder Upgrade Option: If you decide at a later date that the Absolute Encoders will enhance your imaging sessions, you can order the kit to install yourself. This kit will include all of the parts required. The system will be tested as a whole prior to shipment to ensure that all components are functioning properly. In order to complete the assembly, a little manual dexterity is required to position the screws in a restrictive space. We will provide detailed instructions to guide you through this process. This kit will not be available until sometime in 2016.

Back to top

Back to Products

Copyright © 2013-2016, Astro-Physics, Inc. - All Rights Reserved
This page was last modified: October 2, 2017

Astro-Physics, Inc.
11250 Forest Hills Road, Machesney Park, IL  61115, U.S.A.
Phone: 815-282-1513   Fax: 815-282-9847