FAQ: More Information about HEM27

Q & A about HEM27
  1. What does the HEM27 name mean?
    The H stands for Hybrid and Harmonic (Strain Wave gear), EM for Equatorial Mount. The payload of the mount without CW is 13.5 kg, or 29.74 lb. It was initially planned as HEM30, to reflect 30 lbs payload, however, we named it HEM27 leaving some headroom.

  2. Why Hybrid? Is there any advantage/disadvantage?
    The most important and attractive feature of strain wave mount is going counterweight-less for grab-n-go. Strain wave gear works well at imbalanced condition, therefore, a mount made from strain wave gears does not need a CW. Naturally, no CW is necessary on DEC for any equatorial mount, since the payload (telescope) can slide forward/backward easily to achieve self-balance. On the other hand, worm gears naturally have much better PE than strain wave gears, and unlike the solid piece of strain wave gears, the hollow shaft of worm gear structure makes cable management possible. So, a hybrid mount combines the advantages of strain wave and worm gears and can achieve counterweight-less at lower cost. We can then put the savings on to some important features, such as the cable management system and electronic polar scope.

    Strain wave gear has negligible backlash while worm gear varies; some designs without spring loading might have large backlash and others with spring loading backlash free. The DEC unit of HEM27 utilizes belt and spring-loaded gear to achieve zero backlash.

    iOptron might be the only company in the strain wave gear mount business who has a track record making quality and precision telescope mounts (iOptron and its prior entity has a 30-year track record of dedication on telescope mounts, and only telescope mounts). Our design of hybrid strain wave/worm gear is logical and practical with the balance of quality, performance, and price.

  3. What differs the HEM27 from other strain wave mounts in its category?
    HEM27 is very different from other strain wave mounts in its category. HEM27 is hybrid strain wave drive mount; DEC is backlash free worm/belt drive while RA strain wave. If light weight is a primary reason for choosing a strain wave mount, HEM27 is about one third (4 lbs) lighter than the AM5, while about one lb better on payload. HEM27 has a cable management system and PEC function while AM5 has none. Cables of HEM27 are on a stationary platform (no moving/no wrapping), while cables of AM5 rotate with the mount. When power off, telescope/payload on HEM27 stays still while AM5 suffers from scope free swing on DEC.

    HEM27 is almost the same weight and payload capability as RST-135, but less than 50% the cost ($1888 vs. $3895), and HEM27EC 40% less than the RST-135E ($3248 vs. $5495). The price of HEM27 and HEM27EC includes V/D dovetail saddle while RST-135/RST-135E does not. Some features with HEM27 are not available with RST-135, i.e., cable management system, power off brake, power-down memory, PEC, no moving/no wrapping cables, etc. However, RST-135/RST-135E comes with GPS while HEM27/HEM27EC does not.

  4. Why HEM27 is EQ only, will AZ be an option in the future?
    HEM27 is designed as a dedicated EQ mount since the high intrinsic PE of strain wave gear can only be suppressed by autoguiding or high precision encoder. Autoguiding and high precision encoder are not economically feasible for AZ mode. On the other hand, HEM27 comes with cable management system, and AZ mode might compromise the cables through the mount. A multi-function/mode mount sounds good, but there are some compromises to either mode that would prevent the mount to perform the best at either mode.

    If AZ is a primary application, another super portable iOptron mount, the SkyHunter (2.9 lbs with battery, 11 lbs payload) might be a choice at fractional cost. For larger payload, the AZ Mount Pro (13 lbs with CW shaft and battery, 33 lbs payload) is available at much lower cost than the HEM27.

    We will soon introduce a dual-axis strain wave HAZ model as a dedicated AZ mount, primarily for large binoculars and for satellite tracking. Counterweight-less is especially attractive for large binoculars since balancing bino with a traditional AZ mount is really challenging.

  5. Why the payload rating “Max payload (w/o CW) at 200 mm?”
    What really matters to a mount/motor is the torque moment (weight times the distance from the RA rotation center to the telescope center) instead of the weight alone. We can define the mount torque arm as the RA rotation center to the dovetail saddle, and the payload torque arm as the radius of the telescope. The total torque arm relative to the payload is the total of mount and payload torque arms.

    HEM27 has a mount torque arm 90 mm for both Vixen and Losmandy dovetail saddle, therefore, the maximum payload at 200 mm means a telescope of 110 mm radius, or 220 mm diameter. For larger telescope, the maximum payload weight, therefore, should be reduced, or for smaller telescope, increased.

    As a comparison, the mount torque arm of AM5 is 108 mm to Vixen saddle or 113 Losmandy saddle, which leaves less torque arm for telescope/payload.

  6. Will iOptron introduce HEM mount with lower and higher payload?
    Yes, we have other models (with and without high precision encoder) in the pipeline with lower and higher payloads, e.g., 15 lbs and 47 lbs without a counterweight used, to be released in August.

  7. Why HEM27 does not come with PE spec?
    Since the PE of any strain wave drive mount is dictated by the strain wave gear, which is from 3rd party, and dependent on payload, position, and imbalance condition, we don't spec the PE of HEM27 and believe any PE value is inappropriate. On the other hand, almost all strain wave gear mount manufacturers use similar or identical strain wave gears, so one may reference any other mount which comes with a PE value (though we doubt its meaningfulness).

  8. Strain wave gear is very different from traditional gears in term of PE, how your PEC/PPEC works?
    PEC stands for periodical error correction. Permeant PEC means once the PE curve is recorded, it can be saved for PE correction/playback until a new PE curve is recorded. Unlike a conventional telescope mount with worm gears, PEC on strain wave gear might be less efficient and PEC algorithm in practice is much more difficult since the PE curve varies from point to point and condition to condition. With sensors built in the mount to ensure phase matching and some special algorithms, our team could make PEC effective in HEM27.

  9. Why HEM27EC and how it functions?
    Since the strain wave drive is intrinsically with high PE (2-10 times higher than worm gears), autoguiding with short exposure is essential, or a high precision encoder on RA can bring the PE down to a few arc sec. When a high precision encoder is used, the periodical error is corrected in real time by the encoder, or so-called RPEC, and the amplitude of the remaining error is solely dependent upon the encoder accuracy/resolution.

    The high precision encoder used in HEM27EC is the same as in CEM26EC/GEM28EC and should be able to reduce the mount PE significantly, to a level similar to CEM26EC/GEM28EC, and capable for imaging without autoguiding.

  10. What payload HEM27 can undertake with CW applied?
    HEM27 is designed to handle imbalanced payload on RA, up to 13.5 kg. It can also receive optional counterweight for higher payload. We recommend no more than 21 kg payload with proper CW attached.

  11. What is the reduction ratio with the belt and strain wave gear?
    HEM27 uses a belt and strain wave gear in the RA driving train. The belt reduction ratio is 4:1 and strain wave gear 120:1, that give a total reduction 480:1. Higher reduction ratio helps the motor tackling higher payload

  12. Can an HEM27 be controlled by a computer?
    HEM27, as other iOptron mounts, can be controlled by:
    (1) Windows PC via ASCOM/iOptron Commander, through USB or WIFI
    (2) MacOS via direct control/plug-in (SkySafari/The Sky X), or third party INDI driver
    (3) iOS via iOptron Commander Lite, SkySafari Plus/Pro through WIFI connection
    (4) Android via iOptron Commander Lite (pending), SkySafari Plus/Pro through WIFI connection
    (5) Raspberry PI via third party INDI driver

  13. What tripod, pier extension and adapter available for HEM27/HEM27EC?
    HEM27/HEM27EC can seat directly on the CEM26/GEM28 tripod and Carbon Fiber Tripod (8061), Theycan also seat on a Tripier (8034) with an adapter, 8036-026. Customers who have CEM25 tripod may contact us for an adapter which can mate HEM27 with the CEM25 tripod.

  14. Is there an HEM version suitable for low latitude near the equator?
    Yes, we have special version HEM27 and HEM27EC designed for low latitude, 0-22 degrees. They are immediately available at our factory. Special order in USA with 2-month lead time.
    H272L, Low latitude HEM27
    H272AL, Low latitude HEM27 with iPolar
    H274AL, Low latitude HEM27EC


HEM27 Payload Testing
(HEM27 with a C11 Edge HD, ~13 kg)

360 degree display of a HEM27



HEM27EC Autoguiding
(HEM27EC mount, payload 11kg, FL 530mm, iGuider guiding camera with exposure 5s, 20 minutes guiding)