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. Will iOptron introduce HEM mount with lower and higher payload?
    Yes, HEM44 has been released in August, shipping on 11/8/22 in USA. HEM44EC and HEM15 are available for shipping from the factory in Mid-November.

  3. 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.

  4. 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 while AM5 does not. 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, no moving/no wrapping cables, etc. However, RST-135/RST-135E comes with GPS while HEM27/HEM27EC does not.

  5. Why HEM27 does not come with a GPS?
    When designing HEM series, we judged between WiFi and GPS, and chose WiFi instead of GPS. When Commander Lite is in action (WiFi version ASCOM), a GPS module is redundant; one may easily port the GPS info from a smartphone to the mount/hand controller. There is no worry about lacking of internet, WiFi, or cell phone signals in rural areas; Commander Lite can always get GPS info from a smartphone to the HEM mount/hand controller, no dead corner.

  6. 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.

    We are introducing dual-axis strain wave models, HAZ31 and HAZ46, as dedicated AZ mounts, 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. HAZ comes with “Level n Go” function, same as AZ Mount Pro, customers only need to level the mount and power on, the mount can automatically calibrate the directions and ready to goto and tracking.

  7. Will iOptron make dual strain wave gear EQ/AZ mount?
    Yes, we are introducing HAE series strain wave gear mounts, HAE29/HAE29EC and HAE43/HAE43EC. HAE models are EQ/AZ capable, good for 0-90 latitude. Differ from other brands, HAE series are pass- through channel ready for customers putting their cables.

  8. 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.

  9. 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).

  10. Why HEM27EC and how does it function?
    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 for short focal length.

    When guiding strain wave gears, short exposure, e.g., 500ms, is usually suggested. Such short exposure is not sufficient for most stars; hence, it would prevent OAG guiding. However, high precision encoder version often produces the best results with long-cadence guiding, mostly to guide out the low-frequency errors, such as polar alignment error, atmospheric refraction, and flexure, and let the high precision encoder keeping the mount on track. Therefore, with high precision encoder, much longer exposure can be used when guiding, e.g. 5 to 10 seconds. This is especially useful for OAG guiding.

  11. 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.

  12. 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

  13. 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, SkySafari Plus/Pro through WIFI connection
    (5) Raspberry PI via third party INDI driver

  14. 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), They can 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.

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)