Seeking review/feedback of HEM27 iOptron HEM27 Strain Wave Mount · Rajat Kumar · ... · 8 · 288 · 0

Here was my experience- take it as a single data point. I used one for about 6 months. Had it on the stainless steel tripod that comes with the GEM28. Really liked it, but it couldn't handle my largest imaging rig. It was fine with an 80mm apo and an 8" EdgeHD (guiding was around 0.4-0.6 "), but it couldn't handle my 130mm f/7 TS Photoline with imaging gear. The 80mm rig weighed about 16 lbs, the Edge was around 21 lbs but the 130 was slightly more than 31 lbs. Using a counterweight didn't help- I still got elongated stars across the frame of a 294mm pro. I did some troubleshooting with iOptron, and they felt my bigger imaging rig was too long and heavy for the mount. I then switched to a HEM44, and have been completely happy- this mount handles everything I've thrown at it so far.

So- the HEM27 was great for most imaging rigs, I just had a problem when I went long and heavy. YMMV.

Well,  my limited understanding is that the length of the imaging train plus how far the center axis  (an imaginary line through the rig from front to back at the center of balance) is above the dovetail, together with total weight, is what matters.  Apparently having a heavy triplet objective at one end and an ASIAir Plus,  3" flattener/EFW and camera on the other end makes it difficult for the SWG to handle. 

I don't think the type of mount support would matter, since the issue was only in RA, which suggests to me that it wasn't a tripod issue. But I never had the chance to test on a permanent pier, for example, so I can't definitively rule it out.

And since I didn't test any other HEM27s, then I can't rule out that it was only the copy I had that was problematic. I never got a technical explanation for what was causing the problem- they just finally said that rig was too much for the SWG in the mount. And my problem disappeared when I moved to the bigger mount.

I have an HEM27 with iPolar, and I think it’s a very solid mount with a few oddities/downsides. I also have an AM3, which I prefer. Here are some thoughts after a couple years of use:

• The ergonomics are not great. Compared to the AM3, it’s challenging to fine-tune the polar alignment, and the way it mounts to the tripod relies on hand-threading two screws, which can be misery in cold weather as it’s impossible with gloves. Mounting the AM3 is much simpler.
• DEC seems extremely sensitive to balance. Approximate Z-axis balancing is still needed as well. If you’re coming from a GEM27, you may be already used to this, but being able to not really worry about balance on the AM3 makes it much faster to set up.
• Oddly, I find guiding can be a bit better with my heavier rigs (a C6 with the full set of usual equipment attached). I think the heavier load helps keep constant strain on the gears?
• When guiding, I found success using 90-100% aggressiveness in RA with 0.5-1s exposures (as usual for strain wave mounts) but I use 50% with DEC in “North guiding only” mode. This helps avoid large overcorrection swings if the balance is slightly off.
• The saddle is a little finicky and can sometimes seize under heavy loads.
• I _love_ the DC feed-through on the HEM27 saddle, and miss it on the AM3.
• It seems lighter and smaller than the AM3, despite the AM3’s lower max payload.
• It seems to use slightly less power than the AM3, which is nice if you’re running off batteries.
• When at its best, tracking accuracy seems comparable to the AM3, but rarely better.

I generally like this mount. It’s as good as the AM3 in performance, it just falls a bit short in ease-of-use and the amount of work required to set it up.

Thank you for your reply. It seems all the issues with ergonomics of GEM28 are replicated here in terms of mounting/polar alignment.

Pleiotrope makes a number of very relevant comments.  Here are some of my observations to go with his-

• attaching the mount to a tripod requires using a 3mm allen wrench for the mounting bolts. I bought a 3mm t-handle ball allen wrench for that purpose- having the handle, a 9 inch shaft and the ball head makes it really easy to screw the bolts in and out in any weather

• DEC balance is important as it uses a wormscrew in DEC instead of a SWG (like the Zwo mounts), so it does require a bit more work to set up

• the saddle did bind under a heavy load- it turned out there is a screw on the back of the bottom of the dovetail that serves to prevent >180 rotation- this protects the internal wiring connections (DC and USB) from being twisted. However, when a heavy weight is in the dovetail this screw makes contact with the mount in certain positions. iOptron sent me a replacement that didn't protrude as much, and that fixed the issue (I had forgotten about that)

• with both iOptron mounts, I also use 05-1.0 sec guiding exposure with high aggression levels in both RA and DEC to get between 0.4-0.6" . Typically use 85% for RA and 90% for DEC

I've had a number of iOptron mounts over the years (GEM28, GEM45, HEM27 and HEM44) and I like them because they are reliable, even if a bit finicky now and then. In fact, I think all astro gear is made to some unknown standard that requires oddities/awkardness to be incorporated  !

That's very insightful comment. Thank you very much. I have pulled the trigger and hopefully next week, HEM27 will arrive. I think it should be fine for me as am aware of Ioptron eco-system, if one can call it that. As compared to AM5, HEM owners are a quieter bunch. I think both comments here just summarised all the 'reviews' or user reports out there in the internet.

Initially, I was going to purchase a second hand AM5 but due to some issues have opted a new HEM27. It seems both mounts are more or less the same. Balancing Dec does not bother me, albeit it's slightly more work. My primary motive was to have a room to upgrade the telescope and increase the mobility which SW mounts provide. I hope experience with HEM is significantly better than GEM (which has been very good too). Just a hypothetical question - would you mount a 9.25" SCT or a 8" f/4 newt size-ish scope for AP on the mount? Not that I'm planning to mount these scopes, just trying to imagine the outer limits of payload capacity of the mount.